16
Pestic. Sci. 1997, 51, 115È130 Persistence and Mobilit y of Tebufenozide in Forest Litter and Soil Ecos y stems under Field and Laborator y Conditions K. M. S. Sundaram Natural Resources Canada, Canadian Forest Service, 1219 Queen Street East, PO Box 490, Sault Ste. Marie, Ontario, Canada P6A 5M7 (Received 9 April 1996 ; revised version received 28 October 1996 ; accepted 24 April 1997) Abstract : A Ðeld microcosm study was conducted to determine persistence of tebufenozide, an insect growth regulator, in sandy litter and soil. Litter and soil plots (c.4É5m2 each) were sprayed with an aqueous suspension concentrate for- mulation of tebufenozide at rates of 35, 70 and 140 g AI ha~1. Samples were collected at intervals up to 408 days after spraying, and analyzed for tebufeno- zide residues. The data were subjected to regression analysis and half-life (DT 50 , the time required for 50% of the initial residues to disappear) values were com- puted. The was c. 62 days for both substrates treated with the two lower DT 50 dosage rates. At the highest dosage rate, the was 115 days for the litter and DT 50 c. 52 days for the soil, indicating irregular variations in persistence. Downward movement in soil occurred only in trace amounts, suggesting strong adsorption. Laboratory microcosm studies were conducted to investigate the relative importance of rainfall, exposure to light and volatilization on persistence. Verti- cal movement occurred in litter and soil (both sandy and clay types) during rainfall. The amount moved increased with the amount of rainfall, but decreased with the rain-free period. The larger the rain droplets, the greater the downward movement. When the rainwater could move laterally along the surface of the substrate (as would occur on a slope), more lateral movement than vertical movement of tebufenozide occurred. The photolysis study indicated that disap- pearance of tebufenozide was directly related to the duration of exposure to radi- ation and radiation intensity. Volatilization of tebufenozide depended upon the ambient temperature and the duration of air passing through the substrates. Nonetheless, the amount lost by volatilization was much lower than the amount lost after rainfall or exposure to radiation, thus indicating the greater inÑuence of rainfall and sunlight on persistence. In the laboratory microcosm studies, more tebufenozide was lost from the sandy substrates than from the clay substrates. This behaviour was attributed to the greater adsorptive capacity of the clay substrates, thus providing a greater protection against downward mobility and loss due to radiation. Pestic Sci., 51, 115È130, 1997 No. of Figures : 5. No. of Tables : 9. No. of Refs : 16 Key words : tebufenozide, soil, mobility, persistence 1 INTRODUCTION Sustainable development of forestry resources is vital to CanadaÏs economic growth. Protection of these resources requires the use of insecticides to reduce losses caused by insect pests. E†orts are being undertaken to discover and implement selective insect-control methods that are environmentally acceptable, instead of relying on conventional broad-spectrum insecticides. Insect growth regulators have some selectivity, and these com- pounds include juvenile hormones, anti-juvenile hormone analogs, moulting hormone (ecdysone) agon- ists and chitin synthesis inhibitors. The hydrazine derivative, tebufenozide (“MimicÏ ; RH- 5992 ; N-tert-butyl-N@-(4-ethylbenzoyl)-3,5-dimethyl- benzohydrazide) is a novel type of insect growth regulator interfering with the moulting process of lepi- dopteran insects. When ingested, it acts as an agonist 115 1997 SCI. Pestic. Sci. 0031-613X/97/$17.50. Printed in Great Britain (

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Page 1: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

Pestic. Sci. 1997, 51, 115È130

Persistence and Mobility of Tebufenozide inForest Litter and Soil Ecosystems under Field andLaboratory ConditionsK. M. S. SundaramNatural Resources Canada, Canadian Forest Service, 1219 Queen Street East, PO Box 490,Sault Ste. Marie, Ontario, Canada P6A 5M7(Received 9 April 1996 ; revised version received 28 October 1996 ; accepted 24 April 1997)

Abstract : A Ðeld microcosm study was conducted to determine persistence oftebufenozide, an insect growth regulator, in sandy litter and soil. Litter and soilplots (c. 4É5 m2 each) were sprayed with an aqueous suspension concentrate for-mulation of tebufenozide at rates of 35, 70 and 140 g AI ha~1. Samples werecollected at intervals up to 408 days after spraying, and analyzed for tebufeno-zide residues. The data were subjected to regression analysis and half-life (DT50 ,the time required for 50% of the initial residues to disappear) values were com-puted. The was c. 62 days for both substrates treated with the two lowerDT50dosage rates. At the highest dosage rate, the was 115 days for the litter andDT50c. 52 days for the soil, indicating irregular variations in persistence. Downwardmovement in soil occurred only in trace amounts, suggesting strong adsorption.

Laboratory microcosm studies were conducted to investigate the relativeimportance of rainfall, exposure to light and volatilization on persistence. Verti-cal movement occurred in litter and soil (both sandy and clay types) duringrainfall. The amount moved increased with the amount of rainfall, but decreasedwith the rain-free period. The larger the rain droplets, the greater the downwardmovement. When the rainwater could move laterally along the surface of thesubstrate (as would occur on a slope), more lateral movement than verticalmovement of tebufenozide occurred. The photolysis study indicated that disap-pearance of tebufenozide was directly related to the duration of exposure to radi-ation and radiation intensity. Volatilization of tebufenozide depended upon theambient temperature and the duration of air passing through the substrates.Nonetheless, the amount lost by volatilization was much lower than the amountlost after rainfall or exposure to radiation, thus indicating the greater inÑuence ofrainfall and sunlight on persistence.

In the laboratory microcosm studies, more tebufenozide was lost from thesandy substrates than from the clay substrates. This behaviour was attributed tothe greater adsorptive capacity of the clay substrates, thus providing a greaterprotection against downward mobility and loss due to radiation.

Pestic Sci., 51, 115È130, 1997No. of Figures : 5. No. of Tables : 9. No. of Refs : 16

Key words : tebufenozide, soil, mobility, persistence

1 INTRODUCTION

Sustainable development of forestry resources is vital toCanadaÏs economic growth. Protection of theseresources requires the use of insecticides to reduce lossescaused by insect pests. E†orts are being undertaken todiscover and implement selective insect-control methodsthat are environmentally acceptable, instead of relyingon conventional broad-spectrum insecticides. Insect

growth regulators have some selectivity, and these com-pounds include juvenile hormones, anti-juvenilehormone analogs, moulting hormone (ecdysone) agon-ists and chitin synthesis inhibitors.

The hydrazine derivative, tebufenozide (“MimicÏ ; RH-5992 ; N-tert-butyl-N@-(4-ethylbenzoyl)-3,5-dimethyl-benzohydrazide) is a novel type of insect growthregulator interfering with the moulting process of lepi-dopteran insects. When ingested, it acts as an agonist

1151997 SCI. Pestic. Sci. 0031-613X/97/$17.50. Printed in Great Britain(

Page 2: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

116 K. M. S. Sundaram

or mimic of the insect moulting hormone, 20-hydroxyecdysone, by causing cessation of feeding,premature ecdysis and eventual death. The material wasfound to be lepidopteran-speciÐc in laboratorystudies.1,2 This property makes tebufenozide a desirableinsecticide for forestry use to manage lepidopteraninsect populations. In laboratory studies, tebufenozidehas shown appreciable toxicity to the eastern sprucebudworm,3 Choristoneura fumiferana (Clem.), a defolia-tor that causes considerable damage to the spruceÈÐrforests in eastern North America. Recent aerial Ðeldtrials have also shown that the material is e†ective incontrolling the spruce budworm (Cadogan, pers.commun.).

Before an insecticide can be used operationally inCanada, it must be registered under the Pest ControlProducts Act (PCPA), and this requires that the chemi-cal should not have any signiÐcant adverse impact onenvironmental quality or on human health. Literatureinformation is sparse on the environmental chemistryand behaviour of tebufenozide in forest litter and soilunder Canadian conditions. Recently, a Ðeld study wasconducted on persistence and mobility of tebufenozidein clay loam soil and the litter overlay4 (described inthis paper as the “clay soilÏ and “clay litterÏ respectively).The present paper reports a Ðeld microcosm studyinvolving sandy loam soil and the corresponding litteroverlay (referred to as the “sandy soilÏ and “sandy litterÏrespectively). The purpose of this study was to under-stand (a) the persistence of tebufenozide in forest litterand soil of low clay content following application usingsimulated aerial sprays, and (b) vertical mobility in thesoil.

In addition to the Ðeld study, several laboratorymicrocosm studies were undertaken on the same sandylitter and soil, and also on the clay litter and soil report-ed previously.4 The purpose was to examine the e†ectsof (i) rainfall on vertical and lateral mobility of tebufe-nozide in litter and soil samples of two di†erent types,(ii) light radiation on photostability and (iii) volatil-ization from the two substrates on disappearance. Theobjective was to determine the relative importance ofthese three factors under controlled conditions in thelaboratory, and to compare with the Ðeld behaviour oftebufenozide.

2 MATERIALS AND METHODS

2.1 Study IÈBehaviour of tebufenozide in sandy litterand soil ecosystems under Ðeld conditions

2.1.1 Site descriptionThe study was conducted about 5 km east of Greenwa-ter Lake, which is located c. 80 km northwest (46¡53@ N,

84¡03@ W) of Sault Ste. Marie, ON, Canada. The areacontained white spruce [Picea glauca (Moench) Voss]trees of varying heights (2È8 m). The forest Ñoor was Ñatand covered with grass and moss patches. In the openareas of the treatment block (c. 0É8 ha), four litter andfour soil plots (c. 4É5 m2 each) were selected. Three plotswere used for tebufenozide treatment, and the fourthserved as the control. Small objects such as fallenbranches, twigs and small stones were removed from theplots, and the surface was levelled and packed to theoriginal condition. The overlying litter, moss andorganic detritus were also removed to a depth of c.10 cm in the soil plots to fully expose the underlyingmineral soil to spray deposits.

2.1.2 Spray applicationPlots were treated at rates of 35, 70 and 140 g tebufeno-zide ha~1 (operational rates are 70È140 g ha~1).Appropriate amounts of a tebufenozide 240 g litre~1SC (RH-5992 2F; Rohm and Haas Co.) were mixedwith distilled water containing 2 g litre~1 RhodamineWT dye (technical grade, A. S. Patterson, 1110 Shep-herd Ave. East, Suite 404, North York, ON, Canada), toproduce the spray-mixes. A portable shelter(polyethylene sheets Ðxed onto wooden frames) enclos-ing an area of 2É2 ] 2.2 m to a height of c. 0É5 m wasplaced around each plot before spray application, andwas removed about 20 min after spray. The shelterhelped to prevent drift and hence contamination of theneighbouring plots. The volume rate used was 2 litreha~1 for all three dosage rates, and application wasmade on 2 June 1992 between 0630 and 1100 h, using ahand-held, battery-powered rotary atomizer“FlakÏ}(Micron Agri-Sprayers Canada, Walkerton, ON).During spraying, the average temperature, relativehumidity, wind speed and cloud cover were 11É8¡C,85%, 5É2 km h~1 and 1/10 respectively, and there wasno precipitation.

2.1.3 Droplet size spectra and deposit assessmentFor droplet measurements, two cards (K-Kromekote}card ; Intercity Papers Ltd, Mississauga, ON, Canada)(each 5É0 ] 7É5 cm) were mounted side by side on analuminium (Al) sheet (12] 12 cm). For assessment ofspray mass deposit, two glass plates (each having thesame dimensions as the K-card) were mounted onanother Al sheet. The two Al sheets were joinedtogether by a heavy duty masking tape. Four of theseunits (described as the K-card/glass plate units) wereplaced on the ground of each treatment plot, one toeach corner. The samplers were laid out c. 15 min priorto spray application, and were collected at 30 min post-spray. The K-cards were wrapped in Al foil and storedin a desiccator. The glass plates were eluted with ace-tonitrile and the eluates were collected in amber-coloured glass bottles, stored initially on dry ice, andlater at [20¡C in the freezer.

Page 3: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

Persistence and mobility of tebufenozide in soil and litter 117

2.1.4 Collection of sandy litter and soil samplesSandy litter and soil samples were collected from thethree treatment plots at 1 h after spray (described as thezero time) and at 4, 9, 16, 23, 31, 43, 52, 64, 85, 107, 135,169 (prior to snowfall), 357, 370 and 408 days post-spray (after snowmelt in the spring of 1993). Littersamples were taken as single cores per sampling intervalby driving a metal frame, 15É5 ] 15É5 ] 5É0 cm, into theground and removing the contents with a clean trowel.Three samples were taken at each time interval. Eachsample was packed separately in Al foil, stored at[20¡C and brought to the residue laboratory. In thelaboratory, each litter sample was macerated by passingthrough a chopper after removing the stones,Hobart}twigs, roots, etc., passed through a 2-mm sieve, mixedwell and stored at [20¡C until analysis.

Soil samples (sandy loam, depth 0È15 cm) were col-lected by using a heavy duty stainless steel auger devel-oped in our laboratory. The auger was constructed of astainless steel tube (diameter 5É0 cm, length 50 cm) splitopen lengthwise, and had a sharpened bevel at one endas a cutting edge. The two halves were held together bytongue and groove channels along their length andlocked in place with screw clamps. A solid stainless steelcap was placed over the head of the auger and a slidingcylindrical stainless steel rod (length 50 cm, diameter2É5 cm) was passed through a handle. The auger wasdriven into the soil to a depth of 15 cm with a sledgehammer. The auger was then drawn out by twisting andpulling the handle, without disturbing the surroundingsoil, and the tube was split open. The core was slicedwith a knife (cleaned between slices) into four segmentscorresponding to 0È2É5 cm, 2É5È5É0 cm, 5É0È10É0 cmand 10É0È15É0 cm. Three soil cores were taken fromeach of the three dosage (35, 70 and 140 g AI ha~1)plots at each sampling period. The auger was cleanedbetween sampling to avoid contamination. The soil coresegments belonging to the same dosage level and depthwere pooled to form a composite sample for residueanalysis. The samples were sieved (2-mm openings) toremove stones, twigs, roots, etc., wrapped in Al foil,placed in containers with dry ice, and brought to theresidue laboratory where they were stored at [20¡Cuntil analysis.

2.1.5 Extraction and analysisAnalytical grade tebufenozide (purity[ 99É6%) wasprovided by Rohm and Haas Company. The sampleswere processed as described by Sundaram,5 andanalyzed for residues using a high-performance liquidchromatographic (HPLC) method.6 BrieÑy, aliquots ofthe samples were extracted with a mixture of acetoneand water ; the crude extracts were solvent-partitioned,concentrated and cleaned by using orFlorisil}

columns. After fractional elution of thePrepsep}-NH2columns with hexane] ethyl acetate as the eluent, theeluates were analyzed by HPLC using a diode-array

UV detector set at 236 nm, and an RP-8, 10-km columnwith a mobile phase of acetonitrile ] dioxane] water.Mean recoveries after fortifying the pre-spray andcontrol samples with tebufenozide at 0É05 to 1É0 kg g~1ranged from 94 to 102%, with coefficients of variationfrom 5 to 10%. All measurements were corrected forrecovery efficiency. The limit of detection (LOD) andlimit of quantiÐcation (LOQ) for both substrates were0É020 and 0É050 kg g~1 respectively. None of the pre-spray or control samples contained any tebufenozide,and there was no interference from co-extractedmaterials. To investigate the storage stability of tebufe-nozide, processed pre-spray and control sample extractswere fortiÐed with varying levels of the analyte andstored at [20¡C for 60 days. During this period, ali-quots were taken and analyzed.6 The recoveries werequantitative for a 30-day period, beyond which agradual decrease in recovery with time was noted,ranging approximately from 5% on the 42nd day to 8%on the 60th day. Radiotracer studies using[14C]tebufenozide showed negligible adsorption ontopolyethylene, glass or surfaces. MoistureTeÑon}content of the substrates was determined7 by taking5É0-g aliquots of the processed samples (n \ 3) anddrying them in a thermostatic oven at 120¡C.

2.1.6 Analysis of droplet stains on Kromekote cards(K-cards)The droplet stains on the K-cards were analyzed asdescribed by Sundaram et al.8,9 using an AmericanOptical Microscope at magniÐcations of 25], 40] and100]. The minimum detection limit (MDL) was 25 kmfor the stains on the K-cards (corresponding to 11 kmin diameter for the spherical droplets). The data from 24cards [(four cards] three litter plots) ] (four cards] three soil plots)] were grouped according to sizecategory, to calculate the maximum and minimumdiameters and respectively), number and(Dmax Dminvolume median diameters and respectively)(DN.5 DV.5and droplet density (droplets cm~2).

2.1.7 Deposit recovery on glass platesThe deposits obtained on glass plates were extractedwith acetonitrile, and the extracts were concentrated byÑash-evaporation prior to analysis of tebufenozide.6The data obtained for the litter and soil plots were aver-aged and converted into g AI ha~1.

2.2 Study IIÈBehaviour of tebufenozide in sandy/claylitter and soil samples in laboratory microcosms

Samples of sandy litter and soil were collected fromthe same location as in Study I, processed and packedin Al pans (household turkey-roasting pans,

Page 4: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

118 K. M. S. Sundaram

38 ] 27 cm ] 7 cm high), to a depth of 4 cm. The panswere covered with plastic sheets to minimize evapo-ration of moisture from the samples, and stored in acold room at [ 10¡C until ready for use. Samples ofclay litter and soil were collected from a location4 inLaird Township Municipality (46¡23@ N, 84¡01@ W;about 30 km southeast of Sault Ste. Marie, ON,Canada). The samples were processed, packed in Alpans and stored as described above.

Aliquots of the formulation, RH-5992 2F, were mixedwith water to provide a concentration of 5 mg AI ml~1.Calculated amounts of the resulting suspension weremixed thoroughly with litter samples (4 cm deep) in oneof the Al pans to provide an initial concentration of10 mg AI kg~1 wet weight. Two Al pans containinguntreated litter (4 cm deep) were taken out of the coldroom and allowed to warm up to room temperature (20(^1É5)¡C). Over the 4-cm untreated litter sample, thetebufenozide-treated litter was packed to a height of2 cm. Replicate pans were prepared just before use tominimize storage problems. The Al pans containinguntreated forest soil samples (4 cm deep) were alsopacked in a similar manner with a 2-cm layer of treatedsoil containing tebufenozide at 10 mg kg~1 wet weight.

2.2.1 E†ect of cumulative rainfall on vertical mobility oftebufenozide in litter and soilPrior to the investigation, test trials were conducted todetermine persistence of tebufenozide in the top 2-cmlayer of litter and soil (both clay and sandy types) in theAl pans kept in darkness at 15¡C in an environmentalchamber without any rainfall. No loss occurred even upto 12 days after treatment, with little vertical mobilityinto the bottom untreated layers.

To determine vertical mobility of tebufenozide due torain, simulated rainfall was generated in a spray

chamber (4É3 ] 0É9 ] 3É05 m) using a 8002Veejet}nozzle having an oriÐce of 0É90 mm and a spray angleof 74¡ (Spraying Systems Co., Wheaton, Illinois, USA).The rainfall application parameters (spray pressure180 kPa, Ñow rate of rain water 0É58 litre min~1, rain-fall intensity 5 mm h~1) were chosen by repeated trialsto provide 20 and 40 mm amounts of rain over the Alpans containing the litter and soil samples (Fig. 1), at arain-free period of 2 days after tebufenozide treatment.Two types of application were used : (i) continuous rain-fall for 4 and 8 h to provide 20 and 40 mm cumulativerain respectively and (ii) intermittent rainfall at fourtimes (with an interval of 3 h in between), each time for1 and 2 h to provide 20 and 40 mm rain respectively. Arain gauge was used to monitor the amount of rainfall,and rain droplet size spectra (diameter range 80 to1100 km, 445 (^85) km, 780 (^95) km) wereDN.5 DV.5measured by collecting the droplets in castor oil.10Immediately after the rain, the Al pans were removedfrom the spray chamber. Samples (cores, 6 cm deep and1 cm in diameter) were taken from the pans and slicedinto top 2-cm, middle 2-cm and bottom 2-cm layers.The cores from corresponding layers were pooled foranalysis of tebufenozide. Some of the samples werefound to be soaked with the rain water, and they wereÐltered under suction using PTFETM membrane Ðlters[1-km pore, Product No. TF-1000 (66159), Gelman Sci-ences, Ann Arbor, MI 48106, USA] to remove theexcess water. The Ðltered water and the moist litter/soilsamples were both analyzed for tebufenozide residues.6

2.2.2 E†ect of rain-free period on vertical mobility oftebufenozide in litter and soilThe Al pans containing the litter and soil substrates asabove were subjected to 30 mm rainfall at two rain-free

Fig. 1. Arrangement of litter/soil substrates in the spray chamber for exposure to simulated rainfall.

Page 5: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

Persistence and mobility of tebufenozide in soil and litter 119

periods of 3 and 12 days after treatment. After the rain,samples were processed and analyzed for tebufenozideresidues.6

2.2.3 E†ect of rainfall intensity on vertical mobility oftebufenozide in litter and soilLitter and soil substrates were prepared as above andsubjected to 15 mm rain with two di†erent droplet sizespectra (i.e., rainfall with two intensities) after a rain-freeperiod of 6 days post-treatment. The smaller rain drop-lets (diameter range 30 to 340 km, 155 (^25) km,DN.5

315 (^25) km) were produced by using aDV.5 Veejet}8001 nozzle with an oriÐce of 0É65 mm and spray angleof 72¡, at a pressure of 160 kPa and Ñow rate of 0É24litre min~1, and the duration of the rain was 15 h con-tinuously at an intensity of 1 mm h~1. The larger raindroplets were produced in the same manner asdescribed in Section 2.2.1, and the 15 mm rain wasapplied over 3 h at an intensity of 5 mm h~1. Samplesof litter and soil were collected, processed and analyzedfor tebufenozide residues.6

2.2.4 E†ect of cumulative rainfall on lateral mobility oftebufenozide in litter and soilAn apparatus (Fig. 2) was constructed using a stainlesssteel circular pan (labelled CC-I, 30 cm ID, 6 cm high),and four concentric circular frames (CC-II, 24 cm ID,7É5 cm high ; CC-III, 18 cm ID, 12 cm high ; CC-IV,12 cm ID, 16 cm high ; CC-V, 6É5 cm ID, 18 cm high)made of stainless steel gauze with mesh size 0É2 mm.The pan, CC-I, was Ðlled with the litter or soil substrate(untreated) up to a height of 3 cm, and the four circularframes were Ðlled with the same substrate (alsountreated) to a height of 6, 9, 12 and 15 cm. Two suchapparatus were used, and replicate samples werepooled.

Fig. 2. Apparatus used to investigate the lateral mobility oftebufenozide due to simulated rainfall.

To determine lateral mobility of tebufenozide due torainfall, the substrate (12 cm high) in the innermost cir-cular frame, CC-V, was topped with a 3-cm layer oftreated litter or soil containing an initial concentrationof 10 mg AI kg~1. Two replicate units of the apparatuswere placed in the spray chamber in the same manneras the two Al pans used for the vertical mobility study(Fig. 1), and the substrates were subjected to continuousrainfall of 20 or 40 mm as described above. Immediatelyafter the rain, both units were removed from the spraychamber and replicate cores (each 3 cm deep, 1 cm indiameter) were taken from the pan, CC-I and circularframes, CC-II to CC-V. The cores were sliced into threelayers, each 1 cm deep, and replicate cores were pooledto provide three composite samples. The excess waterwas Ðltered using the same membrane Ðlters asdescribed above (PTFETM, 1-km pore, GelmanSciences). The substrates (and the Ðltered water) werethen analyzed for tebufenozide.6

2.2.5 E†ect of light radiation and radiation intensity ondisappearance of tebufenozide from litter and soilPrior to the investigation, the intensity and emissionspectra of the natural sunlight were measured in an areaopen to sunlight between 0900 and 1500 h every day for20 consecutive days using a Spectroradiometer (ModelLI-1800, LI-COR, Inc., Lincoln, Nebraska). The wave-length ranged from 320 to 1100 nm (Fig. 3), with anaverage intensity of 678 W m~2 and a mean tem-perature of 16É5¡C. Similar measurements made insidean environmental chamber (where the investigation was

Fig. 3. Light intensity (W m~2) versus wavelength (nm) ofradiation during photostability studies : (A) Intensity with four100-W lamps ; (B) Intensity of natural sunlight ; (C) Intensity

with two 100-W lamps.

Page 6: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

120 K. M. S. Sundaram

carried out) at a height of 40 cm below two householdlamps (100 W each) indicated the same wavelengthrange (Fig. 3), but with an intensity of 495 W m~2(c. 73% of the value in the open sky), and a mean tem-perature of 15¡C.

Litter and soil samples, treated with RH-5992 2F at10 mg AI kg~1, were packed (1 cm deep) in Al trays(24 ] 16 cm ] 1 cm deep), and stored in the environ-mental chamber in darkness until exposure to lightradiation. Two lamps (100 W each) were placed at40 cm above the substrates. The substrates wereexposed to radiation (intensity, 495 W m~2) for 6 and12 days after a radiation-free period of 5 days post-treatment, and samples (cores, 2 cm in diameter and1 cm in depth) were analyzed for tebufenozide residues.6

Treated litter and soil samples were also packed in Altrays as above, and subjected to radiation at two di†er-ent intensities (Fig. 3), after a radiation-free period of 8days post-treatment. One set of samples was exposed toradiation intensity of 495 W m~2 as above. To obtainlight radiation of higher intensity, two additional house-hold lamps (100 W each) were used (i.e., four lamps intotal). This increased the radiation intensity from495 W m~2 to 815 W m~2 (c. 120% of the value in theopen sky). The duration of exposure was maintained at8 days for both intensities. After exposure, the samples(cores, 2 cm in diameter and 1 cm in depth) wereanalyzed for tebufenozide residues.6

2.2.6 E†ect of volatilization of tebufenozide from litterand soil on disappearanceVolatilization of tebufenozide from treated substrateswas conducted in an apparatus shown in Fig. 4. A 200-galiquot of each substrate, containing tebufenozide at aconcentration of 10 mg kg~1, was taken in a Buchner}Ñask (500 ml capacity). Air was allowed to pass throughthe substrate at a Ñow rate of 0É75 litre min~1, and col-lected in two impingers,11,12 containing acetonitrile asthe trapping medium (Fig. 4). The apparatus (induplicate) was placed in darkness in two environmentalchambers maintained at 7 and 30¡C respectively, andtebufenozide vapour was collected for 10 days contin-

uously. The impingers were removed every 24 h, thesolvent in the two impingers was pooled into onesample, and a new set of impingers containing fresh ace-tonitrile was used. At the end of 10 days, the 10 acetoni-trile samples were pooled into Ðve composite samples,i.e., those collected on days 1 and 10 into sample 1 ;days 2 and 9 into sample II ; days 3 and 8 into sampleIII ; days 4 and 7 into sample IV and days 5 and 6into sample V. These were concentrated by Ñash-evaporation at 27¡C prior to analysis of tebufenozide.6

Another set of experiments was conducted to deter-mine the increase in volatilization of tebufenozide whenthe time interval of passage of air through the substrateswas increased. Air was allowed to pass through treatedsubstrate samples (kept in darkness at a constant tem-perature of 15¡C) for eight and 20 days respectively, andtebufenozide vapour was collected. The acetonitrilesamples were processed as above and tebufenozide resi-dues were determined.6

3 RESULTS AND DISCUSSION

3.1 Study IÈBehaviour of tebufenozide in sandy litterand soil ecosystems under Ðeld conditions

3.1.1 Residues of tebufenozide in sandy forest litterThe residues in sandy forest litter are given in Table 1.The data, measured on a dry weight basis, were adjust-ed to the constant, pre-spray moisture level of 27%. Theinitial average concentrations measured at 1 h post-spray (referred to as zero day in Table 1) increased inproportion to the dosage rates. Residues decreasedgradually during the 408-day persistence study,although some Ñuctuations were observed in the con-centration levels. This behaviour is difficult to explain,although rainfall occurring during the course of thestudy could have played some role. The long persistenceobserved in litter could be due to the strong bindingbetween tebufenozide and particulates in the littermatrix. The high organic matter content (c. 90%), lipo-philic nature, low acidity (pH 5É4) etc. (Table 1) could

Fig. 4. Apparatus used in the volatilization study for trapping tebufenozide vapour.

Page 7: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

Persistence and mobility of tebufenozide in soil and litter 121

TABLE 1Study IÈDissipation Kineticsa of Tebufenozide in Sandy Forest Litterb

Rate applied (g AI ha~1)

35 70 140

T ime after spray (days) Concentration in litter (kg g~1 wet weight) (^SD)c

0 0É176 (^0É076) 0É338 (^0É162) 0É637 (^0É232)4 0É133 (^0É048) 0É232 (^0É081) 0É531 (^0É179)9 0É098 (^0É022) 0É226 (^0É072) 0É580 (^0É159)

16 0É091 (^0É031) 0É166 (^0É113) 0É535 (^0É138)23 0É076 (^0É033) 0É189 (^0É162) 0É533 (^0É198)31 0É099 (^0É046) 0É229 (^0É086) 0É497 (^0É266)43 0É079 (^0É058) 0É172 (^0É066) 0É472 (^0É137)52 0É084 (^0É031) 0É164 (^0É037) 0É456 (^0É219)64 0É069 (^0É008) 0É134 (^0É032) 0É452 (^0É269)85 0É062 (^0É005) 0É169 (^0É094) 0É396 (^0É112)

107 0É055 (^0É020) 0É144 (^0É071) 0É335 (^0É205)135 0É082 (^0É026) 0É147 (^0É006) 0É313 (^0É057)169 0É052 (^0É015) 0É131 (^0É050) 0É237 (^0É103)357d Te 0É089 (^0É024) 0É291 (^0É112)370f NDg 0É083 (^0É019) 0É242 (^0É141)408h ND 0É072 (^0É021) 0É164 (^0É081)Constant B 0É139 0É287 0É603Coe†. detr. (R2) 0É796 0É877 0É959Rate constant C 0É001 11 0É002 31 0É002 63DT50 (days) 63É1 62É3 115

a Dissipation kinetics of residues followed the three equations, Y \ B[ Ct, Y \ 0É5B\ B[ C and B/C.DT50 DT50 \ 0É5b Organic matter c. 90% of total mass ; of remainder : sand 57%, silt 27%, clay 16%,pH 5É4.c Calculated from six measurements of three replicate samples.d Date of sampling \ 25 May 1993.e T\ Traces, 0É02È0É049 kg g~1.f Date of sampling \ 7 June 1993.g ND \ Not detected (LOD, 0É02 kg g~1).h Date of sampling \ 15 July 1993.

have favoured adsorption and immobilization of tebufe-nozide to the litter matrix, thus acting as a micro-sink.13

The residual concentrations (individual valuesmeasured) obtained during the initial 85 days afterspray were subjected to regression analysis, and werefound to Ðt into eqns (1) to (3) :

Y \ B[ Ct (1)

Y \ 0.5B\ B[ C DT50 (2)

DT50 \ 0.5B/C (3)

where Y \ residue concentration at time t, B\theoretical initial residue level, C\ rate constant of thelinear decline of residues, and the half-life (theDT50 ,time required for 50% of the initial residue todisappear) of persistence. Values of the constants, B, Cand are given in Table 1. The values for theDT50 DT50

plots treated at the three dosage rates were all high andranged from 62É3 to 115 days. These high values suggestthat tebufenozide could even overwinter after aerialapplication. However, it is worth noting that initial con-centrations were high in the present study, probablybecause the material was applied close to the litterplots. High concentrations in forest substrates areknown to take a much longer time to dissipate than lowconcentrations.13 In aerial applications, however, theamount deposited on the forest Ñoor is expected to bemuch lower than in the present study, and this couldinÑuence the values. Further studies are requiredDT50before any deÐnite conclusions can be drawn on theundue persistence of tebufenozide in forest litter and soilsamples.

3.1.2 Residues of tebufenozide in sandy forest soilThe concentrations of tebufenozide in sandy soil (coresof 0È2É5 cm depth), adjusted to the constant pre-spray

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122 K. M. S. Sundaram

TABLE 2Study IÈDissipation Kineticsa of Tebufenozide in Sandy Forest Soilb

Rate applied (g AI ha~1)

35 70 140

T ime after spray (days) Concentration in soil (0È2É5 cm layer) (kg g~1 wet weight) ( ^ SD)c

0 0É154 ( ^ 0É133) 0É338 ( ^ 0É100) 0É696 ( ^ 0É206)4 0É116 ( ^ 0É084) 0É232 ( ^ 0É079) 0É461 ( ^ 0É266)9 0É089 ( ^ 0É084) 0É226 ( ^ 0É033) 0É685 ( ^ 0É186)

16 0É088 ( ^ 0É024) 0É166 ( ^ 0É108) 0É527 ( ^ 0É274)23 0É094 ( ^ 0É055) 0É189 ( ^ 0É074) 0É357 ( ^ 0É131)31 0É084 ( ^ 0É062) 0É229 ( ^ 0É081) 0É262 ( ^ 0É104)43 0É066 ( ^ 0É032) 0É172 ( ^ 0É088) 0É305 ( ^ 0É088)52 0É069 ( ^ 0É033) 0É164 ( ^ 0É048) 0É414 ( ^ 0É284)64 0É056 ( ^ 0É050) 0É134 ( ^ 0É049) 0É384 ( ^ 0É095)85 0É057 ( ^ 0É028) 0É169 ( ^ 0É069) 0É180 ( ^ 0É085)

107 Te 0É051 ( ^ 0É034) 0É096 ( ^ 0É039)135 T T 0É063 ( ^ 0É023)169 T T 0É070 ( ^ 0É037)357d NDg ND 0É078 ( ^ 0É041)370f ND ND T408h ND ND TConstant B 0É121 0É297 0É616Coe†. detr. (R2) 0É802 0É938 0É935Rate constant C 0É000 98 0É002 40 0É005 88DT50 (days) 62É2 62É1 52É4

a, c, d, e, f and g : See footnotes to Table 1.b : Organic matter 3É9% of total mass ; of remainder : sand 51%, silt 42%, clay 7%, pH 5É9.

moisture level of 24É3%, are given in Table 2. The initialconcentrations were similar to those measured in theequivalent litter plots (Table 1). The residues decreasedgradually with time and lingered up to 408 days post-spray in samples collected from the plots sprayed at140 g AI ha~1. The 169-day post-spray samples stillcontained about 10% of the initial deposits of tebufeno-zide, whereas samples from other plots, treated with thetwo lower dosages, contained only “traceÏ (T) levels(0É02È0É049 kg g~1) (Table 2). Similar to the Ðndings inlitter samples, there were noticeable Ñuctuations in thesoil concentrations over the post-spray period.

The residual concentrations (individual measure-ments) obtained during the initial 85-day post-sprayperiod were found to Ðt into eqns (1) to (3), and DT50values were computed (Table 2). The values were aboutthe same (62É2 and 62É1 days respectively) for the twodosage rates of 35 and 70 g AI ha~1, and were similarto those (63É1 and 62É3 days respectively) obtained forthe litter (Table 1). At the highest dosage rate of 140 gAI ha~1, the soil samples showed a slightly lower DT50of 52É4 days, whereas the litter samples showed a highervalue of 115 days. The reason for this discrepancy isunclear. The sandy litter was more lipophilic and con-tained more organic matter (about 90%) than the sandysoil (OM 3É9%), and therefore the values might beDT50expected to be higher in litter than in soil. In the present

study, however, the observed Ñuctuations in the DT50values could be due to irregular variations in rainfall,exposure to sunlight, volatilization and microbialcontent in the substrates, all of which would a†ect thepersistence of tebufenozide under Ðeld conditions.

The high values in soils, similar to those inDT50litter, indicate that tebufenozide is likely to persist in thematrix for a long time. However, as mentioned above,further investigations are required using aerial applica-tions before any deÐnite conclusions can be drawn onthe undue persistence of tebufenozide in forest soilmatrices.

3.1.3 V ertical downward mobility in sandy forest soilNo tebufenozide was detected (LOD, 0É02 kg g~1) insoil samples collected from 2É5È5É0 cm depth duringzero to 85 days post-spray (Table 3) irrespective ofdosage rate. Samples from the plot sprayed at thehighest dosage rate contained 0É05 kg g~1 on the 107thand 408th day post-spray. All the other samples (2É5È5É0 cm cores) including the ones from 5É0È10 and 10È15 cm had no detectable amounts of tebufenozideexcept for the occasional incidence of “traceÏ amounts(Table 3). The lack of downward mobility observed inthe present study could be due to strong adsorption oftebufenozide onto the sandy soil.

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Persistence and mobility of tebufenozide in soil and litter 123

TABLE 3Study IÈVertical Downward Mobility of Tebufenozide in Sandy Forest Soil

Rates applied (g AI ha~1)

35 70 140

T ime after spray (days) Concentration (kg g~1 wet weight) in soil cores from 2É5 to 5É0 cm deptha

0È85 NDb ND ND107 Tc T 0É05135 ND T T169 ND T T408 ND T 0É05

a : No quantiÐable residues in 5È10 and 10È15 cm cores.b : ND \ Not detectable (LOD, 0É02 kg g~1).c : “TÏ\ Trace (0É02È0É049 kg g~1).

3.1.4 Droplet size spectra and deposit assessmentThe average droplet parameters obtained on Krome-kote cards for the litter and soil plots are given in Table4. Neither nor the dropletDmax , Dmin , DN.5 , DV.5 ,density showed marked di†erences among them,although the percentage distribution of droplet numberor volume in di†erent size categories indicated some dif-ferences, as shown in Fig. 5. However, such di†erencesdid not inÑuence the or dropletDmax , Dmin , DN.5 , DV.5density values markedly.

The average deposit of tebufenozide on glass platesranged from about 88 to 95%, indicating a high level ofdeposition (Table 4). This is expected because spray was

TABLE 4Study IÈPooled Mean Valuesa of Droplet Parameters andSpray Mass Deposits Obtained on ArtiÐcial Samplers inLitter and Soil Plots After Application of Tebufenozide atThree Dosage Rates With a Constant Volume Rate of 2 litre

ha~1

Rates applied (g AI ha~1)

Parameters 35 70 140

Droplet parameters on Kromekote cards :Dmax (km) 135 143 140Dmin (km) 16 17 11DNÕ5 (km) 79 78 76DVÕ5 (km) 84 97 93Droplets cm~2 57 45 53

Spray deposits on glass plates :Deposit oftebufenozide(g AI ha~1) 30É75 65É65 132É9Deposition (%) 87É86 93É79 94É93

a : Pooled mean values refer to the means of data obtained inlitter and soil plots at each of the three dosages of 35, 70 and140 g AI ha~1 respectively.

applied close to the ground level (\0É5 m) within apolyethylene enclosure.

3.2 Study IIÈBehaviour of tebufenozide in sandy/claylitter and soil samples in laboratory microcosms

3.2.1 E†ect of cumulative rainfall on vertical mobility oftebufenozide in litter and soilAfter continuous and intermittent rainfall (Table 5)tebufenozide moved downward more in the sandy litterand soil than in the clay litter and soil. This behaviourwas noted after both continuous and intermittent rain-fall of 20 and 40 mm. However, continuous rainfallcaused more downward movement than intermittentrainfall. In the 3 h rain-free interval between rainfalls,tebufenozide was probably adsorbed onto the moistsubstrates, thus decreasing downward mobility duringthe following rainfall. With the continuous rainfall,however, there seems not to have been enough time fortebufenozide to become adsorbed onto the substrates.

As expected, the 40 mm rain caused more downwardmobility than did the 20 mm. This was noted with bothcontinuous and intermittent rainfall. No residues werefound in the bottom layer of either sandy or clay sub-strates after 20 mm rain, compared to those found (0É17to 0É45 kg g~1) after 40 mm rain (Table 5). With theintermittent rain, however, the middle 2-cm layershowed no measurable residues after 20 mm rain, andresidues were present only after 40 mm rain. Thebottom layer did not show any residues after 20 or40 mm rainfall.

The amount of tebufenozide found in the Ðlteredwater was probably due to its solubility [c. 0É83 mglitre~1 (0É83 mg kg~1, density of water B1É00)].However, it is worth noting that the residues found inwater were lower than the solubility limit, and rangedfrom 0É101 to 0É437 mg kg~1 depending on the sub-strate layer from which the water was Ðltered. It

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124 K. M. S. Sundaram

Fig. 5. Droplet number and volume distribution percentages according to size category for tebufenozide spray mixtures applied tolitter and soil plots at 35, 70 and 140 g AI in 2 litre ha~1.

appears that during the time of rainfall, tebufenozidenever reached its maximum solubility limit in water.

3.2.2 E†ect of rain-free period on vertical mobility oftebufenozide in litter and soilThe data in Table 6 indicate that downward mobilityoccurred more after a three-day rain-free period thanafter a 12-day period. For example, more residues werepresent in the middle layer when a 30-mm rainfalloccurred at three days post-treatment than at 12 days.In fact, residues in the bottom 2-cm layer were still mea-surable after a three-day rain-free period, but werebelow the detection limit when the same amount of rainoccurred at 12 days post-treatment. This was notedboth with sandy and clay substrates. It appears that thelonger the rain-free period, the greater the time avail-able for adsorption of the chemical onto the active sitesof the substrates.

Similar to the Ðnding after the 20 and 40 mm cumu-lative rainfall, more downward mobility occurred after30 mm rainfall with the sandy substrates than with the

clay substrates. This is probably due to the greateradsorptive capacity of the clay substrates than that ofthe sandy substrates, in agreement with the Ðndingsreported previously.14,15

3.2.3 E†ect of rainfall intensity on vertical mobility oftebufenozide in litter and soilAfter a cumulative rain of 15 mm, downward mobilityoccurred more with the rainfall of higher intensity (i.e.larger droplet size with 5 mm h~1) than with the sameamount of rainfall of lower intensity (i.e. smaller dropletsize with 1 mm h~1) (Table 6). This behaviour wasnoted with both the sandy and clay substrates. Forexample, no residues were found in the middle 2-cmlayer of the substrates after the 15-mm cumulative rain-fall with of 315 (^25) km (intensity of 1 mm h~1),DV.5whereas residues were measurable after the sameamount of rain with a of 780 (^95) km (intensityDV.5of 5 mm h~1) (Table 6). The larger the rain droplet size,the greater the impact velocity of the droplets, resulting

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Persistence and mobility of tebufenozide in soil and litter 125

TABLE 5Study IIÈE†ect of Cumulative Rainfall (Continuous and Intermittent) on Vertical Downward Mobility of Tebufenozide from

Treated Sandy/Clay Litter and Soil Layers into Bottom Untreated Layers

Sandy littera,b Sandy soila,b Clay littera,b Clay soila,bParameter (Filtered water)c (Filtered water)c (Filtered water)c (Filtered water)c

Pre-rain residues (mg kg~1)Top 2 cm 10É0 10É0 10É0 10É0

(NA)d (NA) (NA) (NA)Middle 2 cm NDe ND ND NDBottom 2 cm ND ND ND ND

Post-rain residues (mg kg~1) after 20 mm continuous rainfall at a rain-free period of two days after treatmentTop 2 cm 8É23 7É14 9É33 8É56

(0É421) (0É430) (0É411) (0É395)Middle 2 cm 0É85 1É05 0É72 0É97

(0É377) (0É369) (0É382) (0É375)Bottom 2 cm ND ND ND ND

(NSf) (NS) (NS) (NS)

Post-rain residues (mg kg~1) after 40 mm continuous rainfall at a rain-free period of two days after treatmentTop 2 cm 6É54 5É33 7É82 6É65

(0É405) (0É403) (0É391) (0É433)Middle 2 cm 1É35 1É45 0É93 1É19

(0É365) (0É347) (0É387) (0É357)Bottom 2 cm 0É36 0É45 0É17 0É43

(0É101) (0É123) (0É134) (0É127)

Post-rain residues (mg kg~1) after 20 mm intermittentg rainfall at a rain-free period of two days after treatmentTop 2 cm 9É13 8É44 10É0 9É77

(0É404) (0É412) (0É437) (0É427)Middle 2 cm ND ND ND ND

(NS) (NS) (NS) (NS)Bottom 2 cm ND ND ND ND

(NS) (NS) (NS) (NS)

Post-rain residues (mg kg~1) after 40 mm intermittentg rainfall at a rain-free period of two days treatmentTop 2 cm 7É74 6É87 8É19 7É47

(0É429) (0É403) (0É411) (0É391)Middle 2 cm 0É95 1É23 0É75 0É99

(0É399) (0É338) (0É326) (0É343)Bottom 2 cm ND ND ND ND

(NS) (NS) (NS) (NS)

a : For organic matter, sand, silt, clay and pH values of sandy litter and soil, see Tables 1 and 2.The corresponding values for the clay litter : OM c. 80%, sand 21%, silt 35%, clay 44%, pH 5É2 ; and for the clay soil : OM c. 3%,sand 18%, silt 40%, clay 42%, pH 5É3.b : Residue data refer to the mean of three replicate samples.c : Values in parenthesis refer to the amount found in the Ðltered water.d : NA\ not applicable.e : ND \ not detectable (L OD, 0É020 mg kg~1).f : NS \ samples were not soaked in rain water, because the rain water did not reach the bottom layer.g : The 20 and 40 mm rainfall were applied in four applications, each of 5 or 10 mm respectively.

in a forceful movement of the tebufenozide from the topto the middle layer.

3.2.4 E†ect of cumulative rainfall on lateral mobility oftebufenozide in litter and soilThe data in Table 7 indicate that after the 20 mm rain-fall, little downward movement of tebufenozide

occurred in the innermost frame, CC-V, from the top1-cm layer to the middle and bottom layers (1 cm each)because the residue concentration measured in themiddle 1-cm layer (9É27 to 9É95 mg kg~1) was close tothe initial level treated (10É0 mg kg~1). A similar Ðndingwas also noted after the 40 mm rainfall. Nevertheless,appreciable amounts of tebufenozide were found in thetop and middle 1-cm layers of the untreated substrates

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126 K. M. S. Sundaram

TABLE 6Study IIÈE†ect of Rain-Free Period and Rainfall Intensity on Vertical Downward Mobility of Tebufenozibe from Treated Sandy/

Clay Litter and Soil Layers into Bottom Untreated Layers

Sandy littera,b Sandy soila,b Clay littera,b Clay soila,bParameter (Filtered water)c (Filtered water)c (Filtered water)c (Filtered water)c

Pre-rain residues (mg kg~1)Top 2 cm 10É0 10É0 10É0 10É0

(NA)d (NA) (NA) (NA)Middle 2 cm NDe ND ND NDBottom 2 cm ND ND ND ND

Post-rain residues (mg kg~1) after 30 mm rainfall at a rain-free period of three days after treatmentTop 2 cm 7É39 6É22 8É13 7É67

(0É391) (0É402) (0É398) (0É435)Middle 2 cm 1É22 1É01 0É71 0É82

(0É394) (0É366) (0É377) (0É345)Bottom 2 cm 0É16 0É25 0É09 0É23

(0É090) (0É109) (0É123) (0É118)

Post-rain residues (mg kg~1) after 30 mm rainfall at a rain-free period of 12 days after treatmentTop 2 cm 9É44 8É33 9É92 9É01

(0É451) (0É437) (0É403) (0É383)Middle 2 cm 0É38 0É69 0É27 0É36

(0É335) (0É344) (0É307) (0É327)Bottom 2 cm ND ND ND ND

(0É097) (0É131) (0É076) (0É094)

Post-rain residues (mg kg~1) after 15 mm rainfall with intensity of 1 mm h~1 at a rain-free period of six daysafter treatment

Top 2 cm 9É03 8É00 9É83 8É77(0É424) (0É419) (0É405) (0É444)

Middle 2 cm ND ND ND ND(NSf) (NS) (NS) (NS)

Bottom 2 cm ND ND ND ND(NS) (NS) (NS) (NS)

Post-rain residues (mg kg~1) after 15 mm rainfall with intensity of 5 mm h~1 at a rain-free period of six daysafter treatment

Top 2 cm 7É84 6É43 8É99 7É77(0É459) (0É409) (0É424) (0É372)

Middle 2 cm 1É25 1É33 0É86 1É03(0É353) (0É341) (0É317) (0É322)

Bottom 2 cm ND ND ND ND(NS) (NS) (NS) (NS)

a : For organic matter, sand, silt, clay and pH values of sandy litter and soil, see Tables 1 and 2.For those of clay litter and soil see Table 5.b, c, d, e and f : See footnotes to Table 5.

of CC-IV and CC-III. This must have been carried bywater passing laterally through the gauze surroundingthe treated layer in CC-V and falling onto the surface ofCC-IV and then CC-III. After 40 mm rainfall, this wassufficient to give measurable residues even in thebottom layer in CC-IV. Thus appreciable lateral move-ment of tebufenozide can occur under the inÑuence ofrainwater moving away from the treated area, forexample, down a slope.

The residues found in the Ðltered water after 20 and40 mm rainfall, were similar to those observed in thevertical movement investigations, and ranged from

“tracesÏ (0É02 to 0É049 mg kg~1) to 0É457 mg kg~1.These values, once again, are below the solubility limitof tebufenozide in water.

3.2.5 E†ect of light radiation and radiation intensity ondisappearance of tebufenozide from litter and soilThe data in Table 8 indicate that loss of tebufenozideoccurred more from sandy substrates than from claysubstrates. This behaviour was noted regardless of theintensity of radiation and duration of exposure. Onceagain, this is probably due to the greater adsorptivecapacity of the clay substrates than that of the sandy

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Persistence and mobility of tebufenozide in soil and litter 127

TABLE 7Study IIÈE†ect of Cumulative Rainfall on Vertical and Lateral Mobility of Tebufenozide from Treated Sandy/Clay Litter and

Soil Layers into Untreated Layers

Sandy littera,b Sandy soila,b Clay littera,b Clay soila,bParameter (Filtered water)c (Filtered water)c (Filtered water)c (Filtered water)c

Post-rain residues (mg kg~1) in CC-V after 20 mm rainfall at a rain-free period of four days after treatmentTop 1 cm 8É79 7É89 9É05 8É35

(0É454) (0É423) (0É413) (0É457)Middle 1 cm 9É85 9É43 9É95 9É27

(0É232) (0É156) (0É176) (0É275)Bottom 1 cm 10É0 9É95 10É2 9É99

(NS)f (NS) (NS) (NS)

Post-rain residues (mg kg~1) in CC-IV after 20 mm rainfall at a rain-free period of four days after treatmentTop 1 cm 5É03 4É50 4É97 4É03

(0É394) (0É409) (0É385) (0É414)Middle 1 cm 0É15 0É23 0É13 0É22

(Th) (T) (T) (T)Bottom 1 cm NDe ND ND ND

(NS) (NS) (NS) (NS)

Post-rain residues (mg kg~1) in CC-III after 20 mm rainfall at a rain-free period of four days after treatmentTop 1 cm 2É85 3É75 2É99 3É67

(0É359) (0É380) (0É383) (0É395)Middle 1 cm T T T T

(T) (T) (T) (T)Bottom 1 cm ND ND ND ND

(NS) (NS) (NS) (NS)

Post-rain residues (mg kg~1) in CC-V after 40 mm rainfall at a rain-free period of four days after treatmentTop 1 cm 7É24 6É63 8É12 7É17

(0É414) (0É408) (0É435) (0É422)Middle 1 cm 8É95 7É97 9É85 8É77

(0É331) (0É304) (0É276) (0É173)Bottom 1 cm 10É38 9É91 10É28 9É09

(NS) (NS) (NS) (NS)

Post-rain residues (mg kg~1) in CC-IV after 40 mm rainfall at a rain-free period of four days after treatmentTop 1 cm 6É14 5É03 5É55 4É99

(0É404) (0É319) (0É365) (0É324)Middle 1 cm 0É95 1É23 0É75 0É96

(0É222) (0É195) (0É176) (0É134)Bottom 1 cm 0É25 0É37 0É12 0É23

(0É077) (0É089) (0É088) (0É096)

Post-rain residues (mg kg~1) in CC-III after 40 mm rainfall at a rain-free period of four days after treatmentTop 1 cm 3É95 4É43 2É92 3É70

(0É427) (0É309) (0É314) (0É302)Middle 1 cm 1É25 1É33 0É86 1É03

(0É358) (0É371) (0É333) (0É379)Bottom 1 cm T T T T

(T) (T) (T) (T)

a : For organic matter, sand, silt, clay and pH values of sandy litter and soil, see Tables 1 and 2. For those of clay litter and soil seeTable 5.b, c, e, and f : See footnotes to Table 5.h : Traces\ values between 0É02 and 0É049 mg kg~1.

substrates, in agreement with the Ðndings reported pre-viously.14,15

The disappearance of tebufenozide showed a directrelationship with the duration of exposure, since a

greater amount disappeared after exposure to light ofintensity 495 W m~2 for 12 days than for six days.Similarly, higher amounts of tebufenozide were lostfrom the substrates exposed to light of high intensity

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128 K. M. S. Sundaram

TABLE 8Study IIÈE†ect of Light Radiation on Disappearance of Tebufenozide from Treated

Sandy/Clay Litter and Soil Layers

Sandy littera,b Sandy soila,b Clay littera,b Clay soila,b

Pre-radiation residues (mg kg~1)10É0 10É0 10É0 10É0

Post-radiation residues (mg kg~1) after exposure to 495 W m~2 for six daysat a radiation-free period of Ðve days after treatment

8É17 7É56 8É85 7É89

Post-radiation residues (mg kg~1) after exposure to 495 W m~2 for 12 daysat a radiation-free period of Ðve days after treatment

7É40 6É66 7É83 7É09

Post-radiation residues (mg kg~1) after exposure to 495 W m~2 for eight daysat a radiation-free period of eight days after treatment

6É74 5É69 7É28 6É01

Post-radiation residues (mg kg~1) after exposure to 815 W m~2 for eight daysat a radiation-free period of eight days after treatment

6É04 5É06 6É51 5É44

a : For organic matter, sand, silt, clay and pH values of sandy litter and soil, see Tables1 and 2. For those of clay litter and soil see Table 5.b : See footnote to Table 5.

(815 W m~2) than to light of low intensity(495 W m~2), after the same duration (eight days) ofexposure.

3.2.6 E†ect of volatilization of tebufenozide from litterand soil on disappearanceThe data in Table 9 show that volatilization of tebufe-nozide occurred more at 30¡C than at 7¡C, indicatingthat the process is related to temperature. However,when the temperature was maintained at 15¡C, a longerduration of passage of air caused a greater loss of tebu-

fenozide. This behaviour was noted with both types ofsubstrate, i.e., sandy and clay types. Nonetheless, tebufe-nozide volatilized more from substrates of the sandytype than those of the clay type, again probably becauseof the greater adsorption occurring onto the clay sub-strates than onto the sandy substrates.14,15

4 CONCLUSIONS

The present investigation indicated that, under Ðeldconditions, tebufenozide persisted in sandy litter and

TABLE 9Study IIÈE†ect of Volatilization on Disappearance of Tebufenozide from Treated Sandy/Clay

Litter and Soil Samples

Sandy littera,b Sandy soila,b Clay littera,b Clay soila,b

Pre-volatilization residues (mg kg~1)10É0 10É0 10É0 10É0

Post-volatilization residues (mg kg~1) in samples kept in darkness at 7¡C for 10 days9É49 9É01 9É85 9É46

Post-volatilization residues (mg kg~1) in samples kept in darkness at 30¡C for 10 days8É89 8É29 9É02 8É45

Post-volatilization residues (mg kg~1) in samples kept in darkness at 15¡C for eight days8É56 7É66 9É08 7É92

Post-volatilization residues (mg kg~1) in samples kept in darkness at 15¡C for 20 days7É33 6É77 8É29 7É06

a : For organic matter, sand, silt, clay and pH values of sandy litter and soil, see Tables 1 and 2.For those of clay litter and soil see Table 5.b : Residue data refer to the mean of Ðve replicate samples.

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Persistence and mobility of tebufenozide in soil and litter 129

soil for a considerable length of time. The valuesDT50were c. 60 days for both the substrates treated with thedosage rates of 35 and 70 g AI ha~1. At the highestdosage rate (140 g AI ha~1), however, the wasDT50higher for the litter (115 days) than for the soil (c. 52days), indicating irregular variations in persistence. Thesandy litter was more lipophilic and contained moreorganic matter (c. 90%) than the sandy soil (OM c. 4%),and therefore tebufenozide is likely to persist for alonger period in the litter than in the soil. Nonetheless,the variations observed in the present study could bedue to Ñuctuations in rainfall, exposure to sunlight,volatilization, and microbial content in the two sub-strates during the post-spray period. The valuesDT50reported in the literature for another Ðeld study4 were52É9, 51É7 and 60É3 days in clay litter treated with thedosage rates of 35, 70 and 140 g AI ha~1 respectively.The corresponding values for the clay soil were lower,34É0, 32É9 and 32É1 days respectively. The clay litter andsoil had an organic content of c. 80 and 3% respectively,and therefore the observed longer persistence of tebufe-nozide in clay litter than in clay soil was probably dueto the higher organic matter content. Nevertheless, the

values obtained in the present study for the sandyDT50substrates are in disagreement with the values reported4for the clay substrates. It appears that several otherfactors (see above) could have played a role in a†ectingthe persistence. Vertical mobility of tebufenozide fromthe top 2É5-cm layer of the sandy soil to the bottom2É5È5É0-cm layers was minimal, and only traces werefound during the post-spray period of 107 to 408 days.This observation is in agreement with that reportedpreviously4 for the clay soil, since tebufenozide residueswere below the quantiÐable limit (0É05 kg g~1) in layers2É5È7É5 cm.

The laboratory microcosm studies indicated thatmeasurable concentrations of tebufenozide moved fromthe top 2-cm layer to untreated layers beneath as aresult of simulated rainfall. Downward mobilityincreased when the cumulative rainfall increased from20 to 40 mm, and also when the rain was continuousrather than intermittent. Movement of tebufenozide wasalso inÑuenced by the rain-free period (i.e., the timeinterval between tebufenozide treatment and occurrenceof rainfall), because the shorter the rain-free period thegreater the downward movement. Rainfall intensity (i.e.,rain droplet size spectra) also contributed to downwardmobility, because more residues were found in theuntreated bottom layers when the rain droplet sizeswere large 780 km) than when they were small(DV.5 ,

315 km). When rainwater could move laterally(DV.5 ,along the surface, as would occur in practice on asloping site, this caused more lateral movement of tebu-fenozide (i.e. along the surface) than vertical movementinto the soil.

The Ðndings of the present laboratory study on verti-cal mobility in sandy soil are in disagreement with the

observation made in the Ðeld study, since little tebufe-nozide moved from the top 2É5-cm soil to bottom layers(2É5È5É0, 5È10 and 10È15 cm). Similar disagreementcould also be found with Ðndings of the Ðeld studyreported in clay soil.4 The reason could be that tebufe-nozide was probably more strongly adsorbed to bothsandy and clay soils under Ðeld conditions than underlaboratory conditions, thus resisting downward move-ment. Or, rainfall probably did not occur in the Ðeldstudy for a long time after spraying tebufenozide, thusproviding a longer time interval for adsorption. It isalso likely that the rainfall intensity was lower in theÐeld study than in the laboratory study.

The laboratory investigation on photolysis indicatedthat the disappearance of tebufenozide showed a directrelationship with duration of exposure and also withradiation intensity. This Ðnding is in agreement withthat reported in the literature,16 since tebufenozide wasfound to be susceptible to degradation after exposure tosunlight and ultraviolet radiation. The study on volatil-ization of tebufenozide under controlled conditionsindicated that the amount lost depended on the ambienttemperature and duration of air passing through thesubstrates. Nonetheless, the amount lost by volatil-ization was much lower than the amount lost by rainfalland exposure to radiation, thus indicating the greaterimportance of rainfall and sunlight radiation on Ðeldpersistence, than of volatilization.

In all of the laboratory microcosm studies, moretebufenozide was lost from the sandy litter and soil thanfrom clay litter and soil. This behaviour was probablydue to the greater adsorptive capacity of the clay sub-strates than that of the sandy substrates, in agreementwith the Ðndings reported previously.14,15

ACKNOWLEDGEMENTS

The author thanks Reg Nott, Johanna Curry, LindaSloane, Jim Synox and Fred Albert for their technicalassistance in Ðeld sampling and laboratory analysis oftebufenozide residues. Thanks are also due to Dr. AlamSundaram for providing data on droplet size spectra ofsprays obtained on cards in the ÐeldKromekote}study, and for her help in generation of simulated rain-fall and rain droplet sizes in the laboratory study.

REFERENCES

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2. Hurt, S. S., Bulletin on RH-5992 Toxicology. Rohm andHaas Company, Independence Mall West, Philadelphia,Pennsylvania, 1990, p. 2.

Page 16: Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

130 K. M. S. Sundaram

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