Proceedmqe of the 7th International Working Conference on Stored-product Protection - Volume 1

The efficacy of silicaceous dust alternatives toorganophosphorus compounds for the control of storage mites

D. M. Armitage, D. A. Collins, D. A. Cook and J. Belli

Abstract

Mites are Important and destructive pests of storedproducts. They cause direct damage by destruction of thegerm, cause taints and allergies and transmit fungi andbacteria and have been implicated in the transmission ofpnons Conventional organophosphorus pesticides are theonly chemicals approved for use on stored gram and oilseedsin the UK but are not very effective against mites.Silicaceous dusts are one alternative to organophosphorus

pesticides WhICh have proved effective against storageinsects as structural treatments and as surface treatmentsand have been registered for use in countries mcluding theUSA, Canada, Australia. The experiments reported hereinvestigated the effect on developing mite populations ofdusts based on freshwater and saltwater diatoms as well asamorphous silica.At 15'C, 75% r. h., doses of 1 - 5 g/kg for all four

silicaceous dusts generally gave > 99% control of Acarussiro , With 3 - 5 g/kg required against Tyrophaguspuireeceniuie and Glycyphagus destructor, the latterspecies being the least susceptible. ThIS high dose precludesuse as an admixture treatment due to cost and effect on grainflow but IS acceptable as a top-dressing as part of anintegrated storage strategy or as a fabric treatment.

Introduction

MItes are very common pests of stored cereals and oilseed inthe U. K. As well as the damage they cause to storedcommodities, there are potential health risks to workers bythe development of serious allergies following exposure tothe pests. MItes have also been implicated m thetransmission of fungi, bacteria and prions. In the U. K.,stored grain protection has relied heavily on bulk adrruxturewith pesticides. The only contact pesticides approved for useon stored grain and Oilseed are the organophosphorus (OP)compounds pirimiphos-methyl, etnmfos and chlorpynfos-methyl. However, none of these pesticides are now

1Central Science Laboratory, Muustry of Agriculture, Fisheries andFood, Sand Hutton, York Y04 ILZ

particularly effective against field strains of mites andresistant populations have been detected. As well asconcerns regarding the development of resistant pests, thecosts and consumer resistance to toxic chemicals in food,have led to increasing pressures for a reduction in pesticideuse, and the need to find alternative compounds.Inert dusts have been used traditionally as a stored grain

protectants, and there is increasing interest in their use asalternatives to chemical control measures. A number ofstudies on the efficacy of inert dusts against stored productinsects have been reported and they have proved effective asgrain protectants (Desmarchelier and Dines, 1987), asstructural treatments m empty stores (Bridgeman, 1994)and as surface treatments in conjunction with aeration(Nickson et al, 1994). Diatomaceous earths (DEs) havebeen registered for storage use in USA, Canada, Australia,Japan, Indonesia and Saudi Arabia.The products are based on inert materials such as silica gel

or diatomaceous earth and contam no insecticide or knock-down agents. They are pesticide-free, effective againstchemically resistant species, and are stable at high and lowtemperatures (Mclaughlin, 1994). In contrast to chemicalinsecticides which induce rapid immobilisation and kill, theaction of inert dusts is slower so extended exposure periodsof 20 days or more, may be required to eliminate an insectpopulation (Mclaughlin, 1994). Most products, at theappropriate concentration, provide protection for at least 12months (Mcl.aughhn, 1994).Inert dusts act by physical means, with insect mortality

thought to occur by desiccation as a result of the dustadsorbing lipids from the cuticle (Ebeling, 1971).Several studies have investigated the efficacy of mert

dusts against parasitic mites but knowledge of the effects ofmert dusts against stored product mites is very limited.Cook and Armitage (1996) investigated the efficacy of'Dryacide.' , applied to wheat at Ig/kg and 3 g/kg, againstAcarus siro L. and Glycyphagus destructor Schrank. At14% moisture content (me) and 17. 5'C, both doses werecompletely effective at controllmg both species. At 16% meand 17.5'C, 3glkg was fully effective against A. siro butnot against G. destructor. Fields and Timlick (1995)found that the product' Super Insecolo' reduced predaceousand grain feeding mite species by over 98% when applied towheat at 50 ppm.

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Proceedings of the 7 th Internaiumal Working Conference on Stored-product Protection. - Volume 1

The use of inert dusts as grain protectants may becomemcreasmgly more significant as alternatives to conventionalchemicals are sought. Their advantages include lowmammalian toxicity, cost effectiveness, persistence and lackof toxic residues on grain Disadvantages have included thevery high treatment rates required, which affect thephysical properties of the gram, particularly bulk density,angle of repose and flow rate (Jackson and Webley, 1994).Inert dusts are also less effective at high relative

humidities and moisture contents, which IS a majorconsideration for their use as grain protectants m UK. Thehigher doses that may be required for control may have anadverse effect on the physical properties of the grain.However, these disadvantages would be minimised if dustswere used as a surface treatment, in conjunction withcoolmg as part of an integrated storage strategyDue to the particle SIZeof some dusts they are considered

to be respirable and therefore represent a potential hazard tousers (Golob, 1997) However, dusts of larger particleSIzeSare relatively safe to use With minimum protectiveequipment (dust mask). Diatomaceous earths may be ofmarine or freshwater origm and usually contam about 90%Si(h (Golob, 1997).Manne diatoms contain high quantities of crystalline

sihca, which can result in silicosis and other respiratorydiseases. The formulations of the mert dusts' Protect-It'(Super Insecolo), from Canada and Insecto, from the USA,were of marine origin (Subramanyan, m press). Crystallmesilica shows hmited carcmogenic effect (Anon, IPDACItedinQuarles, 1992).Dryacide ISbased on freshwater diatoms but coated WItha

sihca aerogel (Subramanyan, m press) SIlica aerogels areeffIcaCIOUSat lower rates than diatom-based products. Theyare produced by drymg aqueous solutions of sodium SIlicatebut their low dust density has prevented their use alonebecause of potential mhalation hazards (Golob, 1997)'RID IP', the UK product, IS amorphous preciprtated

silica. Compounds of tlus description usually have a SI(hcontent of 98 % or more (Golob, 1997). According to IARCamorphous sihca ISnot carcinogenic.The aIm of these ImtIal expenments was to evaluate four

sIhcaceous dusts for the protection of stored gram agamstmIte mfestatIon Because the aim was to IdentIfy poSSIblereplacements for OPs as admIxture treatments, the mostappropnate method of evaluation would be to assess thecompounds when apphed dIrectly to gram. EffIcacy neededto be evaluated agamst the most common mIte pests found mstored gram under typIcal U. K storage conditions. Also,because different mIte stages may have different tolerances,the compounds were evaluated agamst mIxed stages andassessed for penods long enough to mclude the passing of atleast 2 generations

Materials

Wheat: The gram used was pesticide-free, English milhngwheat with a moisture content of about 15%, as determmedusing the oven method (BS4317), by drying m a ventilatedoven at l30°C for 2 hours. The wheat was stored in plasticbags m a freezer for at least 21 days prior to use to ensureany nutes coming m on the gram were killed.Mites: The mites used were laboratory susceptible strainsof A eiro , Tyrophagus putresceniuie Schrank and Gdestructor. All had been reared at the Central SCIenceLaboratory (CSL) m constant conditions of 15°C and 75%rh WIthout exposure to pesticides. Mixed stages of unknownage were used.

Methods

Preparatum. and treatment of gram: Batches of 493 g ofwhole wheat and 7 g of kibbled wheat were weighed out andmixed together into 1.5 I 'Kilner' jars, prior to apphcationof the dusts. The kibbled wheat served as an nutial foodsupply for the mites to atd establishment on the whole grainsand prevent population crashes.The sihcaceous dusts were apphed at 5, 3, 1 and 0.5 g

kg-I by weighing out the appropriate amount and addingdirectly to the gram in Jars. One batch remained untreatedto act as a control. After treatment the batches of treatedand control gram were mixed on a tumbler for 15 minutes toensure an even distribution and stored at room temperatureovernight The Jars were then tumbled for a further 10minutes. Each batch of treated and control gram was dividedinto approximately 50 g lots, put into 120 ml WIde-neckedbioassay Jars and closed WIth filter paper hds. SIXrephcateJars were prepared for each treatment and each mitespecies The Jars were left to equilibrate in the testconditions of 15°C and 75% rh for 24 hours.Bioassay: A heaped spatula contammg approximately 0.01 gof mixed stage rrutes was placed into each Jar and the Jarswere re-closed with a filter paper lid. The jars wereincubated m the test conditions for periods long enough to bemclude the passmg of at least two generations. Since thedIfferent mIte speCIes have different developmental ratesfrom egg to adult, exposure penods vaned for each speCIes.Therefore, assessments of the FI and F2 generations tookplace at approXImately the following number of days aftertreatment:

MIte speCIesAcarus stroTyrophagus putrescentiaeGlycyphagus destructor

F1 assessment F2 assessment305045

70110100

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Proceedings of the 7th International Working Conference on Stored-product Protection - Volume 1

Three replicate jars were assessed at each assessmentperiod. The contents of each jar were sieved over a 710 urnmesh and the sievings examined under a low power binocularmicroscope. The numbers of live mites were counted usingthe technique of Solomon (1962) using a disc divided intoareas. After the assessments the moisture content of thegrain was re-checked using an oven method (BS 4317) .The mean percentages of inhibition were calculated by

comparing numbers of mites in the treatments with numbersin the untreated controls.

Results. and Discusson

All the dusts inhibited the F1 generation of the flour mite,Acarus siro and the mould mite, Tyrophagusputrescentiae, by more than 95% at all doses between 1and 5 g kg I, However the cosmopolitan food mite,Glycyphagus destructor proved less susceptible and a doseof 3 g / kg was required to inhibit the population by between88% and 100 % after one generation (Figs. 1 - 4 ). Of thefour products tested, the amorphous silica, 'RID IP'performed consistently best at the F1 assesment. For

100

90

80

70

60

50

40

30

20

10

oA.siro, Control=4875/50 g, F1 =

29d

T.putrescentiae,Control = 870 150g, F1 = 50 d

instance, at O.5 glkg, against G. destructor, the F1populations were 1027 (592 - 1360), 1536 (1512 - 1872) ,3445 (2024 - 4384), 4048 (2464 - 5088) and 4875 (4088-5752) for' RID IP' , Protect-it. Insecto, Dryacide and thecontrol respectively.The populations of all three species of mites in the

controls declined between the F1 and F2 assessments. This isbecause the high populations of mites would have exhaustedtheir limited food supply (Solomon, 1969). In theseconditions it is likely that the mites hollow out the cerealgerm and cannot access the endosperm through the dividingcellular layer (Parkinson, 1990). In addition, there wasconsiderable variation between replicates, especially at theF2 assessments. Both these factors mean that the estimates

of control were unduly pessimistic.

Acknowledgements

This work was funded by the Home-Grown CerealsAuthority. Thanks are due to the manufacturers of theproducts: Rentokil, Headley Industries, Dryacide' andInsecto' .

G.destructor,Control = 2472 I 50

g, F1= 43 d

m5g1kg

B3g/kg

D 19/kg

mO.5g1kg

Fig.l. Percent inhibition by four doses of 'RID IP' of populations of three mite species (expressed as a proportion of populationson untreated grain) at 75 % r. h., 15t:. F1 assessmemt.

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Proceedings of the 7th International Working Conference on Siored-proiuct Protection - Volume 1

100

90

80

70

60

~ 500

40

30

20

10

0A. siro Control =4875/509, F1

=29d

T.putrescentiaeControl = 870 1509, F1 = 50 d

G. destructorControl = 2472 1509 F1 = 43

days

as g/kg83 g/kg

01 g/kg

mO.5 g/kg

Fig.2. Percent inhibition by four doses of 'Protect-it' of populations of three mite species (expressed as a proportion ofpopulations on untreated grain) at 75% r. h. , 15t. F1 assessmemt.

100

90

80

70

60

~ 500

'40

30

20

10

0A. siro Control =4875/50 9, F1

=29d

as g/kg.3 g/kg

01 g/kg

ElO.5g/kg

G. destructorControl = 24721509 F1 = 43

days

Fig.3. Percent inhibition by four doses of 'Dryacide' of populations of three mite species (expressed as a proportion ofpopulations on untreated grain) at 75% r. h. ,15°C. F1 assessmemt.

T.putrescentiaeControl-= 870 150g, F1 = 50 d

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Proceedings of the 7th International Working Conference on Stored-product Protection - Volume 1

100

90

80

70

60

"#. 50

40

30

20

10

o

1lIlI5 g/kg

.3gtkg01 g/kg

&:;l0.5g/kg

G. destructorControl = 2472 I50 9 F1 = 43

days

Fig. 4. Percent inhibition by four dosesof 'Insecta' of populationsof three mite species (expressed as a proportionof populationson untreated grain) at 75% r. h. , 15t F1 assessmemt.

A. siro Control =4875 I 50 g. F1

=29d

T.putrescentiaeControl = 870 I50g, F1 =50d

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on Stored-Products Protection, Canberra, Australia 1994,2~ 635 - 637.McLaughlin, 1994. Laboratory trials on desiccant dustinsecticides. In: Highley E. , Wright, E. J. , Banks, H. J.and Champ, B. R. Eds. Proc. 6th International WorkingConference on Stored-Products Protection, Canberra,Australia 1994, 2, 638 - 645.Nickson, P. J. , Desmarchelier, J. M. and Gibbs, P. 1994.Combination of cooling with a surface application ofDryacide to control insects. In : Highley E., Wright, E.J. , Banks, H. J. and Champ, B. R. Eds. Proceedings 6thInternational Working Conf. Stored-Products Protection,Canberra, Australia 1994, 2 638 - 645.Parkinson, C. L. 1990. Population increase and damage bythree speciesof mites on wheat at 20°C and two humidities..Experimentaland AppliedAcarology,8,179 - 193.Quarles, W. 1992. Diatomaceous earth for pest control.The IPM Practitioner, 14 (5/6) : 1 - 1.Solomon, M. E. 1962. Notes on the extraction andquantitative estimation of the Acaridae (Acarina).Progress in soil zoology. Proceedings of a colloquium onresearch methods in soil zoology. Rothamstead. 1, 305-307.Solomon, M.E. 1969. Establishment, growth and decline ofpopulations of the grain mite (4.carus siro L.),Proceedings 2nd. International Congress Acarology 1967,255 - 260.Subramanyan, Bh.Gingera, G.]. Downey, D. and Sonterre,T. In press. Comparative efficacy of diatomaceous earthdusts on stored-product insects. Journal EconomicEntomology.

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