Development of Management Strategy Against Potato ...philjournalsci.dost.gov.ph/images/pdf/pjs_pdf/vol131no2/pdfs/... · (0-35%). Other factors contributory to the outbreak status

Development of Management Strategy AgainstPotato Leafminer, Liriomyza huidobrensis

(Blanchard) in Cordillera, Philippines

Lita Molitas-Colting1, Bonie S. Ligat1, Rogelio D. Colting1,Jocel C. Perez1, Teresita K. Mangili2, Maritess Alimurung1 and Casiano S. Pagadan1

Philippine Journal of Science131 (2): 99-117, December 2002ISSN 0031 - 7683

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1College of Agriculture, Benguet State University, La Trinidad Benguet2Bureau of Plant Industry, Baguio National Crops Research

and Development Center, Baguio City

Keywords: Solanum tuberosum, yellow sticky trap, pest outbreak, pest management

Potato leafminer, Liriomyza huidobrensis (Blanchard), is a new pest of potato (Solanum tuberosum Linn.) in the Philippines. It triggered an outbreak in December 1999 in Benguet province causing an estimated crop loss of Php333 M. The extensive use of yellow sticky traps with corn (non-host) and carrots, cabbage or beans (rotation/intercrops), planting of Igorota potato variety and need-based use of Cyromazine insecticide significantly reduced the pest population to a manageable level of 50%. Need-based use of insecticide gave a return of investment of 88.32% which is 34% higher than the traditional practice of calendar insecticide application apart from lessened non-exposure to insecticides. Insecticide was applied only during the critical stage of the crop, (vegetative to tuber initiation stage), moderate larval damage (25% injury); and the mean number of leafminer adults in four yellow sticky traps is 10 per sq cm. The appropriate time to monitor adult population is between 10:00 a.m. to 2:00 o’clock p.m. Use any yellow sticky trap and surround the field if the pest population is high, if not use chrome yellow plastic plate (25 cm diam) with motor grease no. 3 to be staked 30 to 60 cm above or beside the crop and not more than 15 sq m. apart. Six species of leafminers occur in Benguet, with L. huidobrensis as the most dominant with 49 host plants and four parasitoids with a low rate of parasitism (0-35%). Other factors contributory to the outbreak status of potato leafminer are: warm and humid conditions, introduction of infested chrysanthemum plants, extensive planting of potato and misuse of insecticides.

The occurrence of insect pest outbreak on potato in Benguet, in Northern Philippines occurs in a cycle of ten years. Aphids became a problem in the 1960’s; whiteflies in the 1970’s; thrips and potato tuber moth in the 1980’s and lately, the potato leafminer, Liriomyza huidobrensis (Blanchard) particularly at Buyacaoan, Buguias in December 1999. The leafminer caused yield loss as high as 100% estimated at PhP 333M in less than one month (Domoguen, 2000). The larva totally mined the leaves which dried-up and eventually defoliated the plants (Plates a-b).

PLM is a problem pest in many counties worldwide where it was introduced (Mujica and Cisneros, 1997; and Weintraub and Horowitz, 1995) and some of the management strategies they employed (Chavez and Raman, 1987 and Parella and Keil, 1984) served as basis in containing the outbreak at Buguias. The bioecology of the pest and results of verification trials on the strategies are presented in this paper. The possible factors that triggered the outbreak of the pest are also included.

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Molitas-Colting et al.

Materials and Methods

Identification of Leafminers and its Natural Enemies

Immature and adult specimens of leafminers and its natural enemies were collected from potato and other crops infested with leafminers by the Department of Agriculture (2001). The leafminers were shipped to the United States and identified by Dr. Sonja J. Scheffer, a molecular systematist (Agromyzidae) of the Systematic Entomology, USDA-ARS while the natural enemies were given to the Commonwealth and Industrial Research Organization (CSIRO) of Australia and identified by Dr. John La Salle. Replicates of the specimens are deposited at the Department of Agriculture, Guisad, Baguio City.

Factors that Favored Outbreak

Status of PLM

Potato farmers at the outbreak area and in other potato growing areas in the province were interviewed along with actual field surveys. The interviews focused on factors known to contribute to the development of pest outbreak as follows: crops and variety planted, insecticide used (name, dosage, frequency of application, and mixtures), change in temperature and relative humidity, new plants introduced in the locality, and other cultural management practices. To determine if there were natural enemies, leaf and topsoil samples were taken at random from infested potato crops and nearby weeds and observed for the emergence of possible larval-pupal parasitoids. Sweep nets around the fields were also conducted to collect the adults. Climatic data was taken at La Trinidad Valley about 100 km away from the site of pest outbreak.

Life Span, Host Range and Distribution

The life span of PLM was studied on potted potato plants at temperatures ranging from 17 to 21oC and 84-90% RH. The plants were exposed in the field for one day for egg laying and later put individually in cages. Some leaves with eggs were plucked and put in moistened petri plates. These were placed under the microscope and the eggs were marked and observed until they hatched. The lifespan of the larva was noted on the potted plants by following the mines until they pupated. The pupae were caught as they fall at the base of the plant using moistened brown papers and put in petri plates to allow the adults to emerge. Thirty specimens were used. The body length and distinct color of each stage were also taken.

To determine other host crops, representative sample plants with mines were collected throughout the province. These were put in cages and the leafminer adults emerging from them were identified using morphological keys (Parella, 1996; and Weintraub and Horowitz, 1995). The data was also used to identify the area of distribution of the pest.

Degree of Damage

Weekly field sampling in four identified potato fields were done in two barangays in Buguias; Loo Valley – lower elevation (warmer) and Natubleng – higher elevation (cooler) for six months from the start of the pest outbreak (December 1999) and monthly thereafter when the degree of damage became moderate. In cases where there were no potato crops, sampling was done on nearby alternate host crops including weeds. The general rating index used in assessing insect pest damage was followed ranging from 1 (sound) on plants without mines and punctures to 9 (very severe damage) on plants with more than 75 to 100 leaves with mines.

Development of Monitoring Scheme Using Yellow

Plate 1. Potato fields at Loo Valley, Buguias: a) during the outbreak (December 1999); and b) after the outbreak (December

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Management Strategy Against Potato Leafminer

Sticky Traps

Economic injury level/critical stage of potato. The standard procedure in doing simulated leaf injury study was followed. Leaf injury ranged from 0 to 100% where 0 is without injury (standard), 25% - slight injury, 50% - moderate, 75% - slightly severe, and 100% - very severe. The growth of potato (Granola variety) was divided into four stages starting from plant emergence: seedling stage - 1 to 23 days old, vegetative - 24 to 37 days old, tuber initiation stage - 38 to 51 days old, and tuber bulking stage - 52 to 65 days old. The numbers of big tubers were counted at harvest.

Sampling time . The time to monitor PLM population was based on the flight biorhythm of the adults in view of the difficulty in counting the larva or pupa. Ten YSTs made of yellow polyethylene sheet, 36 cm x 64 cm (standard), were set at random for 15 days in infested potato fields. These were set at 6:00 am to 6:00 pm in the evening and replaced after two hours interval. The total number of trapped adults were counted and analyzed.

Efficacy of YST. The number of trapped PLM adults was counted from YSTs staked by farmers during the pest outbreak and on-station trials. In the YST’s of farmers, the total number of trapped adults in one-week period was counted while in the on-station trial only four sample units were taken per trap using 2.5 sq cm as sample unit and where the adults were most dense. Although the Department of Agriculture provided chrome yellow and SAE 50 as sticky material and yellow polyethylene sheets for YST, these were inadequate hence some farmers made use of other materials. The kind of material, color, size, height from the ground and sticky material used were noted including the stickiness and number of days that the sticky material dried up.

Cultural Management

The recommended cultural management practices in growing potato were followed except when being evaluated. Granola variety (the most commonly grown and severely damaged during the PLM outbreak) was used as standard. Treatments were replicated four times and arranged in randomized complete block design. Sampling on the degree of damage was done at random using 10% of the total crop population per treatment while the total marketable yield was considered.

Irrigation and cropping scheme. Two frequencies of irrigation (3 days and 6 days) using overhead type, three crop rotation, and two rows intercropping using the later (first and third crop rotation) were evaluated. The crop rotation/intercrop study used alternate crops such as carrot, snap beans and cabbage; and non-host

crop such as corn as follows:Hilling-up. Topsoils (at least 7.5 cm thick) were

taken at random from five PLM infested fields one day before and 3 days after hilling up. The soil samples

were put in plastic bowls and the number of PLM pupae was counted by floatation method.

Varietal screening. The protocol in evaluating varieties was followed, i.e. without insecticides and use of generation zero seed tubers to eliminate any effect of bacterial infection that is prevalent in the locality. Two studies were conducted. In the first study, all the entries (Granola, Hermes, NY110, NY112, NY115, Salem, Monona, and Atlantic) except Igorota were susceptible. The second study was conducted to confirm the tolerance of Igorota compared with other local varieties (Raniag, Agria, and Kennebec). Tolerance/Resistance and susceptibility was rated using a range from 1 to 9, where 1 is very tolerant/resistant and 9 very susceptible.

Screening insecticides. Screening of insecticides was done with the consent of the Fertilizer and Pesticide Authority (FPA). A total of 31 insecticides were first evaluated at the start of the pest outbreak following a quick test protocol to identify insecticides in less than one month to contain the pest. Four spray applications at weekly intervals were made in farmers’ fields heavily infested with PLM. Efficacy (%) was computed with the untreated as common factor. Insecticides with an efficacy of 60% and above were given conditional registration by FPA. The promising insecticides (Cyromazine, Cartap and Chlorfenapyr) were further evaluated under greenhouse and field conditions, the standard procedure in evaluating insecticides. Efficacy was based on larval counts, degree of damage and marketable yield.

Results and Discussion

Species of Leafminers in Benguet

Dr. Scheffer of the Systematic Entomology in USA identified the collected leafminers as belonging to the Genus Liriomyza under Family Agromyzidae (Order Diptera). The most dominant species is L. huidobrensis and is very similar to those in South and Central America than in California and Hawaii (Department of

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Agriculture, 2001). Two other leafminer species were also reported: Chromatomyia horticola Goureau on chrysanthemum and Phytomyza horticola Goureau on sweet pea and other crops (Molitas-Colting, 1975).

The Liriomyza species are as follows: Liriomyza chinensis (Kato) on mustard, pechay and radish; Liriomyza huidobrensis (Blanchard) which is polyphagous; Liriomyza sativa Blanchard on beans, tomato, wild tomato, zucchini, cucumber and an unidentified plant; and Liriomyza trifolii (Burgess) on beans, mustard, pechay, pole beans, tomatoes and zucchini.

The Liriomyza species are similar in appearance but can still be differentiated by their size, color markings, number of pores in the posterior spiracles, and site and appearance of mines (Parella, 1996 and Weintraub and Horowitz, 1995). The adult of L. huidobrensis is generally larger, darker with a more pale yellow color in-between the eyes and thoracic pleura, black mesonotum, and brownish yellow antennal segments with the distal end of the third segment sometimes darkened. The eggs are preferably laid near the base of the petiole and the larva prefers to mine along the midrib and lateral veins at the lower leaf surface (Plates 2a-c). The pupa has six to nine pores in the posterior spiracles.

Factors that Favored Outbreak Status

Several factors generally known to enhance the outbreak status of insect pests were noted at Buguias as follows:

1. Absence/Inadequate Population of Natural Enemies

Natural enemies were rarely found (0 to 3 individuals/ten sweeps) in barangays Buyacaoan, Baculongan Norte, Baculongan Sur, Loo Valley, and Bangao all in the municipality of Buguias where the pest outbreak occurred. A similar case was noted at Guinaoang, Mankayan where potatoes were also totally damaged about a month after. In contrast, more natural enemies (up to 10 individuals/ten sweeps) were found from the weeds and potato plants in areas where potatoes showed slight to moderate damage. It is most likely that the pesticides used in the locality killed the natural enemies as in the case in South America (Chavez and Raman, 1987) where L. huidobrensis was under natural biological control until it was secondarily subjected to massive amounts of insecticides. Cahyana (2001) also reported low parasitism (0.9%) on potato in Indonesia where use of pesticide is high.

2. Improper Use of Pesticides

All the potato farmers interviewed had been using insecticides in the past five years in various

Plate 2. Characteristic damage of L. huidobrensis on potato: a) adult punctures and mines by newly hatched larvae, b) larval mines along midrib in upper leaf surface, c) matured leaves showing mines under leaf

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ways such as closer spray intervals from three days (60%) to five days (40%), increased dosage (100%) and cocktail preparations (50%). The common insecticides used include Decis (Deltamethrin), Tamaron (Metamidophos), Selecron (Profenofos), Pegasus (Diafenthiuron), Agrimek (Avermectin), Perfekthion (Dimethoate), Karate (Lambdacyhalothrin), Bulls Eye (Cypermethrin), Hercules (Triazophos), Success (Spinosad), Acetam (Acephate), Basudin (Diazinon) and Magnum (Cypermethrin). Adjuvants such as APSA, Persue and LOC, and detergent such as tide were also used. During the pest outbreak, 70% of the farmers mixed two insecticides such as Tamaron + Selecron (16%), Tamaron + Karate (31%), Tamaron + Decis 100 EC (10%), Selecron + Decis 2.5 EC (21%), Selecron + Malathion (47%), Funguran (a fungicide) + Decis (16%), and Cartap (Cartap) + Siga (Chlorphyriphos) (16%). Diagran (Diazinon) a granular soil insecticide was used alone by 58% of the respondents. Baygon, a household insecticide, was also allegedly used by 2%. The leafminer must have developed resistance to these pesticides as Mujica et al (2001) mentioned this insect is tolerant to many insecticides and is capable of developing resistance to products that were originally effective.

The survey showed that there is a misuse of pesticides in the locality. The ten-year cycle of pest outbreak in Benguet is another proof (Molitas-Colting, 2000) noted in his study that misuse of pesticides are due to the non-compliant attitude of vegetable farmers on product specification including disposal of used pesticide containers, misinformation, sheer negligence and low literacy. In the survey, 68% of the respondents got information on efficacy of pesticides from co-farmers, 53% from pesticide dealers and only 21% from agricultural technicians. The result may likewise imply that there are lapses in the Integrated Pest Management Program of the government because the pest outbreak happened in one of the sites of the Farmers Field School.

3. Monocropping

From 1998 to 1999 potato was planted continuously at Loo Valley by 84% of the farmer respondents where before different crops were planted in succession. The main reason given was potato became very profitable in 1998 reaching as high as PhP80.00/kg for the first time in the history of the potato industry. Those that practiced crop rotation unknowingly must have planted the alternate hosts that are also preferred by the pest.

4. Planting of Susceptible Varieties

Potato is a secondary host of PLM but is highly susceptible in the absence of the major host

chrysanthemum (Braun and Shepard, 1997). The potato Granola variety that is commonly grown in Benguet is highly susceptible and its wide scale plantation contributed to the outbreak status of PLM.

5. Introduction of Infested Planting Materials

The farmers in Buguias claimed that the leafminer on potato originated from the newly introduced chrysanthemum crop at Sitio Tagpaya in 1998. It was in this crop that occurrence of mines on crops was first noted and in the same year potato crops nearby started to show the same symptom although the damage was slight. In 1999, chrysanthemum was not planted and the potato fields started to show moderate damage until it reached its peak in the later part of the year.

6. Change in Climatic Conditions

The outbreak of the pest must have been enhanced also by change in climatic condition brought about by the El Niño phenomenon that affected many parts of the world including Northern Philippines. Local meteorological data from 1998 to 2001 indicated an increase in temperature from 23.50 to 25.61 oC and low rainfall from trace to 48.60 mm while the relative humidity remained high (81 to 90%). Braun and Shepard (1997) claimed that PLM has shorter lifespan when it is warm and humid.

Lifespan, Host Range, Natural Enemies, Distribution and Degree of Damage Lifespan

The lifespan of PLM on potato in Benguet (17 to 25oC and 84-90% RH) ranges from 20 to 31 days from egg laying to adult emergence. The egg turns into a larva in 3 to 5 days, mines the leaves for about 7 to 12 days then pupates in the soil from 10 to 14 days. In view of the short life span, the pest may undergo four to five generations in one potato-cropping season that also last from 4 to 5 months. Since the pupa stays in the soil, the succeeding host crops become infested. The adults may live for 3 days to as long as 27 days if food is available and lay as many as 15 eggs per day. A female lays a total of 200 eggs throughout its lifetime and this accounts for only 10 to 15% of the total punctures thus fewer mines are produced than the punctures on the leaves (Braun and Shepard, 1997).

The egg is oblong (0.3 to 0.4 mm long) and transparent white to yellow-orange. The larva is elongated (2.5 to 3.5 mm long) and changes color from cream to cream yellow when about to pupate. The pupa is 2 to 2.5 mm long and appears cream to dark brown in appearance (Plates 3a-c). The adult body is 2-3 mm long.

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Host Range

About 49 plant species in Benguet serve as host crops for the potato leafminer: 26 species of vegetables, 10 cutflowers and ornamentals, and 14 garden weeds and wild plants. Potato, chrysanthemum, Chinese cabbage, tomato, celery, beans, onions and lettuce were severely damaged during the collection period conforming to the report of Joshi et al (2000). The host crops are as follows:

Natural Enemies

Ten species of larval/pupal parasitoids of PLM reported in other countries (FAO, 2000) were identified from the collections by Dr. J. La Salle of CSIRO as follows: Asecodes deluchii (Boucek), Cirrospilus ambiguous Hanson and LaSalle, Diglyphus isaea (Walker), Hemiptarsenus varicornis (Girault), Neochrysocharis formosa (Westwood), N. okazakii Kamijo, Pnigalio katnois (Ishii) and Quadrastichus liriomyzae (Hanson and La Salle) including unidentified two Opius species (Braconidae). The most common parasitoid that emerged from the collections was D. Iseae, which reported as a registered biological control agent against potato leafminer in Japan apart from Dacnusa sibirica.

Plate 3. Life stages of L. huidobrensis: a) egg, b) larva, and c) pupa.

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General insect predators were also noted such as the staphylinids (most common), coccinelids and earwigs. An adult predator probably Coenosia humilis (Order Diptera) that is reported to be common in Indonesia (Cahyana, 2001), may also be present but this needs confirmation.

Distribution and Degree of Damage

PLM was found in all the 13 municipalities of Benguet (Fig. 1) since its host crops are grown throughout the province. At Loo Valley, La Trinidad Valley and Guinaoang, the damage ranged from slightly severe to very severe. These areas are relatively warmer and criss-crossed by a large river (more humid). Damage was lower (slight to moderate) in the colder areas (Tables 1a-b).

Mines started to appear on mature leaves, first on the leaves then towards the upper leaves as the plant matured (Fig. 2). This is in view of the extrusion of the eggs on young leaves but not in mature leaves (Braun and Shepard, 1997). Severely infested plants showed thousands of pinholes and/or hundreds of mines and succumbed faster to blight infestation. Others were stunted but mostly had dried leaves. Potatoes in the field that were severely mined from vegetative to tuber initiation stage failed to produce tubers. This was confirmed as the critical stages of potato in the simulation study.

Critical Stage of Potato and Pest Monitoring

The critical stage of potato is from seedling up to tuber initiation (Fig. 2) and can only tolerate 25% injury (moderate). With this injury it produced 4 big tubers

Figure 1. Map of Benguet Province showing the distribution and degree of damage caused by L. huidobrensis (December 1999- March 2000).

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Table 1a. Degree of damage of L.huidobrensis on potato at Loo Valley (warmer area) where outbreak oc-curred (Buguias, Benguet; Dec. 1999) to June 2001).

Table 1b. Degree of damage of L.huidobrensis on potato at Natubleng and Sinipsip (colder areas) (Buguias, Benguet; Dec. 1999) to June 2001).

a/ Degree of damage was rated as follows:1 - sound, no leaves with mines and pin holes3 - slight, <25% of leaves with mines and pin holes5 - moderate, >25% to 50% of leaves with mines and pin holes7 - slightly severe, >50% to 75% of leaves with mines and pin holes9 - very severe, >75% to 100% of leaves with mines and pin holes

Figure 2. Growth stages of potato: a) susceptible stages to L. huidobrensis, and b) tuber formation as the plant matures.

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and none when damage was very severe (Fig. 3). This finding corroborates with the report of Braun and Shepard (1977) that potatoes can tolerate 30% injury.

The adult population can also be used to monitor pest damage by using the formula Y = 0.5227 + 0.3157 where Y is the degree of damage and X is the number of adults trapped in the YST. If there are 10 adults moderate damage is expected. This formula was taken from the correlation and regression analysis of degree of damage and number of adults trapped in four sq inch of at least four YST staked in the field for at least 30 min. It is best to monitor the adult population between 10:00 o’clock a.m. to 2:00 o’clock p.m. on sunny days and calm breeze when the adults are most active in flying as shown in (Fig. 4).

Cultural Management Studies

Irrigation and Fertilizer Management

There was regular rainfall from the middle part of the study until the crops reached maturity although it was dry season hence the result on irrigation is

considered preliminary. In the fertilizer trial (Table 2), those plants with processed chicken manure (PCM) combined with T-14 as basal were significantly taller, moderately damaged and had the highest marketable yield (12.60 t/ha). On the contrary, those with PCM + foliar got stunted (14.47 cm), with moderately to slightly severe damaged, dried earlier and produced the least marketable yield (6.31 t/ha). Probably the application of foliar made the plants more favorable for the pest to lay eggs and/or feed.

Crop rotation/Intercropping

The effect of corn as rotation crop and intercropping of carrots, cabbage and beans that are least preferred alternate hosts (FAO, 2000) on the degree of damage, growth, and yield of potato was insignificant indicating that these crops are effective lowering the pest population. The potato and all the other crops were slightly and moderately damaged. The planting of corn (not a host crop) and least preferred crops as intercrops created a condition where there was no abundant food source. The result also showed that corn does not affect the yield of potato in succession.

Figure 3. Critical stage of potato to injury in a simulation study under greenhouse condition.

Figure 4. Mean number of adults of L. huidobrensis trapped in YST staked at certain times of the day in potato fields at Loo Valley (sunny, 16 to 21°C and calm wind).

6:00-8:00am

8:00-10:00am

10:00-12:00nn

12:00-2:00pm

2:00-4:00pm

4:00-6:00pm

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Hilling-up

Hilling-up potato and other vegetables in Benguet is a common practice intended to cover fertilizers that are side dressed during the vegetative stage of the crop and in the case of potato, to provide more space for the growing tubers so that these will not be exposed to the sun before these are harvested. In the study it showed that hilling-up could reduce pest population whose pupa stays in the soil like PLM. Figure 5 shows that the pupa of leafminer in the soil was reduced by more than 50% through hilling-up. Second hilling-up may be recommended if the pest population is high in order to kill the pupa of the second generation.

Evaluation of Potato Varieties

The Igorota variety, a selection of the Northern Philippines Rootcrop Research and Training Center

Table 2. Effect of irrigation and fertilization on the degree of damage L. huidobrensis and on growth and yield of potato (Loo Valley, Dry Season 2001)

a/ Means with the same letter in a column are not significantly different at 5% level of significance using DMRT.b/ Moderate damagec/ Moderate to slightly severe damage

(NPRCRTC) was the most tolerant/resistant as shown in Table 3. Leaf damage ranged from slight to moderate up to two months after emergence similar to Agria. Unlike Igorota, Agria became severely damaged thereafter while Igorota regenerated new leaves thus the marketable tuber was higher by about half tons. Granola and the other varieties were highly susceptible to leaf damage. These were moderate to severely damaged during their vegetative to tuber initiation stage resulting in the production of lesser marketable yield except Columbus. Columbus gave the highest yield (23.83 tons/ha) but not significantly different than that of the Igorota. The high yield of Columbus despite being susceptible may be attributed to the quality of planting materials. Columbus was newly imported and it has been observed in previous trials at NPRCRTC that imported certified

Table 3. Response of potato varieties to L. huidobrensis infestation (Loo Valley, dry season crop 2001).

a/ Rating Index1 - very resistant/tolerant, no leaves with mines and pinholes3 - slightly resistant/tolerant, <25% of leaves with mines and

pinholes5 - moderately resistant/tolerant, >25% to 50% of leaves with

mines and pinholes7 - slightly susceptible, >50% to 75% of leaves with mines and

pinholes9 - very susceptible, >75% to 100% of leaves with mines and

pinholesb/ MAE - month after emergencec/ Means with the same letter are not significantly different at 5% level of

Figure 5. Effect of hilling-up on the pupal population of L. huidobrensis on potato.

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seeds generally produce higher yields when planted for the first time.

The YST as Tool in Managing PLM

Color of Sticky Trap

Among the different colors evaluated, yellow was the most preferred followed by green, red, blue, white and orange while the least preferred was black (Fig. 6). This confirms that yellow attracts more adults of PLM compared with the other colors. This result though implies that other colors may be used in case there are no yellow materials. It also explains why YSTs with sticky material that blackens the yellow background were less effective. In a separate study, the adults did not discriminate the shades of yellow. Chrome yellow, however, is recommended since it does not fade easily.

Distance in Staking YST

The influence of distance on the efficacy of YST was insignificant in another study indicating that YST’s may be staked up to 15 sq m apart or even more. Closer distance, however, is recommended to catch the most number of adults (Table 4) especially when the pest population is high. Surrounding the fields totally with YST may also be done to prevent entry of PLM adults from adjoining fields as was observed in farmers’ fields.

Material, Design and Size

Any material could be made into a leafminer trap as long as it uses sticky substances as shown in Plates 4a-j. During the pest outbreak, farmers used varied kinds of YST most of which were recycled materials and

Figure 6. Color preference of L. huidobrensis using sticky traps staked in potato fields (Sunny, 15 to 24°C and calm wind).

Table 4. Mean number of L. huidobrensis adults trapped in yellow plastic YST (25 cm diam) installed one week at vari-ous distances in a potato field (La Trinidad Valley, dry season

a/ Means followed by the same letter are not significantly different at 5% level of significance using DMRT.

yet they caught thousands of adults. These were drum covers, empty drums, galvanized iron, empty fungicide containers, rice sacks, plastic sheets, kitchen plates, and even plastic drinking straws used for soft drinks. Some painted yellow the stone-walls around potato fields, outer walls of cemented irrigation tanks, and even nearby walls. Some farmers totally surrounded their farms with yellow plastic sheets or rice sacks, to keep away stray animals in the field and also serve as windbreak.

Table 5 however shows that the efficacy of the YSTs differed significantly. The most effective was the smallest material, the soft drink straw YST. It trapped the most number of adults with 531 in one week compared with recycled fungicide bag (501 adults) and the polyethylene sheet YST (standard) with only 40 adults. A similar result was noted at the on-station trial. Based on the fecundity rate of 200 offspring per female, the soft drink YST trapped more than 20% or even more of the emerging adults in the first generation. The differences may be attributed to the placement of the traps rather than the kind and size of material. The

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Plate 4. Kinds of YST used by farmers a) drum cover, b) yellow plastic plates (recommended), c) GI sheet, d) plywood (recom-mended), e) close up of plywood YST showing trapped black PLM adults, f) empty drum, g) kerosene can, h) plastic bottle, i) plastic sheet, and j) rice sacks (surrounding the crops) and individual yellow plastic sheets.

a b c

d e

f g h

i j

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Table 5. Mean count of adults of L. huidobrensis on YST staked in potato fields during the pest outbreak (2000) and after the pest outbreak (2001) (Loo, Valley, dry season crop).

a/ Means followed by the same letter per study not significantly different at 5% level of significance using DMRT.

soft drink straw and the fungicide bag YST (501 adults) were staked just beside the plants and edge of potato fields thus trapping the adults emerging from the soil and those coming from adjoining areas, respectively. Moreover, the straw was hanged so it rotated freely as it is blown by the wind and thus all sides become an effective trap. The inefficiency of the polyethylene sheet is may be due to its friability and somewhat waxy

surface while that of the yellow pan trap (5.21 adults) may be due to the flight pattern of PLM adult. This may be the same reason why the drum cover YST that was laid flat on the ground surface also trapped fewer adults despite its being bigger than the straw.

Effective Height of YST

Figure 7 shows that the number of adults that will be

Figure 7. Mean number of trapped adults of L. huidobrensis on YST (25 cm diam plastic plate) staked in a potato field at different heights above the ground (La Trinidad Valley, 2001)

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Table 6. Mean count of PLM adults trapped on YST (450 sq cm) set at various heights from the ground/crop canopy (La Trinidad Valley, July to August 2001).

a/ Means followed by the same letter are not significantly different at 5% level of significance using DMRT.

Figure 8. Number of trapped adults of L. huidobrensis in YST (25 cm diam) using different sticky materials staked in potato fields (La Trinidad Valley, March 2001).

trapped decreases as the height of the YST increases indicating that PLM adults do not fly high above the crop canopy probably except when these are carried by strong wind current. It is best to stake YST 30 cm above the crop canopy than when it is at 60 cm and beyond. The statistical analysis in a different study (Table 6) confirmed such result. Staking the YST beside the plant more or less traps adults emerging from the soil especially if the area was previously planted with a host crop while the YST just above the crop canopy traps those adults coming from adjoining fields.

Sticky Materials

The sticky materials used by farmers during the outbreak period varied: used motor grease, kerosene and motor oil. The motor oil were SAE 40, SAE 50 and SAE 90 while the motor grease were No. 1 and No. 3. All these sticky materials were able to trap PLM adults but with varying extent with Motor grease No. 3 as the best followed by SAE 40. Motor Grease No. 3 and SAE 40 can be spread readily using ordinary paintbrush, they are somewhat transparent so they do not discolor the trap, and do not drip easily. These may be the reasons why the YST with these materials caught

many leafminer adults. They can be easily washed off so that the trap material can be reused. The other two brands of Motor grease are difficult to spread on the trap especially when it is cold. The used motor oil blackens the yellow color of the trap and drips when the sun is hot. Kerosene on the other hand, is not as sticky as the other materials. It is therefore necessary to consider the kind of sticky material with respect to the temperature of the place. Figure 8 shows that Motor grease No. 3 is better than SAE 40 and the other motor greases under the conditions in La Trinidad.

Efficacy of YST

The high density of trapped adults in the different YST used by farmers in Plates 4a-j showed that they reduced PLM adults during the pest outbreak that in about three months the succeeding alternate hosts were moderately damaged. This was coupled with the uprooting and burning of infested leaves, and planting of non-host crops. Apart from the reduction in the number of females that will lay eggs in the next generation was the reduction in the puncture injuries on the leaves. The computed number of adults that can be trapped in a 30 x 40 cm YST is about 14,000

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and this size will be enough to kill the emerging adults in the second generation.

In Table 7, the trapping of adults had a carry over effect on the reduction of the larval population by 2.30% and the pupal population by 9.00%, and resulting in the increased market yield by 33.00%. YST also provides complimentary control to insecticide. In the case of Cartap the larval population gathered from the treatments without trap was greater by 67.74% compared to the treatments without YST. YST may then be necessary especially if the insecticide is an ovicide or larvicide.

The computed amount to be spent per hectare in using yellow plastic plate YST (25 cm diam) is PhP7,245 and if used in combination with an insecticide (Cyromazine) the return of investment (ROI) is 88.32%. Compared to the traditional practice of weekly application of insecticides the expenses per hectare is PhP120,000 with lower ROI of 58.33% this is apart from the possible risk to health due to exposure to insecticides. Apart from being effective (it is being used by chrysanthemum growers to control leafminer) this kind of YST can be reused for two or more cropping periods because it is sturdy and can be washed. There

Table 7. Effect of YST and insecticides on the population of larva and pupa of PLM and on computed yield of potato (Loo Valley, dry season 2001)a/

a/ Means with the same letter in a column are not significantly different at 5% level of significance using DMRT.

is no added cost for paint and it can also be hanged like the soft drink straw YST made to rotate and trap adults from every direction. It is also available in the local market at PhP8.00 and can be easily obtained by farmers far from the city.

Need-based Used of Insecticide

Using the data on the economic injury level (more than 25 to 50% damage or moderate damage), and the critical stage of the crop (one week to 1 1/2 month or vegetative stage to tuber initiation stage), the return of investment to be derived when insecticide is applied only when necessary is 88.32% as shown in Table 8. In the traditional method where there is calendar application of insecticide at weekly interval even if the pest population is below the economic injury level and the plant reached bulking stage, the return of investment is 58.33%. In the current computation, there will be an added income of PhP20,000 if the need-based use of insecticide is followed. This is apart from the added benefits of not being exposed to insecticides.

Efficacy of InsecticidesIn the initial study (Table 9) where 26 insecticides were

Table 8. Computed return of investment on potato (Granola variety) following a need-based use of insec-

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Materials/Inputs Required

Table 9. Efficacy of different insecticides evaluated against PLM larvae in a quick response research during the pest outbreak (Loo Valley, dry season 2000)

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Figure 9. Degree of damaged caused by L. huidobrensis at different growth stages of potato treated with insecticides under greenhouse condition (La Trinidad Valley, Dry season 2000).

Figure 10. Computed yield (t/ha) on potato treated with insecticides against L. huidobrensis under green-house condition (La Trinidad Valley, Dry season 2000).

Figure 11. Degree of damage (%) caused by L. huidobrensis at different growth stages of potato treated with insecticides under field condition (La Trinidad Valley, Dry season 2001)

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Figure 12. Computed yield (t/ha) of potato treated with insecticides against L. huidobrensis under field condition (La Trinidad Valley, Dry season 2001)

evaluated Cyromazine (Trigard 75 WP), Chlorfenapyr (Kutetso) and Cartap (Dimotrin, Vegetox and Cartap) were found effective. The follow-up studies under greenhouse and field conditions showed similar results (Figs. 9-12). The result on Cyromazine conformed to the report (Ciba, 1994) that it is highly effective against PLM. It also mentioned that Cyromazine is compatible with IPM since it is harmless to the parasitoid D. iseae that also occurs in the locality.

ReferencesBraun AR & Shepard M. 1997. Leafminer fly: Liriomyza

huidobrensis Technical Bulletin. CIP, Lima, Peru.

Cahyana W. 2001. Vegetable leafminers in Indonesia. In Proceedings of National Leafminer Management Workshop, NTC-ATI, La Trinidad, Benguet, July 406, 2001. Pp. 28-39.

CIBA. 1994. Trigard: Technical product information. 24 pp.

Chavez GL & Raman KV. 1987. Evaluation of trapping and trap types to reduce damage to potatoes by the leafminer Liriomyza huidobrensis (Diptera: Agromyzdiae). Insect Sci. Appl. 8:369-372.

Department of Agriculture. 2001. “Friendly wasps” fight leafminers in the Cordilleras. Aggie Trends. 14(2):4.

Domoguen RL. 2000. Status of leafminer infestation in Benguet. BSU Res. Digest. 2(1):4,5,8.

FAO. 2000. Progress Report: Vegetable IPM Program in Indonesia (1998-2000).

Joshi RC, Scheffer SS, Verzola EA & Baucas NS. 2000. Primer on leafminers of vegetables in the Cordillera Administrative Region, Philippines.

DA-RFU-CAR, Baguio City. 23 pp.

Mason GA, Johnson MW & Tabashnik BE. 1989. Effects of biological and operational factors on development of insecticide resistance in Liriomyza (Diptera: Agromyzidae). J. Econ. Entomol. 82:369-373

Molitas-Colting L, Verzola EM, Ligat BS, Mangili TK, Sabularse DC & Saavedra NT. 2000. The leafminer (Liriomyza spp.): A serious threat to the vegetable industry in the Cordillera; A model for cooperative and integrated approach in pest management (Abstract). Phil. Entomol. 15(2):200

Mujica N, Pravantiner M & Cisneros F. 2001. Effectiveness of Abamectin and plant-oil mixtures on eggs and larvae of the leafminer fly, Liriomyza huidobrensis Blanchard. Scientist and Farmer: Partners in Research for the 21st Century. Program Report 1999-2000. International Potato Center, Lima, Peru. Pp. 161-166.

Mujica N & Cisneros F. 1997. Developing IPM components for leafminer fly in the Cañete Valley of Peru. Program Report 1995-1996. International Potato Center, Lima, Peru. Pp. 177-184.

Parella MP. 1996. Leafminer. Part I: Accurate identification. Grower Talks. October. Pp. 34-43.

Parella MP & Keil CB. 1984. Insect pest management: The lesson of Liriomyza.

Bull. Entomol. Soc. Amer. 30:22-25.

Weintraub PG & Horowitz AR. 1995. The newest leafminer pest in Israel, Liriomyza huidobrensis. Phytoparasitica. 23(2):177-184.

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