Entomology

Insecticide Efficacy Monitoring in Greenhouse Whitefly Populations

Dr. Nick C. Toscano, Principal Investigator, Department of Entomology, University of California, Riverside, CA 92521 ph: 909-787-5826 e-mail: [email protected]

Collaborators: Dr. Frank G. Zalom, Dept. of Entomology, UC Davis (Co-PI), Jian Bi, Dept. of Entomology, UC Riverside; Greg Ballmer, Dept. of Entomology, UC Riverside; Shu-Jen Tuan, Taiwan Agricultural Chemicals and Toxic Substances Research Institute, Wu-Feng, Taichung, Hsien, Taiwan

Dr. Nick C Toscano, Dr. Greg Ballmer and Dr. Jian Bi (left to right.) Department of Entomology University of California, Riverside

Summary: Susceptibility of adult greenhouse whitefly (Trialeurodes vaporariorum) on strawberries to five broad-spectrum insecticides was monitored in three different regions of southern California using a yellow sticky card technique. Methomyl (Lannate), at a rate of 0.25 times the label recommended application rate, killed nearly 100% of adult whiteflies, whereas fenpropathrin (Danitol), endosulfan (Thiodan), malathion, and chlorpyrifos (Lorsban) at 1, 2, and 4 times the label recommended application rates killed less than 50%, 70% and 100% of adult whiteflies, respectively. Populations of GHWF at Irvine and Oxnard were also tested for susceptibility to the insect growth regulators (IGRs) pyriproxyfen (Esteem) and buprofezin (Applaud). Pyriproxyfen caused 100% egg mortality at 0.025 times the label recommended application rate and Buprofezin killed more than 98% of nymphs at half of the label recommended application rate.

Resúmen: Susceptibilidad del adulto de la mosquita blanca de los invernaderos(Trialeurodes vaporariorum) en la fresa cuando se estudiaron cinco insecticidas de amplio espectro fue monitoreado en tres diferentes regiones del sur de California usando la técnica del cartón amarillo con pegamento. El Methomyl (Lannate), a una concentración de 0.25 veces la dosis de aplicación recomendada en la etiqueta, se obtuvo aproximadamente el 100% de mortalidad de la mosquita blanca adulta, mientras que fenpropathrin (Danitol), endosulfan (Thiodan), malathion, y chlorpyrifos (Lorsban) a 1, 2, y 4 veces la dosis de aplicación recomendada en la etiqueta se obtuvieron porcentages de mortalidad menores al 50%, 70% y 100% de mosquita blanca adulta, respectivamente. Las poblaciones de la mosquita blanca de los invernaderos(GHWF) en Irvine y Oxnard fueron también probadas por la susceptibilidad del insecto a los reguladores de crecimiento (IGRs): pyriproxyfen (Esteem) y buprofezin (Applaud). El pyriproxyfen causo 100% de mortalidad en huevecillos a 0.025 veces la dosis de aplicación recomendada por la etiqueta y el porcentage de mortalidad de ninfas con Buprofezin fue de más del 98% usando la mitad de la dosis de aplicación recomendada por la etiqueta

Introduction: The greenhouse whitefly (GHWF) is a major pest of strawberries (Fragaria X Annanassa L.) and vegetable crops in coastal southern California (Bi et al, 2001b, 2002b). Damage to strawberries includes potential virus infection, yield reduction, contamination with honeydew excretions and sooty mold, and lowered fruit quality by reducing sugar and both ascorbic and citric acid content (Bi et al, 2003). Because GHWF immatures are on the underside of leaves and the sedentary nymphs have a waxy covering, conventionally applied broad-spectrum insecticides are more effective against adults and first instar crawlers. Populations of GHWF were exceptionally dense during 2002 and early 2003, but much lower in the second half of 2003. The cause of these fluctuations is uncertain, although it may stem from changes in the use of insecticides, including recent wider use of IGRs and systemic neonicotinoids, together with climatological factors.

Insecticides are important components of integrated pest management programs designed to suppress GHWF. Our recent research focuses on investigating the effectiveness against GHWF of new insecticides with novel modes of action and monitoring the effectiveness of other insecticides currently available to growers. We previously evaluated and demonstrated the effectiveness of novel and standard insecticides against GHWF on strawberries in the Oxnard area (Bi et al, 2000, 2001a, 2002a, 2002c). We also demonstrated the importance of early application (preferably at planting) of neonicotinoid compounds, such as imidacloprid (Admire), and thiamethoxam (Platinum) (Bi, et al, 2001a) and that control persisted for at least three months (Table 1). Here we report our 2002-03 bioassay test results on susceptibility of GHWF in strawberries to seven insecticides in Ventura, Orange and San Diego Counties. These include five conventional broad- spectrum insecticides (plus one combination of two compounds) and two IGRs.

Materials and methods: Adult GHWF Bioassay tests on adult GHWF were conducted simultaneously at locations near Oxnard, Irvine, and Vista in Ventura, Orange, and San Diego Counties, respectively, using yellow sticky cards coated with insecticides. The sticky card technique offers a rapid means of determining current tolerance/resistance levels of local GHWF populations to contact insecticides. This is especially useful because insecticide resistance levels are often labile and vary according to local insecticide selection pressures. Our tests were conducted using fenpropathrin, endosulfan, malathion, chlorpyrifos and methomyl at approximately monthly intervals from March 2002 to May 2003 in strawberry fields having sufficiently high GHWF populations. GHWF populations later in 2003 were too low to employ this technique.

Sticky cards were prepared using 3 X 5 inch yellow vinyl plastic cards coated with a thin layer of Tanglefoot© adhesive on one side. A six-inch plastic pot label stake was stapled to each card to facilitate handling. Chemicals were applied to the sticky side of cards with a Potter Spray Tower© using commercial formulations diluted in water to simulate tank mixes (at 20 gallons of tank mix per acre) of recommended maximum application rates, and multiples or fractions thereof. Cards were prepared with chemicals applied at 0 (control), 0.03, 0.06, 0.125, and 0.25 times the label recommended field application rate for methomyl; all other materials were applied at 0 (control), 1/4, 1/2, 1, 2, 4, and 8 times the label recommended field application rate. The label recommended rates were 10.66 oz, 1.33 qt, 1 qt, 1 qt, and 1 lb per acre for danitol, endosulfan, malathion, chlorpyrifos and methomyl, respectively. Half the label recommended rates each for fenpropathrin and malathion in combination were used for the “label recommended rate” for that mixture. We used 4-6 replicates of each rate for a given test. Cards were prepared within a few hours of use and transported to the field in insulated ice chests to which a small amount of water was added to maintain high humidity. A styrofoam platform into which the pot stake card handles were inserted facilitated storage. All cards for each chemical were kept together with an untreated control card in separate ice chests for each chemical.

Sticky cards were exposed to whiteflies in the field by holding them near infested strawberry plants, which were gently disturbed to induce flight. Whiteflies were attracted to the cards and adhered to the sticky coating. When the cards had collected sufficient whiteflies (50-100 in the central portion), they were replaced in the ice chests and returned to the laboratory, where they were incubated for 24 hours at room temperature (ca 70° F). Whitefly mortality was determined by inspecting test cards, using a dissecting microscope, at 24 hours after the start of exposure. A needle was used to prod the whiteflies if they were not obviously alive; individuals that could move at least one leg or wing were rated alive; all others were considered dead.

Dosage-mortality data was analyzed using the POLO log probit software package to determine effective concentrations of insecticides needed to achieve 10, 50, and 90 % mortality.

Immature GHWF Bioassay tests of IGRs on immature GHWF were conducted in the laboratory using potted strawberry plants (cv. Camarosa) and GHWF from two locations near Oxnard and one site in Irvine. In this procedure, forty adult whiteflies collected at test sites were confined per clip-cage for 24h on the underside of the middle leaflet of the youngest fully expanded trifoliate to induce oviposition. The adults were then removed.

For pyriproxyfen tests, the leaflets with GHWF eggs were sprayed with pyriproxyfen solution until run-off to ensure complete coverage of the leaf surface. Test concentrations of pyriproxyfen were 0 (control), 0.02, 0.2, 0.8, and 3.2 ppm (mg AI/ml); the last concentration represents one-tenth of the label recommended application rate (0.54 lb AI/a in 20 gal H2O). All treatments were prepared in water containing 0.01% Triton X-100, and the control plants were sprayed with water containing 0.01% Triton X-100 only. Five leaves were used for each treatment rate and the control. The numbers of eggs on each leaf were counted before the application of pyriproxyfen. Leaves with a minimum of 30 eggs were used for the tests. Egg mortality was assessed from the 7th day to four weeks after treatment and was calculated by subtracting the number of live first instar nymphs from the total number of eggs laid. Mortality of immatures was assessed 28 days after treatment by comparing the numbers of live and dead nymphs. Nymphs were considered dead if they were dry and could be separated easily from the leaf surface.

For buprofezin tests, egg hatch and number of 1st instar nymphs were monitored daily. By the 12th day post-oviposition, almost all eggs had hatched. Leaflets were then sprayed with buprofezin solution until run-off to ensure complete coverage of the leaf surface. The concentrations of buprofezin were 0 (control) 16.4, 65.6, 262.5, and 1050 ppm (mg AI/ml); the highest concentration represents half of the label recommended application rate (0.35 lb AI/a in 20 gal H2O). All solutions were prepared in water containing 0.01% Triton X-100, and the control plants were sprayed with water containing 0.01% Triton X-100 only. Five leaves were used for each treatment rate and the control. Immature mortality was assessed by daily counting of dead nymphs from 7 to 28 days after treatment.

Results: Our results showed that fenpropathrin, endosulfan, malathion and chlorpyrifos were much less toxic to adult whiteflies in all locations than was methomyl. At 1, 2, and 4 times the label recommended application rates, fenpropathrin, endosulfan, malathion and chlorpyrifos killed less than 50%, 70% and 100% of adult whiteflies, respectively (Tables 2, 3). Malathion in combination with fenpropathrin yielded marginally higher mortality than either material alone. Methomyl, at a rate of one quarter the recommended field application rate, killed nearly 100% of the whiteflies (Tables 2, 3). Some of the observed temporal fluctuations in chemical toxicity in fields sampled multiple times may be attributed to differences in the age of and variable conditions to which whiteflies were exposed prior to testing, as well as to real genetic differences responsible for chemical tolerance.

Our data indicate that GHWF immatures were highly susceptible to pyriproxyfen and buprofezin. Pyriproxyfen caused complete inhibition of egg hatching at 0.025 times the label recommended application rate (0.8 mg AI/ml) and 50.4-69.8% egg mortality at 0.0006 times the label recommended application rate (0.02 mg AI/ml) (Fig. 1, Table 4). Pyriproxyfen also caused 82.3- 94.7% mortality of 1st and 2nd instar nymphs from surviving eggs at 0.02 ppm (Table 5).

Buprofezin killed more than 88% and 98% of nymphs, respectively, at 0.125 and 0.5 times the label recommended application rates (262.5 mg AI/ml and 1050 mg AI/ml, respectively) (Table 6). Pyriproxyfen is currently registered for use on strawberries, while Buprofezin is not.

Discussion: Our work demonstrates that the susceptibility of adult GHWF to conventional broad-spectrum insecticides used on strawberries varied greatly by material and location. Susceptibility of GHWF to these compounds was generally stable or decreased somewhat over the observation period at given locations. It is remarkable that very high concentrations of most materials (with methomyl being a notable exception) were needed to kill 100% of test subjects within 24 hours. These results do not necessarily indicate the efficacy of the test compounds under field conditions, where variables of temperature, humidity, sunlight intensity, and contact with GHWF may affect both their toxicity and persistence.

The demonstrated effectiveness of the two IGRs tested is a potent argument for their inclusion in the arsenal of chemicals available for a GHWF IPM program. At this time, only pyriproxyfen is registered for use on strawberries. These materials, together with the neonicotinoids admire and platinum, are much less likely to adversely affect beneficial predators and parasitoids than are the traditional broad-spectrum insecticides. Also, because they represent entirely different classes of compounds, they are well suited for inclusion in an IPM program in which rotation of insecticides having different modes of action is crucial to forestalling insecticide resistance.

References: Bi, J. L., N. C. Toscano, and G. R. Ballmer, 2000. Field evaluation of various insecticides against the greenhouse whitefly on fall-planted strawberries. The Pink Sheet, California Strawberry Commission 00-09, Watsonville, CA.

______2001a. Field evaluation of novel insecticides against the greenhouse whitefly on summer-planted strawberries. The Pink Sheet, California Strawberry Commission 01- 06, Watsonville, CA.

______2001b. Seasonal population dynamics of the greenhouse whitefly Trialeurodes vaporariorum in Oxnard area. The Pink Sheet, California Strawberry Commission 01- 12, Watsonville, CA.

______2002a. Greenhouse and field evaluation of six novel insecticides against the greenhouse whitefly Trialeurodes vaporariorum on strawberries. Crop Prot. 21: 49-55.

______2002b. Seasonal population dynamics of the greenhouse whitefly Trialeurodes vaporariorum (Homoptera: Aleyrodidae) on strawberries in Southern California. J. Econ. Entomol. 95 (6): 1179-1184.

______2002c. Field evaluations of novel chloronicotinyls and insect growth regulators against the greenhouse whitefly on strawberry. Hort Science 37 (6):914-918.

Bi, J. L., S. J. Tuan, G. R. Ballmer, and N. C. Toscano, 2003a. Susceptibility of the greenhouse whitefly in different locations of southern California to insect growth regulators Esteem and Applaud. The Pink Sheet, California Strawberry Commission 03-05, Watsonville, CA.

Bi, J. L., S. J. Tuan, G. R. Ballmer, and N. C. Toscano, 2003a. Impact of novel insecticide applications against the greenhouse whitfly on strawberry fruit quality. The Pink Sheet, California Strawberry Commission 03-09, Watsonville, CA.

Tables and Figures

Table 1. Duration of activity of neonicotinoid and IGR insecticides for control of the greenhouse whitefly on summer-planted strawberries.

Insecticide Rate/ac When Reduction of Whitefly Suppression in Applied (% & range in days) weeks adults 1st-2nd 3rd-4th Imidacloprid 32 oz 3 weeks 58-90 78-93 42-86 All stages -11 (Admire 2F) post- plant “ “ pre-plant 71-83 58-74 52-74 “ Thiamethoxam 11 oz 3 weeks 58-80 78-93 48-80 Adults, 1st, 2nd (Platimun 2F) post- instars - 6 plant 3rd-4th instars -10 Buprofezin 8 oz 6 weeks 25-81 61-92 45-100 All stages-7 (Applaud 70 WP) post-plant Pyriproxyfen 8oz “ 40-73 51-100 37-87 “ (Esteem 0.86 EC)

Table 2. Susceptibility of adult greenhouse whiteflies in Oxnard area to fenpropathrin, malathion, fenpropathrin + malathion, chlorpyrifos, endosulfan, and methomyl.

Field Material Date Effective dose* location LD10 LD50 LD90 Hailes fenpropathrin March 2002 0.4 9 220 Rd. “ April 2002 1.8 6 23 “ October 2002 0.3 1.5 8 “ May 2003 4 16 72 malathion April 2002 0.6 5.3 46 “ October 2002 0.1 0.56 2.9 “ May 2003 2.9 13 54 fenpropathrin + October 2002 0.1 0.7 4.7 malathion “ May 2003 2.1 9 40 chlorpyrifos April 2002 0.5 1.8 6 “ May 2002 1 1.9 3.5 “ October 2002 1.0 3.6 13 endosulfan May 2002 1.8 3 5.6 “ October 2002 0.9 3.3 12 methomyl April 2002 0.01 0.05 0.2 “ October 2002 0.02 0.1 0.3 “ May 2003 0.02 0.1 0.4 Laguna fenpropathrin June 2002 0.5 2.1 8.5 Rd. malathion “ 2 5.4 15 chlorpyrifos “ 1 2.6 6 endosulfan “ 0.3 1.8 11 methomyl “ 0.03 0.08 0.2 Hueneme fenpropathrin June 2002 0.8 3 11 Rd. malathion “ 0.5 1.65 5 chlorpyrifos “ 0.7 2.6 9.4 endosulfan “ 0.8 3 11 methomyl “ 0.01 0.07 0.3 Rice Rd. fenpropathrin March 2003 2.2 13 81 Malathion “ 3 18 111 fenpropathrin + “ 1.3 8 49 malathion * Expressed as multiples of field application rate

Table 3. Susceptibility of adult greenhouse whiteflies in Irvine and Vista areas to fenpropathrin, malathion, fenpropathrin + malathion, chlorpyrifos, endosulfan, and methomyl.

Location Material Date Effective dose* LD10 LD50 LD90 Sand fenpropathrin March 2002 0.1 3 10 Cyn. Rd, “ April 2002 0.1 2 31 Irvine “ May 2002 0.7 2 7 “ June 2002 0.9 2.4 6 “ March 2003 1.8 9 47 malathion April 2002 0.25 5.4 118 “ May 2002 0.5 5 48 “ June 2002 0.6 3 18 “ March 2003 2.4 14 89 fenpropathrin + July 2002 0.04 0.3 2.5 malathion “ March 2003 1.3 8 48 chlorpyrifos March 2002 0.4 2 10 “ April 2002 0.6 2.4 9.5 “ May 2002 0.7 2 6 “ June 2002 0.8 4 25 endosulfan May 2002 0.27 1 5 “ June 2002 0.15 0.8 4.5 methomyl March 2002 0.004 0.015 0.06 “ April 2002 0.001 0.016 0.18 “ May 2002 0.006 0.035 0.20 “ June 2002 0.008 0.035 0.16 “ March 2003 0.06 0.14 0.35 River fenpropathrin April 2002 0.5 4.5 40 ranch, “ May 2002 0.25 4.9 94 Vista area chlorpyrifos April 2002 0.7 1.8 - “ May 2002 0.5 2 8.6 endosulfan April 2002 0.13 0.9 7.3 “ May 2002 0.3 3.1 33 · Expressed as multiples of field application rate.

Table 4. Hatching rate of greenhouse whitefly eggs treated with pyriproxyfen from three locations.

Mean % egg-hatching rate Rate (ppm) Sand Canyon Rd. Hailes Rd. Las Posas Rd. (Irvine) (Oxnard) (Oxnard) 3.2 0 0 0 0.8 0 0 0 0.2 6.1 4.3 3.9 0.02 30.2 49.6 41.0 Control 98.4 98.4 98.8

Table 5. Mortality of immature greenhouse whiteflies treated with pyriproxyfen from three locations.

Mean % mortality Rate (ppm) Sand Canyon Rd. Hailes Rd. Las Posas Rd. (Irvine) (Oxnard) (Oxnard) 0.2 100 100 100 0.02 94.7 82.3 85.9 Control 7.9 9.9 7.6

Table 6. Mortality of immature greenhouse whiteflies treated with buprofezin from three locations.

Mean % mortality Rate (ppm) Sand Canyon Rd. Hailes Rd. Las Posas Rd. (Irvine) (Oxnard) (Oxnard) 1050 98.28 99.29 98.22 262.5 88.82 88.40 93.21 65.6 73.08 75.24 82.90 16.4 66.26 66.41 77.77 Control 25.48 24.41 32.65

Control 100 0.02 ppm 90 0.2 ppm 80 0.8ppm 70 60 50 40 30

Egg hatching rate (%) 20 10 0 -10 7D 14D 21D 28D Days post application

Figure 1 Efficacy of pyriproxyfen at various concentrations on cumulative egg hatching rate of the greenhouse whitefly.