Section C: Chemical Control, Biorationals and Pesticide Application Technology - 2000

Section C (1999)

Investigator’s Name(s): D. H. Akey and T. J. Henneberry.

Affiliation & Location: USDA, ARS, Western Cotton Research Laboratory, Phoenix AZ 85040-8803.

Research & Implementation Area: Section C: Chemical Control, Biorationals and Pesticide Application Technology

Dates Covered by the Report: June-September 1999

Azadirachtin (as Bollwhip ™ ), a Biorational Agent Against the Silverleaf Whitefly, Bemisia argentifolii, in Field Trials in Upland Cotton in Arizona

Deltapine NuCOTN 33^B was planted and furrow irrigated in plots 109 ft. in length and 12 rows across (40-in. rows). Plots were separated by 2 fallow rows and 20 ft alleys. Spray applications were made by a ground boom with 5 nozzles/row, 1 overhead, and 2 swivel nozzles angled upward on a drop on each side of the row, at 250 psi and 30 gal/ac. Eight sprays were applied weekly beginning July 22 and ending September 9. The 1999 cotton season was a good year in respect to abiotic factors favorable to growing cotton. Silverleaf whitefly populations were present in cotton from mid to late season.

Azadirachtin as Bollwhip ™ (Thermo Trilogy Corp.) was used in a 4.5% formulation at 6 oz product/ac. This treatment was part of a 10-treatment random block design that included a "Best Agricultural Practice" (BAP) treatment, and an embedded control treatment, plus a single 1-ac block control. Buprofezin (Applaud ™ 70 WP, AgrEvo, 0.35 lb. AI/ac) was the 1^st BAP treatment applied followed by pyriproxyfen (Knack ™ 0.86 EC, Valent USA, 0.054 lb. AI/ac).

Whitefly eggs, small nymphs, and large nymphs were sampled from one leaf taken from each of 10 plants per plot, from the 5^th main-stem node down from the 1^st expanded terminal leaf. Each sample was counted from a 2.22 cm diameter disk taken from the leaf between the main (central) and the adjacent lateral vein. All whitefly adults were counted on the 5^th main-stem leaf abaxial surface sampled from 30 leaves/plot, using the leaf-turn method; the first 10 were from the same plants used for immature samples. Weekly sweeps (25/plot) were taken in all plots for predators, parasites, the thrip, Frankliniella occidentalis, and Lygus (primarily hesperus ).

Azadirachtin as Bollwhip ™ had seasonal efficacies (as % reduction from block control) against whitefly immatures as follows: eggs, 37 %; small nymph, 32 %; and large nymphs, 66 %, respectively. These efficacy rates were significant at P<0.001 by ANOVA and P< 0.05 mean separation by LSD. Buprofezin application followed by pyriproxyfen 2 weeks later provided season-long control with egg, small nymph, and large nymph efficacies of 42, 75, and 95 %, respectively. The mean number of large nymphs in the Bollwhip ™ plots did not exceed treatment threshold for the season (University of AZ recommendations).

Investigator’s Name(s): D. H. Akey and T. J. Henneberry.

Affiliation & Location: USDA, ARS, Western Cotton Research Laboratory, Phoenix AZ 85040-8803.

Research & Implementation Area: Section C: Chemical Control, Biorationals And Pesticide Application Technology

Dates Covered by the Report: June-September 1999

Effect on Lygus of Biorationals (Insect Growth Regulators and Entomopathogenic Fungi) Used for Control of the Whitefly Bemisia argentifolii, in Field Trials in Upland Cotton in Arizona

Deltapine NuCOTN 33^B was planted and furrow irrigated in plots 109 ft. in length and 12 rows across (40-in. rows). Plots were separated by 2 fallow rows and 20 ft alleys. Spray applications were made by a ground boom with 5 nozzles/row, 1 overhead, and 2 swivel nozzles angled upward on a drop on each side of the row, at 250 psi and 30 gal/ac. Eight sprays were applied weekly beginning July 22 and ending September 9. The 1999 cotton season was a good year in respect to abiotic factors favorable to growing cotton. Silverleaf whitefly populations were present in cotton from mid to late season and Lygus (primarily hesperus) was present during the entire season. Weekly sweeps (25 per plot) were taken in all plots for Lygus, predators, parasites, and the thrip, Frankliniella occidentalis.

Biorational entomopathogenic fungi used included: Beauveria bassiana, as Naturalis ® L (Troy Biosciences Inc.) 10 oz. Product/ac, 2.3x 10⁷ conidia/ml; as Mycotrol ® ES (Mycotech Corp.), 0.5 pt/ac, 2 x 10 ¹³ spores/qt, and Paecilomyces fumosoroseus as PFR- 97 ™ (Thermo Trilogy Corp.), 0.025 lb /gal., 1x 10 ⁹ CFU (spores)/ gm equivalent 20% product. All three of these products were used at full rate for multiple applications.

Biorational insect growth regulators used were at full rate as single or multiple applications and included: azadirachtin as Bollwhip ™ (Thermo Trilogy Corp.), 4.5% formulation 6 oz product /ac.(note, other action modes also); buprofezin as Applaud ™ 70 WP (AgrEvo), 0.35 lb. AI/ac; and pyriproxyfen as Knack ™ 0.86 EC (Valent USA) 0.054 lb. AI/ac. These treatments were part of a 10-treatment random block design that included a "Best Agricultural Practice" (BAP) treatment, and an embedded-control treatment, plus a single 1-ac block control. Treatment efficacy was measured as mean percent reduction from the block control.

Effects of biorational entomopathogenic fungi on Lygus nymphs and adults, respectively, were as follows: Naturalis ® L, efficacies of 11 and 17 % (neither significant); Mycotrol ® ES, efficacies of 0 and 22 % (latter significant at P<0.05 by ANOVA and LSD), and PFR- 97 ™ , efficacies of 0 and 22 % (latter significant at P<0.05 by ANOVA and LSD).

Effects of biorational insect growth regulators on Lygus nymphs and adults, respectively, were as follows: Bollwhip ™ , efficacies of 14 (not significant) and 15 %, (significant at P< 0.05 by ANOVA, ), Applaud ™ , 0%-no efficacy, and Knack ™ , efficacies of 0 and 36% (neither significant).

None of the treatment efficacies sufficiently controlled Lygus. The damage resulted in severe yield losses. However, the Lygus populations were very high in the entire region and convention chemical control was also unable to stop Lygus and most if not all cotton in the region suffered substantial yield losses. The treatments reported here need to be retested in trials with more moderate Lygus populations representative of endemic rather than epidemic levels.

Investigators’ Name(s): Frank J. Byrne¹, Nilima Prabhaker¹, Nick C. Toscano¹, Ralf Nauen² & Steve Castle³

Affiliation & Location: ¹Department of Entomology, University of California, Riverside, CA 92521; ²Bayer AG, Agrochemicals Division,. Research Insecticides, Institute of Insect Control, D-51368 Leverkusen, Germany; ³USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management, and Application methods.

Dates Covered by the Report: 1999

Studies on Imidacloprid Resistance in Bemisia Whiteflies

Imidacloprid is a nicotinic acetylcholine receptor agonist belonging to the chloronicotinyl class of insecticides, and is one of the most effective insecticides currently available for the control of Bemisia infestations. However, as with any insecticide, over exposure of insect populations can result in the development of resistance. Resistance to imidacloprid has now been reported in both field and laboratory populations. In Almeria in Southern Spain, heavy reliance on imidacloprid to control the spread of geminivirus diseases in tomatoes has resulted in the development of resistant populations. Resistance has also developed in populations maintained in the laboratory under continuous selection pressure.

Thus far, there are no accounts of biochemical mechanisms involved in conferring resistance to imidacloprid in Bemisia. In our laboratory, we are investigating the potential role of MFO-based metabolism using radiolabelled (¹⁴C) imidacloprid. An assay has been developed which will detect metabolites of imidacloprid. Microsomal preparations of susceptible Bemisia collected from two locations in Imperial Valley have so far been compared with similar preparations from housefly abdomens. We detected no metabolites in experiments using these field strains, whereas the houseflies readily produced significant amounts of mono-hydroxy derivatives (resulting from hydroxylation of the imidazolidine ring at positions 4 and/or 5), and to a lesser extent the olefin.

Our preliminary data suggest that there is little potential for the metabolism of imidacloprid by susceptible whitefly populations. The next stage of the study will involve comparisons of resistant whitefly populations from both laboratory and field locations.

Investigator’s Name(s): ¹C. C. Chu, ¹T. J. Henneberry, ²B. E. Mackey, & ³H. H. Perkins.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ; ²USDA--ARS--PWA, Albany, CA; ³USDA--ARS--CQRS, Clemson, SC.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management, and Application Methods.

Dates Covered by the Report: 1993 - 1996

Effects of Silverleaf Whitefly Infestation on Upland Cotton Yield and Honeydew Lint Contamination and Establishment of Action Threshold in the Imperial Valley, California

In 1993 and 1994, we conducted studies to determine the effect of chemical control on silverleaf whitefly populations, upland cotton yield and honeydew contamination. Different population densities of silverleaf whiteflies were established with fenpropathrin-acephate insecticide mixture treatments during the growing season at the Irrigated Desert Research Station, Brawley, CA. Regression analyses showed that the highest cotton lint yields and lowest lint stickiness occurred when silverleaf whitefly densities were 0.3 and 1.3 nymphs/cm² of leaf disk, respectively or 4.1 and 7.5 adults per leaf-turn from 5th main stem node leaves from terminals, respectively. In 1995 and 1996, we verified the 4 adults per leaf-turn (the highest cotton lint yields) action threshold with 15 adults (the highest economic return) per leaf-turn and an untreated control. Results showed that initiating chemical control at 4 adults per leaf-turn produced higher lint yields and less lint stickiness compared to initiating chemical control at 15 adults per leaf-turn. Higher lint yields and lower lint stickiness occurred at both treatment levels compared to untreated cotton. Initiating treatments at 15 adults per leaf-turn required 2 to 3 applications and initiating control at 4 adults per leaf-turn required 5 to 6 applications during 1995 and 1996, respectively. Economic returns based on insecticide costs and lint yield were highest when cottons were treated at 4 adults per leaf-turn. Cotton lint stickiness was considered in the analyses and discounts for sticky cotton could significantly reduce net monetary returns.

Investigator’s Name(s): Gary W. Elzen.

Affiliation & Location: USDA, ARS, Kika de la Garza Subtropical Agricultural Research Center, Beneficial Insects Research Unit, Weslaco, TX.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management, and Application Methods.

Dates Covered by the Report: 1998-1999

Laboratory Toxicity of Selected Insecticides to Silverleaf Whiteflies (Preliminary Results)

Formulated insecticides tested were abamectin [Agri-Mek 0.15 emulsifiable concentrate (EC)], tebufenozide [Confirm 2.0 flowable (F), imidacloprid [Provado 1.6 F}, chlorfenapyr [Pirate 3 suspension concentrate (SC), neonicotinyl [CGA-293343 25.0 wettable granule (WG), pymetrozine [Fulfill 50 WG], endosulfan (Phaser 3 EC)], and buprofezin {Applaud 70.0 wettable powder (WP)].

Silverleaf whitefly eggs on cotton, cabbage, and sweet potato were treated with recommended rates of insecticides using a laboratory spray chamber. Rates were selected based on recommendations in an appropriate field guide or from manufacturer’s recommendations in the case of newer or non-registered materials. Development was followed for ten days post-treatment. One application of tebufenozide, imidacloprid, or chlorfenapyr to cotton resulted in mortality exceeding 80%. However, treatment with abamectin resulted in less than 9% mortality on cotton. Results varied by type of host plant treated. Further experiments are planned.

Investigator’s Name(s): Tong-Xian Liu.

Affiliation & Location: Texas Agricultural Experiment Station, Texas A&M University, 2415 E. Highway 83, Weslaco, TX 78596-8399.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management, and Application Technology.

Dates Covered by the Report: 1999

Efficacy and Persistence of Platinum, Actara and Admire for Management of Silverleaf Whitefly on Cantaloupe in South Texas

The silverleaf whitefly, Bemisia argentifolii Bellows & Perring, continues to be one of the most important pests on cucurbits in south Texas. One of the most effective insecticide, imidacloprid (Admire), has been used for many years. Although it is still effective against the whitefly up to 9-11 weeks when it is applied at planting, growers cannot rely on only one insecticide. Alternative materials are essential needed. Thiamethoxam (Platinum and Actara) is one of the newer insecticides effective against the whitefly. The objective of this experiment was to determine the efficacy and persistence of these materials against silverleaf whitefly on melon in south Texas. The cantaloupe was seeded in the field on February 9, 1999. Each plot was 30 ft long with two separate rows (80 in wide), and 30 plants each. The plots were arranged in a randomized complete block design with 4 replications. The seven treatments were 1. Platinum (0.90 fl. oz/1000 ft) dripped at planting; 2. Platinum (0.65 fl. oz/1000 ft) dripped at plating; 3. Actara (38.9 g ai/ac) sprayed for 3 times; 4. Admire (1.1 fl. oz/1000 ft) dropped at planting; 5. Admire (1.1 fl. oz/1000 ft) dripped in mid-season; 6. Admire (1.1 fl. oz/1000 ft) dripped at planting and mid-season; and 7. Untreated control. Platinum were dripped 15 cm (6 in) deep in the soil through the irrigation system at planting on February 22, 1999. There were three treatments for Admire, dripped at planting, at mid-season on April 13, and dripped both at planting and mid-season. Actara was sprayed 3 times, on March 24, April 7, and 22, 1999. Sampling was initiated 4 weeks (on March 8) after planting. Ten plants per plot were randomly selected, whitefly adults from the third leaf from the apical meristem were counted by leaf turn method. Plants were sampled in 7-day intervals for 11 times. When plants were younger than 6 leaves, nymphs (all instars), and pupae (re-eyed nymphs) per 4 leaf-discs (2-cm diameter) per leaf from the oldest leaf were counted. When plants have >6 leaves, nymphs on the 4th~5th leaf proximal to the base of the plant were counted. Plant damage, including sooty mold on foliage, overall plant damage, fruit weight and soluble sugar, was evaluated.

At the early season, whitefly population was high on untreated treatments (in treatment 5, Admire was not used until the mid-season on April 13, and Actara was not sprayed until March 23). Silverleaf whitefly population was significantly lower on the plants treated with Platinum (2 rates) and Admire (dripped at planting). Actara, after the first application on March 23, whitefly population dropped immediately to a low level (about 1 adult per leaf), in 1 week, and kept at low level throughout the season. All insecticide-treated plants had significantly lower whitefly population than those on untreated plants. At late or the end of season, whitefly population on insecticide-treated plants was often greater than on the untreated plants because the whiteflies killed the untreated plants, causing the whiteflies to relocate their feeding plants to the treated, green plants. Number of eggs on untreated plants was significantly greater than that on all insecticide-treated plants from the mid- to late, and to the end of season. Numbers of nymphs and pupae of silverleaf whitefly increased at a relatively slow pace before the mid-season. Nymphs and pupae on untreated plants increased sharply from late April, whereas those on insecticide-treated plants increased, but at a very slower pace. Among the insecticide treatments, Actara, Admire at planting and at planting and mid-season, Platinum at higher rate and Admire at mid-season had slightly fewer nymphs and pupae than those on the plants treated with Platinum at lower rate. Sooty molds on leaves were lowest on the plants treated with Actara, Admire at planting and at planting- mid-season, Platinum at higher rate, followed by Platinum at lower rate and Admire at mid-season. In the untreated plots, almost all leaves and melons were covered by sooty mold and honeydew, and the leaves dried at the end of the season. Plants treated Platinum and Admire resulted significantly higher yields, and higher percentage of sugar than those of untreated plants. Numbers of melon per plot varied greatly and were significantly different. Plots treated with Platinum had the highest number of larger melons than other treatments, followed by Actara, and Admire treatments. In contrast, untreated plots had fewest large melons and highest number of small melons. Similarly, treated plants had higher yield than untreated plants.

Investigator’s Name(s): Steven E. Naranjo¹ & Nilima Prabhaker².

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ and ²Department of Entomology, University of California, Riverside, CA.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management and Application Methods.

Dates Covered by the Report: January 1999 - December 1999

Toxicological Studies of Two Insect Growth Regulators on the Predator Geocoris punctipes

Laboratory experiments were conducted to evaluate the direct effects of two insect growth regulators (IGR), buprofezin (chitin synthesis inhibitor) and pyriproxyfen (JH analog), on survival and reproduction of Geocoris punctipes. Topical and contact residue assays were performed on both 5th instar nymphs and on 5-10 day old adults. Over a wide range of doses buprofezin had no effect on survival or reproduction of 5th instar nymphs or adults based on topical or contact residue assays. Topical assays with pyriproxyfen on 5th instar nymphs yielded an LD50 and LD90 of 3520 and 47,186 ppm, respectively. Concentrations of field applications are ca. 2700 ppm. At doses > 2700 ppm most nymphs molted to the adult stage but many had wing deformities that prevented successful mating and reproduction. Contact residue assays on 5th instar nymphs yielded an LD50 and LD90 of 706 and 2307 ppm, respectively. Again, most of the affected nymphs molted to the adult stage but had wing deformities. Pyriproxyfen did not affect adult survival in either topical or contact residue assays over a wide range of doses. Reproduction also was unaffected at doses up to field application rates. Egg viability, but not rate of oviposition, declined slightly at doses >5 times higher than field application rates. Results suggest that buprofezin is benign to G. punctipes but that pyriproxyfen may be slightly to moderately detrimental under ideal exposure. Similar assays are planned for Orius tristicolor, Collops vittatus and Chrysoperla carnea and further studies with IGRs and other insecticides are planned for all species.

Investigator’s Name(s): Steven E. Naranjo.

Affiliation & Location: USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management and Application Methods.

Dates Covered by the Report: January 1997 - December 1998

Effect of an Experimental Insecticide (NI-25) on Natural Enemies of Sweetpotato Whitefly

Replicated small-plot studies were continued in 1998 to test the effects of a new imidacloprid-like insecticide (NI-25, Rhone-Poulenc) on generalist predators and whitefly parasitoids in cotton. Treatments consisted of NI-25 at 0.05, 0.075 and 0.1 lb AI/A, pyriproxyfen (grower standard), and an untreated control. Insecticides were applied according to thresholds and natural enemy populations were monitored weekly from May through September. Rates of parasitism were low (< 12%) and there were no differences in percentage parasitism between any of the treatments and the untreated control on any sampling dates. Densities of predatory beetles were generally low and no treatment differences were detected on any sample date. NI-25 significantly depressed populations of predaceous Heteroptera in comparison with the untreated control and pyriproxyfen on several sample dates and a decline in the density of spiders was detected on 1 sample date in July. Overall, these results are consistent with findings in 1997 studies. Heteropteran predators are facultative plant-feeders and this may place them at higher risk for exposure to NI-25 given its systemic activity.

Investigator’s Name(s): Eric T. Natwick.

Affiliation & Location: University of California Cooperative Extension, University of California Desert Research and Extension Center, 1050 E. Holton Road, Holtville, CA 92250.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management and Application Methods.

Dates Covered by the Report: March 1999 - July 1999

Evaluation of Insecticides for Silverleaf Whitefly Control In Spring Planted Cantaloupe Melons, 1999

A stand of cantaloupe melons, var. Topmark, was established at UC Desert Research & Extension Center 24 March 1999. Nine insecticide treatments and an untreated control were replicated four times in a randomized complete design experiment. Insecticide treatments were as follows: Platinum 2 SC applied via drip irrigation at rates of 0.036 and 0.046 lb ai/acre and Admire 2 F was applied through the drip irrigation at 0.25 lb ai/acre, but was not followed by foliar sprays. Admire 2 F was applied through the drip irrigation at 0.25 lb ai/acre followed by various treatments of foliar sprays: Capture 2 EC + Thiodan 3 EC at 0.1 and 1.0 lb ai/acre, Danitol 2.4 EC + Thiodan 3EC at 0.2 and 1.0 lb ai/acre, Applaud 70 WP at 0.25 lb ai/acre, Applaud 70 WP at 0.37 lb ai/acre. Actara 25 WG was applied at 0.047 and 0.086 lb ai/acre and was not proceeded by any drip irrigation insecticide treatments. Drip irrigation insecticides treatments were applied 21 April. Foliar spray insecticide treatments were applied 10 June. Silverleaf whitefly, Bemisia argentifolii, were sampled by counting adults on the fourth leaf from the terminal of the main stem cane from ten plants at random in each plot via the leaf turn method and whitefly nymphs were counted on 1.65 cm² leaf disks from ten crown leaves extracted from randomly selected melon plants in each plot. Adult silverleaf whitefly and nymphs were sampled on the following dates: 21 & 26 April, 3, 10, 17, 24 May, 1, 7, 15, 21 & 28 June, 1999.

Adult whitefly population levels were suppressed by Platinum 2 SC at 0.036 lb ai/acre for 5 weeks following drip irrigation application. Adult whitefly population levels were suppressed by Platinum 2 SC at 0.046 lb ai/acre and Admire 2 F at 0.25 lb ai/acre for 7 weeks following drip irrigation application. Adult whitefly population levels were not suppressed by foliar insecticide spray applications. Silverleaf whitefly nymphal population levels were suppressed by Platinum 2 SC treatments and by Admire 2 F for 10 weeks following drip irrigation application. Silverleaf whitefly nymphal population levels were suppressed by all foliar treatments for 3 weeks following application.

Investigator’s Name(s): Eric T. Natwick¹, T. J. Henneberry², & D. Brushwood³.

Affiliation & Location: ¹UC Cooperative Extension, UC Desert Research and Extension Center, Holtville, CA, USDA--ARS, ² Western Cotton Research Laboratory, Phoenix, AZ, and ³ USDA--ARS, Cotton Quality Research Laboratory, Clemson, SC.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management and Application Methods.

Dates Covered by the Report: March 1999 - December 1999

Evaluation of Insecticides for Silverleaf Whitefly Control In Cotton, 1999

A stand of cotton, var. DPL 5415, was established at UC Desert ../../../../cotton.htmlension Center 23 March 1999. Eight insecticide treatments and an untreated control were replicated four times in a randomized complete design. Insecticide treatments were as follows: Rimon 10 EC at 0.011, 0.022, and 0.045 lb ai/a applied 15 June, Applaud 70 WP at 0.35 lb ai/a applied 15 June, Danitol 2.4 EC at 0.2 lb al at 0.2 lb ai/a plus Orthene 90S at 0.5 lb ai/a applied 15 June, 14 July and 3 August, Knack 0.86 EC at 0.05 lb ai/a applied 15 June, NI-25 70 WP (Acetamiprid) at 0.044 and 0.1 lb ai/a applied 15 June, 14 July and 3 August Silverleaf whitefly adults were sampled from ten plants at random in each plot via the leaf turn method using the fifth main stem leaf from the terminal on 27 May, 3, 10, 14, 18, 22 & 29 June, 5, 9, 13, 16, 21 & 26 July. Silverleaf whitefly nymphs were counted on 1.65 cm² leaf disks from 5th position, main-stem terminal leaves extracted from ten randomly selected plants in each plot on 10, 18, 22 & 29 June, 5, 9, 13, 16, 21 & 26 July. Seed cotton was hand picked from 0.002 acre per plot and yield data were recorded on 8 and 9 September 1999. Seed cotton samples were ginned at the USDA--ARS, Western Cotton Research Laboratory in Phoenix, AZ and lint samples were sent to the USDA--ARS, Cotton Quality Research Station in Clemson, SC for stickiness and sugar analysis.

The seasonal nymphal means for the insect growth regulator (IGR) treatments Rimon 10 EC, Applaud 70 WP and Knack 0.86 EC were not different than the non-treated control. The nymphal means for the IGR treatments Rimon 10 EC, Applaud 70 WP and Knack 0.86 EC were not different than the non-treated control on any of the sampling dates with the exception of Knack 0.86 EC on 26 June, 11-days after treatment. The adult means for the IGR treatments Rimon 10 EC, Applaud 70 WP and Knack 0.86 EC were not different than the non-treated control on any of the sampling dates with the exception of Rimon 10 EC on 18 and Rimon 10 EC, Applaud 70 WP and Knack 0.86 EC on 22 June. The NI-25 70 WP treatments and Danitol + Orthene treatment provided the highest levels of control for silverleaf whitefly nymphs with means lower than the non-treated control on all post-treatment sampling dates, P # 0.05. The NI-25 70 WP treatments and Danitol + Orthene treatments were variable by sampling date for control of silverleaf whitefly adults , but usually did not lower population levels below the non-treated control and the seasonal means for these insecticide treatments were not different from the non-treated control. The means for seed cotton yields for NI-25 70 WP treatments and the Danitol + Orthene treatment were greater than all of the Rimon 10 EC treatments and the non-treated control. The mean for seed cotton yields for Knack 0.86 EC was not different from the Rimon 10 EC treatments and the non-treated control nor from the means for seed cotton yields for NI-25 70 WP treatments and the Danitol + Orthene treatment.

Investigator’s Name(s): Eric T. Natwick & Keith S. Mayberry.

Affiliation & Location: UC Cooperative Extension, UC Desert Research and Extension Center, Holtville, CA.

Research & Implementation Area: Section C: Chemical Control, Biopesticides, Resistance Management and Application Methods.

Dates Covered by the Report: September 1999 - January 1999

Efficacy of Selected Insecticides for Silverleaf Whitefly Control in Iceberg Lettuce, 1999

Iceberg lettuce var. Desert Queen was sown at UC Desert Research & Extension Center 16 September 1999. Four insecticide treatments and an untreated control were replicated five times in a randomized complete design experiment. Insecticide treatments were as follows: Admire 2F at 0.25 lb ai/acre injected 3 inches below the seed-line pre-plant, Applaud 70 WP at 0.25 lb ai/acre, Applaud 70 WP at 0.38 lb ai/acre, and Phaser 3 EC at 0.75 lb ai/acre. Foliar sprays were applied on 28 September and 13 October, 1999. Silverleaf whitefly, Bemisia argentifolii, were sampled by counting adults via leaf turn of basal leaves on ten plants at random from each plot and nymphs were counted on 1.65 cm² of leaf surface from basal leaves of ten plants at random from each plot on 27 September, 4, 12, 18, & 25 October, 1, 8 & 15 November 1999.

The silverleaf whitefly adult means for the non-treated control were not greater than the adult means for any of the insecticide treatments on any of the sampling dates with the exception of Admire 2 F on 1 November. The silverleaf whitefly nymphal seasonal mean for the non-treated control was greater than the Admire 2 F nymphal seasonal mean but was not greater than the nymphal seasonal mean for the other insecticide treatments. The nymphal means for Phaser 3 EC were lower that the means for the non-treated control on 1 and 8 November but not on other sampling dates. The nymphal means for Applaud 70 WP at 0.25 lb ai/acre and Applaud 70 WP at 0.38 lb ai/acre were not lower that the means for the non-treated control on any of the sampling dates.

Investigator's Name(s): N. Prabhaker¹, N. C. Toscano¹, and T. J. Henneberry².

Affiliation & Location: ¹Department of Entomology, University of California, Riverside, ²USDA-ARS, Western Cotton Research Lab, Phoenix, AZ.

Research & Implementation Area: Section C: Chemical Control, Biorationals, and Pesticide Application Technology.

Dates Covered By the Report: June 1998 - September 1999

Incorporating Various Neonicotinoids into Chemical Control Practices for Whitefly Management

The impending widespread use of various neonicotinoids currently under development has focused attention on the risk of rapid selection of resistance as well as development of cross-resistance in target insects. We have conducted experimental studies that address these two concerns, the cross-resistance patterns between three neonicotinoids, acetamiprid, imidacloprid and thiamethoxam, and the rate of resistance development to thiamethoxam in whiteflies.

Monitoring for baseline susceptibility was the first step in examining the potential problems that might arise in the field with the use of multiple neonicotinoids. In general, monitoring results will provide comparisons among populations that may reveal cross-resistance patterns to acetamiprid, imidacloprid and thiamethoxam. Monitoring results demonstrated trends in whitefly responses from Imperial Valley, CA, to all three neonicotinoids. Seasonal variations in whitefly responses to acetamiprid, imidacloprid and thiamethoxam were observed. Whiteflies showed higher LC₅₀s during summer when collected on cotton compared to lower LC₅₀s during spring and early fall on melons and cole crops. The LC₅₀s ranged from 2.5 to 139 ppm for thiamethoxam and from 4.7 to 80.2 ppm for acetamiprid. Susceptibility appears to decline during late summer and early fall. This trend was observed to other conventional chemistries and suggests the absence of any clear-cut cross-resistance patterns between the three neonicotinoids. Perhaps both biological and environmental factors can affect a pest's response to insecticides.

Cross-resistance patterns to acetamiprid and thiamethoxam against an imidacloprid-resistant strain (IM-R) varied for each compound with time, for example, thiamethoxam was more toxic to the resistant whiteflies than acetamiprid within 48 h (LC₅₀ = 10 ppm). Acetamiprid was also active against the IM-R strain but was slower in action compared to the toxicity of thiamethoxam (LC₅₀ = 39 ppm in 96 h). Acetamiprid was 10X less active against the IM-R strain in 72 h. This large difference in toxicity between thiamethoxam and acetamiprid is not observed in the field populations suggesting that cross-resistance patterns may not be the same for all populations. The rate and magnitude of resistance development to thiamethoxam was fairly rapid under laboratory conditions. Resistance level was low (7-fold) early in the selection process during the third generation but increased 400-fold by the fifth generation. Although the selection was achieved under laboratory conditions, these results can still be useful predictors of selection effects in the field after widespread use of these three neonicotinoids. In addition, selection of resistant strains in the laboratory allow experimental testing of management tactics to delay resistance to the three neonicotinoids.

Two synergists, DEF and piperonyl butoxide (PB), marginally increased toxicity to thiamethoxam in the Thiam-R strain. The combination of imidacloprid with DEF and PB against the Thiam-R strain showed synergistic activity as indicated by a decrease in LC₅₀s of imidacloprid from 42 ppm to 2.3 ppm with DEF and to 12.8 ppm with PB. The limited effect of the two synergists on thiamethoxam resistance in this strain indicate that this particular resistance might be metabolic. Toxicity was also enhanced to thiamethoxam in the IM-R strain when DEF and PB were applied against the adults. Future work will include metabolism studies to learn the mechanisms of resistance in this strain.

Although the results of this study do not show definite cross-resistance patterns in a number of whitefly populations to the three neonicotinoids, the development of cross-resistance with extensive use is a possibility because of similarity in structure. To preserve the value of this chemistry and to avoid high selection pressure on any one chemical from this group, there is an urgent need for integrating the neonicotinoids into a diversified program of chemical control.