(Guthion| bifenthrin (Capture| buprofezin (Applaud| cypermethrin (Ammo| methyl parathion and thiodicarb (Larvin| with a water control. Parasitoid adults.
ENTOMOPHAGA 40 (2), 1995, 153-162
RESPONSE OF ADULT PARASITOIDS OF BEMISIA TABACI (HOM.: ALEYRODIDAE) TO LEAF RESIDUES OF SELECTED COTTON INSECTICIDES W. A. JONES(i), D. A. WOLFENBARGER(2) • A. A. KIRK (3) (~) Biological Control of Pests Research Unit, Subtropical Agricultural Research Laboratory, ARS, USDA, 2413 E. Hwy. 83, Weslaco, Texas, USA. (2) Crop Insects Research Unit, Subtropical Agricultural Research Laboratory, ARS, USDA, 2413 E. Hwy. 83, Weslaco, Texas, USA. (3) European Biological Control Laboratory, ARS, USDA, Parc Scientifique, Agropolis, 34397 Montpellier, Cedex France.
The contact toxicity of eight insecticides to adults of four parasitoids of the sweetpotato whitefly Bemisia tabaci was evaluated in the laboratory. Two common Texas species, Eretmocerus sp. and Encarsia pergandiella Howard, and two exotic species, Eretmocerus mundus Mercet from Spain and Encarsia formosa Gahan from Greece were tested. Insecticides, applied as sprays to greenhouse-grown cotton plants at recommended rates were amitraz (Ovasyn| azinphosmethyl (Guthion| bifenthrin (Capture| buprofezin (Applaud| cypermethrin (Ammo| methyl parathion and thiodicarb (Larvin| with a water control. Parasitoid adults were confined on discs cut from leaves (1) sprayed the same day and (2) sprayed two days previously. Survival in both treatments was measured two and four days following exposure. Significant differences in toxicity were detected among the insecticides. Buprofezin was not toxic to any of the four parasitoids. When caged on leaves sprayed two days previously, only amitraz of the remaining compounds allowed significant general parasitoid survival after two days. E. mundus exhibited the greatest overall tolerance to insecticides, with 40 % or more surviving 48 hr after confinement on leaves sprayed with amitraz, thiodicarb and cypermethrin. Survival was generally much reduced after 96 hr. In a separate test, fresh residues of endosulfan (Thiodan | were highly toxic at the two rates tested, but two day old residues at the lower rate allowed 76.7 % survival of E. mundus and 35 % survival of E. pergandiella after 48 hrs. KEY-WORDS: biological control, Encarsia, Eretmocerus, Aphelinidae, insecticide resistance, parasitoid
The recent dramatic increase in the economic importance of the sweetpotato whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) in the United States and elsewhere has been attributed to the appearance and rapid spread a new biotype (biotype B) (Brown etal. 1995), which is considered by some to be a new species, the silverleaf whitefly B. argentifolii Bellows & Perring (Bellows et al. 1994). Since the taxonomic status of the species has not yet been fully resolved, we will refer to the insect as B. tabaci. This whitefly is a year-round pest in subtropical areas where the most prominent host crops are spring and fall cucurbits and cole crops, and cotton in summer. Each of these susceptible
W. A. JONES, D. A. WOLFENBARGER, A. A. KIRK
crops has one or more other pests that usually require insecticide treatments. Thus, strategies for managing natural enemies of the sweetpotato whitefly in ~multi-crop, multipest systems must take into account the need to control other pests. Most pesticides used in cotton are highly toxic to parasitoids (reviewed by Dowell 1990). Detrimental effects of pesticide applications on parasitoids of the sweetpotato whitefly have been mentioned by Sharaf (1982), Butler &Henneberry (1983), Abdelrahman (1986), Kapadia & Puri (1991) and Price & Schuster (1991). However, Gerling (1967), Shires et al. (1983), Bellows & Arakawa (1988) and Kapadia etaL (1992) reported little reduction in parasitoid activity using certain compounds. Croft (1990) has summarized current knowledge on the known effects of pesticides on natural enemies. The identification of selective or short residual action chemicals, and selection of pesticide-resistant strains of parasitoids are desirable goals for management of whiteflies within an area wide multi-crop system such as the Lower Rio Grande Valley of Texas. We report here the results of laboratory tests designed to evaluate a variety of compounds within different insecticide classes for leaf residue toxicity to adults of native and imported parasitoids that attack B. tabaci. MATERIALS AND METHODS Four species of parasitoids (Hymenoptera: Aphelinidae) were evaluated. The two native species tested are the two most important species in the Lower Rio Grande Valley of Texas, Eretmocerus sp. and Encarsia pergandiella Howard (Carruthers et al. 1993, Jones et al. 1993). The exotic species were Eretmocerus mundus Mercet and Encarsia formosa Gahan. Eretmocerus mundus is the most widespread parasitoid of the sweetpotato whitefly in the Old World (Greathead & Bennett 1981; Gerling, 1986; Lopez-Avila, 1986). The test culture was originally collected from near Murcia, Spain in November, 1991 from B. tabaci in cotton (Mission Biological Control Laboratory Culture No. M92014). The E.formosa culture was collected near Angelohori, Greece from Trialeurodes sp. on Phaseolus vulgaris (L.) (Mission Biological Control Laboratory Culture No. M92017). Both exotic species were collected by personnel and cooperators associated with the USDA, ARS, European Biological Control Laboratory, Montpellier, France. Test insects were provided by USDA, APHIS, Biological Control Laboratory, Mission, Texas, where the cultures were maintained on B. tabaci (biotype B) reared on Hibiscus rosa-sinensis L. var. Kona Pink. Test specimens of the two native species were obtained locally from parasitized B. tabaci (biotype B) on Brassica oleracea L. (Cruciferae). The International Organization of Biological Control (IOBC) has developed standardized methods for screening for the effects of pesticides on a variety of important natural enemies so that results from various researchers can be directly compared (Hassan, 1985 ; Hassan et al. 1994 ). Specific procedures have been established for Encarsiaformosa, an important parasitoid of the greenhouse whitefly (Hoogcarspel & Jobsen, 1984 ; Oomen, 1985). However, we developed the techniques used in the research reported here so that several insecticides and species of parasitoids could be much more conveniently tested simultaneously. Eight insecticides were selected on the basis of their current and potential use for management of key insect pests of cotton and because they represented a variety of chemistry classes. The materials were prepared as water solutions that would be applied at 381 per hectare. Concentrations, in kg active ingredient per hectare (ai/ha) were: amitraz (Ovasyn| at 0.28, azinphosmethyl (Guthion| at 0.28, bifenthrin (Capture | at 0.09, buprofezin (Applaud| at 0.42, cypermethrin (Ammo| at 0.08, methyl parathion at 0.56, and thiodicarb (Larvin| at 0.37. Generally, the toxicity of leaf residues to adults of four
INSECTICIDE EFFECTS ON PARASITOIDS OF BEMISIA TABACI
species of parasitoids was tested. Endosulfan (Thiodan | was tested separately at two rates, 0.56 and 1.125 kg ai/ha to only two parasitoid species. Single plants of cotton variety Stoneville 453 were grown in 15.2 cm-diam, plastic pots in an insect-free greenhouse, three plants per each treatment. When plants were 0.75-1.0 m tall, both surfaces of all leaves of each test plant were sprayed using a trigger-operated hand sprayer. Sprays were applied to both leaf surfaces until the material began to drip from the leaf; the amount of material remaining on each leaf was not measured. Toxicity to parasitoids was measured using discs cut from the sprayed plants as described below. Two insecticide exposure treatments were conducted: parasitoids were exposed to cotton leaves on the same day the insecticides were applied (> 2 hr post-spray), and two days after insecticide application. Water was used as a control treatment. Mortality was assessed two and four days following each exposure. Only E. pergandiella and E. mundus were tested with endosulfan at two rates; all other insecticides were tested with four parasitoid species within recommended field rates. The experimental unit consisted of five newly emerged parasitoid adults confined in a Petri dish containing a disc cut from a treated cotton leaf. The 60 x 15 mm Petri dishes (Bectyon Dickinson and Company, Lincoln Park, New Jersey, USA 17035) were modified by replacing most of the bottom with organdy cloth (subsequently inverted to become the top). The bottom half of a Petri dish was used as a template to cut out a circular section of sprayed leaf. Half of the discs were cut from leaves on the day of insecticide treatment (same-day-exposure); the other half were prepared after two days in a greenhouse. Each leaf disc was fitted into the inner surface of a dish lid, bottom leaf surface exposed. A small amount of honey was applied to the inner side of the dish for parasitoid food. Parasitoids were briefly chilled before being aspirated into the dishes containing the treated leaf discs. The ventilated dish bottom was replaced, becoming the top. Dish halves were secured by a rubber band, then placed randomly in an environmental chamber set at 27 + 1 ~ 55 + 10 % RH and a photoperiod of 16:8 (L:D) under 40 watt Vita-Lite | (Duro-Test Corp., Fairfield, New Jersey, USA 07004) fluorescent lights. There were ten replicates (50 parasitoids) per treatment. Voucher specimens of each parasitoid species were mounted on glass slides; subsamples were placed in liquid nitrogen and are stored at the Mission Biocontrol Laboratory, USDA, APHIS, Mission, Texas. On the day of each assessment for mortality, each experimental unit was removed from the environmental chamber and briefly chilled before opening under a binocular microscope. Dead parasitoids were readily identified by their immobility and shrivelled appearance; living parasitoids recovered quickly from chilling and were readily identified by their activity. An analysis of variance was performed on the survival for each treatment combination (SAS Institute 1988). If treatments were significant at P