Effect of Application Technology on Interactions Between Biorational Insecticides,
Microbial Control Agents and Natural Enemies of Sweetpotato Whitefly
    The first objective of this project was to develop and test bioassay techniques for all stages of SPWF and its key natural enemies, and to use these methods to evaluate selectivity of biorational insecticide formulations, especially insecticidal soaps and oils. A related goal was to evaluate phytotoxicity of these materials.
    We chose first to work with the whitefly and compared the insecticidal and repellant properties of a soap, a mineral oil, and a surfactant-like sucrose ester extracted from Nicotiana gossei Domin., with a pyrethroid (bifenthrin) against silverleaf whitefly Bernisia arnentifolii Bellows & Perring. An extract of seeds from the neem tree Azadirachta indica A. Juss. was also used in some tests. All materials tested by leaf-dip bioassay were highly toxic to young a. argentifolii nymphs, but mineral oil and a synthetic pyrethroid (bifenthrin) were more toxic to all whitefly stages and more repellent to adults than the insecticidal soap or N. gossei extract. Residues of insecticidal soap and N. gossei extract were toxic to adult whiteflies only when wet.
    The same materials were tested against all developmental stages of key predators and parasitoids of B. argentifolfi. The pyrethroid standard, bifenthrin, was toxic to all developmental stages of Nephaspis oculatus (Blatchley).(Coccinellidae) and insecticide soap was toxic to larvae, but not to adults, eggs or pupae. Mineral oil was moderately toxic to eggs, but had no significant effect on other stages. Mixtures of sucrose and glucose esters were innocuous to all life stages of the beetle. Bifentrhin was also toxic topically and residually to adults of the lacewings Chrysoperia rufflabris (Burmeister) and Ceraeochrysa cubana (Hagen) and larvae of C. rufflabris, but was not to eggs of either species or larvae of C. cubana. At field concentrations, neem, mineral oil and insecticidal soap were generally not toxic to larvae or adults of either species whether exposed topically or to residues. Oil was toxic topically to eggs of both species but neem and soap were not.
    Bifenthrin was also the most toxic material tested against immature stages of Encarsia pergandiella Howard (Aphelinidae). Mineral oil dips also caused high mortality of immature parasitoids, and mineral oil residues on plant prevented parasitization by the wasp. Neem extract, insecticidal soap, N. gossei sucrose ester and the synthetic sucrose ester had relatively little effect on E. pergandiella. The phenoxy juvenile hormone analog, pyriproxyfen, also was innocuous to all stages of E pergandiella at concentrations causing near 100% mortality to the whitefly host. However significant reduction in subsequent emergence of E. fonnosa Gahan was observed at the highest rate (1 mg ai/liter).
    These results indicated that the use of bifenthrin would be incompatible with biological control of whitefly by the coccinellid, chrysopids and the aphelinid tested, and that considerable interference with coccinellid activity could be expected from insecticidal soap, depending on the timing of application. On the other hand mineral oil and especially N. gossei extract could be used against whitefly with relative impunity to the coccinellids, although oil could be deleterious to the E pergandiella, depending on coverage.  Pyriproxyfen at concentrations tested proved to be effective against B. argentifolfi, safe to E. pergandiella and E transvena, but relatively toxic to E. fbanosa, especially pupae.
    A household detergent was evaluated and proved to be 4 times more toxic to B. argentifolii than insecticidal soap. However, concentrations as low as 0.5% sprayed twice a week reduced early tomato production, although later production tended to compensate for the loss. Weekly spray intervals produced a less severe phytotoxic effect. Similar experiments with mineral oil produced comparable results, although at twice or more the concentration.
    Another project objective was to evaluate techniques of applying biorational control agents against SPWF Computerized image analysis and a blue tracer dye technique confirmed that distribution of spray deposit on tomato foliage from a tractor-drawn hydraulic boom sprayer in the field, and a moving boom (chain-driven) table sprayer used on dense canopied "Lanai" tomatoes in a greenhouse was comparable. Therefore the table sprayer provided a good model of field application. More material was deposited on leaves in the outer plant canopy than within the plant interior. Best coverage on lower leaf surfaces was achieved with highest flow rates and pressure. Air carrier and electrostatic systems were evaluated in the field and the greenhouse, but no clear advantages were seen over hydraulic systems.
    Toxicity of mineral oil, and to a lesser extent, insecticidal soap to B. argentifolii was greatly reduced when applied with a Potter tower compared to a leaf dip. In contrast, bifenthrin was equally toxic whether sprayed or dipped. Thus, coverage was more critical to the functioning of oil and soap, which depend on topical activity, than to bifenthrin with an internal mode of action. Even more dramatic differences in the mortality response of E pergandiella sprayed-on residues versus residues of mineral oil applied as a dip to glass pipettes in which parasitoids were later confined. Residues applied by the latter method were 7 times more lethal to the parasitoids whereas the toxicity of bifenthrin was the same with either bioassay method. Parasitoids were killed when they became entrapped in droplets of oil which they were able to avoid on the sprayed leaf surface but not in the dipped pipette. Thus, low adult parasitoid mortality could be expected from oil in the field.
    We concluded that insecticides effective again B. argentifolii differed in terms of compatibility its predators and parasitoids, with broad-spectrum such as pyrethroids at one extreme and sucrose esters on the other. Mineral oil and insecticidal soap were relatively innocuous but could be deleterious to certain stages depending on concentration and/or method of application. These results have been utilized in the field and in greenhouses to optimize control obtained with biorational insecticides and reduce negative impacts on natural enemies.            The consept of biorational control as a management system based on biological control and other practices compatible with it, is gaining wide acceptance and may someday be a reality many agricultural systems.
 

Selected Publications.

Liu, T. X. & P. A. Stansly. 1995. Toxicity of some biorational insecticides to Bemisia argentifolfi (Homoptera: Aleyrodidae) on tomato leaves. J. Econ. Entomol. 88(3): 564-568

Liu, T. X. & P. A. Stansly. 1995. Toxicity and repellency of some biorational insecticides to Bemisia argentifolii on tomato plants. Entomol. Exp. Appl. 74: 137-143

Liu, T. X. & P. A. Stansly. 1995. Oviposition by Bemisia argentifolii (Homoptera: Aleyrodidae) on Tomato: Effects of Leaf Factors and Insecticidal Residues. J. Econ. Entomol. 88(4) 992-997.

Liu, T. X. & P. A. Stansly. 1995. Deposition and Bioassay of Insecticides Applied by Leaf Dip and Spray Tower Against Bemisia argentifolii (Homoptera: Aleyrodidae). Pesticide Science. 44:317-322.

Vavrina, C. S., P. A. Stansly & T. X. Liu. 1995. Household Detergent on Tomato:

Liu, T. X. & P. A. Stansly. 1996. Oviposition, development and survivorship of Encarsia pergandiella adults (Hymenoptera:       Aphelinidae) in four instars of Bemisia argentifolii (Homoptera: Aleyrodidae). Ann. Entomol. Soc. Amer. 89(l): 96-102.

Liu, T. X. & P. A. Stansly. 1996. Morphology of Nephaspis occulatus and Delphastus pusillus (Coleoptera: Coocinellidae), Predators of Bemisia argentifolii (Homoptera: Aleyrodidae). Proc. Entomol. Soc. Wash. 98(2): 292-300

Liu, T. X. & P. A. Stansly. 1996. Toxiological effects of selected insecticides to Nephaspis occulatus (Coleoptera: Coocinellidae), a predator of Bemisia argentifolii (Homoptera: Aleyrodidae). J. Appl. Entomol. 120, 00-00.

Liu, T. X. & P. A. Stansly. 1996. Pupal Orientation and Emergence of some aphelinid parasitoids (Hymenoptera: Aphelinidae) of Bemisia argentifolii (Homoptera: Aleyrodidae). Ann. Entomol. Soc. Am. 89(3): 385-390.

Liu, T. X., P. A. Stansly & 0. T. Chortyk. 1996. Insecticidal activity of natural and systhetic sugar esters against Bemisia argentifolii (Homoptera: Aleyrodidae). Journal of Econ. Entomol. 89:1233-1289.

Liu, T. X. & P. A. Stansly. 1996. Effects of Pyriproxyfen on Three Species of Encarsia, Endoparasitoids of Bemisia argentifolii. Journal of Econ. Entomol. 90(2): 404-411

Stansly, P. A. & T. X. Liu. 1996. Selectivity of Insecticides to Encarsia pergandiella (Hymenoptera: Aphelinidae), endoparasitoid of Bemisia argentifolii (Homoptera: Aleyrodidae). Bulletin of Entomological Research. 87: 525-531

Stansly, P. A. & T. X. Liu. 1995. Activity of Some Biorational on Silverleaf Whitefly. Proc.
Fl. State Hort. Soc. 107: 167-172

Stansly, P. A., T. X. Liu and D. J. Schuster. 1995. The rationality of biorational insecticides for control of the silverleaf whitefly (Bemisia argentifolfi). Proc. Third Ann. Prog. Rev. 5-year National Res. & Action Plan for Develop. Manage. & Control Method for Silverleaf Whitefly, USDA-ARS p. 94.

Liu, T. X., P. A. Stansly, & J. M. Conner. 1996. Deposition of Spray Material on Tomato Foliage as Influenced by Volume and Pump Pressure. Proc. Fl. State Hort. Soc. 108: 212-216.

Stansly, P. A., T. X. Liu, D. J. Schuster & D. E. Dean. 1996. Role of Biorational Insecticides in Management of Bemisia. In: Bernisia 1995: Taxonomy, Biology, Damage Control and Management. Andover, Hants, UKD. D. Gerling and R. T. Mayer, Jr. {Eds.} PP: 605-615.