Submitted to: Bemisia International Workshop Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/3/2006
Publication Date: 12/8/2006
Citation: Castle, S.J. & Palumbo, J.C. 2006. Relationship of Bemisia tabaci adult mortalities and population growth to imidacloprid concentrations in cantaloupes. Bemisia International Workshop Proceedings, December 3-8, 2006, Key West, FL., 8:4 p. 81. http://www.insectscience.org/8.04/ref/abstract20.html
Technical Abstract: Decision-making in chemical pest management typically depends upon information concerning how well a candidate pesticide performs against a particular pest species in a particular crop environment. The principal source of such information has traditionally been accumulated field efficacy data produced under variable trial circumstances, often outside the experimenter’s control. Factors such as the type and age of the experimental crop, the degree of pest pressure from resident and immigrant populations, the level of natural mortality due to biotic and abiotic sources, etc., all potentially influence the outcome of each experimental trial. Thus, efficacy profiles for insecticides usually depend upon a consensus evaluation of individual trials conducted over a broad range of conditions. The process of consensus is a practical phenomenon whereby experimental field trial results are combined with the on-farm experiences of consultants and growers to arrive at a general and informal ranking of insecticides and how each performs against particular pest and crop combinations. While this system of evaluating insecticide efficacies has provided pest managers with rough guidelines on what works best in a given situation, it has done little towards identifying the activity profile of an insecticide in a crop over time. Greater confidence in the fate of an application might help reduce ‘insurance’ treatments that pest managers are sometimes compelled to apply because of uncertainty about persistence in the crop and level of control being exerted on a target population. The commercial availability of ELISA kits for quantifying residues of imidacloprid and thiamethoxam has made it possible to monitor titers of each of these systemic insecticides in plant tissues and extracted fluids. Quantification of residues provides a direct measure of activity within a plant in contrast to indirect measures such as monitoring insect densities post treatment. In the case of spring and fall cantaloupes grown in Arizona and California, titers of imidacloprid and thiamethoxam were measured in leaf tissue collected from plants in different stages of growth and under various treatment regimes. One of the choices facing growers each spring is whether to apply only one application at planting, as a sidedress, or as split applications. Results from two consecutive spring trials showed that significantly higher and more persistent titers of imidacloprid were attained when a single application was made at planting compared to a full sidedress or split treatments. Much higher titers (>10-fold) occurred in older, fully expanded leaves compared to younger, growing leaves. Mean titers remained moderately high up to six weeks after planting, but then declined to a level where Bemisia tabaci nymphs began to establish. The relationship of imidacloprid titers to mortality of B. tabaci was further investigated by conducting in-field bioassays of adults attached to leaves with clip cages. Mortality was scored after 48 h, then leaf punches taken from the location where clip cages had been attached. Although adult mortality was generally low in each set of bioassays conducted, even when imidacloprid titers were high in leaves, nymphal densities remained low until later in the season. Differential mortality between adults and nymphs may be the reason why nymphs are controlled in the field at the same time that adults do not readily die. Sub-lethal effects, however, may prevent adults from actively feeding and ovipositioning on treated plants.