Deposit structure and efficacy of pesticide application. 2: Trichoplusia ni control on cabbage with fipronil

Pesticide deposits have a spatial structure having elements of size, number per area and toxicant per deposit. To investigate the relative contributio ns of these elements to the efficacy of the deposit structure, we developed a bioassay using the cabbage looper (Trichoplusia ni), cabbage, and a solu ble concentrate formulation of fipronil [(+/-)-5-amino-1-(2,6-dichloro-alph a,alpha,alpha-trifluoro-p-tolyl)-4-trifluoromethylsulfinylpyrazole-3-carbon itrile]. The bioassay manipulated deposit structure by changing the number, toxicant concentration of the solution, and size of droplets used in creat ing deposits. The bioassay methodology was developed as an extension from s tandard industrial mixture experimental designs. Results from the bioassay led to the following conclusions: (1) Deposit structure plays a major role in toxicant efficacy. (2) The effect of droplet size is roughly equal to th e effect of concentration, while both these factors may have a greater effe ct than droplet number. (3) The interactions between the factors of deposit size, deposit number, and concentration are more important than any single component. (4) Uniform coverage is not the most efficacious deposit struct ure if one is forced to limit application rates, and field persistence. (5) Uniform deposit structures have less variability in their biological effec t than do more heterogeneous structures-though the relationship is not line ar. These bioassay data corroborate the predictions of an earlier paper. (C ) 1999 Society of Chemical Industry.