Deposit structure and efficacy of pesticide application. 1: Interactions between deposit size, toxicant concentration and deposit number

Application of pesticides through a hydraulic nozzle produces deposits on a plant surface which have a spatial structure with elements of deposit size , number per area, and toxin per deposit. To investigate the relative contr ibutions of these elements to the interaction of deposit structure and toxi cant efficacy, we used a stochastic cellular automaton model of diamondback moth feeding on Bacillus thuringiensis (Bt)-treated cabbage - the Pesticid e Dose Simulator (PDS) model. Data were analyzed using a specialized respon se surface approach called a mixture design. The advantage of this design w as that it integrated the effects of deposit size, number per area and toxi n per deposit on toxicant efficacy. Results from PDS simulations led to the following conclusions: (1) Deposit structure plays a major role in toxin e fficacy. (2) Small deposits are not always the most efficacious, (3) Unifor m coverage is not the best deposit structure if one is forced to Limit appl ication rates and field persistence. (4) Since uniform deposit structures a llow an insect to Live longer, uniform deposit structures should result in more insects acquiring sub-lethal doses. This may result in an interaction between 'uniform coverage' and the development of pesticide resistance in i nsect populations. (5) Percentage mortality and the level of crop protectio n are not necessarily correlated. Overall, these results help reconcile lab oratory observations that small droplets are more efficacious with field ob servations that application of small droplets (eg from spinning disk spraye rs) does not necessarily increase field efficacy. (C) 1999 Society of Chemi cal Industry.