THE USE OF TRACERS TO ESTIMATE THE EXPOSURE OF BENEFICIAL INSECTS TO DIRECT PESTICIDE SPRAYING IN CEREALS

A fluorescent spray tracer was used to measure the profile of spray de posits through winter-wheat crop canopies and upon beneficial insects, placed in fixed positions which approximated to their natural distrib utions. Tracer deposition profiles were recorded on a series of occasi ons between G.S. 47 to G.S. 90. Deposition levels declined through the canopy on each occasion but the proportion of the spray reaching the ground varied between growth stages. Deposition rates upon insects fol lowed the same trends but insects on the aerial parts of the plant rec eived a higher volume of tracer per unit area than the part of the cro p that they were placed upon. Insects placed on the ground received a dose equivalent to their ground area. For a given distribution of inse cts, these data permit direct spray exposure to be estimated. The gene ral application of the method to a wide range of crops, to the calcula tion of risk indices, to the standardisation of field tests and to the development of selective spray placement methods are discussed.