Quantitative impact assessment of spray coverage and pest behavior on contact pesticide performance

Standard procedures for evaluation of pesticide performance do not take into account pest behavioral response or incomplete spray coverage. In this study, a series of laboratory experiments was conducted with two non-systemic miticides (propargite and hexythiazox) applied to cotyledon cotton plants, which were subsequently infested with spider mites. The results of these laboratory experiments are discussed through a comprehensive pest population dynamics model.

When cotton leaves were submerged in miticide solutions, both miticides provided effective control of spider mites. In a two-choice test it was demonstrated that propargite was repellent to spider mites, but not hexythiazox. Finally, the spray coverage on cotton plants was varied, and, for both miticides, significantly positive relationships between spray coverage and spider mite mortality were shown. However, propargite required higher spray coverage (20%) than hexythiazox (10%) to control spider mites. A theoretical model showed that, without repellency, the pesticide performance is positively correlated with target pest mobility. If the pesticide is repellent, the probability of exposure decreases, especially for a less mobile pest.

With an experimental and theoretical modeling approach, it was demonstrated how the combination of behavioral avoidance and low spray coverage can markedly reduce pesticide performance.