The Interactive Effect of Pesticides, Heat Stress, and Population on the Performance of a Mite Pest, Tetranychus Urticae

Global warming is a cause for concern and is predicted to diversely affect agricultural systems. One aspect of the predicted effects is the susceptibility of pests to chemical pesticides. It is therefore crucial to study the reaction of pests to different pesticide compounds under different temperatures in order to better predict how those pesticides might function under warming field conditions. In this study, we examined the effect of a combination of two stressors: heat and pesticide exposure, on the highly polyphagous pest mite Tetranychus urticae Koch (Trombidiformes: Tetranychidae). We exposed four populations of mites of different backgrounds to the pesticide pyrimidifen as well as a short exposure to high temperatures, applied either before or after pesticide exposure. We then focused on a single population and exposed it to two more compounds: tebufenpyrad and acequinocyl, again in combination with heat. We assessed the mites' mortality, the number of eggs laid, and attempts to escape from the arena. We demonstrate that each factor, i.e., heat, pesticide, and genetic background, can have a significant effect on the mites' reaction. For example, we show that when heat is applied after pesticide exposure, it can dramatically increase the mortality in all populations tested. Surprisingly, when heat is applied before pesticide, it reduces mortality rates in certain populations. Our experiment indicates the difficulties in predicting the efficiency of pesticides in the field under warming conditions and the interactive effects of population, heat, and pesticides on the mite responses.