Assessing the short‐term impact of an insecticide (Deltamethrin) on predator and herbivore abundance in soybean Glycine max using a replicated small‐plot field experiment

Abstract A greater understanding of the relative impact of insecticide use on non‐target species is critical for the incorporation of natural enemies into integrated pest management strategies. Here we use a small‐plot field trial to examine the relative impact of an insecticide on herbivores and predators found in soybean (Glycine max L.), and to highlight the issues associated with calculating impact factors from these studies. The pyrethroid insecticide (Deltamethrin) caused a significant reduction in invertebrate abundance in the treated plots, and populations did not recover to pre‐treatment levels even 20 days after spraying. To assess the relative impact of the spray on arthropods we first examined the mean difference in abundance in each plot before and after spraying. All herbivores decreased in abundance in the sprayed plots but increased in the control plots after spraying. Most predators (excluding hemipterans) showed a decrease in the control plots but a proportionally greater decrease in the sprayed plots. Next we examined the corrected percentage population reduction calculated using Abbott's formula. All predators (including Araneae) experienced a greater reduction (mean 87% ± 3.54 SE) than herbivores (mean 56% ± 4.37 SE) and Araneae alone (mean 71% ± 8.12 SE). The range in values across the plots varied and made categorising overall impact subjective for some taxa. Despite the constraints associated with small‐plot trials, by using a combination of impact factors and examining community‐level response across time, we did get some indication of the likely impact of this insecticide if used in a commercial situation.