Impact of Insecticides on Cucumber Beetles (Coleoptera: Chrysomelidae) and Spider Predators in Watermelon and Corn

The primary goal of this research study was to provide updated pest management recommendations to growers, including the reduction of insecticide applications on a calendar basis by the use of pest economic thresholds, with the purpose of maximizing insecticide efficacy while minimizing the associated negative impacts on natural enemies and their ecosystem services.

Commercial watermelon (Citrullus lanatus) production in the Midwest typically relies on neonicotinoid and pyrethroid insecticides to manage insect pests, particularly striped and spotted cucumber beetles (Acalymma vittatum Fabricius and Diabrotica undecimpunctata howardi Barber, respectively). The role of arthropod predators in managing cucumber beetles is not well documented, and data on the effects of insecticides on predators in watermelon production are deficient. Common cucumber beetle predators include coccinellid beetles found on plants, ground-dwelling carabid beetles (Coleoptera: Carabidae) and spiders in several families that inhabit the soil surface in watermelon fields. I hypothesize that these generalist predators and the ecosystem services (e.g., pest predation) they provide are at risk from insecticides used for pest management without regard to economic thresholds. My study compared the effect of insecticide use on cucumber beetle pests, spider predators, collembola populations and field pest predation under two treatments: 1) watermelons treated with neonicotinoid soil drench and subsequent pyrethroid sprays, surrounded by corn with neonicotinoid-treated seeds (Conventional), and 2) watermelons treated only with pyrethroid spray when economic thresholds were reached, surrounded by corn with untreated seeds (IPM).

The frequent application of insecticides decreased cucumber beetles in the watermelon plots managed with Conventional pest management; however, they also reduced spider predators, collembola densities, and field pests predation measurements, possibly due to the subsequent pyrethroids applications during the growing season. In addition, our study showed that neonicotinoid seed treatment in corn had no negative impact on any of the above-mentioned response variables measured.

Ultimately, following an IPM strategy and the use of pest monitoring helped to reduce unnecessary insecticides applications, conservation of pest regulatory services provided by natural enemies, and possibly less ecological impact to manage significant insect pests in watermelon plots.