Insecticides are often needed to control pests in vegetable crops, but in crops that require pollinators we often worry about the impact those insecticides may have on those pollinators (Figure 1). In the summer of 2018, a team of researchers at Purdue University explored the effects of insecticide applications on watermelon yield across Indiana, considering their impacts on both pests and pollinators.
Using 5 of the Purdue Agricultural Centers (PACs), pairs of ½ acre watermelons plots were planted, each in the middle of a 15-acre corn field (10 total plots) (Figure 2). The two watermelon plots at each site were assigned either to a conventional or an integrated pest management (IPM) system. The corn surrounding the conventionally managed watermelons had a neonicotinoid seed treatment, the watermelons were given a neonicotinoid soil drench at transplant, and 4-5 pyrethroid sprays were applied throughout the summer regardless of pest pressure. The IPM system had no insecticides applied to corn or watermelons at planting, and foliar sprays were only applied when striped cucumber beetle, reached the economic threshold of 5 beetles per plant. Only the IPM plot at the Pinney PAC (outside Wanatah, IN) ever reached the threshold and received a single pyrethroid application. The other four IPM plots received no insecticide applications. Each pair of watermelon plots had the same fungicide applications throughout the summer using the MELCAST system. Striped cucumber beetle populations were higher in the IPM plots than in the companion conventional plot in all locations. The yields for the IPM plots were higher than the conventional plots with two plots showing significantly greater yields.
How can yields be lower in fields with more applied insecticides and fewer pests? We believe that the crucial difference comes from a non-target effect of insecticides on visiting pollinators. During 5 weekly surveys during peak watermelon flowering, there were 653 pollinators in the IPM plots, as opposed to just 349 in the conventionally managed plots. These visitors included “managed” species such as honeybee and bumblebees along with numerous native sweat bees and syrphid flies. The total number of visits to flowers and number of pollinating events (traveling from a male to a female flower) were recorded for all of the surveyed insects. The IPM plots had twice as many total flower visits and three times as many pollinating events compared to the conventional watermelon plots. Since multiple insect visits are required for watermelon to produce healthy and marketable fruit, the reduced frequency of pollination could be leading to the differences in yield. These trends show that a “less is more” approach may be a superior management strategy when it comes to insecticides in watermelons and the other vegetable crops that rely on bees for pollination.
This work was led by entomology PhD student Jacob Pecenka and he, along with other members of the team will continue to look at these trends for the next few years, examining the effects that insecticides have on managed pollinators like honey bees or bumble bees that are often found near cucurbits. The goal is to provide guidelines for growers who want to employ more sustainable management practices in their farming operations. It seems that a return to an old, and often neglected practice – IPM – may offer a solution.
If you have questions or would like to learn more contact at jpecenka@purdue.edu or check out the project’s website for updates and information: pollinatorprotection.org
Figure 1. Honey bee visiting a male watermelon flower.
Figure 2. Example of research plot design; ½ acres of watermelons within a 15-acre corn field.