What is rindworm? Investigating a poorly understood pest complex in watermelon

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai, Cucurbitales: Cucurbitaceae] hosts a diversity of arthropod pests, but most research focuses on leaf- and root-feeding guilds. Little is known about the pests responsible for causing injury to the fruit surface, a complex colloquially referred to as "rindworm." In this study, we conducted a 2-year survey across 32 commercial watermelon fields in Indiana for arthropod populations and associated damage to the melon rind. In addition to visual surveys, fruit surfaces were photographed and the images analyzed to quantify the spatial extent of rind blemishes. Over both years, we evaluated >10,000 melons from which ca 9% displayed evidence of insect feeding. Based on USDA (U.S. Department of Agriculture) thresholds for rind injury tolerance at individual fruit and field scales, we estimate that 14.9% of harvests risked marketability loss or load rejection. Far more rind damage was documented on the underside of the fruit compared to the top. Similarly, we found significantly more damage on fruits developing on soil compared to plastic and in the center of the field compared to those on the edge. Population counts of the striped cucumber beetle, Acalymma vittatum (Fabricius) (Coleoptera: Chrysomelidae), were a weak predictor of rind injury and only at very high densities. Our data demonstrate that "rindworm" is an understudied but economically important component of pest management for watermelon and other cucurbits. Due to their protected feeding location beneath melons, management efforts need to time insecticide applications to minimize the likelihood of crop loss to this late-season pest complex.