Professor Purdue University West Lafayette, Indiana, United States
Abstract: The striped cucumber beetle (SCB), Acalymma vittatum, is a common pest of Midwest cucurbit crops where it causes damage by feeding on leaves, stems, flowers, and fruits. In watermelon production, these pests are usually controlled either by conventional (CPM) or integrated pest management (IPM). In CPM, imidacloprid is applied as a soil drench at planting followed by pyrethroids applied as regular “calendar” sprays. In IPM, pyrethroids are used only when and if SCBs exceed a threshold of 5 beetles per plant. Tachinid fly parasitoids are among the natural enemies of SCBs. We evaluated SCB parasitism in commercial-scale watermelon plots managed under either CPM (5 fields) or IPM (5 fields) practices throughout the growing season for two years. We collected live cucumber beetles for 1 hour periodically from all fields throughout the season. A total of 4020 adult SCBs were collected, sexed and kept alive in the lab in individual containers maintained with fresh organic zucchini until they died, or a fly larva emerged and pupated. From the CPM plots, 22 tachinid pupae (of 196 total beetles) were collected yielding 14 Celatoria setosa adults and 8 without viable adults, while from the IPM plots, 562 tachinid pupae (of 3824 total beetles) were collected yielding 361 C. setosa adults and 201 without viable adults. Our results showed a marginally significant difference in parasitism between treatments (GLMER, binomial distribution) at 11.2% in CPM vs 14.7% in IPM plots. Significantly more male SCBs (63%) were parasitized than females (36%) across treatments, and parasitism was significantly higher in mid (49.5%) compared with early (14.7%) and late (35.8%) seasons coinciding with the seasonal phenology of their SCB hosts. We conclude that watermelon fields under IPM management maintain more hosts for tachinid parasitoids like C. setosa, compared to fields under conventional pest management. However, not all hosts are optimum for parasitoid development. Host sex and seasonal phenology are better predictors of tachinid parasitism on SCB than pest management practices.
