Influence of weeds on insecticide deposition and soybean looper (Pseudoplusia includens) management

Glycine max fields with Sesbania exaltata and Senna obtusifolia were select ed to evaluate weed effects on insecticide spray droplet deposition within the crop canopy and soybean looper (Pseudoplusia includens) mortality. Weed y and weed-free treatments with and without thiodicarb insecticide applicat ion were included. Soybean looper populations in plots with S. exaltata and S. obtusiolia were approximately half those found in weed-free plots. Thio dicarb reduced soybean looper larval survival in the absence of weeds or wh en weeds were present in three of four experiments. Dye-sensitive cards pla ced in the top, middle, and bottom thirds of the G, max canopy showed dropl et coverage to be highest on cards placed in the top of the canopy. Weeds r educed insecticide coverage in the top of the G. max canopy 26 to 43%. Thio dicarb deposition within the middle and bottom levels of the canopy was not reduced by weeds. Leaflets from the top, middle, and bottom thirds of the crop canopy were collected immediately after insecticide application to bio assay for insecticide deposition. For both years, mortality of soybean loop er larvae on leaflets collected from the middle or top of weedy and weed-fr ee G. maw treated with thiodicarb was equivalent. Assuming that an economic return can be expected, soybean looper management may not need to be alter ed when moderate infestations of S. exaltata and S. obtusifolia are present in G. max in late season.