Suppressing spread of Tomato spotted wilt virus by drenching infected source or healthy recipient plants with neonicotinoid insecticides to control thrips vectors

Field experiments were done to determine whether drenching plants with two systemically active neonicotinoid insecticides, thiamethoxam and imidacloprid, suppresses spread of Tomato spotted wilt virus (TSWV) by thrips vectors. Separate treatments to TSWV 'infector' tomato (source) and healthy lettuce (recipient) plants provided information on the relative importance of targeting control at virus acquisition by nymphs versus virus transmission to healthy plants by adults. Drenches were applied either to seedlings just before transplanting or to soil around plants. The thrips vectors recorded were Frankliniella occidentalis, F. schultzei and Thrips tabaci, but F. schultzei and T. tabaci predominated. Overall ratios of external to internal TSWV spread into and within plots without insecticide ranged from 1:2.3 to 1:2.8 between field experiments. Applying thiamethoxam as a soil drench to both young source plants and recipient seedling transplants suppressed TSWV incidence by 86%, while such application to either young source or recipient seedlings diminished incidence by 67-70%. When thiamethoxam was applied either as a soil drench to old source plants and concurrently as a seedling drench to recipient plants or as a seedling drench to recipient plants alone, incidence was suppressed by 65-73% and 54-73%, respectively. Thiamethoxam applied as a soil drench to old source plants diminished incidence by only 33% or not significantly. Imidacloprid applied either as a soil drench to old source plants and concurently as a seedling drench or as a seedling drench alone, suppressed TSWV incidence by 90-92% and 80% respectively. Although adult vector thrips and nymph numbers were low, fewer adults and/or nymphs were sometimes recorded due to insecticide application. Drenching healthy seedlings with neonicotinoid insecticides just before transplanting can be an effective chemical control measure to include in integrated disease management strategies to suppress TSWV epidemics in short-duration crops like lettuce.