Temporal efficacy of neonicotinoid seed treatments against Frankliniella fusca on cotton.

Reports of neonicotinoid seed treatment (NST) failure against Frankliniella fusca in the mid-south and southeastern USA led to the discovery of widespread resistance in these insect populations. Previous studies of NSTs in other crops have shown the concentration of the insecticide to change over time, which could reduce its efficacy. To understand this temporal effect in cotton with F. fusca, our study examined how plant age alters the effects of NSTs (imidacloprid, imidacloprid + thiodicarb and thiamethoxam) by examining larval establishment at multiple seedling ages during the period of cotton seedling susceptibility to this insect. Additionally, we used F. fusca populations with differing neonicotinoid sensitivity levels to understand how resistance impacts this changing efficacy. Greenhouse studies showed that larval numbers were significantly greater on older NST-grown cotton seedlings. The population with elevated neonicotinoid resistance had a more rapid increase in larval number on thiamethoxam-treated plants over time. NSTs reduce the number of F. fusca larvae on younger seedlings, but this effect declines as seedlings age. The duration of efficacy is shorter against neonicotinoid-resistant populations. Neonicotinoid resistance in cotton-infesting F. fusca populations may be accelerated by this time-dependent decrease in efficacy, which likely encourages low-dose exposure to these insecticides. © 2018 Society of Chemical Industry.