Dispersal of boll weevil toward and within the cotton plant and implications for insecticide exposure.

Background: Immature stages of boll weevil complete development endophytically leaving only the adult stage accessible for chemical control. We tested the hypothesis that boll weevil colonization of the cotton plants significantly affects their exposure to sprayed insecticides. We determined the adult dispersal toward and within cotton plants, lethal time (LT), and residual control by recommended insecticides (malathion, carbosulfan, thiamethoxam, fipronil, beta-cyfluthrin, lambda-cyhalothrin, and thiamethoxam + lambda-cyhalothrin) through dried residue exposure and residual control regarding the leaf position in the upper and the lower thirds of the treated plant canopy.

Results: Newly emerged adults from fallen buds reached the cotton plants by walking (80%) and most of the time settling on cotton bolls in the lower part of the plants (78%). Irrespective of sex and mating status, adults released on the upper part of the plant remained longer on the same release site than the lower part, with some individuals remaining up to 50 h on the same flower bud. The shortest LT was found with thiamethoxam (106 h). Fipronil and malathion, respectively, provided the longest (>144 h) and shortest (24 h) residual control times and caused boll weevil mortality above 80%.

Conclusions: These findings suggest that weevils accessing the cotton plants exhibited within-plant distribution that minimizes their contact with insecticide residue on plant foliage. Furthermore, short residual control with malathion, the most used insecticide against boll weevil, and the low susceptibility exhibited by the tested population to pyrethroids highlight the current challenges faced for boll weevil control in Brazilian cotton fields. © 2020 Society of Chemical Industry.