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Walking activity and dispersal on deltamethrin- and spinosad-treated grains by the maize weevil Sitophilus zeamais

Abstract:

The maize weevil (Sitophilus zeamais) is a serious worldwide pest of stored products frequently requiring control measures to reduce its infestation and minimize grain losses. The fumigant phosphine and insecticides including pyrethroids and organophosphates have been the most used management tools against this stored product pest, but the over-reliance on these chemical products has led to insecticide resistance in maize weevil populations. A recent alternative, the actinomycete-based insecticide spinosad, has been considered highly effective against several stored grain pests, including maize weevils, and has sparked increasing attention. Nonetheless, the sublethal effects of spinosad have yet to be studied, particularly regarding its potential effect on insect dispersal within contaminated grain masses since exposure may be either enhanced or compromised. Thus, we carried out laboratory experiments using a digital tracking system and commercial insecticide formulations with the objective to assess if deltamethrin- or spinosad-contaminated grains may affect the walking activity and dispersal movements of adult maize weevils. The overall activity of groups of weevils was enhanced with insecticide exposure. However, insecticide exposure did not affect resting time and distance walked, but velocity and number of stops were affected compromising walking in individual walking bioassays. Insecticide avoidance by either irritability and repellence were not detected. Dispersal of adult maize weevils on treated grains differed among insecticidal treatments with individuals spending more time at the edges of arenas in non-treated and deltamethrin-treated grains. In contrast, insects on spinosad-treated grains were more evenly distributed within the arena. These findings reinforce the potential of spinosad as an alternative for managing maize weevils because this insecticide minimizes the risk of dispersal away from treated grains.