Horizontal transfer of methoprene by contact and/or cannibalization in Tribolium castaneum

Aerosol applications using reduced risk insecticides, such as methoprene mixed with synergized pyrethins, are becoming more commonly used to manage Tribolium spp. in food facilities, but have limited ability to penetrate into hidden refugia typically exploited by Tribolium spp. Horizontal transfer of insecticide could occur as treated individuals move into hidden refugia and encounter untreated individuals. Our previous laboratory studies have demonstrated that horizontal transfer can occur, but the mechanism was not identified. Here, two potential mechanisms, contact and cannibalization, are evaluated. In the contact experiment, late-stage T. castaneum (Herbst) larvae were exposed to methoprene treated adults for 24-hours and mortality, physical deformities, and adult emergence were measured over 30 days. Comparisons between the controls and methoprene exposed treatments showed no significant differences (P>0.05) for percent mortality and adult emergence (2x2 chi-square comparisons) and deformities (proc glm, pair-wise comparisons). In the cannibalization experiment, late-stage T. castaneum larvae are allowed to consume a dead methoprene exposed T. castaneum pupa for 48 hours and the development was measured and analyzed as above. The statistical analysis showed similar results to the contact experiment (P>0.05). However, the comparison between flour and no food (control) had significant differences in percent mortality (P< 0.05) but this is likely due to starvation. These results suggest that horizontal transfer can occur but that contact and cannibalization are not likely to be the primary mechanism but could be secondary mechanisms to substrate contamination. However, the impact of horizontal transfer under more realistic conditions still needs to be evaluated.