Presence of flour can influence the efficacy of pyrethrin aerosol spray against flour beetles

Aerosol insecticides are used in food industry pest management programs, but there is limited information on their efficacy. We evaluated efficacy of aerosol pyrethrin insecticide against different life stages of red flour beetle, Tribolium castaneum, and confused flour beetle, Tribolium confusum. To evaluate different ways that insects could be exposed to insecticide within a flour mill the following treatments were created for each life stage: insect treated with aerosol and added to untreated flour, insect untreated and added to aerosol treated flour, treated insect added to treated flour, untreated insect added to untreated flour, and insects and flour combined and both treated together. The pyrethrin aerosol was effective against both species and all life stages when insects were directly treated with aerosol irrespective of transferring them to treated or untreated flour, with mortality generally >88%. However, the mortality was significantly reduced when insects were either treated together with flour or untreated insects were transferred to treated flour (indirect exposure to aerosol spray). Additionally, moribund insects initially observed in both indirect exposure treatments were better able to recover over time than moribund insects in direct exposure treatments. For T. castaneum exposed together with flour there were no differences in mortality among life stages, but for T. confusum there was greater egg and pupae mortality. Results show the importance of getting the aerosol directly to the insects instead of treating flour and how sanitation conditions in flour mills can significantly affect the efficacy of aerosol applications.