Comparative efficacy and studies of mode of action of minerals from diatoms against three species of filth flies

Filth flies pose a health risk because of the microbes they carry outside and inside of their bodies to humans and animals. Mostly synthetic chemical insecticides are used for fly control. Alternative approaches with a different mode of action are needed because of increasing fly resistance to pesticides. We used a modified World Health Organization cone test to determine the efficacy of the minerals produced by diatoms (diatomaceous earth) as a mechanical insecticide against adults of the house fly, Musca domestica , the secondary screwworm, Cochliomyia macellaria, and the grey flesh fly, Sarcophaga bullata , under low and high humidity. The use of mechanical insecticides as an alternative to kill filth flies has received minimal consideration. However, recent research showed that expanded perlite, a new mechanical insecticide made from volcanic rock, was highly efficacious against mosquitoes and flies. Mortality for diatomaceous earth in this paper at 30°C and 50% relative humidity was observed as early as 2 h after exposure with 50% and 90% mortality observed at 3.5 and 4.6 h (the LT 50 and LT 90 , respectively) in M. domestica . The LT 50 and LT 90 increased as the size of the fly species increased ( M. domestica to C. macellaria to S. bullata ) and increased for all three species when the relative humidity increased from 50 to 70%. These results suggest dehydration was the mode of action. Scanning electron micrographs of C. macellaria adults 2 h after diatomaceous earth exposure, showed the flies were evenly self‐covered with the mineral with no obvious damage to the cuticle. Proof of concept was demonstrated that diatomaceous earth as a residual surface treatment could be used as an alternative for filth fly control.