Laboratory evaluation of particle size, food contamination, and residual efficacy of pyrethrin + methoprene aerosol

Efficacy of aerosol applications of pyrethrin + methoprene was assessed through emergence of morphologically-normal adults exposed as larvae and through a developmental index. Concrete arenas were treated with aerosol droplets (particles) averaging 2 μm or 16 μm, food material was placed on the arenas at 1-8 weeks post-treatment, and 3-4-week-old larvae of one of four stored product insect species placed on the arenas at the same time. No exposed Lasioderma serricorne (F.) larvae reached the adult stage at either particle size, though they developed further when exposed to residual deposits of the 2 μm aerosol particles compared to residual deposits of 16 μm particles. No exposed larvae of either Tribolium confusum Jacqueline duVal or Trogoderma variabile (Ballion) reached the adult stage when exposed to residues of the 16 μm particles, but most of the larvae exposed to the 2 μm residues completed development. Exposed Dermestes maculatus (DeGeer) larvae completed development to the adult stage after exposure to both particle sizes, indicating low susceptibility to methoprene residues. When food material in the arenas was also treated with the two particle sizes and bioassays conducted using only larvae of L. serricorne, T. confusum, and T. variabile, results were similar to those described above. In a final experiment, only the 16 μm particle size aerosol was used to treat arenas with or without food, with arenas without food having it added one day after treatment. At 1-8 weeks post-treatment, the food was transferred to new untreated arenas and food was replaced in the arenas originally treated with aerosol. Bioassays were conducted using only larvae of T. confusum and T. variabile. The food material blocked deposition of the aerosol on the arenas or absorbed some of the residues, indicating transference of the methoprene residues from the treated surfaces onto the food.