Potential of flavesone as a grain protectant: Long-term efficacy and residues for controlling the lesser grain borer, Rhyzopertha dominica (F.), in stored wheat

A study was undertaken to evaluate the synthetic biopesticide, flavesone, as a potential grain protectant for the lesser grain borer, Rhyzopertha dominica (F.), in wheat. The search for new grain protectants is critical because of the propensity of R. dominica to develop genetic resistance to various insecticides such as organophosphates (OPs), carbamates, synthetic pyrethroids and juvenile hormone analogues. Wheat was treated with 0, 60, 90 or 120 ppm of flavesone as it passed through an auger into large (1 tonne) storage bags. The treated wheat was stored for 13 months under sheltered ambient conditions in southeast Queensland, and samples were also collected 1 week after treatment and stored for 13 months in the laboratory at 30 °C and 55% RH for comparative purposes. Bioassays of wheat stored under laboratory and ambient conditions, showed that an application rate of 60 ppm provided protection for at least 13 months from a susceptible strain, based on high levels of suppression of the F₁ generation. Untreated wheat stored under ambient conditions became heavily infested with R. dominica, while none were detected in any of the treatments, thus further confirming the potential of flavesone as an effective grain protectant. However, bioassays showed that the 60-ppm rate only provided 3 months protection from a strain that was resistant to OPs and pyrethroids, indicating potential non-target site resistance present in this strain. Higher application rates of 90 and 120 ppm were required for 13 months protection. As reported in some other grain protectant studies, initial flavesone residues were lower than the targeted rates, suggesting potential for improving formulation and application methods. Flavesone residues remained relatively stable throughout the study period. We believe that the data generated through this study will provide foundation in establishing field application rate for this new molecule and its possible registration for use by industry to mitigate resistance problems.