Effects of insecticides on lesser mealworm (Alphitobius diaperinus) – bioaccumulation, mortality, and growth

The lesser mealworm (LMW, Alphitobius diaperinus (Panzer); Coleoptera: Tenebrionidae) is increasingly used as a novel food and feed source. The feed materials on which this insect species is reared may contain residues of insecticides from agricultural use. The objective of this study was to determine whether selected insecticides, spiked to the substrate, could bioaccumulate in LMW and/or affect growth or survival. Insecticides were selected and spiked at concentrations equal to the legal limit in EU. Tested substances were chlorpyrifos, propoxur, imidacloprid, spinosad, tebufenozide, fipronil, pirimiphos methyl, cypermethrin, and the synergist piperonyl butoxide (PBO). Cypermethrin and PBO were also tested together. Insecticide concentrations in the spiked substrate and larvae were determined by LC-MS/MS. Concentrations in the larvae were largely below the limit of quantification, meaning that bioaccumulation did not occur. Significant reductions in total yield were observed for spinosad (present in the substrate at 1.6 mg/kg) and imidacloprid (0.12 mg/kg). Spinosad is one of few insecticides that is permitted to be used in organic agriculture, which raises questions over the safety of organic produce for insect rearing. More research on the safety of organic produce treated with this insecticide for rearing of LMW is therefore highly recommended. The reported harmful effects of insecticide residues on LMW need to be accounted for by lowering the current legal limits or reconsidering approval for use. More research is advised particularly on sublethal effects of insecticide residues on LMW on adult beetles.