Comparative toxicity of spinetoram to Trialeurodes vaporariorum Westwood and its parasitoid Encarsia formosa Gahan

The role of selective new generation bioisecticides, beside their effectiveness against key pests, relies on their safety to beneficial arthropods. Spinetoram, a semi-synthetic analogue of the microbial-derived bioinsecticide spinosad is registered worldwide for application in numerous crops, but assessment of its ecotoxicological risk to beneficial arthropods has scarcely been documented. Moreover, this is the first report on toxic effects of spinetoram on a pest, the greenhouse whitefly Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae), and/or its successful biocontrol agent, the parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae). Under laboratory conditions, we assessed the acute toxicity of spinetoram insecticide (25% a.i.) to adults, nymphs and eggs of the greenhouse whitefly, as well as to parasitoid adults and pupae. In all concentration-response bioassays, the spinetoram insecticide was applied to tobacco leaves settled onto 1% agar layer in ventilated Petri dishes using a Potter spray tower. The parameters of spinetoram acute toxicity to adults of both the pest and the parasitoid were evaluated in residual contact bioassays, while whitefly eggs and nymphs, and parasitoid pupae were topically treated with a series of spinetoram concentrations, covering a range of 10-90% mortality. Lethal spinetoram effects on the parasitoid E. formosa were assessed through selectivity ratio (SR) estimations, showing the ratios beetween median lethal concentrations (LC50s) estimated for the parasitoid, and LC50s estimated for the pest. The following LC50 values were obtained: 4.593, 15.027 and 11.73 mg A.I./l for whitefly adults, nymphs and eggs, respectively, and 0.686 and 1.715 mg A.I./l for parasitoid adults and pupae, respectively. The calculated SR estimations were below 1, indicating that spinetoram insecticide is non-selective to both tested stages of the parasitoid E. formosa. A more detailed understanding of spinetoram impact on E. formosa in whitefly integrated management requires further evaluation of sublethal effects and greenhouse trials, with an emphasis on population-level responses.