Behavioural effects of the neonicotinoid insecticide thiamethoxam on the predatory insect Platynus assimilis

Little information is available regarding sublethal effects of neonicotinoids on insect predators, many of which perform important roles in ecosystem functioning and biocontrol. In this study, dose-dependent sublethal effects of a dietary administered neonicotinoid insecticide thiamethoxam on two basic behaviours, locomotion and feeding, were quantified in the carabid Platynus assimilis (Coleoptera, Carabidae) using automated video-tracking and weighing of consumed food, respectively. Acute toxicity tests showed that, when orally administered, the LD₅₀ of thiamethoxam for P. assimilis beetles was 114.5 ng/g. Thiamethoxam at 108.1 ng/g caused a short-term locomotor hyperactivity within several hours of treatment. Next day after exposure to the insecticide, all the beetles were in a state of locomotor hypoactivity independent of the administered dose ranging from 1.1 to 108.1 ng/g. Reduction in clean food consumption rate (CFCR) is another altered behavioural endpoint of poisoned insect predators as first demonstrated in this study. On the first day of thiamethoxam administration, a remarkable reduction in feeding only occurred in beetles treated at 108.1 ng/g but on the next day, this negative effect appeared even at doses ten to a hundred-fold lower. Recovery from locomotion abnormalities and reduced feeding took several days. Both locomotor activity and CFCR are sensitive and valuable ecotoxicological biomarkers of carabids which should be taken into account in Integrated Pest Management programs where optimal combination of reduced insecticide use and biological control by predatory insects is crucial to achieve best results.