Abstract
Pesticides and pathogens are known drivers of declines in global entomofauna. However, interactions between pesticides and viruses, which could range from antagonistic, over additive to synergistic, are poorly understood in ants. Here, we show that in ants the impact of single and combined pesticide and virus stressors can vary across castes and at the colony level. A fully-crossed laboratory assay was used to evaluate interactions between a sublethal dose of the neonicotinoid thiamethoxam and Acute bee paralysis virus (ABPV) in black garden ants, Lasius niger. After monitoring colonies over 64 weeks, body mass, neonicotinoid residues and virus titres of workers and queens, as well as worker behavioural activity were measured. ABPV, but not thiamethoxam, reduced activity of workers. Neonicotinoid exposure resulted in reduced body mass of workers, but not of queens. Further, thiamethoxam facilitated ABPV infections in queens, but not in workers. Overall, virus exposure did not compromise detoxification and body mass, but one colony showed high virus titres and worker mortality. Although the data suggest additive effects at the level of individuals and castes, co-exposure with both stressors elicited antagonistic effects on colony size. Our results create demand for long-term holistic risk assessment of individual stressors and their interactions to protect biodiversity.
Highlights
The global entomofauna is affected by precipitous declines in both abundance and diversity[1,2,3]
We investigated sublethal effects of the neonicotinoid insecticide thiamethoxam and Acute bee paralysis virus (ABPV) on L. niger at the colony and individual queen/worker level in a fully-crossed laboratory experiment
There was no significant difference among the four treatments (controls, neonicotinoid = chronic exposure to thiamethoxam (30 ppb), virus = feeding regime with ABPV, combined = exposure to thiamethoxam and ABPV; each N = 10; log-rank test, X2 = 0.0, df = 3, p = 1.0)
Summary
The global entomofauna is affected by precipitous declines in both abundance and diversity[1,2,3]. Chronic exposure of soil-dwelling organisms to fluctuating concentrations and mixtures of neonicotinoids in the range of 1 to > 100 ppb in arable farmlands are likely[17,18] They may induce long-term inadvertent sublethal effects on insect physiology and behaviour[7,14,15]. Colonies of eusocial insects are characterized by overlapping generations, cooperative brood care and reproductive division of labour between castes, whereby one or a few female individuals (queens) reproduce, and the rest of the usually non-reproductive females (workers) perform all other tasks necessary to maintain the c olony[26] These colonies are superorganisms, with individual insects being analogous to cells in multicellular organisms, acting as a functional unit[29,30]. It seems plausible that susceptibility and stressor interactions vary between the two castes
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