Environmental levels of neonicotinoids reduce prey consumption, mobility and emergence of the damselfly Ischnura elegans

Abstract

Freshwaters are among the most endangered ecosystems in the world as a result of anthropogenic interference such as pollution. Pollution in the form of neonicotinoids has been intensively studied, but data thus far is often conflicted by contrasting responses between laboratory and field experiments. In addition, toxicity data are scarce and contradictory for insects such as Odonates (dragonflies and damselflies) and a potential risk to them may therefore be overlooked. We investigate the potential risk of neonicotinoids to Odonates by exposing nymphs of the blue-tailed damselfly Ischnura elegans to environmentally relevant concentrations of the neonicotinoid thiacloprid. We consider I. elegans as an indicator species for other Odonates as it is an abundant, widespread and eurytopic species. We analyse the effects of thiacloprid on multiple endpoints (survival, consumption, growth, molting, mobility and emergence), using cage-experiments as well as controlled field observations in naturally colonized experimental ditches. In addition, we assess sensitivity by either feeding the damselfly nymphs with lab-cultured prey or by letting them feed freely on natural aquatic invertebrates. All sublethal endpoints of I. elegans are affected to some degree, and strongly depend on the food offered; free-feeding nymphs are more sensitive than culture-fed nymphs. Environmental relevant concentrations of thiacloprid strongly reduce the emergence of I. elegans and this effect is more substantial in the natural populations compared to the caged damselflies. This is likely explained by exclusion of additional biotic pressures such as predation in the caged experiment. Policy implications. Literature reports that one out of seven Odonates is threatened and 24% of the species have declining populations. Our observations show that current risks of neonicotinoids to Odonates are underestimated in laboratory experiments as the toxicity is governed by multiple biotic factors such as food quantity/quality and predation. Given the widespread abundance of blue-tailed damselfly Ischnura elegans, the observed sensitivity to neonicotinoids and current population trends of this species, these results indicate neonicotinoids play a central role in the Odonate decline in general.