Gregarines modulate insect responses to sublethal insecticide residues.

Throughout their lifetime, insects face multiple environmental challenges that influence their performance. Gregarines are prevalent endoparasites in most invertebrates that affect the fitness of their hosts, but are often overlooked in ecological studies. Next to such biotic factors, a current common challenge is anthropogenic pollution with pesticides, which causes a major threat to non-target organisms that are readily exposed to lethal or sublethal concentrations. In a laboratory study, we investigated whether the presence of gregarines modulates the food consumption and life history traits of a (non-target) leaf beetle species, Phaedon cochleariae, in response to sublethal insecticide exposure. We show that the larval food consumption of the herbivore was neither affected by gregarine infection nor sublethal insecticide exposure. Nevertheless, infection with gregarines led to a delayed development, while insecticide exposure resulted in a lower body mass of adult males and a reduced reproduction of females. Individuals exposed to both challenges suffered most, as they had the lowest survival probability. This indicates detrimental effects on the population dynamics of non-target insects infected with naturally occurring gregarines that face additional stress from agrochemical pollution. Moreover, we found that the infection load with gregarines was higher in individuals exposed to sublethal insecticide concentrations compared to unexposed individuals. To counteract the global decline of insects, the potential of natural parasite infections in modulating insect responses to anthropogenic and non-anthropogenic environmental factors should be considered in ecological risk assessment.

Direct and delayed effects of exposure to a sublethal concentration of the insecticide λ-cyhalothrin on food consumption and reproduction of a leaf beetle.

Anthropogenic pollution such as the application of pesticides poses a major threat to many (non-target) organisms. However, little is known about the persistence of harmful effects or potential recovery in response to a period of exposure to a sublethal insecticide dose. Adults of the mustard leaf beetle, Phaedon cochleariae (Coleoptera: Chrysomelidae), were either exposed to a sublethal concentration of the pyrethroid λ-cyhalothrin for two weeks or kept unexposed as control. During, immediately after and at a delayed time after the exposure, consumption and reproduction, i.e., number of eggs laid and hatching success, were assessed. In addition, long-term effects on unexposed offspring were investigated. Exposure to λ-cyhalothrin reduced the consumption during the insecticide exposure, but led to compensatory feeding in females at a delayed time after exposure. The reproductive output of females was impaired during and directly after λ-cyhalothrin exposure. At the delayed time point there was no clear evidence for a recovery, as the reproduction of heavier females was still negatively affected, while lighter females showed an enhanced reproduction. Persistent negative effects on unexposed offspring had been found when collected from parents directly after a λ-cyhalothrin exposure period. In contrast, in the present experiment neither negative effects on life-history traits nor on consumption were observed in unexposed offspring derived from parents at the delayed time after λ-cyhalothrin exposure. Moreover, eggs of offspring from insecticide-exposed parents showed a higher hatching success than those of offspring of unexposed parents, which may indicate transgenerational hormesis. Our results highlight that λ-cyhalothrin exposure has persistent negative effects on fitness parameters of the exposed generation. However, offspring may not be harmed if their parents had sufficient time to recover after such an insecticide exposure. Taken together, our study emphasises that the time-course of exposure to this anthropogenic pollution is crucial when determining the consequences on life-history.