Saija Piiroinen

Although the evolution of insecticide resistance has received a lot of attention, less is known about sublethal effects of insecticide stress experienced by the preceding generations on the performance of pest populations. We investigated whether three generations of parental exposure to a deltamethrin insecticide influences physiological traits and overwintering success of offspring in Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae). Beetles descending from insecticide‐exposed parents had lower adult body mass but higher relative lipid content and resting metabolic rate than those descending from non‐insecticide‐exposed parents. Also, a higher proportion of beetles descending from insecticide‐exposed parents than from control parents overwintered on the soil surface, which was associated with very low overwintering survival. When burrowed into the soil for overwintering, both groups had similar probability to survive the overwintering period. Parental insecticide exposure can reduce overall overwintering survival in the next generation by disturbing overwintering behaviour. Although beetles descending from insecticide‐exposed parents were small, the overwintering conditions seem to select for high lipid content, which could override the negative effects of small weight, and possibly improve fitness in the following growing season. Thus, insecticide application may have unintended consequences, which should be considered in pest management.

Invasive pest species offers a unique opportunity to study the effects of genetic architecture, demography and selection on patterns of genetic variability. Invasive Colorado potato beetle (Leptinotarsa decemlineata) populations have experienced a rapid range expansion and intense selection by insecticides. By comparing native and invasive beetle populations, we studied the origins of organophosphate (OP) resistance-associated mutations in the acetylcholinesterase 2 (AChE2) gene, and the role of selection and demography on its genetic variability.

Adaptation to stressful environments is one important factor influencing species invasion success. Tolerance to one stress may be complicated by exposure to other stressors experienced by the preceding generations. We studied whether parental temperature stress affects tolerance to insecticide in the invasive Colorado potato beetle Leptinotarsa decemlineata. Field‐collected pyrethroid‐resistant beetles were reared under either stressful (17°C) or favourable (23°C) insecticide‐free environments for three generations. Then, larvae were exposed to pyrethroid insecticides in common garden conditions (23°C). Beetles were in general tolerant to stress. The parental temperature stress alone affected beetles positively (increased adult weight) but it impaired their tolerance to insecticide exposure. In contrast, offspring from the favourable temperature regime showed compensatory weight gain in response to insecticide exposure. Our study emphasizes the potential of cross‐generational effects modifying species stress tolerance. When resistant pest populations invade benign environments, a re‐application of insecticides may enhance their performance via hormetic effects. In turn, opposite effects may arise if parental generations have been exposed to temperature stress. Thus, the outcome of management practices of invasive pest species is difficult to predict unless we also incorporate knowledge of the evolutionary and recent (preceding generations) stress history of the given populations into pest management.