Mode of overwintering of invasive Harmonia axyridis in the Netherlands

After establishment of Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) in Europe, population densities of native ladybird species have decreased. The post-hibernation onset of female reproduction, a key characteristic influencing population dynamics and competition with related species, was studied. Hibernating individuals were collected and transferred to outdoor cages to continue overwintering. Every two weeks a sample of individuals was transferred to long-day, warm conditions. Intensity of dormancy was studied by determining the pre-oviposition period and ovarian development. Pre-oviposition periods were short throughout our observations, indicating that Harmonia axyridis was not in diapause but in a quiescent state. H. axyridis becomes active rapidly when temperature rises in spring but is not active earlier in the year than native species. Neither the mode of overwintering, nor the onset of spring activity can explain the invasion success of H. axyridis.