Long Term Monitoring in Switzerland Reveals That Adalia bipunctata Strongly Declines in Response to Harmonia axyridis Invasion

SIMPLE SUMMARY: The harlequin ladybird, Harmonia axyridis, is an Asian species that has invaded Europe and other continents, where it is suspected to cause the decline of native ladybirds through competition and predation. In north-western Switzerland, ladybirds were monitored for 11 years in four habitats (broadleaved hedges, meadows, pine and spruce stands) to assess the decline of native ladybirds following the invasion of the harlequin ladybird. These surveys showed that, on broadleaved hedges, the harlequin ladybird quickly became the most abundant species, representing 60–80% of all specimens collected in this habitat. One species, the two-spot ladybird, Adalia bipunctata, almost disappeared during this period, whereas it was the most abundant ladybird on broadleaved trees and shrubs when this study started. The other native species did not show any clear sign of decline. The harlequin ladybird was the second most abundant species in pine stands, and was not abundant in meadows and in spruce stands. The total number of ladybirds feeding on aphids did not decline during this period, suggesting that the arrival of the harlequin ladybird did not affect the predation pressure on aphids. Nevertheless, the severe decline of the two-spot ladybird deserves further investigations. ABSTRACT: A long-term monitoring was conducted at 40 sites in four different habitats in north-western Switzerland to observe changes in populations of native ladybirds, following the invasion of the Asian harlequin ladybird, Harmonia axyridis. From 2006 to 2017, the same trees and meadows were sampled at least seven times per year using standard protocols. On 15 broadleaved hedges, H. axyridis quickly became the dominant species, representing 60 to 80% of adult ladybirds collected. It was second in abundance at five pine (Pinus sylvestris) stands and was a minor component of the ladybird complex at five spruce (Picea abies) stands and in 15 meadows. This survey revealed the severe decline of Adalia bipunctata, which was the most abundant native ladybird on broadleaved trees in 2006–2009 and has almost disappeared since 2010. So far, other native ladybirds do not seem to decline significantly, including species occupying the same ecological niches as H. axyridis. The total number of aphidophagous ladybirds did not decline either, suggesting that the biological control function of ladybirds on aphids living in these habitats has not been affected by the arrival of H. axyridis. Recommendations are given to further assess the impact of H. axyridis on native ladybirds and aphids.