The harlequin ladybird (Harmonia axyridis Pallas) in Denmark: spread and phenology during the initial phase of invasion.

The harlequin ladybird, Harmonia axyridis (Pallas), is an invasive species new to the Danish insect fauna. A few adult specimens were recorded in 2006. By October 2007 huge numbers of larvae, pupae and adults were found in greater Copenhagen and aggregations of overwintering adults were reported from indoor locations. Through personal observations and records by the public we have mapped the spread and habitat colonization of H. axyridis since October 2007 and have made the first observations on the biology of the species in Denmark. Verified records exist from the southern and eastern part of the country with Hvide Sande as the northernmost location by April 2009, and the species is well established in Copenhagen, in Funen and in South-East Jutland. So far, most records have been made from urban habitats, such as parks and gardens. A number of records also exists from reed beds, ruderal vegetation, cereal fields, meadows and heathland and from wash-up on sea shores, particularly in the southern and eastern part of Denmark. Based on the observed phenology and on calculations of thermal sums for complete development, we suspect that H. axyridis is bivoltine in Denmark with a possible third generation in warm years. The harlequin ladybird emerges from overwintering sites from mid-April onwards. In 2008 eggs were observed from the beginning of May, and the new generation emerged after approximately 5 weeks. Generations are overlapping, and in mild autumns with ample food supply as in 2007, larvae and adults can be active until mid-December. Long-term studies of possible adverse effects of H. axyridis on the native fauna have been initiated.