Aarhus University Seal / Aarhus Universitets segl

Swarming ladybirds invade buildings in droves

The exotic harlequin ladybird, which made its first appearance in Denmark six years ago, is now seriously starting to make its presence noticed in Danish homes. It is not a particularly pleasant houseguest.

2012.11.06 | Janne Hansen

The harlequin ladybird has already moved indoors. Photo: Susanne Harding

The harlequin ladybird, which is an invasive species to Denmark, is quite happy to hibernate in Danish homes. This year it has crept indoors to a much greater extent than usual.

 

It started gradually in 2006 with the discoveries of individual harlequin ladybirds, which are native to Asia. It has now become very widespread in Copenhagen and surrounding area, and is also seen in other parts of the country. This year it seems to have changed its behaviour when it comes to looking for suitable places to hibernate

 

- The harlequin ladybird takes flight when the temperature rises above 16° C, and can crawl from place to place at temperatures above 12° C. At this time of year they usually reside in the treetops of maple trees, to mention just one. Here they stockpile energy by eating the season's last aphids. This autumn, however, there are very few aphids on the trees and bushes. At the same time we have had more nights with freezing temperatures followed by sunny days with light winds and very high temperatures. From abroad it is known that such weather conditions stimulate the ladybirds into taking flight and forming swarms, seeking out suitable winter homes, explains senior scientist Tove Steenberg from Aarhus University.

 

Tove Steenberg has followed the ladybird's spread since it migrated to Denmark in 2006. In the USA the harlequin ladybird has been nicknamed the Halloween ladybird, as it finds its way into people's homes by the thousands right around Halloween.

 

- It is my estimate that there are no more harlequin ladybirds than usual at this time of year, but due to the specific conditions this year with the lack of food in the treetops combined with suitable ”swarming weather” the ladybirds have been able to seek out winter homes in buildings to a much greater extent than before, says Tove Steenberg, who from the 18th to the 22nd of October received countless inquiries about mass gatherings of ladybirds on building exteriors and indoors in the capital area.

The exotic little insect is not particularly welcome in such places. The ladybirds can cause odour issues, can soil furnishings and cause allergic reactions when there are large numbers of them.

The harlequin ladybird has been seen as a helping hand for gardeners and farmers, as it has a great predilection for aphids. It has unfortunately become apparent that it is capable of outcompeting other insect species, including Danish ladybird species.

It is not just in Copenhagen that the ladybirds have settled.

- I haven't observed ladybirds in such large numbers in Aarhus, but I saw the first larvae and pupae six weeks ago, say Tove Steenberg. When in Aarhus, she has a habit of wandering over to the town hall park to look for ladybirds. Here there are linden and maple trees, which are some of the ladybirds' favourite feeding grounds, as they usually contain large numbers of aphids throughout the growth season.


Further information: Senior Scientist Tove Steenberg, Department of Agroecology, Telephone: +45 8715 8123, email: Tove.Steenberg@agrsci.dk

 

 

Research, Public / media, Agriculture and food, Pests