March of the Harlequins

1 March 2008

Stephen Montgomery explains why ladybirds are not cute little bugs, but rather a potential pest that can make you sick, kill your computer, stink up your room and commit crimes of infanticide…

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This past winter my room has been frequented by a number of uninvited guests–27 to be exact. I know because they currently reside in a jam jar on my desk; they are ladybirds. Of these, only two were British, the other 25 are an invasive Asian species called the Harlequin ladybird. Up and down England others have had the same visitors. The Harlequin is on the march; it arrived in England in 2004 and has just reached Scotland and Northern Ireland, causing problems not just for native ladybirds but also for people.

Ladybirds are popular beetles, often featuring on children’s clothes, books or TV programs. Even among adults they get good press; they are, after all, “the gardener’s best friend”. But behind this affable public front they are actually vicious critters: infanticide, siblicide, they do it all. It’s a bug-eat-bug world. So the arrival of the Harlequin is bad news for our British species; it’s big, aggressive and it comes armed with a potent chemical defence.

When different types of ladybirds meet, one will often get eaten–usually the smaller one. Given that Harlequins are over twice the size of most native species, British ladybirds won’t fare well in a fight. Harlequins also have the advantage of being more plastic in their development, so they have greater adaptability, allowing them to spread quickly across a range of environments and conditions. For example, their pupae range from a light colour to almost black. The colour depends on the temperature the larvae are exposed to just before pupating. Darker pupae survive better in colder conditions by absorbing more heat; similar links between environment and development of characteristics are seen in adults. They also have a greater range of diet and can eat up to 65 aphids a day, but if aphids are rare they have a taste for other insect eggs. The combination of a varied diet and habitat type coupled with their size and adaptability makes the Harlequin an incredibly adept invasive force.

In America, where they were originally introduced as a biological control in 1988, they are now the most common ladybird and their success has brought about a downfall in the fortunes of many native American insects. The future of British ladybirds, which are more particular in their tastes, is therefore in danger. What’s more, the Harlequin is expected to affect biodiversity in general as it increases its dominance of the aphid-eating world–especially now as the effects of global warming kick in, disturbing the favoured conditions of British species.

In America, insects aren’t the only ones suffering as a result of the Harlequin. Humans too are finding them an unwelcome addition to the country’s fauna. In winter ladybirds look for a warm place to stay until spring. Harlequins often find houses ideal, hence their current predicament in my jam jar. Once inside, they are a bit of a nuisance: they tend to be quite active as our central heating warms them up and when disturbed they excrete a foul-smelling, bitter “reflex blood”, which can stain clothes and furniture. People can also be allergic to the ladybirds, and they even bite! As if all of that isn’t bad enough, there have also been a number of reports of Harlequin’s climbing into computers (nice and warm), causing them to crash.

Outside the home they can form swarms as they migrate to their over-wintering sites. During their journey they tend to be attracted to large, light coloured objects on the horizon, like a wall or shed; they can engulf the whole side of a house! Given their probable impact in Britain, the spread of Harlequin ladybirds is being monitored closely by a group of researchers, including some from the Cambridge Genetics Department, with the help of the public.

Monitoring the spread and effects of the Harlequin invasion will allow the team to assess the effects on native species and provide a useful case study on the effects of invasive insects, which will become more and more prevalent with the changing climate.