On the other hand, it is only 5mm to 7mm long and will keep your garden free of greenfly.

The culprit is Harmonia axyridis, known in English as the Harlequin ladybird. Harmonia is native to a part of the world stretching from the central Asian republics all the way to China and Japan. The very characteristics that present such a problem now are part of the reason it was introduced here artificially in the late 1980s: It has a voracious appetite and was used by commercial growers to tackle aphids and other scale insects in greenhouses, fields and gardens.

However, as with some house guests, it proved easier to invite in than to ask to leave. Now Harmonia is turning out to be a serious threat to the native ladybirds in regions where it was introduced.

According to a study carried out by the Flemish Research Institute for Nature and Forest (Inbo), together with institutions in five other countries over a period of 13 years, native species like the two-spot ladybird (Adalia bipunctata) are being wiped out by Harmonia.

“There are now 30% fewer two-spot ladybirds in Flanders than there were in 1999,” says Tim Adriaens of Inbo. “In England, 44% have disappeared.” To perform the survey, Inbo, a Flemish government agency, had the help of about 1,000 volunteers, who together gathered about 200,000 sightings of the species.

Harmonia first made its appearance in Flanders in 2004, and a longrunning survey showed its presence in 269 of the 365 one-kilometre squares studied. Over the period of the survey, the presence of six native species of ladybird declined, including Adalia. Only two suffered no decline: the 22-spot and the more common seven-spot.

Two-pronged attack

Harmonia’s effect on native species is two-fold: The intruder is tough and rapacious and competes successfully with native species for limited food resources. In addition, Harmonia is partial to eating the larvae of its competitors and, in the case of the smaller Adalia, even adult insects. In a paper published in the journal Diversity and Distributions, the study reports that in Europe Adalia is “near the threshold of detection”, or, in other words, almost too few to count.

And Harmonia is bringing about a reduction in the biodiversity in Flanders, something that reduces the ability of an ecosystem to react to changes from outside, such as climate change. “The seven-spot ladybird is just about holding its own, probably because it’s as big as the Asian species and doesn’t live in trees,” Adriaens says. “The 22-spot ladybird is doing better, as it feeds on fungi and doesn’t eat aphids. All the rest are in decline.”

The seven-spot, the paper explains, eats vegetation rather than aphids, so it doesn’t compete with Harmonia. The 16-spot ladybird, meanwhile, also doesn’t compete for food, but its larvae can fall prey to Harmonia in times when aphids are scarce, such as in autumn.

Pest control

The problem is not limited to native ladybirds: Harmonia can also be a pest for humans. They hibernate in the winter and use pheromones to attract more of their own kind, so if you’ve experienced a sudden plague of the beasts in the autumn, it was probably Harmonia. It uses a defensive chemical to deter predators and will also deposit it if scared or crushed, leaving a bad smell and a stain.

The gatherings seem to be unique to Harmonia: otherwise it isn’t easy to identify, ranging in colour from red and orange to yellow and black, and with anything from zero to 20 spots. Occasionally it’s been known to bite, though the bite is more irritating than harmful. If confronted with such a home invasion, the experts at the US Department of Agriculture (Harmonia is also a problem in the United States) advise using a vacuum cleaner with a nylon stocking tied over the nozzle to collect the ladybirds, rather than allowing them to go into the cleaner’s bag. Prevention is better than cure, including closing off cracks in walls and joints where they can enter.

Unfortunately, science seems to be some way from a more lasting remedy for the problem. “We can only hope that the natural enemies of our native species develop a taste for the Asian species,” Adriaens says. “But it will take some time for evolution to get to grips with that. In the meantime, our local predators are not interested in them at all.”

www.inbo.be