Over the last few days I have spotted an increasing number of harlequin ladybirds. Normally I would be delighted to see a ladybird but the harlequin is not one of our native species. Instead, it is a potentially troublesome and invasive addition to our fauna, reaching our shores because of our own actions. Let me explain. The harlequin ladybird is a native of eastern Asia but it has been introduced to a number of countries as a biological control agent. Scientists, looking for an alternative to pesticides, have used the harlequin to control crop pests, such as aphids and scale insects, by introducing them into glasshouses, crops and even gardens. Unfortunately, as has been the case for a number of species used in this way, the harlequin has gone on to become a pest in its own right. It first established itself here in Britain in 2004, with individuals arriving from the Continent (where they had been introduced) thanks to their remarkable powers of dispersal or by gaining assistance in shipments of plants and other foodstuffs.
The harlequin ladybird is pretty catholic in its choice of habitats but does seem to favour deciduous trees like sycamore and maple. Here it feeds on aphids, scale insects, the eggs of various moths and a range of other insects. This species has a clear competitive advantage over most of our native ladybirds because it has a much greater breeding potential. The pre-adult stage lasts just 20 or so days (although this is temperature dependent) and, once they reach adulthood, the females can begin to lay eggs after just five days. This means that a single female can produce more than 1,000 eggs during her lifetime.
At the moment, the numbers of harlequins in Britain have not reached the pest proportions seen elsewhere in the world. In North America, many tens of thousands may overwinter in a single home and, given their tendency to bite, they are not the most welcome of lodgers. They are also a threat to our native species, especially when aphid numbers are low, as the species is likely to feed on other ladybirds and their larvae. Even wine production could be threatened. Harlequins are attracted to soft fruit, like grapes, and may occur in such numbers that they taint the resulting wine production with the alkaloid chemicals they carry to deter potential predators.
Fortunately, the Government has provided funding to the Harlequin Ladybird Survey Project (see www.harlequin-survey.org) to collate information on sightings and to undertake research to assess the level of the threat. It is also hoped to determine how the species may be controlled to prevent it becoming a pest. Let’s hope the research comes up with a solution.