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The high cost of living with ants

December 09, 2002

A new sexually transmitted disease has been found in British ladybirds, ecologists have discovered, and it is their close association with wood ants that is to blame. Speaking at the British Ecological Society's Winter Meeting, being held at the University of York on 18-20 December 2002, Dr John Sloggett of the University of Bayreuth in Germany and Dr Greg Hurst of University College London will say that they have found sexually-transmitted Coccipolipus mites on the scarce seven-spot ladybird Coccinella magnifica.

Ecologists have known for some time that ladybirds suffer from sexually-transmitted diseases, and Coccipolipus mites generally occur in southern and eastern Europe and the tropics, but this is the first time that the mites have been found on ladybirds in Britain.

According to Dr Sloggett: "Such mites are transferred when adult ladybirds mate and the most important cost is that they render hosts sterile. In general, Coccipolipus mites only occur in the tropics and in southern and eastern Europe. The colder spring and summer seasons of north-western Europe, including Britain, mean that parental and offspring generations of ladybirds do not often occur together (and therefore inter-generational matings do not take place). Since the mites are parasites of adults, this means they cannot be transferred between generations of ladybirds in the UK."

In Britain, the scarce seven-spot ladybird C. magnifica only occurs in association with wood ants (Formica rufa), and Dr Sloggett believes that it this association which explains why only this species of ladybird is affected in Britain. "I checked a lot of species of ladybirds for Coccipolipus mites, but didn't find any, which suggests that there is something unusual about C. magnifica compared with other British ladybirds," he says.

Wood ants are large and aggressive, and build huge mound nests out of pine needles in woodland. C. magnifica lives with the ants because the ladybird eats the aphids that are "farmed" by the ants for their sugary honeydew secretion. Using previously collected data, Dr Sloggett compared the length of C. magnifica's breeding season with that of other UK ladybirds, and found that it is much longer: 12 weeks, compared to 6-8 weeks for most other species. "The much longer breeding season means that many C. magnifica offspring reach adulthood before the end of the breeding season and they sometimes mate with the old adults of the parental generation. Thus the mite can spread from generation to generation, unlike in other UK ladybirds," Sloggett explains.

"C. magnifica's breeding season is so long because of those ant-farmed aphids. Ant-tended aphids are protected by their ant 'farmers' from most predators (C. magnifica excepted), and aphid colonies survive much later into the year than aphids without ants. So the ladybird can go on breeding for longer. This study also illustrates a possible cost of living with ants, in terms of enhanced disease transmission. There are many other insects which are specialised to living with ants: it is possible that some of them may also harbour diseases specifically related to this mode of life," Sloggett says.

Dr Sloggett will present his full findings at 15:40 on Wednesday 18 December 2002.

British Ecological Society (BES)