Sexually transmitted diseases (STDs) such as HIV/AIDS and syphilis can ravage humans--but STDs in wild animal populations have gone largely unstudied. Now, a team of researchers has found a recurring epidemic of a sexually transmitted mite in a wild population of ladybugs, suggesting that STDs might play a larger role in controlling wild populations than once thought.

Ladybugs are known to be among the most promiscuous insects, and studies have found that STDs run rampant where they live in high densities. A sexually transmitted mite, for example, is suspected to be particularly prevalent in a two-spot ladybug population in Poland. The mite doesn't normally kill the insects but sterilizes all infected females. Scientists didn't understand how such a virulent infection could persist at high levels without wiping out the whole population.

To find out, a team led by ecologist Mary Webberley of the University of Western Australia in Perth, recorded the mating habits of the ladybugs. They found that the insects typically mated every two days during the mating season, each time with a new partner. This extreme promiscuity allowed the mite to sweep through the population. Disease peaked in late spring, with the infection rate racing from 20% to 80% of the population in only two weeks, the researchers report online in the 17 October Journal of Animal Ecology.

"Normally, if you've got an infection this nasty, it sweeps everything else out, and it's gone," Webberley says. However, in this case, the ladybugs and the mite both seem to persevere thanks to some good timing: A new generation of ladybugs was born just before the disease peaked, the researchers discovered. That generation provided a new pool of fertile females to propagate the ladybug population--and a fresh batch of individuals for the mite to infect, allowing the cycle of infection to continue. The system is in balance, but just barely, Webberley says.

Such a delicate balance could be upset by climate change, says ecologist Rob Knell of Queen Mary, University of London. He points out that another two-spot ladybug population in Britain doesn't suffer from mite infections, possibly because the cooler weather affects the timing of reproduction. If climate change alters that timing, Knell says, the effect on ladybug populations could go either way.

Related sites

Insects, sex, and parasites group (researchers' Web page)

Knell's home page