Dying aphids obey wasp's commands.

Dying aphids obey wasp's commands

Pity the poor, parasitized aphid. A tiny wasp, Aphidius nigripes, has injected a batch of eggs into the hapless bug's body. For days the eggs develop: first into larvae that consume their host's innards and eventually kill it, then into dormant pupae that incubate in the aphid mummy before hatching as adult wasps.

Playing the perfect, edible host for some wasp's larvae semingly would drive any insect mad. In fact, aphids often jump to their death soon after becoming parasitized -- a "host suicide behavior" entomologists attribute not to psychosis but to an aphid's instinctive sense that by dying immediately, it will kill the wasp eggs too, reducing the chances of other aphids becoming infected. Now researchers have documented an even more complex set of behavioral changes in parasitized aphids, but in this case working to the wasp's advantage. The findings, described in the April 14 SCIENCE, highlight the subtleties of host-parasite interactions among even the tiniest insects.

Jacques Brodeur and Jeremy N.McNeil of the Universite Laval in Sainte-Foy, Quebec, examined the behavior of A. nigripes-parasitized potato aphids living on greenhouse plants. Adult A. nigripes usually emerge from aphid mummies after a two-week incubation. But if the days are short enough, indicating autumn, they remain in the mummies for months and emerge in the spring. Brodeur and McNeil manipulated "day lengths" with artificial lighting to get overwintering and non-overwintering varieties of wasps. The found that aphids infected with the overwintering variety of wasp generally wandered from their plants to die in protected places ideal for the wasp pupae's long hibernation. Aphids infected with non-overwintering wasps remained on plant leaves to die. the researchers suggest the parasites somehow induce host behavioral changes beneficial to pupal survival.

Researchers know of several chemical and hormonal changes induced by insect parasites, and some may trigger specific behaviors in bugs, says Bradleigh Vinson, an entomologist at Texas A&M University in College Station. "The evidence here suggests that the parasitoids are in control in some ways."

Art Shapiro, a zoologist at the University of California, Davis, notes that in an evolutionary sense, host-parasite relationships are in a constant, competitive flux resembling an arms race. He says the new finding complements observations of host suicide behaviors in other aphids and represents a moment in evolutionary time where the parasite seems to have the upper hand.