Bugs Cuddle Up to Dead Comrades for Protection

Matt Kaplan
for National Geographic News
March 25, 2009

Most humans—and animals—don't cuddle up with corpses.

Not aphids, though. The insects snuggle with fallen comrades as a way to evade parasitic wasps, a new study suggests.

A parasitic wasp typically lays its eggs inside an aphid. After hatching, the young wasp—only one egg makes it to wasphood—eats the aphid from the inside out before breaking free and flying away.

Normally, when a nonpredator, like a deer or a rabbit, encounters a bunch of dead animals, its instinct is to flee.

That's what Yannick Outreman, of France's Agrocampus Ouest university, and his colleagues expected aphids to do when presented with a pile of aphid corpses that had been killed by parasitic wasps.

Aphids reproduce quickly and can produce either winged or wingless offspring—scientists don't know exactly how. When in areas clearly threatened by wasp predation Outreman expected the aphids to quickly give birth to winged young that could fly away to a new, safer location.

The answer is that they do not actually know. The researchers comment in their paper that they believe it to be a "chemical cue" emanating from the aphid mummies that induced the production of winged offspring. They add "Further experiments are needed to precisely identify the cues that trigger the aphids response observed here."

Yet the aphids gave birth to wingless offspring.

"We noticed that parasitic wasps tended to pass over plants that had corpses on them, while coming in for a close look when corpses were absent," Outreman said.

When wasps see aphid corpses, the team thinks, "the wasps assume the area has already been overly used by other wasps and move on," Outreman said.

The team found that aphids near corpses were attacked 30 percent less often than aphids on plants without corpses.

The researchers argue that staying near the dead increases an individual aphid's chances for survival and that aphids are stimulated by the presence of corpses to behave in this way.

Findings detailed online in the February 12 issue of the journal Behavioral Ecology.

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