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Wasp Wars - When Sisters Kill Brothers

New research turns up deadly secrets in the hidden world of insects

  • Roger Di Silvestro
  • Oct 01, 2007
WHEN YOU READ about an animal in an article that uses such terms as polyembryonic encyrtids and ecdysteroid titers, you know you're not dealing with such familiar subjects as lions, tigers and bears. Nevertheless, once you get past the scientific lingo, you'll be drawn into the story of a brother-killing wasp that, though common in occurrence, is so little known to the general public that it lacks a common name. Scientists refer to it as Copidosoma floridanum, but a layperson might want to call it something like "the looper wasp."

Why "looper?" Well, this wasp is a parasite--it can reproduce only by laying its eggs inside the eggs of looper moths, such as soybean loopers and cabbage loopers. So much for the origins of potential nomenclature. Now for the biology: Inside the looper moth egg, the wasp eggs form a microscopic grapelike cluster. As the moth egg gives rise to a caterpillar, the wasp egg produces up to 3,000 larvae. Wasps lay only one or two eggs per looper moth egg. If a wasp lays one egg, it will hatch into either all male or all female wasp larvae. If it lays two eggs, one will hatch all female and one will hatch all male young, which is when things get even more interesting.

Once they hatch inside the caterpillar, the wasp larvae start feeding on their host, rather like an unwelcome guest at your lake house during holiday season. Only a fifth of an inch long, the maggotlike larvae feed on caterpillar blood. Or about 80 percent of them do. The rest develop into snakelike creatures with rasping jaws--killer larvae that, instead of feeding on bug blood, attack other wasp larvae.

It gets curiouser and curiouser. The vampire larvae grow and begin eating the caterpillar's organs until it dies as an empty husk. Then these larvae mature and fly off as moths. The killer larvae, however, don't. They are dead ends. They cannot escape the host body. When the host is dead, and the vampire larvae take off, the killer larvae die.

Recently, scientists have begun to sort out the details of how this system works. It seems--"seems" is the operative word here, as investigators have not yet nailed down all the inner workings of this reproductive strategy--that the killer larvae may actually benefit the vampires by attacking the larvae of other wasp species that have laid eggs in the same caterpillar. The killers function as evolutionary death squads, weeding out the competition. The killers may not be able to reproduce, but they help ensure the survival of the vampire larvae, which can breed and which share the same exact set of genes as the killers. The killers in some way can tell which larvae are relatives and which are not. Woe to those that are not.

But that isn't the whole story. When a looper wasp lays two eggs in a single caterpillar--one that will produce all females and one that will produce all males--a war between the sexes results. The sisters are all genetically identical, but they share only a portion of their genes with their brothers. So the killer female larvae attack their brothers. After all, the female breeders will stand a better chance of producing another wasp generation if they don't have to compete with all of the males for food. And, since one male can breed with many females, the males are relatively expendable.

Researcher Mike Strand, at the University of Georgia, and Andrew Gardner, at the University of Edinburgh in Scotland, are the scientists turning up these new details about what is going on in the secret world of the caterpillar body. They also have found that killer larvae that develop early tend to attack their own siblings, while late-developing killers attack the larvae of other wasp species. But understanding what it all means is still a work in progress.

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