Egg-counting lets parasitic wasps know if a host is ‘taken’

Like most mothers, female parasitic wasps (Leptopilina heterotoma) want the best for their offspring. For the tiny, dime-sized insects, this means finding a caterpillar—the host—in which to lay their eggs. Each egg develops into a larva that slowly kills the host by eating its interior tissue. But only one larva can grow per host. How is a parasitic wasp to tell which caterpillars are “free” and which have already been parasitized? The insects make this distinction with their egg laying organs, needle-like structures called ovipositors that they insert into the caterpillar (visible above between the hind legs of the parasitic wasp). Within seconds, the wasps can tell whether some other mother has laid her eggs inside the host. Now, researchers report in PLOS ONE that the wasps can even count the number of eggs that have been previously laid. To find out how, the scientists stimulated the minute, taste budlike structures on the tip of the wasp’s ovipositor with the blood of fruit fly (Drosophila) larvae. The larvae were either free of parasitical eggs, or had one or two eggs. The wasp’s ovipositor’s taste buds are equipped with six neurons, which send signals to the insect’s brain. The scientists recorded these signals as they exposed the wasps’ ovipositors to the larval blood. Their analysis showed that these signals differ depending on whether there were no other eggs, one egg, or two eggs inside the host. Thus, using a simple taste-test, the wasps are able to count—and select—the best home for their eggs.