If you keep a vegetable garden, there’s a fair chance you’ll encounter a grisly sight this summer. Some poor caterpillar will be clutching a leaf, with the pupae of parasitic wasps sprouting off its back. It has just died in a most grotesque way. A wasp has zeroed in on the caterpillar and injected eggs into its body. The eggs hatched, and the larvae devoured their hosts from within, keeping it alive until they were ready to emerge.
What makes this sight all the more grotesque is the fact that the plant the caterpillar is sitting on may have been an accomplice to the crime. When caterpillars nibble on plants, the plants sometimes respond by releasing a distinctive cocktail of chemicals. This odor then attracts parasitic wasps. The plants are not just releasing a sort of chemical scream. Wasps are very precise in the species of caterpillars they choose, and they can tell these odors apart.
But the caterpillars are not entirely helpless in this struggle. After all, they are in a sense parasites as well, and like other parasites, they have evolved ways of evading the defenses of their hosts. In the journal Public Library of Science Biology, Japanese scientists demonstrate that caterpillars are eavesdropping on the signals plants are sending and shifting their behavior to make it less likely they’ll become food for wasps.
The scientists studied a species of moth (Mythimna separata) that eats corn. Previous research had shown that the catepillars were nocturnal, munching on corn at night and then slinking into leaves during the day. But the questioned remained what sort of cues the insects were responding to. Was it the rising and setting of the sun, or was it the scent released by injured corn plants–which they only release in the day?
The Japanese researchers set up an experiment to see what was driving the insects in and out of hiding. They put caterpillars into cups covered in nylon, and gave them a folded paper tent to hide in as they wished. The scientists then observed how the caterpillars responded to different combinations of signals. They exposed them to light and dark, to the scent of uninjured plants, and to the scent of plants that had been nibbled. If the caterpillars had a supply of artificial food, they didn’t respond much to light and dark on their own. The odors released by the corn plants had a much stronger effect, regardless of the lighting.
There’s a very good reason why a corn plant should release its distress calls during the day: that’s the only time when parasitic wasps are active. Manufacturing this fragrance at night would be a waste of effort. The caterpillars have responded, it appears, by evolving to hide when they smell the odor. The ones that respond this way are the ones that tend not to turn into inspiration for science fiction movies.
This new find is just the latest addition to a list of adaptations that have evolved in caterpillars against the threat of parasites. My all-time favorite is the way some caterpillars fire their droppings out of an anal cannon to avoid building up stinking piles that will give their location away. You can read about this hygienic howitzer here, in my book Parasite Rex, and in a recent scientific review here.
Update 5/16 9 am: Link to PLOS paper added. Plus, catepillars grow up to become caterpillars