Subsisting on only one food is a poor survival strategy for humans, but a great one for caterpillars. Caterpillar species with very specialized diets are less likely to be plucked from their leaves by hungry birds, scientists have discovered. The less picky eaters are more apt to die (even if their moms praise them in the meantime). The finding is important not just for bugs and birds, but even for the health of the trees they inhabit.
Wesleyan University biologist Michael Singer and his colleagues tested a hypothesis that’s been around for a while: that among insects, more selective eaters are safer. Since these bugs spend all their time on one or a few host plants, the reasoning goes, they may be better adapted to hide on those plants. Insects that wander between many different plant species—with different colors and textures of leaves and branches—may be easier for predators to spot.
The researchers tested this by creating an experiment in the wild. In the forests of central Connecticut, they tied mesh bags around tree branches or small saplings. The mesh allowed insects to come and go, but kept birds out. Each branch or sapling was paired with a nearby one of similar size, which the scientists didn’t wrap up. Over the course of four years, they counted caterpillars on the bagged and unbagged branches to see how many were normally eaten by birds.
Then they broke down their results among the 41 most common caterpillar species crawling on those trees. The species spanned a wide range of eating styles, from specialists that live and feed on a single plant to unfussy generalists that live on many different kinds of plants.
Picky caterpillars are safer, the scientists found. Birds prey more heavily on generalist species.
Some of this has to do with camouflage: specialists are more likely to have evolved coloration that helps them blend in with their host plants. (To measure how well different insects camouflage themselves, the scientists asked human subjects to look at black-and-white images of caterpillars on plants and try to spot the bugs.) Specialists were also more likely to hang out in one place on their host plants—always on the underside of leaves, for example, or on a tree trunk. This might help the camouflaged species stay hidden.
But the protective effect of picky eating also worked for the 8 caterpillars in the study with “warning” coloration—bright colors, stripes, or spikes that tell predators you might be poisonous. These bugs aren’t trying to blend in; just the opposite. Sticking to one kind of plant, and even one part of that plant, might help these caterpillars because birds get used to always seeing them in the same context. This could help send a clear “stay away” signal to predators, so they don’t get confused and take a taste anyway.
Because specialization is good for caterpillars, it’s also bad for trees. The authors looked at insect damage to 15 tree species, both in their study and previous ones. They found that trees hosting more species of picky caterpillars had more damage to their leaves.
That makes sense if birds are ignoring these caterpillars in favor of the easier-to-spot generalists. And it means that insect pickiness could be an important factor in the health of ecosystems, one that people haven’t considered much. The effect on ecosystems of children who eat only plain spaghetti, on the other hand, is yet to be determined.
Image: by Lara (via Flickr)
Singer, M., Lichter-Marck, I., Farkas, T., Aaron, E., Whitney, K., & Mooney, K. (2014). Herbivore diet breadth mediates the cascading effects of carnivores in food webs Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1401949111