Why are caterpillars hairy?

Why is it advantageous to be a hairy caterpillar? One answer can be found in a beautiful paper by Sugiura and Yamazaki (2014). They put five species of caterpillars with various amounts of hairiness in containers with carabid beetles that naturally prey on caterpillars. The carabid beetles were always successful when attacking the smooth/hairless caterpillar species, and it usually only took beetles one try to successfully catch the caterpillar. When attacking a short-haired caterpillar species, the carabid beetles were still always successful, but it took them more tries. And when attacking a long-haired caterpillar species, the carabid beetles were only successful ~50% of the time. Even when they were successful, it took beetles more attempts to catch the caterpillars. Therefore, it looked like long hair protected caterpillars from beetle attacks. To test that idea, Sugiura and Yamazaki (2014) gave the long-haired caterpillars haircuts, so that the hairs were shorter than the beetles’ mandibles. The beetles were then way more successful at attacking the caterpillars with haircuts than the long-haired caterpillars! Now that is sexy science.

So, long-haired caterpillars are out there multiplying like crazy while their short-haired neighbors are getting mown down by beetles, right? Actually, having hairs may be a trade-off. Hairy caterpillars are more likely to be attacked by parasitoids, and a higher diversity of parasitoids attack hairy caterpillars than smooth caterpillars (Stireman and Singer 2003)! It might be beneficial for parasitoids to stick their eggs in hairy caterpillars because the eggs+caterpillars will be less likely to be eaten by a predator before the parasitoid emerges than if the caterpillar is smooth. Or it may be that hairy caterpillars – which are usually not cryptic – are easier for parasitoids to find. Either way, these papers have changed my life.

References:

Stireman, J.O., and M.S. Singer. 2003. Determinants of parasitoid-host associations: insights from a natural tachinid-lepidopteran community. Ecology 84(2): 296-310.

Sugiura, S., and K. Yamazaki. 2014. Caterpillar hair as a physical barrier against invertebrate predators. Behavioral Ecology 25(4): 975–983.