Scientists have found that hawkmoths avoid plants where they can smell hawkmoth caterpillar poop, to avoid competition for food for their own offspring. When Jin Zhang and colleagues wanted to see how well hawkmoths could smell, they conducted experiments on plants hosting three-lined potato beetle larvae. They wanted to see if the hawkmoths could recognise the beetles as food competitors or if they ignored them. To their surprise, they found that hawkmoths actually preferred plants infested by beetle larvae.
Manduca sexta, tobacco hornworm, is used by biologists as a model organism to see how invertebrates work. They prefer to lay their eggs on Datura wrightii, Jimson weed. Earlier work found that the moths avoid plants that already have M. sexta larvae so that the younger eggs have fewer competitors for food. However, D. wrightii is also home to three-lined potato beetle, Lema daturaphila. If the moths avoid competition for their offspring, you could expect them to avoid the beetle larvae. The team set plants in wind tunnels to test whether they would choose uninfested plants over beetle-infested plants. Oddly, they chose the plants where beetles were already living.
Zhang and colleagues knew from earlier work that moths preferred to lay eggs on plants that were enriched in nectar. So to test the beetle preference, they enriched the Datura flowers and gave the moths a new choice. Once again, they chose the beetle-infested plants. The moths were not selecting the plants with the best eating. They had something else in mind.
“To be honest, we were a little frustrated at first because we had expected the experiments to confirm our initial hypothesis, namely that egg-laying tobacco hawkmoths avoid food competitors. In our case, however, the unexpected observation suddenly made sense when we realized that beetle-infested plants smell quite differently and that parasitoid wasps, which often use plant odors to find their host insects, are less efficient at detecting tobacco hornworms on beetle-infested plants,” explained study leader Markus Knaden in a press release.
Life is tough for an M. sexta caterpillar. For many other animals, it’s food, and it’s particularly attractive to Cotesia congregata wasps. C. congregata looks for a host for its larvae, but the wasp is after meat. It lays an egg inside the caterpillar, where it lives as a parasitoid. The wasp uses its sense of smell to find the caterpillars, so hiding among the beetles is good camouflage for the caterpillar. While the moth likes to hide its young among beetle larvae, a reverse experiment shows that beetles don’t return the compliment.
The research adds evidence to other studies that show insects trade off food against defence when deciding which plants to attack. Quite why the Datura plants help the moths decide by emitting different odours is unclear at the moment. Zhang and colleagues note that the two larvae attack the plant by chewing, so they should be causing similar damage. Their best guess at the moment is that microorganisms in the larva’s saliva affect how the plant reacts. They suggest that studying the microbiology of plant herbivory would show how complex herbivory is.
Zhang, J., Komail Raza, S.A., Wei, Z., Keesey, I.W., Parker, A.L., Feistel, F., Chen, J., Cassau, S., Fandino, R.A., Grosse-Wilde, E., Dong, S., Kingsolver, J., Gershenzon, J., Knaden, M. and Hansson, B.S. (2022) “Competing beetles attract egg laying in a hawkmoth,” Current Biology. https://doi.org/10.1016/j.cub.2021.12.021