On the trail of a surprising preference: Researchers have uncovered why a butterfly species prefers to lay eggs for food plants that are already infested by competing beetles. This protects the offspring from an enemy. Because a parasitic wasp species, which targets the larvae of the tobacco hawk, avoids the smell of beetle-infested plants. That is why the tobacco hawk has evidently developed a “fine nose” for egg-laying for host plants that are already occupied, according to the study.
Amazing interrelationships and sophisticated adaptations – the fascinating complexity of nature also includes the numerous strategies of living beings to provide their offspring with optimal chances of survival. In the case of insects, the choice of where to lay their eggs plays a crucial role. An important aspect is: the larvae should find sufficient food after hatching. Using the tobacco hawk as an example, researchers have already shown in an early study that some insects take care of this in a complex manner: The moths are reluctant to lay their eggs on plants where they smell the feces of their fellow species. “So they obviously want to avoid food competition. So we first asked ourselves whether the moths also avoid plants that are infested by other herbivorous insects, ”says Jin Zhang from the Max Planck Institute for Chemical Ecology in Jena.
Paradoxical initial result
To investigate this, he and his colleagues carried out experiments with female tobacco hawks and plants of the thorn apple Datura wrightii, which were infested by the Colorado beetle species Lema daturaphila. The results, however, initially appeared illogical: the female tobacco hawks did not avoid these plants, but even preferred to lay their eggs there than on specimens that were not colonized by the beetles. “To be honest, we were a little frustrated at first because we had expected that the experiments would confirm our initial hypothesis, namely that egg-laying female tobacco hawks avoid potential food competitors,” reports senior author Markus Knaden.
But then the researchers suspected: the behavior could have something to do with another well-known aspect of the choice of where to lay eggs – protecting the offspring from predators. In the case of the tobacco hawk, the small parasitic wasp species Cotesia congregata came into question. It lays its eggs in the caterpillars of the moths. After the larvae develop in them, the victims die and young wasps hatch from them. Often a large part of the butterfly brood falls victim to the parasites.
To investigate their suspicions, the researchers therefore also examined the wasps’ reactions as part of their study. As expected, the parasites were attracted to the smell of plants that their victims – the tobacco hawk caterpillars – ate. The experiments showed that this attraction was significantly weaker when the plants were also attacked by Colorado beetles. The researchers explain that this was due to the special smell that arises from this infestation. “So the initial result suddenly made sense,” says Knaden. The female moths therefore deliberately accept the disadvantage that the competition creates for their offspring. Because it is evidently surpassed by the protective function against the parasitic wasps.
Example of biological complexity
As the scientists were able to show through further analyzes, this advantage has led to the tobacco hawk to develop a “fine nose” for the beetle-infested plants. Comparative studies also provided indications of what the moths react to: In the scent bouquet of the plants infested by beetles, the researchers found increased levels of the substance alpha-copaen. Using molecular biological methods, the team was then able to show that a special odor receptor in tobacco enthusiasts reacts to this substance. It probably influences the females’ choice of where to lay their eggs.
According to the researchers, the system is an interesting example of the often astonishingly complex relationships in biology: “Our results show that simple explanations are often not enough in nature. Tobacco enthusiasts not only have to consider whether a plant is suitable as food for their offspring, but also whether there are already potential competitors, and whether their presence even helps to avoid the dangers of predators, ”says co-author Bill Hansson, summing up the discovery .
Source: Max Planck Institute for Chemical Ecology, Article: Current Biology, doi: 10.1016 / j.cub.2021.12.021