Plants that emit an airborne distress signal in response to herbivory may actually attract more enemies, according to a new study published by a team of researchers from Switzerland. They found that the odor released by maize plants under attack by insects attract not only parasitic wasps, which prey on herbivorous insects, but also caterpillars of the Egyptian cotton leafworm moth Spodoptera littoralis, a species that feeds on maize leaves.
When damaged, many plants release volatile organic compounds, similar to the compounds that cause the characteristic smell of freshly cut grass. These compounds are known to be attractive to parasitoid wasps that lay their eggs inside other insects. Plants appear to use this strategy to fight back against herbivorous insects by calling for their enemies' enemies. In contrast, herbivorous insects such as adult moths and butterflies tend to avoid food plants that are under attack by conspecifics. However, Spodoptera littoralis caterpillars are actually attracted to the odor of damaged maize plants, even when this odor is mimicked in the laboratory with a mix of synthetic compounds
To determine what kind of odors the caterpillars preferred, the researchers let the caterpillars chose among several odors by placing them in an olfactometer, a device consisting of tubes connected to a central chamber, with each tube introducing an airflow carrying a different odor. The caterpillars were more than twice as likely to crawl towards the odor from maize plants under attack by conspecifics than towards undamaged plants, especially if the damage was recent and the caterpillars had already fed on maize.
The question is what might be the advantage to the caterpillars of moving towards plants that are already infested also given the risk of being attacked by parasitoid wasps. The advantage seems to be that fallen caterpillars can quickly rediscover the plant on which they fed by moving towards volatile organic compounds released by damaged maize plants. When they drop from a plant they become highly vulnerable to predators and pathogens in the soil, as well as to starvation. On damaged plants the competition may be more intense, but at least the caterpillars are assured of a suitable plant. The team also observed that the caterpillars feed less and move more when exposed to high concentrations of the volatiles. By moving away from freshly damaged sites, they can minimize risk of predation and avoid competition.
Adult moths, on the other hand, are much more mobile and explore the environment to discover the best food source. As a result they avoid maize that is already under attack