By Paloma Salmeron-O’Brien
Plants can’t run away from a caterpillar looking for a meal, so scientific studies have explored the ways in which plants use physical and chemical defenses to fight back against herbivores. It turns out that corn seedlings can go a step further and call upon an unlikely ally: parasitic wasps! In a study conducted by the U.S. Department of Agriculture, researchers explored the relationship between the chemical language of plants and the observed ability of parasitic wasps to learn. They experimented on the ability of Ioana corn seedlings to release chemicals called terpenoids when damaged. They also studied the impact of beet armyworm caterpillar saliva on terpenoid production and the behavior of female Cotesia marginiventris parasitic wasps. Researchers aimed to gain a better understanding of how connected plants are to their environment, exploring communication that extends beyond plant-herbivore relationships.
Researchers first constructed four treatment groups, allowing three seedlings to be damaged by caterpillars while artificially damaging seedlings with a razor blade in a fashion mimicking caterpillar damage. Containing three seedlings each, the treatment groups were larval damaged seedlings, artificially damaged seedlings treated with caterpillar gut contents, untreated artificially damaged seedlings, and treated undamaged seedlings. The plants, along with a control group of undamaged, untreated seedlings, were left overnight in a glass system to collect released terpenoids. Researchers found that larval damaged seedlings released significantly more terpenoids relative to untreated, artificially damaged seedlings. Additionally, artificially damaged seedlings treated with gut contents released more terpenoids than untreated ones, and undamaged seedlings, both treated and untreated, released virtually no terpenoids. These results suggest that damaged seedlings exposed to caterpillar excretions (such as saliva) increased terpenoid production. As feeding caterpillars trigger increased terpenoid production, seedlings likely release terpenoids as a direct chemical defense against caterpillar feeding.
The effect of terpenoid production was then tested on female parasitic wasps, as it is the females that seek out hosts to lay their eggs. In a series of choice tests, wasps were released into a tube and seedlings from two different groups were placed at either end. Researchers examine the wasps’ preference for different plants by recording how many wasps flew to either choice offered. They found that females preferred caterpillar damage over artificial damage and gut content treated artificially damaged seedlings over untreated artificially damaged seedlings. These results align with those of the previous experiment, as both of these choices favored plants that produced more terpenoids. Additionally, wasps showed no preference between caterpillar damage and artificial damage treated with gut contents, as both plants produced similar terpenoid levels. Furthermore, wasps preferred artificially damaged plants, which would release terpenoids, over treated and undamaged plants, which released virtually no terpenoids. In all choice tests, if there was a significant difference in terpenoid production between the plants offered, the parasitic wasps would prefer the plant producing more terpenoids.
Combining the findings from both experiments, parasitic wasps prefer plants producing more terpenoids, and plants produce the most terpenoids when caterpillars feed on them, so plants under attack can use increased terpenoid production to attract the wasps to prey upon the feeding caterpillars. But how do we know that the wasps are attracted to the terpenoids specifically? Researchers tested this by applying terpenoids to paper, and sure enough, the parasitic wasps were attracted to the paper. To further assess the wasps’ affinity for the terpenoids, a group of wasps with prior experience of caterpillars feeding on corn seedlings and a group of inexperienced wasps were both given a choice between natural terpenoids and an artificial equivalent. Researchers found that while inexperienced wasps showed no preference, experienced wasps preferred the natural terpenoids, supporting the wasps’ ability to learn to associate plant-produced terpenoids with the presence of their hosts.
Corn seedlings are just one of numerous agricultural plant species grown and consumed by humanity and just one example of plants communicating beyond their herbivorous adversaries. With more research, scientists might find ways to harness the natural partnership of agricultural plants and their insect parasite allies for our benefit. Instead of widespread chemical pesticides, we might be able to employ battalions of bugs to keep our crops safe. The results of these experiments broaden our understanding of how plants communicate with the wider world, extending beyond plant-herbivore relationships. They also add depth to parasitic wasp behavior, defending their inherent complexity by exploring their ability to learn.
References:
Turlings, T. C. J., Tumlinson, J. H., Lewis, W. J., T. C. J. TurlingsInsect Attractants, B. and B. B. R. L., J. H. TumlinsonInsect Attractants, B. and B. B. R. L., & W. J. LewisInsect Biology and Population Management Research Laboratory, A. R. S. (1990, November 30). Exploitation of herbivore-induced plant odors by host-seeking parasitic wasps. Science. Retrieved November 4, 2021, from https://www.science.org/doi/10.1126/science.250.4985.1251.