Effects of early season defense induction in Lima bean (Phaseolus lunatus) on a seed beetle and its larval parasitoid

In nature, plants are exposed to a number of herbivores that my attack them at the same or different times during the season. Studies have shown that a first attack can induce chemical changes in a plant that affect the performance of subsequent herbivores. This is usually studied for insects that feed on the same plant tissue. Using wild lima bean plants, we tested the hypothesis that prior induction of plant defenses as a response to damage inflicted to leaves, will later during the season, alter the suitability of seeds for the bruchid Zabrotes subfasciatus and its parasitoid Stenocorse bruchivora. We simulated early season herbivory by damaging leaves and applying jasmonic acid (JA), one of the main hormones known to be implicated in induced plant defense. Control plants were left untreated. Plant performance was monitored along the season until seed production and we determined the effects of early induction on bruchid and parasitoid abundance. Finally, we harvested seeds and determined in the laboratory the performance of bruchids and their parasitoids on seeds from plants that were treated with JA at different times during the season and control plants.

Results showed that early defense induction significantly reduced infestation rates by bruchids and parasitoids. Laboratory experiments confirmed that the performance of bruchid beetles and their parasitoids was lower on seeds that originated from treated plants.

These results are discussed in the context of plant-mediated interactions between insects of different guilds and trophic levels.