A plant's induction of defenses can help decrease the damage caused by herbivores. While these chemical or physical defenses may directly decrease herbivory, they can also have indirect consequences. In a tri-trophic system consisting of a plant, an insect, and an insect pathogen, plant based trait-mediated indirect effects (TMIEs) may alter host-pathogen interactions and affect disease transmission. To examine these potential indirect effects, I experimentally induced soybean (Glycine max) plants by spraying Jasmonic Acid (JA) on their leaves.
In a series of field experiments where I placed fall armyworm (Spodoptera frugiperda) larvae along with a lethal baculovirus (SfMNPV) on JA-induced and non-induced plants, I found that induction decreased overall transmission. The decrease in transmission occurred not because of a decline in the disease transmission rate but because of an increase in the population's heterogeniety regarding susceptibility to the disease. The increase in heterogeneity was driven by changes in feeding rates and larval responses to infectious doses. In general, the direction and magnitude of the TMIE was precipitated by changes in the resource quality of the plant on which the herbivore host feeds. The importance of resource quality in determining a host's response to potential infections can have important consequences for determining infectious disease dynamics.