Costs and benefits of induced responses in soybean

Abstract

Herbivorous insects are known to negatively impact plant fitness, such that plants have evolved defense strategies to reduce the preference and performance of herbivores on those plants. However, a plant’s investment in defense may be costly when herbivores are absent from the environment. Defense traits that are induced only upon herbivory can mitigate costs associated with defense maintenance. Although costs and benefits of induced responses are generally assumed, empirical evidence for them is equivocal. We examined the fitness costs and benefits of jasmonic acid-induced responses by soybean (Glycine max L. Merr.) in the absence and presence of soybean loopers (Chrysodeix includens Walker) (Lepidoptera: Noctuidae), an economically important pest of soybeans in the southern United States. In a greenhouse experiment we demonstrated that induction was costly to soybeans, affecting all components of soybean fitness. Jasmonic acid-induced plants produced 10.1% fewer seeds that were 9.0% lighter, and had 19.2% lower germination rates than control plants. In contrast, induction provided only modest benefits to soybeans. In a choice experiment, soybean loopers exhibited a significantly greater preference for leaves from control plants consuming 62% more tissue than from jasmonic acid- induced plants. Soybean loopers that did feed on induced soybean plants matured at the same rate and to the same size as soybean loopers that fed on control plants. However, at high conspecific density, soybean looper survivorship was reduced by 44% on induced relative to control plants. Negative effects of induction on soybean looper preference and survivorship did not translate into fitness benefits for soybeans. Our study is the first evaluation of costs and benefits of soybean induced responses.