Print PagePlants must protect their vital tissues from herbivores that would consume them. To cope with this pressure, plants have evolved ways of defending themselves from herbivory through either physical or chemical traits. Plants often respond to herbivore damage by turning on inducible secondary metabolites, which have been shown to directly, negatively affect some herbivores.
Previous work in the Eubanks laboratory has shown that aphid herbivory of cotton plants induces the production of defenses associated with the salicylic acid and jasmonic acid pathways. In turn, aphid induction of cotton plants reduces host plant quality for other herbivores. For example, beet armyworm caterpillars, Spodoptera exigua, grow slower, develop into smaller pupae, and are more likely to die before pupation when they consume plants induced by aphid feeding.
We tested the hypothesis that female moths avoid ovipositing eggs on aphid-induced plants. We induced plants by several treatments, including infesting leaves with aphids, volatile induction with methyl-jasmonate, and a root-drench of salicylic acid. We looked at the effect of aphid induction and the chemical treatment with jasmonic acid and salicylic acid on the oviposition preferences of adult beet armyworm females. We hypothesized that moths would prefer naïve plants over those induced by aphids or treated exogenously with salicylic acid or methyl-jasmonate.
Results/Conclusions
In a no-choice field study, beet armyworms laid significantly more eggs on naïve cotton plants than any of the treated cotton plants. Similarly, in a lab choice test, beet armyworm moths preferred to lay eggs on plants that had not been previously infested with aphids. Interestingly, female moths did not show a strong preference between naïve and salicylic acid treated plants in lab.
We also tested the hypothesis that beet armyworm caterpillars could make decisions based on host-plant quality. A choice test with these caterpillars showed that they preferred naïve cotton plants to salicylic acid treated plants.
The oviposition choices made by adult beet armyworms have ecological consequences that strongly influence arthropod community structure in cotton fields. As beet armyworms prefer hosts that have few induced secondary metabolites, plants will receive protection through the induction of defensive chemicals from previous bouts of herbivory. Previous aphid feeding may increase plant fitness by inducing secondary defensive chemicals that repel more damaging chewing herbivores, such as the beet armyworm.
