Many plant species respond to herbivory via morphological or chemical changes in plant quality. Following damage, secondary compound concentrations, physical defenses, or volatile emissions may increase. In addition to directly and indirectly resisting the attacking herbivore, induced responses can affect subsequent interactions with organisms that use the plant as food. Cotton aphids, Aphis gossypii, have weak negative effects on cotton, Gossypium hirsutum, growth and fitness. However, we have found evidence that cotton aphids alter plant chemistry to make the cotton plants less palatable to other, more-damaging herbivores. We hypothesized that aphid herbivory induces cotton defenses which then negatively impacts S. exigua growth and survival. In our first experiment, a range of cotton aphid densities were placed onto 50 cotton seedlings. Aphid densities were monitored bi-weekly for three weeks, after which a 2nd instar S. exigua larva was allowed to feed on one newly formed, aphid-free leaf for 48 hours. S. exigua relative growth rate and amount of damage was measured. We also measured gossypol containing pigment gland density and defensive protein concentration in the cotton leaves. In a second experiment, S. exigua larvae were reared on cotton leaves that had either been previously fed upon by cotton aphids or control leaves that had not experienced herbivory.
In our first experiment, we found that as aphid density increased, S. exigua relative growth rate and amount of herbivory decreased. Plants experiencing high aphid densities also exhibited higher defensive protein concentrations, including peroxidase, trypsin inhibitor and chitinase. In our second experiment, S. exigua reared on aphid-induced cotton leaves displayed reduced survival, development rate and pupal mass compared to larvae reared on control leaves. Together, these results suggest that cotton aphids can potentially protect cotton plants from more-damaging herbivores via aphid-induced changes in plant chemistry and have a net beneficial effect on cotton growth and yield.