The intensity and spatial dispersion of within-plant herbivory drive trait-specific resistance and tolerance in wild cotton.

PREMISE: Herbivory induces complex plant defensive responses that may vary depending on the amount and spatial dispersion of damage on the plant; however, of the two variables, damage dispersion has received much less attention. To address this gap, we studied wild cotton (Gossypium hirsutum) defensive responses to different herbivory amounts and degrees of dispersion among leaves using the specialist caterpillar Alabama argillacea (Erebidae).

METHODS

Four herbivory treatments differing in the amount and spatial distribution were applied to 4-month-old plants: no herbivory (controls), low (two caterpillars: one on each of two leaves), high-concentrated (four caterpillars: one pair on each of two leaves), and high-dispersed (four caterpillars: one on each of four leaves). We measured extrafloral nectar (EFN) and phenolics, both resistance-related traits, and as a proxy for tolerance, regrowth capacity (new leaf biomass post-herbivory). To differentiate local from systemic induction, we measured EFN and phenolics separately for damaged and undamaged leaves.

RESULTS

EFN was significantly and more consistently induced in damaged leaves (i.e., stronger local induction) and in response to high-dispersed damage, but phenolics were unaffected by the treatment. Biomass of new leaves produced post herbivory was similar to that of controls for most treatment groups, except for high-concentrated damage (lower than controls), suggesting this damage treatment incurred higher costs.

CONCLUSIONS

These results highlight the importance of studying induced plant defenses under various herbivory patterns and suggest that the spatial distribution of within-plant damage has contrasting effects on resistance- (namely EFN) versus tolerance-related induced responses of importance for wild cotton growth and ant-plant interactions.