Herbivore-responsive cotton phenolics and their impact on insect performance and biochemistry

Insect damage to plants can have several impacts on plant biochemical composition and such changes may influence the insect performances. Here, we examined herbivore-responsive alterations in the phenolic profile of cotton plant using three cotton genotypes, Bc-68-2, Gcot-16 and Coker-312, belonging to G. hirsutum sp. during their infestation with two biting and chewing insects viz., Helicoverpa armigera and Spodoptera litura, separately for 96 h. Botfi insects had similar effects on phenolic induction in cotton plants (Gossypium sp.) at 96 h post infestation (hpi) however, the phenolic accumulation was genotype specific. We measured 13 phenolic compounds, out of which four phenolic acids viz., gallic acid (GA), cinnamic acid (CA), p-coumaric acid (PA) and salicylic acid (SA) displayed higher accumulation in response to insect herbivory at 96 hpi. The subsequent impact of four herbivore-responsive phenolics on insect performance was evaluated through diet incorporation assay. Insect larvae displayed significant reduction in larval weights and substantial mortality on 10th day in case of CA and PA incorporated diets at higher concentrations. Physiological examination of treated larvae on 5th and 10th day post feeding at CA and PA containing diets revealed oxidative stress, reduced lactate dehydrogenase enzyme activity, altered glucose and protein level and enhanced glutathione-S-transferase enzyme activity in larvae feeding on treatment diets as compared to controls. Our results suggest that the defensive chemistry of phenolics has important effect on the insect fitness and physiology and the flavonoids such as CA and PA can be utilized in pest control programmes. (C) 2017 Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society. Published by Elsevier B.V. All rights reserved.