Physiological and dietary influences on midgut redox conditions in generalist lepidopteran larvae

Abstract

We examined the effects of host plant foliage and redox active compounds on the midgut physicochemistry of several species of lepidopteran larvae. Midgut pH varied among Helicoverpa zea, Heliothis virescens, Pseudoplusia includens and Hyphantria cunea but was not affected by the host plant they fed upon (cotton, clover, wild geranium or soybean). However, redox potential was influenced by both plant and lepidopteran species. Midgut redox potentials closely matched the redox potential of foliage homogenized under alkaline, but not neutral pH, suggesting that alkalinity has a strong influence on the reducing strength of foliage in the gut lumen. Artificial diet containing redox active compounds, including thiol reducing agents and plant allelochemicals, significantly lowered midgut redox potential of H. zea when ingested. Juglone induced a 205% drop in redox potential in the lumen, whereas caffeic acid, chlorogenic acid, pyruvate, cysteine, dithiothreitol and ascorbic acid had more moderate effects (34–44% reduction). Quebracho, tannic acid, o-coumaric and p-coumaric acids had no significant effects. These findings suggest that much of the variation in midgut redox conditions observed in folivorous caterpillars is due to the redox activity of host plant chemicals at the alkaline pH of the gut.