Ascorbic and uric acid responses to xanthotoxin ingestion in a generalist and a specialist caterpillar

Abstract

For herbivorous insects, dietary sources of low molecular weight non-enzymatic antioxidants, such as ascorbic acid, may influence performance in the presence of phototoxic plant constituents. We examined responses of Trichoplusia ni, a broad generalist, and Depressaria pastinacella, a specialist on furanocoumarin-containing plants, to variation in dietary ascorbic acid availability in the presence and absence of xanthotoxin, a phototoxic furanocoumarin. In T. ni, dietary ascorbic acid significantly increased levels of this compound in body tissues (approximately 7-fold, 5-fold, and 8-fold in hemolymph, gut, and fat body, respectively). In the presence of xanthotoxin, however, the amount of ascorbic acid accumulated significantly decreased. This decrease was not due to antifeedant effects of xanthotoxin and may instead have resulted from depletion of ascorbic acid due to its radical scavenging activity. In contrast, ascorbic acid levels in D. pastinacella were less affected by variation in dietary levels of either xanthotoxin or ascorbic acid, although uric acid, another potential water-soluble nonenzymatic antioxidant, increased in response to dietary ascorbic acid, as it did in T. ni. Thus, for generalists, such as T. ni, that lack specialized detoxification mechanisms against phototoxins such as furanocoumarins, dietary ascorbic acid may play an important role in antioxidant defense, and, for caterpillars in general, uric acid may also contribute to antioxidant defenses. Arch. Insect Biochem. Physiol. 42:26–36, 1999. © 1999 Wiley-Liss, Inc.