Effects of dietary ascorbic acid on larvae of the Egyptian cotton leafworm, Spodoptera littoralis

Abstract

Ascorbic acid deficiency symptoms of Spodoptera littoralis larvae were studied. The moulting cycle failure and smaller weight gain of the ascorbic acid-deprived larvae resulted from a smaller head capsule per instar which, in turn, impaired ecdysis and mouth-part function for food ingestion. This caused the larvae to die of hunger. Ascorbic acid was shown to be associated with the moulting stage. Haemolymph ascorbic acid was depleted at the pharate stage with concomitant appearance in the moulting fluid, where active, soluble O-diphenoloxidase was also determined. This enzyme was active against catechol, dopa, dopamine, and N-acetyl dopamine. The high catecholase activity of this enzyme was inhibited in vitro by ascorbates and sulfhydryl compounds. On the other hand, these sulfhydryl-reducing compounds, cysteine, and glutathione-SH, did not replace the dietary effect of ascorbic acid to prevent moulting cycle failure, nor were they detected in the moulting fluid. It appears that the insufficient cuticle expansion in the ascorbic acid-deficient larvae is due to premature hardening of the cuticle. The possible function of ascorbic acid to control cuticle melanization and sclerotization is discussed.