SUBLETHAL EFFECTS OF DIETARY CYFLUTHRIN ON NUTRITIONAL PERFORMANCE AND GUT HYDROLASE ACTIVITY IN LARVAE OF THE EGYPTIAN COTTON LEAFWORM, SPODOPTERA-LITTORALIS

The effects of LD15 of cyfluthrin incorporated to artificial diet on larval growth, food consumption and utilization, and gut hydrolase (carbohydrase and carboxylesterase) activity were investigated during the fifth larval instar of the Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae). As long as the larvae were fed on cyfluthrin-containing diet (24 hr), their growth rate was significantly reduced as a consequence of feeding inhibition (reduction in relative consumption rate (RCR)) due to knockdown and slight repulsive or antifeedant effects. The ability of dietary cyfluthrin to reduce weight gain and larval growth was also due to reduced efficiency of conversion of ingested food (ECD). Cyfluthrin in the diet did not decrease the approximate digestibility (AD), indicating that the quality of the food and the ability to digest material from the diet and to assimilate nutrients were unaffected. These observations were confirmed by measurements of gut glycosidase activity which showed enhanced hydrolysis of maltose and starch in cyfluthrin-exposed larvae. Other glycosidase activities were not affected. It was therefore envisaged that cyfluthrin might also affect growth by diverting part of the energy usually allocated to growth to cover metabolic costs associated with detoxication of the insecticide. However, gut carboxylesterase activity was not appreciably affected by dietary cyfluthrin when 1-naphthyl-acetate or 2-naphthyl-acetate were used as substrates. As soon as they were provided with insecticide-free diet. S. littoralis larvae showed dramatic increases in all indices (RCR, AD, ECD, and RGR), as an effort to compensate for retarded growth. Prolonged development and reduced final body weight indicated that these increases were insufficient to compensate entirely for the growth delay occurring during feeding on cyfluthrin-containing diet. Implications of sublethal effects of dietary pyrethroids on growth and food consumption of insect pests in crop protection are discussed. © 1993 Academic Press.