Ishaaya, I., The Volcani Institute of Agricultural Research, Division of Entomology, Bet Dagan, Israel
Moore, I., The Volcani Institute of Agricultural Research, Division of Entomology, Bet Dagan, Israel
Joseph, D., The Volcani Institute of Agricultural Research, Division of Entomology, Bet Dagan, Israel

In the larvae of the Egyptian cotton worm, Spodoptera littoralis, the optimum for protease activity was found to be a reaction mixture of pH 11·0 plus 0·75% casein at 37°C for 60 min. For amylase activity the optimum was a reaction mixture of pH 9·5 plus 0·25% starch at 37°C for 30 min. Both protease and amylase are adapted to high temperatures in vitro. Proteolytic enzymes lose their activity above 55°C and amylase above 65°C. An artificial diet of high protein content (8 per cent instead of ∼2·5 per cent in clover) strongly stimulates the proteolytic activity, especially in the midgut wall, where it becomes four to five times greater than in larvae reared on clover. On the other hand, only a alight increase of 10 to 15 per cent in amylase activity was found in larvae reared on the artificial diet. Increasing the protein level from 3·6 to 7·6 per cent in an artificial diet, stimulated both proteolytic activity and larval weight. Amylase activity rose four- to fivefold when the protein content increased from 3·6 to 5·6 per cent, but levelled off at 7·6 per cent. Enzymatic activity increased when the temperature rose from 10 to 32°C. At 10°C (i.e. below the threshold of larval development), both proteolytic and amylolytic activity in the midgut wall were less than 10 per cent of that obtained at 32°C. © 1971.