EFFECTS OF SOYBEAN PROTEASE INHIBITORS ON THE GROWTH AND DEVELOPMENT OF LARVAL HELICOVERPA-ARMIGERA

Soybean Kunitz trypsin inhibitor (SBTI) and soybean Bowman-Birk trypsi n-chymotrypsin inhibitor (SBBI) were incorporated into artificial diet and fed to newly moulted 3rd-instar larvae of Helicoverpa armigera, t he corn earworm. 1. Neither SBTI nor SBBI affected larval survival for 14 days. After this time, many of the larvae fed diets with high leve ls of inhibitor suddenly died. This effect was most marked with SBTI d iets. 2. When compared with the effect of the control diet (no inhibit or), inhibitors in the diet reduced total larval biomass and mean larv al weight. This effect on larval growth was much greater with dietary SBTI than with SBBI. The relative effectiveness of the inhibitors in v ivo is in agreement with previous data showing that SBTI was a more ef fective inhibitor of the H. armigera trypsin like gut protease in vitr o. 3. The presence of inhibitor in the diet reduced the mean larval we ight at moult, suggesting a possible response of ecdysis to semi-starv ation. This effect was greater with SBTI than with SBBI. 4. Faecal out put was used as a measure of food intake. By day 10 of the feeding tri als, the mean faecal output/larva was similar for larvae fed the contr ol and SBBI diets, but it was significantly lower for larvae fed dieta ry SBTI. However, when faecal output was corrected to account for deve lopmental states and weights of larvae, no differences were found betw een the effects of control and inhibitor diets. 5. Feeding larvae a di et containing 0.234 mM SBTI significantly reduced the trypsin-like enz yme activities found in their gut contents when compared with the leve ls found in larvae fed the control diet. The possible mechanisms of ac tion of SBTI and SBBI in vivo are discussed, as are their relative pot entials for protecting crop plants against damage by larval H. armiger a.