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

Soybean Kunitz trypsin inhibitor (SBTI) and soybean Bowman-Birk trypsin-chymotrypsin inhibitor (SBBI) were incorporated into artificial diet and fed to newly moulted 3rd-instar larvae of Helicoverpa armigera, the corn earworm. 1. Neither SBTI nor SBBI affected larval survival for 14 days. After this time, many of the larvae fed diets with high levels of inhibitor suddenly died. This effect was most marked with SBTI diets. 2. When compared with the effect of the control diet (no inhibitor), inhibitors in the diet reduced total larval biomass and mean larval weight. This effect on larval growth was much greater with dietary SBTI than with SBBI. The relative effectiveness of the inhibitors in vivo is in agreement with previous data showing that SBTI was a more effective inhibitor of the H. armigera trypsin like gut protease in vitro. 3. The presence of inhibitor in the diet reduced the mean larval weight at moult, suggesting a possible response of ecdysis to semi-starvation. This effect was greater with SBTI than with SBBI. 4. Faecal output was used as a measure of food intake. By day 10 of the feeding trials, the mean faecal output/larva was similar for larvae fed the control and SBBI diets, but it was significantly lower for larvae fed dietary SBTI. However, when faecal output was corrected to account for developmental states and weights of larvae, no differences were found between the effects of control and inhibitor diets. 5. Feeding larvae a diet containing 0.234 mM SBTI significantly reduced the trypsin-like enzyme activities found in their gut contents when compared with the levels found in larvae fed the control diet. The possible mechanisms of action of SBTI and SBBI in vivo are discussed, as are their relative potentials for protecting crop plants against damage by larval H. armigera.