THE EFFECTS OF FEEDING LEAF SAP FROM OATS AND WHEAT, WITH AND WITHOUTSOYBEAN TRYPSIN-INHIBITOR, ON FEEDING-BEHAVIOR AND DIGESTIVE PHYSIOLOGY OF ADULT MALES OF MELANOPLUS-SANGUINIPES

Newly-fledged adult male grasshoppers (Melanoplus sanguinipes Fab.) we re fed measured quantities of freshly prepared sap from the leaves of seedlings of wheat (Triticum aestivum L. cv. Katepwa) or oats (Avena s ativa L. cv. Harmon), or sap from each plant to which soybean trypsin inhibitor (STI) was added. Different patterns of feeding with respect to movement and position of the antennae and palpi were recorded on oa t sap compared to wheat sap. All grasshoppers were allowed to feed ad lib twice daily on each diet and the volumes of sap imbibed were recor ded. The volumes of sap imbibed were not significantly different durin g the first day of feeding, but on subsequent days they were significa ntly smaller in grasshoppers fed oat sap compared to those fed wheat s ap, The addition of STI to oat sap had no effect on the volume imbibed . Meal volumes were more or less consistent from day to day in grassho ppers fed wheat sap, but the addition of STI to wheat sap stimulated a significant increase in the volume imbibed. After 5 days of feeding i n this manner the grasshoppers were weighed, stimulated to regurgitate the crop contents, then dissected. The complete alimentary tract was removed from each grasshopper and assayed for trypsin and chymotrypsin activity. A significantly higher gain in fresh body weight was record ed in grasshoppers fed wheat sap + STI than in those fed wheat sap alo ne. In contrast, grasshoppers fed oat sap lost weight during the cours e of the experiment, and a greater loss was recorded from those fed oa t sap to which STI was added. Protein concentration in the regurgitate varied significantly with diet, and was in the following order: wheat sap + STI > wheat sap > oat sap + STI = oat sap. Trypsin and chymotry psin activities were in the same order as protein concentration. Small increases in trypsin activity were observed in the regurgitate from g rasshoppers fed sap + STI from either plant, compared to the sap alone . A 20-fold increase in chymotrypsin activity was observed in the regu rgitate of grasshoppers fed wheat sap + STI compared to the regurgitat e in those fed wheat sap. A 4.5-fold increase in chymotrypsin activity was observed in the regurgitate of grasshoppers fed oat sap + STI com pared to the regurgitate of those fed oat sap alone. Protein concentra tion in the gut tissues varied significantly with diet and paralleled those observed in the regurgitate. Small non-significant increases in trypsin activity were observed in the grasshoppers fed sap + STI from either plant, compared to the sap alone. A 6-fold increase in chymotry psin activity was observed in the residual gut tissues from grasshoppe rs fed wheat sap + STI compared to wheat sap, but activity of this enz yme was lower in grasshoppers fed oat sap + STI compared to oat sap al one. The results are discussed in the context of protein availability and its allocation, and the dynamic interaction between antifeedants, phagostimulants and antibiosis factors and their influence on the impa ct of proteinase inhibitors.