The accumulation of haemolymph proteins and activity of digestive proteinases of grasshoppers (Melanoplus sanguinipes) fed wheat, oats or kochia

Lesser migratory grasshoppers, Melanoplus sanguinipes (Fab.), were fed single plant diets of wheat, oats or kochia foliage in the instar preceding sampling, from the 2nd to 5th instars. Haemolymph proteins, tryptic and chymotryptic activity in the gut and total gut protein were determined. Nutritional indices were calculated from data collected from 2nd-instar grasshoppers. In this instar, grasshoppers consumed more than twice as much kochia as either wheat or oats, and produced correspondingly greater amounts of frass. The duration of the instar was in the order wheat < kochia < oats. Approximate digestibility was significantly lower in grasshoppers fed oats than those fed either wheat or kochia. The efficiency of conversion of digested food was significantly lower and the relative consumption rate significantly higher in those fed kochia than either wheat or oats. The efficiency of conversion of ingested food was in the order wheat > oats > kochia. Haemolymph concentrations of both putative apoprotein I and the comigrating complex of apoprotein II and larval storage protein (LSP) subunits (70-80 kDa mol.wt) were significantly higher in grasshoppers fed the standard colony diet of wheat germ, lettuce and wheat in all instars than for those grasshoppers fed single-plant diets. Among grasshoppers fed the single-plant diets, those fed wheat had the highest levels of putative apoprotein/LSP. On an individual grasshopper basis, there was no correlation between the concentrations of apoprotein/LSP in the haemolymph and the levels of gut protein, trypsin or chymotrypsin. Total protein in whole-gut homogenates was significantly higher in kochia-fed grasshopper in the 2nd instar and significantly lower in those fed oats in the 5th instar. In all instars, grasshoppers fed oats had higher total trypsin activity than those fed wheat; ratios of trypsin activity in oat- vs wheat-fed grasshoppers were 1.8 : 1, 1.9 : 1, 3.8 : 1 and 1.9 : 1 for the 2nd, 3rd, 4th and 5th instars, respectively. Total chymotrypsin activity was significantly higher in grasshoppers fed oats in the 4th and 5th instars, and in those fed kochia in the 5th instar. The ratios of chymotrypsin activity in oat- vs wheat-fed grasshoppers were 1.5 : 1, 1.0 : 1, 2.5 : 1 and 1.9 : 1 for the 2nd, 3rd, 4th and 5th instars, respectively. The present study confirmed that both the kochia and oats diets were inferior in supporting grasshopper growth, probably because of lower protein acquisition and hypermodulation of digestive proteinases. However, the levels of activity of these enzymes were considerably lower in the earlier instars than those previously reported in adults of this species. The results are discussed in the context of hormonal activity and suggest that protein is allocated differently in nymphs compared to adults. Comparison of the physiological and biochemical responses of grasshopper to wheat and oats and their variably resistant cultivars provides a valuable model for studying such interactions.