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Amino acid requirements of Schistocerca gregaria (Forskal) Williams, David Colin


The development of a chemically defined artificial diet for Schistocerca gregaria (Forsk.) is described,. The diet that permitted the best growth of S. gregaria was used to determine the amino acid requirements of this animal. Further studies were performed to determine whether amino acids were required as nutrient per se or as phagostimulants. The moist artificial diets initially used in growth trials showed signs of deterioration after 20-days storage at -15°C. This deterioration was evidenced by reduced growth of animals on 20-day-old diets, and by the loss of ascorbic acid from such diets. Freeze-or oven-drying diets increased their storage life and their stability under experimental conditions. Growth trials showed that dried diets were stable for at least 2 months when stored in vacuum desiccators. Little ascorbic acid was degraded in diets kept under experimental conditions (30°C, 55% E.H.) for 2 days, suggesting that such diets could be kept under these conditions for at least 2 days without being replaced. S. gregaria showed poor growth on artificial diets containing either an ad hoc mixture of amino acids or a mixture of amino acids based on analysis of lettuce protein. However, growth of animals was improved by using an amino acid mixture based on analysis of cabbage proteins. Diets could be prepared mere rapidly if the vitamin solutions used in compounding the diets were replaced by vitamins triturated in sucrose. The physical properties of the diet were important, and fine-powder diets caused heavy mortality of S. gregaria hoppers.. Powder diets had to be formed into granules or tablets before they could be utilized by S. gregaria. Although few animals reached the adult stage on artificial diets, the best diet did allow S. gregaria hoppers to develop to the 5th instar (with a mean weight of approximately 550 mg) after 33-days growth. To determine which amino acids were essential for S. gregaria, the growth of animals reared on test-diets lacking an individual amino acid was compared with growth of animals on control diets containing the full complement of amino acids. If the removal of an amino acid had no effect on the growth of animals it was classed as an inessential amino acid; if it had an marked detrimental effect it was classed as an essential amino acid, and if it only had a marginally detrimental effect it was termed a semi-essential amino acid. The results of growth trials indicated that tyrosine, alanine, aspartate, glutamate, cystine, serine and proline were inessential amino acids for S. qreqaria, whereas lysine, phenylalanine, isoleucine, valine, threonine, tryptophan, leucine, histidine, methionine and arginine were essential, and glycine was semi-essential. Although the growth of animals on arginine deficient diets was poor enough to warrant arginine being classed as an essential amino acid, S. qreqaria's requirements for this amino acid did not appear to be as stringent as its requirements for the ether essential amino acids. Ihe semi-essential nature of glycine, and the suprisingly good growth of animals on arginine-deficient diets are discussed in relation to S. qreqaria1s amino-acid metabolism. It is suggested that the poor growth of animals on glycine-deficient diets is a result of glycine not being synthesized rapidly enough to meet S. gregaria’s metabolic requirements, and that the growth of animals on arginine-deficient diets is due to limited synthesis of this amino acid via the ornithine cycle. Feeding trials indicated that the failure of animals to show good growth on diets lacking any of the 10 essential amino acids was due to the reduced feeding activity of animals reared on these diets. Individual removal of any of the other 8 amino acids (i.e. the 7 inessential amino acids and glycine) had no effect on food consumption. The role of amino acids as phagostimulants is discussed in relationship to current theories concerning host-plant selection by phytophagous insects, and it is suggested that food selection is based on a learned-aversion response to the metabolic effects associated with the ingestion of an imbalanced ratio of nutrients.

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