Dietary Selection Behaviour in Spodoptera Littoralis: The Effects of Conditioning Diet and Conditioning Period on Neural Responsiveness and Selection Behaviour

ABSTRACT This paper forms part of our study of the behavioural and neural mechanisms involved in dietary selection behaviour of larvae of the polyphagous noctuid Spodoptera littoralis (Boisduval). Larvae were conditioned on one of four chemically defined artificial diets for 4, 8 or 12 h. Diets differed in their proportions of protein and digestible carbohydrate: the PC diet contained 20% protein and 10% carbohydrate; the P diet contained 20 % protein but the 10 % carbohydrate was replaced by indigestible cellulose; the C diet contained 10% digestible carbohydrate and the protein was replaced by cellulose, and the 0 diet lacked both protein and carbohydrate. After conditioning, the larvae were exposed to either behavioural or electro-physiological experiments. In the behavioural choice experiment, larvae were provided with both P and C diets for 9 h. The conditioning diets P and 0 influenced which of the two choice diets was first contacted; significantly higher proportions of the larvae conditioned on the P diet initially contacted the C choice diet and significantly higher proportions of larvae conditioned on the 0 diet initially contacted the P choice diet. Larvae fed on the diet they first contacted and the probability of stopping the first meal was greater when that diet was P, regardless of the conditioning diet. The duration of the first meal on the C choice diet was significantly longer for larvae conditioned on diets lacking digestible carbohydrate (P and 0) than for those conditioned on either PC or C diets. Larvae were observed to compensate for a deficiency in protein by increasing the frequency of feeding contacts with the P choice diet but not the duration of these contacts. To compensate for a deficiency in digestible carbohydrate, the larvae increased the duration of their feeding contacts with the C choice diet. In the electrophysiological experiment, the maxillary styloconic sensilla were stimulated with an amino acid mixture and sucrose. Neural responses were recorded from two groups of larvae, one after they had been conditioned on one of the four diets for 4, 8 or 12 h and the other after they had subsequently been exposed to the choice diets for 9h. Larvae in the first group that had been conditioned on diets lacking sucrose (P and 0) were more responsive to sucrose than larvae conditioned on diets containing sucrose (PC and C). The reverse occurred with the response to the amino acid mixture, with larvae conditioned on diets containing protein (PC and P) being less responsive to the amino acid mixture than those conditioned on diets lacking protein (C and 0). After the 9h choice period, the responsiveness of sensilla was no longer related to the conditioning diet.