Nutritional and Allelochemic Insect-Plant Interactions Relating to Digestion and Food Intake: Some Examples

Digestive enzymes in insects are generally adapted to the diet on which the species feed (Wigglesworth, 1965). Blowfly larvae that feed on animal tissues exhibit high protease and lipase activities (Hobson, 1931). Polyphagous insects such as Spodoptera littoralis larvae secrete high levels of protease, amylase, and invertase (Ishaaya et al., 1971, 1974), whereas hemipterous insects such as aphids and scales, which usually feed on plant fluid, exhibit a relatively high level of invertase activity (Ishaaya and Swirski, 1970, 1976). Nutritional and environmental factors affect digestive enzymes; the digestive proteolytic and amylolytic activities of Spodoptera littoralis, for example, are closely correlated with the protein level in the diet or with the environmental temperature (Ishaaya et al., 1971). In some cases digestive enzymes can be used as parameters for assessing anti-feeding activity (Ascher and Ishaaya, 1973; Ishaaya and Casida, 1975; Ishaaya et al., 1974, 1977, 1980, 1982), or phagostimulation (Ishaaya and Meisner, 1973). Despite availability of ample information concerning biochemical properties of digestive enzymes in various insects (House, 1974; Wigglesworth, 1974), relatively little is known about their role in 410insect feeding and insect-host compatability. Trehalase, another carbohydrase that in insects degrades trehalose to glucose for internal energy supply (Wyatt, 1967), is used as a biochemical parameter for assessing the adaptability of the black scale Saissetia oleae to various host plants (Ishaaya and Swirski, 1976). In Aphis citricola, two types of aphid trehalase have been identified: one is water-soluble and the other is a membrane-bound enzyme. An increased level of the soluble trehalase in the alate morphs of Aphis citricola indicates the importance of this enzyme system in the energy supply needed for aphid flight (Neubauer et al., 1980).