Antifeedant activity of citrus limonoids against Colorado potato beetle: comparison of aglycones and glucosides

The importance of plant allelochemicals in plantherbivore interactions is well known. Although some of these phytochemicals act as phagostimulants for specific herbivores, the majority of allelochemicals examined appear to function primarily in plant defense, acting as insect antifeedants, growth regulators, and/or toxins (Jermy, 1966; Bernays & Chapman, 1977). As such, these chemicals have great potential for use in pest management, either directly, as natural plant protectants, or as models for the development of synthetic products. Citrus limonoids, a group of terpenoid allelochemicals found in the plant family Rutaceae have previously been shown to act as insect antifeedants (Klocke & Kubo, 1982; Alford et al., 1987; Mendel et al., 1991) as well as mammalian anticarcinogens (Lam & Hasegawa, 1989; Miller et al., 1989). They are also useful as chemotaxonomic markers (Hasegawa & Miyake, 1996). Thus, citrus limonoids are important functional chemicals in agriculture and medicine. Limonoid aglycones, which are present in young and growing citrus tissues, are converted to their respective 17β-D-glucoside derivatives during late stages of fruit growth and maturation (Hasegawa et al., 1991; Fong et al., 1992). Limonoid glucosides, such as limonin 17β-D-glucopyranoside, are accumulated in mature fruit tissues and seeds as major secondary metabolites (Fong et al., 1989; Hasegawa et al., 1991). Thirty-six limonoid aglycones and 17 limonoid glucosides have been isolated from Citrus and its hybrids, yet few have been investigated for their biological activities. None of the limonoid glucosides have previously been examined for antifeedant activity against insects. In this study we compared the antifeedant activity of limonoid aglycones and glucosides against Colorado potato beetle, Leptinotarsa decemlineata (Say), larvae.