INTERACTIONS OF ALLELOCHEMICALS WITH DIETARY CONSTITUENTS - EFFECTS ON DETERRENCY

That variation in the water and nutrient content of plant tissues affects allelochemical toxicity to insects is well established. However, little is known about how these dietary constituents influence allelochemical deterrency. In vertebrates, deterrency of particular allelochemicals increases with dietary water, and decreases with an increase in dietary nutrients. We determined if these findings were relevant to phytophagous insects through an experimental design that allowed us to vary independently the content of water (70-90% fresh mass, fm, with nutrient level at 10% fm) and nutrients (10-30% fm with water level at 70% or 80% fm) in an artificial diet through use of alphacel, a non-nutritive cellulose fibre. We examined the effect of these dietary manipulations on allelochemical deterrency by comparing larval feeding responses of two noctuid species, the oligophagous Anticarsia gemmatalis Huebner and the polyphagous Spodoptera frugiperda (J. E. Smith), to a control diet and an allelochemical-treated diet in two consecutive, no-choice tests. These tests were restricted to 3 min to minimize post-ingestive influences. From an initial twelve compounds tested, all but two were excluded for the following reasons: (i) failure to elicit an intermediate level of deterrency at less-than-or-equal-to 1% fm (i.e. albizziin, amygdalin, hordenine, ouabain, pipecolic acid, salicin, sinigrin and umbelliferone); (ii) an apparently rapid toxic effect (nicotine hydrogen tartrate); and (iii) adsorption to alphacel (quinine hydrochloride), which may have reduced deterrency. The deterrency of caffeine and linamarin increased with dietary water but was unaffected by nutrient content for both species. The similar results for an alkaloid and a cyanogenic glycoside, with two species differing considerably in feeding habits, suggest that dietary water is likely to influence the defensive efficacy of a broader range of deterrent allelochemicals to a variety of plant-feeding insects.