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Many researchers investigating plant-insect interactions maintain their insect colonies on artificial diet and assume that salivary enzymes and elicitors remain representative of natural situations. These salivary elicitors, such as the enzyme glucose oxidase (GOX), play important roles in influencing plant defense responses. In fact, GOX has been implicated in suppressing induced nicotine-production in tobacco plants (Musser et al., 2002). In this study, we investigated the effect of artificial or plant diet on the GOX activity in caterpillars of the beet armyworm, Spodoptera exigua. In the later developmental stages, whole body GOX of S. exigua caterpillars reared on a wheat germ-based artificial diet is over ten times higher than when insects were fed plants of the legume, Medicago trunctula. Labial salivary GOX accounted for most of this whole body activity in 4th instar caterpillars (57.8%), with the remaining activity present in the carcass. Hemolymph GOX levels were below the detection limits of the o-dianisidine-peroxidase assay used to measure activity. Labial salivary GOX activity was significantly higher in 4th larval instars reared on artificial diet compared with plant-fed caterpillars (U/mg per pair labial salivary gland, p = 0.0062), suggesting that diet effects GOX activity. When 4th instar plant-fed caterpillars were transferred to artificial diet, increased labial salivary GOX activity is closely correlated with the amount of time spent feeding on artificial diet. This study shows that the labial salivary GOX activity of S. exigua caterpillars is dependent on diet and developmental stage and that caution must be exercised in the design of plant-insect experiments.