Response of Insect Herbivores to Multiple Allelochemicals Under Different Thermal Regimes

Temperature and food quality are well known to constrain the growth of insect herbivores, but less is known about how these factors may interact. We examined the effects of three tomato allelochemicals (chlorogenic acid, rutin, and tomatine) and two thermal regimes (21 ° :10 ° C and 26 ° :15 ° C, representing spring and summer respectively) on three insect herbivores: a Solanaceae specialist, Manduca sexta, and the polyphagous Helicoverpa zea and Spodoptera exigua. The alkaloid tomatine slowed development of the polyphagous H. zea, but the two phenolics had little effect. Development of the polyphagous S. exigua was faster with tomatine at the cool thermal regime, and slower with the two phenolics together at the warm thermal regime. All three of the tomato allelochemicals slowed development of the specialist M. sexta. Values of approximate digestibility (or assimilation efficiency) for the two polyphagous species spanned a range twice that of the Solanaceae specialist, increasing when allelochemicals were added to diet. Our techniques permitted us to separate pre— and post—ingestive effects of allelochemicals on mass gained, but we found complex interaction of allelochemicals and of temperature and allelochemicals. The effects of an allelochemical on larval performance often depended on the combination of other allelochemicals in the diet. But the allelochemicals did not always have a negative effect, and multiple allelochemicals did not always have a more negative effect than a single allelochemical. Interaction between thermal regime and the allelochemicals was also common. For example, whether rutin had a negative, neutral, or positive effect on relative growth rate of S. exigua depended on thermal regime and presence or absence of chlorogenic acid. Often the negative effect of an allelochemical was greater at the warm regime than at the cool regime, but not consistently so. Another experiment showed that thermal regime did not affect allelochemical concentration in the diets. Over time (7 d), only tomatine concentration declined. Therefore, the interactive effects of thermal regime and allelochemicals on the insect herbivores were not an artifact of the experimental procedures. Overall, the results indicated that insect performance is not a simple function of temperature and food quality because interactions occur between these factors. They also suggest that the array of chemical defenses exhibited by a particular plant species may be shaped in part by temperature altering the efficacy of allelochemicals.