Combined effects of night-time temperature and allelochemicals on performance of a generalist insect herbivore

To assess the pattern of temperature influencing the effect of allelochemicals on growth of insect herbivores and to examine the potential effect of warmer nights due to global warming, we examined the simultaneous effects of allelochemicals and warmer night-time temperatures on an insect herbivore (Spodoptera exigua; Lepidoptera: Noctuidae). Dietary chlorogenic acid, rutin and tomatine levels reflected those occurring naturally in the leaves of tomato, a hostplant of this herbivore. We compared the effects of four thermal regimes having a daytime temperature of 26 degrees C, with the night-time temperature increased from 14 to 26 degrees C by increments of 4 degrees C. The effect of a particular allelochemical on developmental rate was similar among the four thermal regimes. Chlorogenic acid and tomatine each reduced final larval weight, but there was no effect of night-time temperature. In contrast, rutin had no effect on final weight, whereas final weight declined with increasing night-time temperature. Night-time temperature did not influence amount eaten. Larvae ate less when chlorogenic acid or tomatine was in the diet. For each allelochemical, there were no allelochemical by thermal regime interactions. In addition, we compared the effects of allelochemicals and the thermal regime of 26:14 degrees C and constant 20 degrees C, which was the average temperature of the 26:14 degrees C regime. Developmental rate was lower at the constant 20 degrees C regime, chlorogenic acid and tomatine each depressed developmental rate, and there were no allelochemical by thermal regime interactions. Thus, regardless of the specific allelochemical or amount, the pattern of response at the fluctuating regime was similar to that at the constant temperature. In contrast, comparison of the thermal regime of 26:22 degrees C and constant 24 degrees C, which was the average temperature of the 26:22 degrees C regime, showed several allelochemical by thermal regime interactions. At the 26:22 degrees C regime, developmental rate was disproportionatly higher at the maximal rutin concentration compared to that at constant 24 degrees C. At the constant 24 degrees C, final larval mass was disproportionately lower at the moderate tomatine concentration compared to that at the 26:22 degrees C regime. Because these results differ from that of other studies examining another species, it appears that the response to incremental changes in night-time temperature will reflect the allelochemicals and insect species tested. The contrast between the constant 24 degrees C and 26:22 degrees C regimes indicates that even small fluctuations (+/-2 degrees C) in temperature over 24 h can yield differences in the response to an allelochemical.