Entomologia Experimentalis et Applicata 88: 81–96, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 81 Response of five insect herbivores to multiple allelochemicals under fluctuating temperatures Nancy E. Stamp & Tod L. Osier Department of Biological Sciences, Binghamton University, State University of New York, Binghamton, NY 13902– 6000, USA Accepted: April 23, 1998 Key words: chlorogenic acid, Heliothis virescens, Manduca sexta, Pseudoplusia includens, rutin, Spodoptera frugiperda, temperature, tomatine, Trichoplusia ni Abstract Analysis of the combined effects of allelochemicals on insect herbivores is useful because there may be adverse additive or even synergistic effects. Analysis of the simultaneous effects of temperature and alleochemicals is also necessary because these factors may interact. We examined the effects of three allelochemicals found in tomato (chlorogenic acid, rutin and tomatine) and thermal regime (21:10 ◦ C and 26:15 ◦ C, representing spring and summer respectively) on five insect herbivores (a Solanaceae specialist, Manduca sexta, and the polyphagous He- liothis virescens, Pseudoplusia includens, Spodoptera frugiperda and Trichoplusia ni). There were allelochemical interactions and thermal regime-allelochemical interactions for all species, and so the patterns were complex. In some cases, paired allelochemicals or the combination of three allelochemicals showed adverse additive effects on insect performance. But that was not always the case, and there were only a few examples of synergism. Negative effects of the allelochemicals were sometimes, but not always, damped by the cooler thermal regime. Comparing the growth rates of the five species in this study with those of a previous study (a total of seven species) revealed five patterns. For two of three pairs of closely-related species, the paired species had distinctly different patterns. For example, for H. virescens, tomatine prevented development and chlorogenic acid slowed growth, whereas for Helicoverpa zea, tomatine just slowed growth and the phenolics had little effect. The specialist Manduca sexta had a pattern that was midway between patterns of the generalists; it was not the most tolerant of the allelochemicals. Introduction When insect herbivores feed on plant material, they ingest an array of phytochemicals. Individually, for example in artificial diet, some of these chemicals can have negative effects on insect feeding, diges- tion, developmental rate and weight gain. Potentially, combinations of those chemicals could have additive effects or even synergistic effects. For example, some combinations of furanocoumarins result in an additive adverse effect on insect herbivores and some others result in an adverse synergistic effect (Diawara et al., 1993; Berenbaum & Zangerl, 1993, 1996). Also, inter- active effects among nutrients can alter performance by insect herbivores (Clancy & King, 1992), as can in- teractive effects between nutrients and allelochemicals (Campbell & Duffey, 1981; Stamp, 1994). Few studies have examined these possibilities. In this study, we will focus on the combined effects of allelochemicals on the performance of insect herbivores. Although both thermal conditions and food quality affect consumption and growth of insect herbivores (Scriber & Slansky, 1981; Raupp & Denno, 1983; Knapp & Casey, 1986), relatively little is known about the simultaneous effects of temperature and food qual- ity on insects (Stamp, 1993). When thermal conditions and food quality have been examined together, it has been shown that the effect of food quality on insect herbivores and insect predators can be a function of temperature (Schramm, 1972; Ali et al., 1990; Stamp, 1990; Stamp & Bowers, 1990; Thomas, 1991; Stamp