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Evolutionary traps arise when organisms use novel, low-quality or even lethal resources based on previously reliable cues. Persistence of such maladaptive interactions depends not only on how individuals locate important resources, such as host plants, but also the mechanisms underlying poor performance. Pieris macdunnoughii (Remington) (Lepidoptera: Pieridae) lays eggs on a non-native mustard, Thlaspi arvense (L.) (Brassicaceae), which is lethal to its larvae. However, in the 150 years since T. arvense invaded this butterfly continues to recognize and oviposit on the invasive mustard. I evaluated two possible constraints on the evolution of decreased preference within an invaded population. First, an evolutionary response to selection may be constrained by low heritable genetic variation for preference. Second, evolutionary traps are expected to persist when overlapping cue sets (cue similarity) link decreased preference for the novel, unsuitable plant with decreased preference for the historical, high-quality resources. I determined that while preference for the nonnative host over the native host is heritable, sex-linked, and varies considerably in the population, it is unlikely that this preference is correlated with preference for native hosts with similar defensive chemical profiles. Thus, neither a lack of heritable genetic variation nor an increased risk of excluding good host plants when avoiding T. arvense are likely to be constraining escape from this evolutionary trap. Instead, our results suggest behavioral v plasticity may buffer populations from innate preference for the lethal host. Finally, I tested the mechanisms underlying poor performance of neonate larvae on the novel host. Larvae were less likely to start eating T. arvense and starvation was a primary cause of mortality, indicating a pre-ingestive feeding deterrent. A primary oviposition stimulant, the glucosinolate sinigrin, increased this deterrent effect and mortality when added to T. arvense and native host plant leaves. Pre-ingestive deterrents, even those familiar to herbivorous insects, may significantly contribute to the persistence of evolutionary traps. vi
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