Fitness costs of butterfly oviposition on a lethal non-native plant in a mixed native and non-native plant community.

Non-native plants may be unpalatable or toxic, but have oviposition cues similar to native plants used by insects. The herbivore will then oviposit on the plant, but the offspring will be unable to develop. While such instances have been described previously, the fitness costs at the population level in the wild due to the presence of the lethal host have not been quantified, for this or other related systems. We quantified the fitness cost in the field for the native butterfly Pieris macdunnoughii in the presence of the non-native crucifer Thlaspi arvense, based on the spatial distributions of host plants, female butterflies and eggs in the habitat and the survival of the larvae in the wild. We found that 2.9% of eggs were laid on T. arvense on average, with a survival probability of 0, yielding a calculated fitness cost of 3.0% (95% confidence interval 1.7-3.6%) due to the presence of the non-native in the plant community. Survival probability to the pre-pupal stage for eggs laid on two native crucifers averaged 1.6% over 2 years. The magnitude of the fitness cost will vary temporally and spatially as a function of the relative abundance of the non-native plant. We propose that the fine-scale spatial structure of the plant community relative to the butterflies' dispersal ability, combined with the females' broad habitat use, contributes to the fitness costs associated with the non-native plant and the resulting evolutionary trap.