When life-history doesn't matter

Most insect herbivores are specialists, adapted for feeding on plants with phylogenetically conserved secondary plant compounds (allelochemicals). Many generalists are able to feed on many unrelated plants with highly different allelochemistries, although they usually perform worse than specialists on these same plant diets. Some generalists become locally adapted to specific plants via frequency-dependent selection and may eventually perform better on these plants than naive generalists (a process known as composite specialization). Here, we compared performance (survival, egg-to-pupal development time and pupal mass) in a generalist herbivore, the cabbage moth Mamestra brassicae reared on a wild plant, Rumex acestosa, and a cultivated plant, cabbage Brassica oleracea. Two genotypes of M. brassicae were compared, one collected as eggs in the field from R. acestosa and another that had been collected >20 years ago from cabbage plants in the field and reared on B. oleracea var. Cyrus ever since. Despite being reared on cabbage for more than 100 consecutive generations, both strains of M. brassicae performed significantly better on R. acetosa than B. oleracea plants. More interestingly, the difference in plant quality was actually greater for the lab strain than for the field strain, despite the long rearing history of the former on cabbage. These results contrast with expectations of habituation and adaptation of the lab strain to cabbage (based on rearing history), and suggest that in some herbivores physiological responses to certain diets and potentially fixed over extended periods of time.