Responses of pest and non-pest Colias butterfly larvae to intraspecific variation in leaf nitrogen and water content

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Abstract
This study examined the effects of intraspecific variation in leaf nitrogen and water content on the growth, consumption, conversion efficiency and nitrogen use of Colias butterfly larvae. Pest and non-pest Colias philodice eriphyle larvae and Colias eurytheme larvae were fed field-collected alfalfa (Medicago sativa) and vetch (Vicia americana) leaves in laboratory experiments. In all treatments, at least one indicator of larval growth performance was positively correlated with leaf nitrogen content, which supports the view that nitrogen is a limiting nutrient for larval growth. The benefits associated with eating leaves with high nitrogen content included higher growth rates, conversion efficiencies, nitrogen accumulation rates and larval nitrogen contents. Over the ranges examined in this study, variation in leaf nitrogen content (2.8–7.0% dry wt) affected larval growth more than variation in leaf water content (66–79% fresh wt). Pest and non-pest C. p. eriphyle responded alike to variation in the leaf nitrogen content of vetch, but there were differences between populations on alfalfa. Pest larvae were more sensitive to variation in leaf water content than non-pest larve. The differences between these populations may be due to specific adaptations resulting from the shift to alfala by pest Colias. It is suggested that herbivores' responses to intraspecific variation in leaf nitrogen content may have important consequences for the evolution of plant defenses and nutrient allocation patterns, and for agricultural pest management.