Variability in host-plant quality for the larvae of a polyphagous insect folivore in midseason: the impact of light on three deciduous sapling species

We examined the effects of light availability on the suitability of foliage from red maple (Acer rubrum L.) (Aceraceae), black cherry (Prunus serotina Ehrhart) (Rosaceae), and sassafras Sassafras albidum (Nuttall) Nees (Lauraceae)] for the larvae of the promethea moth, Callosamia promethea Drury (Lepidoptera: Saturniidae), in midseason. We identified replicate sun- and shade-grown saplings of red maple, black cherry, and sassafras from naturally growing populations in the field. Foliage collected from the experimental saplings was bioassayed using early instars of the promethea moth and assayed for nitrogen and carbon content. Promethea moth-larval performance and survivorship was highest on sassafras, intermediate on black cherry, and lowest on red maple. Larvae feeding on foliage from plants grown in the sun performed better than from those grown in the shade; the effect of light did not depend on sapling species. Foliar nitrogen content varied among the sapling species and was higher, overall, in foliage from plants grown in the sun. Nitrogen concentration related strongly and positively with larval performance and accounted for a great deal of the variation in performance both among the sapling species and between the sun and shade treatments. During midseason, foliar nitrogen content is determined by light availability, it varies among sapling species, and it is likely the primary constituent determining host quality for folivores on these sapling species.