Limiting effects of low leaf-water content on the nitrogen utilization, energy budget, and larval growth ofHyalophora cecropia (Lepidoptera: Saturniidae)

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Abstract
Hyalophora cecropia larvae were reared on leaves of wild cherry,Prunus serotina, which contained variable amounts of leaf water but otherwise did not differ in fiber, total nitrogen, and caloric content. Larvae which were fed leaves low in leaf water grew more slowly and were less efficient at utilizing plant biomass, energy, and nitrogen than those larvae fed leaves which were fully supplemented with water. Experiments were performed using excised leaves under different regimes of relative humidity and leaf water supplementation in climatic control chambers maintained at identical temperatures and photoperiod. Foodplant biomass utilization efficiencies were severely reduced by decreasing amounts of leaf water. Growth rates were halved and the efficiency of conversion of assimilated dry matter into larval biomass was reduced from 82% in the treatment with fully supplemented leaves to 34% in the driest treatment. The nitrogen utilization efficiency (N.U.E.) was reduced from 75–80% to 48%, and the relative accumulation rate of nitrogen (N.A.R.) was suppressed nearly 2-fold for larvae on low-water leaves. Relative maintenance costs (calories expended in respiration/mg tissue/day) of larvae were nearly five times higher on dry leaves than on fully supplemented leaves. Larvae on leaves which were low in water content were themselves more desiccated, and metabolized greater portions of assimilated energy, perhaps in an attempt to supplement body water with metabolic water derived from respiration. The larval rates of consumption of biomass, energy, and nitrogen were the same for all treatments, indicating that leaf water affected larval growth primarily by restricting the efficiency of utilizing these nutrients. Where water was limiting (as in tree leaves), an increased consumption rate did not appear to be a successful means of increasing growth rates. There were daily and seasonal differences in leaf water content between different trees of the same species. Although absolute differences in leaf water exist between different trees and between young and old (fully expanded) leaves of a single tree, these differences are proportional and parallel each other through daily and seasonal cycles. In spite of evolutionary adaptations of herbivores to acquire adequate water and avoid desiccation, the leaf water content naturally encountered by cecropia larvae on cherry leaves may limit their growth, especially if the R.H. is low.