Acquiring nutrients from tree leaves: effects of leaf maturity and development type on a generalist caterpillar
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The rapid growth and prolific reproduction of many insect herbivores depend on the efficiencies and rates with which they acquire nutrients from their host plants. However, little is known about how nutrient assimilation efficiencies are affected by leaf maturation or how they vary between plant species. Recent work showed that leaf maturation can greatly decrease the protein assimilation efficiency (PAE) of Lymantria dispar caterpillars on some tree species, but not on species in the willow family (Salicaceae). One trait of many species in the Salicaceae that potentially affects PAE is the continuous (or “indeterminate”) development of leaves throughout the growing season. To improve our understanding of the temporal and developmental patterns of nutrient availability for tree-feeding insects, this study tested two hypotheses: nutrients (protein and carbohydrate) are more efficiently assimilated from immature than mature leaves, and, following leaf maturation, nutrients are more efficiently assimilated from indeterminate than determinate tree species. The nutritional physiology and growth of a generalist caterpillar (L. dispar) were measured on five determinate and five indeterminate tree species while their leaves were immature and again after they were mature. In support of the first hypothesis, caterpillars that fed on immature leaves had significantly higher PAE and carbohydrate assimilation efficiency (CAE), as well as higher protein assimilation rates and growth rates, than larvae that fed on mature leaves. Contrary to the second hypothesis, caterpillars that fed on mature indeterminate tree leaves did not have higher PAE than those that fed on mature determinate leaves, while CAE differed by only 3% between tree development types. Instead, “high-PAE” and “low-PAE” tree species were found across taxonomic and development categories. The results of this study emphasize the importance of physiological mechanisms, such as nutrient assimilation efficiency, to explain the large variation in host plant quality for insect herbivores.
KeywordsNutrient assimilation Herbivore Leaf maturity Leaf development Protein
This work is dedicated to four of the outstanding mentors of RVB: Paul Feeny, Douglas Futuyma, Elizabeth Bernays, and Michael Martin. In addition, we thank Chris Andrews for statistical consultation, Christine Lokerson and Hannah Nadel (USDA) for providing L. dispar eggs, David Borneman, Kerry Gray, Robert Grese, and Marvin Pettway for permitting the use of trees in the Ann Arbor area, and Jennifer Thaler and two anonymous reviewers for many helpful comments. Support for Sara Kileen was provided by a Winnifred B. Chase Fellowship, K.L. Jones Award, and Program in Biology Fellowship, and support for Caleb Nusbaum was provided by the University of Michigan Undergraduate Research Opportunities Program.
Author contribution statement
RVB conceived and designed the experiments. RVB, MK, SK and CPN performed the experiments. RVB analyzed the data. RVB wrote the manuscript. MK, SK and CPN provided editorial advice.
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