Bryophyte use by an insect herbivore: does the cranefly Tipula montana select food to maximise growth?

1. Growth rates of immature invertebrates can be of profound importance to subsequent reproductive success in the adult stage. If diet selection in he rbivores is a principal means of maximising growth and rate of development, individuals should select food of the highest quality. 2. If disparities are found between performance and preference, these may b e the result of individuals balancing nutritional gain against other demand s on their behaviour, such as a need for shelter. The responses of the cran e-fly herbivore Tipula montana Curtis (Diptera: Tipulidae) to a range of mo ss genera from an upland environment were investigated to determine whether diet selection was determined by food quality alone, as measured by its ef fect on larval growth and survival. 3. Larvae were reared individually on single moss genus diets, and their gr owth, development, and survival were measured. Each of the mosses supported growth and development although the mean weights achieved differed by a fa ctor of two between food genera. In two-choice preference experiments, indi viduals broadly chose bryophyte foods as expected from their quality. One n otable exception was the moss Pleurozium schreberi, which gave the best gro wth performance but was among the least preferred. 4. The two methods of assessing preference gave different results: observat ions of larvae showed relatively greater selection for dense mosses than fa ecal pellet analysis. A strong dietary preference for an angiosperm, the se dge Curer bigelowii, over all mosses supported the interpretation that cran e-fly larvae benefit from bryophytes as a refuge, and that this factor may override dietary quality.