Effect of Litter Type on Colonization and Desertion of Food Patches by a Litter-Shredding, Limnephilid Caddisfly

Larvae of Pycnopscyche guttifer, a large, leaf-shredding caddisfly, have become common in rivers of northern Nova Scotia, Canada, where they may greatly accelerate decomposition rates of leaf litter. How the larvae find litter patches in the complex environment of a stream bottom, and how they decide which litter types to feed on, have not been fully examined. In field and laboratory experiments, I determined the preference of P. guttifer to conditioned leaf litter from four species of deciduous trees ranging from recalcitrant and nutrient poor (red oak) to soft and nutrient rich (speckled alder). In field experiments, larvae exhibited a strong preference toward speckled alder and red maple litter, while trembling aspen and especially red oak litter were rarely used. The abundance of larvae on red maple litter varied according to whether aspen or oak litter was nearby. In laboratory choice experiments, larvae searched actively, moving in straight lines in random directions. They deserted less-favoured litter types sooner than favoured litter types, but sometimes abandoned favoured litter types as well. In a complex environment containing many kinds of litter in scattered patches, a restless larva that periodically searches for new food patches may maximize its intake of energy or essential nutrients and obtain a selective advantage over those that remain on acceptable patches.