Abstract

Forest tent caterpillars ( Malacosoma disstria ) are ubiquitous defoliating insects in North America. In Canada, trembling aspen is their favourite host tree, which can produce an inducible protein, trypsin inhibitor (TI), to inhibit trypsin, a major proteolytic enzyme in caterpillars' midguts. To determine the digestion-inhibiting effect of TI on M. disstria, this study focused on the performance of second instar larvae with different levels of TI on both balanced and low protein diets. Indices of performance were growth, food consumption, nutrient utilization efficiencies and nitrogen concentration in the frass under the treatments of different diet quality and different levels of TI extracted from both soybean and aspen. Both protein deficiency and higher levels of TI impaired the growth of caterpillars, but the impairing effect of TI only appeared on the caterpillars fed on balanced diet. Also, both soybean and aspen TI reduced food consumption and increased the nitrogen concentration in the frass, which was likely due to undigested protein in the frass. But only the aspen TI affected the nutrient utilization efficiencies in terms of decreased approximate digestibility (AD) and increased efficiency of conversion of ingested materials (ECI), which suggests a strategy used by caterpillars to utilize the nutrient more efficiently under aspen TI inhibition of the protein digestion. This study confirmed the digestion-inhibiting function of TI as a defense of aspen against forest tent caterpillars. Also, it suggested that caterpillars were able to regulate their growth under severe protein deficiency, possibly as an adaptation to this defense