Metabolism during overwintering in two species of cold-hardy goldenrod gall insects
Metabolism was examined in the larvae of two cold-hardy goldenrod gall insects, the freeze tolerant fly Eurosta solidaginis (Fitch) (Diptera, Tephritidae) and the freeze avoiding moth Epiblema scudderiana (Clemens) (Lepidoptera, Olethreutidae) during overwintering. Acclimatization resulted in specific changes in enzyme activities and cellular metabolites. Enzyme activity data suggested metabolic shifts concurrent with the requirements of both species. Larvae shifted metabolism towards cryoprotectant synthesis in the fall, as witnessed by increased activities of glycerol and sorbitol producing enzymes, and towards the removal of these polyols in the spring with increased activities of catabolic enzymes. Measured increases in lipid unsaturation in the fall helped to maintain membrane fluidity during cold-exposure. Decreased activities of enzymes of the tricarboxylic acid cycle paralleled the reduced demands on oxidative metabolism over the winter. In Eurosta solidaginis, decreased oxidative metabolism and the anoxic state of the animal when frozen further resulted in the lack of lipid reserve depletion over the winter, whereas freeze avoiding Epiblema scudderiana larvae appeared to use lipid reserves to support basal metabolism over the winter. Decreased activities of antioxidant enzymes in Eurosta solidaginisover the winter suggested that these larvae do not experience increased reactive oxygen species formation during freezing and thawing. Increasing antioxidant enzyme activities in Epiblema scudderianaover the winter suggested that these larvae may be subject to oxidative stress, perhaps due to the maintenance of aerobic metabolism during cold exposure.