Fatty acid content and enzymes of fatty acid metabolism in overwintering cold-hardy gall insects

Fatty acid content and enzymes of fatty, acid metabolism were studied in overwintering larvae of two cold-hardy gall insects, the freeze-tolerant fly, Eurosta solidaginis and the freeze-avoiding moth Epiblema scudderiana. Both species increased the proportion of unsaturated fatty acids during the winter. Whereas total lipid content did not change in Eurosta solidaginis, a decrease in total lipids over the winter in Epiblema scudderiana suggested the use of fat reserves to maintain basal metabolism. Changes in the activities of enzymes of fat oxidation correlated with these observations in Eurosta solidaginis: hydroxyacyl-CoA dehydrogenase, carnitine-palmitoyl transferase, and acetoacetyl-CoA thiolase activities all decreased during overwintering. In Epiblema scudderiana the same activities were constant decreased, or increased. These activities were, however, higher in the fat-oxidizing, freeze-avoiding species than in the freeze-tolerant larvae. Lipid content in Epiblema scudderiana increased again by early, spring, possibly, indicating this pool as the fate of carbon derived from the spring clearance of the cryoprotectant glycerol pool. Decreased activities of malic enzyme and ATP-citrate lyase suggested decreased potential for fatty acid synthesis in both species over the winter, consistent with the cessation of feeding in the fall. The potential for ketone body metabolism, measured as the activity of beta-hydroxybutyrate dehydrogenase, increased greatly in both species during overwintering; however, levels of beta-hydroxybutyrate remained less than 0.35 mu mol/g wet mass throughout the study period. These data indicate that changes to storage lipid profiles in order to maintain fluidity and to lipid-metabolizing enzyme activities may play important roles in insect cold hardiness.