Variability of Norway spruce (Picea abies L.) needles; performance of spruce sawflies (Gilpinia hercyniae Htg.).
Larvae of the diprionid Gilpinia hercyniae were reared on whole branches of Norway spruce trees (Picea abies) in the laboratory at 25°C, 60% RH and LD 18:6. By choosing flowering and nonflowering branches from the top of the trees and nonflowering lower branches, needles with very different nutritional status were presented to the insect. Growth and development of the larvae from the 2nd instar onwards were best on top branches from the flowering trees, and poorest on branches from wind-blown, partly derooted and heavily flowering trees. Analysis of some 30 biochemicals in needles and faeces was performed. Although new needles had the highest concn of some nutrients (total nitrogen and amino acids), larval mortality was 100% on newly flushed needles, probably because they also contained the highest concn of the secondary compound, quinic acid. Old needles showed high variability in concn of nutrients and secondary compounds and were fed upon by the larvae. Regression analysis demonstrated that the total amount of carbohydrates (glucose, fructose and sucrose) was significantly correlated with the linear growth rate of larvae, larval weight, cocoon weight and the number of eggs per female. In these performance variables, 72-88% of the variation could be explained by 5 biochemicals. Survival rates of larvae or pupae and the instantaneous growth rate could not be explained by the biochemical variables. Only weak correlations were found between nitrogen or amino acids and performance variables. Assimilation of the various biochemicals was calculated and showed high rates (90-97%) in hexoses and lower rates (38-65%) in total nitrogen and amino acids.