Effects of tree age and stand thinning related variations in balsam fir secondary compounds on spruce budworm Choristoneura fumiferana development, growth and food utilization

1 The effect of tannins and monoterpenes on the development, mortality and food utilization of spruce budworm Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae) was investigated under laboratory conditions using an artificial diet. Tannins were extracted from balsam fir foliage of thinned and unthinned stands to reproduce stand thinning related variations in tannins. A mixture of synthetic monoterpenes was utilized to simulate the concentration found in young and old balsam fir trees. 2 Longer development time and lower pupal weight were observed for insects fed on diets with a lower nitrogen concentration and a higher tannin concentration (unthinned treatment). Tannins induced higher insect mortality at a low nitrogen concentration compared with the diet with a higher nitrogen concentration. 3 Approximate digestibility was higher for larvae fed on diets with high concentrations of nitrogen at both low and high concentrations of tannins. Efficiency of conversion of digested food (ECD) decreased with an increase in tannin concentration. Tannins reduced both the relative consumption and growth rate (RCR and RGR). 4 Monoterpenes increased spruce budworm mortality and this mortality reached almost 50% under concentrations of monoterpene typical of the young trees compared with 20% under monoterpene concentrations found in old trees. 5 A higher digestibility was observed for larvae fed on diet with a higher concentration of monoterpenes, whereas efficiency of conversion of ingested food (ECI), ECD, RCR, and RGR decreased with an increase in monoterpenes in the diet. 6 The results obtained in the present study are consistent with the defensive role of secondary compounds such as tannins and monoterpenes in the spruce budworm–balsam fir system.