ONE AND 2 YEARS IMPACT OF COMMERCIAL THINNING ON SPRUCE BUDWORM FEEDING ECOLOGY AND HOST TREE FOLIAGE PRODUCTION AND CHEMISTRY

Field rearing experiments of spruce budworm, Choristoneura fumiferana (Clem.), were conducted in conjunction with foliar chemical analyses, one and two years after a commercial thinning (removal of 25% stand ba sal area) in a 50-year-old balsam fir, Abies balsamea (L.) Mill., stan d. The first year after thinning, spruce budworm larvae reared on the residual trees developed five days faster and removed 43% more foliage than those reared on control trees, but in the second year they devel oped two days faster and removed 37% more foliage. The increase in lar val development rate was related to an increase in foliar soluble suga rs while a reduction in foliar monoterpenes caused by the thinning app arently accounted for the greater amount of foliage ingested by the la rvae. The first year after thinning, trees were more vulnerable to spr uce budworm because there was no increase in foliage production and th e trees were more heavily defoliated. However, in the second year tree s were less vulnerable to the insect because there was an increase in foliage production that exceeded the increase in defoliation, hence a net gain in foliage. Results from this study showed that commercial th inning could reduce the vulnerability of balsam fir trees to spruce bu dworm if thinning is conducted two years prior to budworm outbreak, bu t the same silvicultural procedure could increase the vulnerability to the insect if it is conducted during an outbreak.