Spruce budworm, Choristoneurafumiferana (Clemens), defoliation of balsam fir, Abiesbalsamea (L.) Mill., was studied in thinned stands of the Lower St. Lawrence region of Quebec. The object of the study was to quantify defoliation levels in treated and control plots in order to determine what effect, if any, thinning had on the susceptibility of balsam fir.Three main forest types were studied: (a) coniferous, (b) mixed, and (c) hardwood with a fir understory. The coniferous forest type was subdivided into three types: (1) pure balsam fir; (2) balsam fir – white spruce, Piceaglauca (Moench) Voss; (3) balsam fir – hardwoods. Defoliation levels were estimated using two current techniques: the Dorais–Hardy (1976) and the Fettes (1950) methods.Susceptibility of balsam fir to spruce budworm defoliation was not affected after thinning in coniferous and hardwood forest types; however, susceptibility was increased in mixed stands when part of the hardwood cover was removed. In any situation, defoliation of fir was found to be more intense with an increase of the basal area of the coniferous species (balsam fir, red spruce, Picearubens Sarg., and white spruce) while an increase of the basal area of hardwoods resulted in lower defoliation levels; likewise, a higher number of stems per hectare brought higher defoliation levels of fir. Defoliation was also found to vary with stand composition; susceptibility of fir in the three main cover types decreased in the following order: (a) coniferous, (b) mixed, (c) hardwood. Stand age was not a factor influencing the susceptibility of balsam fir for the two age classes studied (30 and 50 years).

No abstract

A hazard rating map is obtained in Quebec by superimposing the map of the annual damage caused by the spruce budworm, Choristoneurafumiferana (Clem.), during the past 4 years on that forecast for the next year. The various levels of risk are based on the physiological value of each year of foliage production and the ability of balsam fir, Abiesbalsamea Mill., to resist repeated budworm attacks. Applied to the forest of Quebec, this hazard map can lead to a more rational use of pesticides against the budworm.

An index (I) based on the presence of apical buds and defoliation of the current growth is proposed to measure the vigor of balsam fir, Abiesbalsamea Mill., trees attacked by the spruce budworm, Choristoneurafumiferana (Clem.). This index is used to evaluate in terms of foliage protection the efficacy of aerial spray programs conducted against the spruce budworm. By comparing the index (I) as measured before and after treatment, the annual loss of vigor (PVA) observed on balsam fir is established, while a comparison of the annual loss of vigor inside and outside the spray areas leads to an appreciation of the overall protection (P) to the trees by the spray operation.

La tordeuse des bourgeons de l'épinette, Choristoneura fumiferana (Clem .), un lépidoptère de la famille des Tortricidae est sans contredit le pire ennemi des forêts du Québec. Ce sont les larves qui, par leurs habitudes alimentaires, sont responsables des dommages infligés au sapin baumier, Abies balsamea (L .) Mill. et à l'épinette blanche, Picea glauca (Moench) Voss, l'épinette rouge, P. rubens Sarg. et l'épinette noire P. mariana (Mill .) B .S .P. A partir du deuxième âge, les larves consomment les aiguilles de leurs hôtes d'abord en minant le vieux feuillage pour ensuite manger les pousses de l'année aussitôt que les bourgeons débourrent. Le sixième et dernier âge larvaire est celui au cours duquel l'insecte consomme le plus de feuillage et par conséquent entraîne les plus grands dommages. Dans des conditions extrêmes, le sapin peut être tué après trois années d'attaque, mais cette essence résiste ordinairement à quatre ou cinq années de défoliaison consécutive avant de mourir. Pour leur part, l'épinette blanche et l'épinette rouge, qui sont pourvues d'une plus grande quantité de feuillage, supportent de cinq à sept ans de défoliaison sévère avant de mourir tandis que l'épinette noire peut facilement résister à dix années ou plus de défoliaison.