• Lee, Thomas H.
  University of Maine
  
• Field, David B.
  University of Maine
  

During the past half century the spruce budworm, Choristoneura fumiferana (Clem.), has been responsible for initiating the destruction of over 300 million cords of balsam fir (Abies balsamea (L.) Mill.) pulpwood and saw timber (Basham 1959). The budworm has caused periodic damage in the spruce-fir forests of North America for hundreds of years, but because balsam fir, its primary host, has been considered a valuable species for no more than 80 years, little attention was paid to either the budworm or its effect on balsam fir prior to 1900. The demand for balsam fir has increased continually since then. As consumption increased, companies using balsam fir became more concerned about a continuing wood supply. Eventually it was concluded that budworm control would be necessary to ensure such wood supplies for the forest industries of North America.

Prior to the Maine budworm outbreak which ended about 1920, standing dead timber was rarely cut for use as sawtimber or pulpwood. There was enough good living wood to harvest without taking the financial risk of cutting potentially low-grade trees. But, later infestations were thought to constitute a real threat to the wood supply and potential economic impacts of large areas of standing dead timber could not be dismissed. This attitude has continued to the present time. In the 1970's, more emphasis is being placed on the utilization of recently killed timber than ever before.

Spruce budworm infestations can cause trees to be weakened, die in part (top killing, branch killing), or die completely. Economic losses may include lost growth, lost wood quality and, at the extreme, lost wood fiber. There is an economic threshold, difficult to define, beyond which damaged timber can be harvested only at a loss. Intelligent planning and evaluation of control programs require information on the nature, rate, and extent of budworm-infested damage.

This report is based on a literature search concerning work done on the degrade decay of (primarily) balsam fir. Although the research reports covered were not all concerned with trees attacked by spruce budworm, the response of trees to different environmental stresses is similar in many cases, so information gained from related studies may be pertinent to the evaluation of budworm damage. Similarities and differences will be noted where appropriate.

The information presented in this report in intended to be summary of what is known about spruce budworm-caused degrade and decay of balsam fir and, to a lesser extent, spruce. In many cases, especially those pertaining to degrade, the picture is not complete. In other cases, especially those pertaining to degrade, the picture is not complete. In other cases, the information collected was contradictory. This is an indication of the difficulty of carrying out such studies and the confusing effects of environmental-climatic differences throughout the geographic range involved. In particular, most of the work done on decay of balsam fir has been conducted in Canada. Its applicability to the spruce-fir forests of Maine is uncertain due to differences in climate and geography.