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Eastern spruce budworm

Quick facts

  • Spruce budworm activity has been observed in Minnesota every year since at least 1954.

  • Management strategies include commercially thinning healthy stands to retain trees with greater than 40% live crown ratio.

  • Using pesticides to control spruce budworm is expensive and not very successful at treating forests, but is an option for saving yard trees.

  • If dead and dying trees are not removed, they become a fire hazard around homes and buildings and contribute to increased fire risk.

Managing spruce budworm in Minnesota forests

The eastern spruce budworm (Choristoneura fumiferana) is a native forest insect of concern across Minnesota’s coniferous forests. Spruce budworm is responsible for defoliating or killing vast acreages of balsam fir and spruce annually in Minnesota.

Despite its name, balsam fir trees are most susceptible to budworm while spruces are moderately susceptible. Fortunately, these important conifer species can be protected through effective forest management.

The impact of spruce budworm

Large-scale spruce budworm outbreaks in eastern Canadian provinces and northern New England typically occur every 30 to 40 years.

In Minnesota, budworm activity has been observed every year since at least 1954, representing an endemic budworm population for over 60 years. Budworm outbreaks in Minnesota typically occur in the same area every 25 to 40 years.

Estimates from the Department of Natural Resources suggest that annual budworm defoliation averaged 94,500 acres of Minnesota’s forests from 2010 through 2014.

The eastern spruce budworm is a native insect that has evolved with forests across the U.S. Great Lakes region.

Life cycle

Spruce budworm larvae

Moths lay up to 10 egg masses on spruce and balsam fir needles in July. Larvae soon hatch from the eggs and spin down on a silk tread which can blow a considerable distance by wind. After overwintering, young larvae emerge just before balsam fir budbreak in the spring.

  • Budworm larvae feed on new foliage growth through May and June then pupate and emerge as moths by mid-July.
  • Adult moths will mate, lay eggs, and eventually die in the summer.
  • Moths are effective fliers and use wind currents to disperse over long distances.
  • Populations remain in the outbreak stage in a forest stand until much of its food source, such as mature and overmature balsam fir and spruce, is killed.
  • When an outbreak of budworm occurs, it typically will remain in that area defoliating trees for the next 8 to 10 years.

Tree symptoms and vulnerability

Spruce budworm damage to new growth on white spruce
  • Budworm feeding damage is first noticed on outer branch shoots in the upper crowns of spruce and fir trees.
  • Partially eaten needles are webbed onto branch tips and turn a reddish-brown color.
  • Long-term damage of budworm defoliation can result in top kill in 2 to 3 years for balsam fir or 3 to 5 years in white spruce.
  • Additional years of feeding cause tree mortality.

Species susceptibility

Repeated defoliation of Balsam fir gives the tree a low chance of survival and it will usually die after 3 to 4 years of defoliation.

White spruce has a better survival rate than balsam fir. Repeated defoliation gives the tree a moderate chance of survival, but it will usually die after 5 to 7 years of defoliation.

  • An entire stand will rarely die following a budworm outbreak, but patches of trees within stands may die.
  • Although trees can tolerate repeated defoliations (white spruce more so than balsam fir), tree growth is reduced.
  • Smaller-diameter trees in an intermediate or suppressed crown position within the stand are more likely to die first.
  • Many years of repeated defoliation can result in death of the majority of overstory and understory fir trees.
  • Dense stands made up mostly of balsam fir and that have fewer non-host species are most vulnerable to budworm damage. Less susceptible stands have more non-host species and are comprised of healthy and vigorous trees (indicated by large live crown ratios—over 40% in white spruce).

Management strategies

Forest management actions can reduce losses from budworm damage.

If you start to notice budworm populations in stands of balsam fir, make plans to have them harvested as soon as possible to maintain their value. Don't wait to harvest trees.


Authors: Matthew Russell and Michael Albers

Reviewed in 2024

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