Extension Logo
Extension Logo
University of Minnesota Extension
extension.umn.edu

Managing pine bark beetle damage

Quick facts

  • Bark beetles attack pole-sized and mature red pine forests.
  • Outbreaks are commonly observed during drought years.
  • Thinning reduces competition and lowers the likelihood of bark beetle outbreaks.
  • Time thinning and other forestry operations during the fall and winter months when bark beetles are not active.
  • Don't thin during a drought and for one year following a drought. 
  • Avoid wounding trees during thinning and harvesting operations.
  • Consult a forestry professional to determine the appropriate time of year to thin stands to minimize bark beetle damage.
Pine engraver beetle, a native pine bark beetle in Minnesota. Photo: bugwood.org

Pine bark beetles (including the pine engraver species Ips pini and Ips grandicollis) are responsible for killing many pine trees in Minnesota.

Minnesota’s red pines are a commercially and ecologically important species throughout the state. Pine engraver beetles usually infest pole-sized (21 to 50 years) and mature red pine forests (older than 50 years).

Bark beetle activity occurs in variable-sized pockets. Outbreaks are commonly observed during drought years.

Beetles develop tunnels beneath the bark, which reduces the flow of water in the tree, ultimately contributing to death in an unhealthy tree.

Life cycle

Exit holes from pine engraver beetles. Photo: Bugwood.org
  • Bark beetles overwinter in the duff layer beneath trees, emerging as adults in April-May.
  • After boring into bark, adults form galleries beneath the inner bark to lay eggs.
  • Eggs will eventually become adults over several weeks and will bore outside of the bark, forming small BB-sized exit holes.
  • When numerous, this resembles damage from a “shotgun blast”.
  • A first generation of adults emerges in June and a second in August.
  • Depending on conditions, a third generation can develop within the same year.

Tree symptoms and vulnerability

  • Bark beetles attack standing pines, freshly-cut pine logs, and slash left on the forest floor.
  • On standing pines, needle color often turns yellow and appears reddish-brown by the late summer.
  • Outbreaks often occur in small pockets of 3 to 5 trees.
  • The stem and branches can be inspected for beetles:
    • Look at sawdust-like material near holes.
    • Remove bark over the holes to check for beetle larvae and galleries.
Discoloration of needles on red pine trees from bark beetles. Photo: Bugwood.org

Red pines are particularly vulnerable to bark beetles that attack weakened trees during drought years.

Pines can overcome bark beetle attacks by producing pitch, but pitch cannot be produced in sufficient quantities when stressed under drought.

Trees showing damage from bark beetles are also often subject to additional stressors such as Armillaria root rot.

Beetle and disease lookalikes

Other wood borers

Wood borer larvae are 1 inch or more in length, while bark beetle larvae are 3/16 inches long. Wood borers are not a threat to living pines.

Mountain pine beetle (Dendroctonus ponderosae) is a western bark beetle species but has not been found in Minnesota’s forest as of 2018. States with mountain pine beetles are prohibited from bringing their pine wood with bark into Minnesota. 

Disease lookalikes

Diplodia is a widespread disease across Minnesota that causes shoot blight. Needles will turn straw colored on pines which may mimic symptoms of bark beetle damage. Tiny, black spores found at the base of dead needles, on cone scales, twigs and branch bark can confirm Diplodia.

Management strategies

Red pine is managed across Minnesota for goals such as timber production and wildlife habitat. This creates a wide variation in forest structure and composition, with some forests receiving several management treatments and others more natural treatments.

Red pine forests producing high-quality saw logs are generally managed in even-aged plantations.

Long-term management strategies that sustain tree vigor can prevent or minimize the risk of beetle outbreaks.

Thinning

Thinning is one tool that reduces competition and lowers the likelihood of bark beetle outbreaks.

Red pine responds robustly to various thinning regimes and intensities. The type and intensity of thinning will depend on the site and stand conditions. The Revised Managers Handbook for Red Pine in the North Central Region, published by the USDA Forest Service in 2006, provides management recommendations based on site conditions.

Proactive treatments are the best strategy for increasing resistance to bark beetles. Time thinning and other forestry operations during the fall and winter months when bark beetles are inactive.

Do not thin during drought and for one year following a drought. Drought conditions may also hinder successful tree regeneration following forest management treatments.

  • Remove logs greater than 4 inches in diameter before June 1 if thinning or cutting in the late winter or early spring.
  • Remove logs greater than 4 inches in diameter within three weeks of thinning or cutting in the late spring or summer. 
  • Remove or destroy woody debris greater than 4 inches in diameter.
  • Avoid wounding trees during thinning and harvesting operations.

The MN DNR updated these guidelines in 2015 by allowing timber sale administrators to allow or stop harvesting operations on state lands to minimize damage from beetle infestations.

MN DNR guidelines to minimize bark beetle damage during pine thinning

Thinning February 1 through April 30:

Haul away logs more than 4 inches in diameter before June 1. For areas in northern Minnesota, if March through May is colder than average, logs and slash can remain on site until June 15 without threatening residual pines.

Thinning May 1 through August 31:

Haul away logs more than 4 inches in diameter within 3 weeks of damage or thinning. Bark beetles can survive in branches down to about 1 inch in diameter. The smaller the log’s diameter, the fewer beetles will reproduce in it. The 4-inch diameter limit represents the typical smallest diameter that is merchantable (inside the bark on the small end of the log). If hauling logs away is not possible, they could be chipped or burned (as long as nearby healthy pines aren’t damaged from burning).

Thinning September 1 through January 31:

Leave logs and slash on site. Bark beetles will feed on pine logs cut during this period. However, regional forest health specialists are not aware of any instance in Minnesota or Wisconsin where pine logs and slash, cut during this period, served as sources of outbreaks for residual pines.

Yard trees

  • If freshly cut conifer firewood or woody debris from conifer species is lying near healthy pines, move the wood away from the trees.
  • Debris can be moved to an approved wood waste disposal site or chipped.
  • Water trees during times with low rainfall and continue to monitor their health.

Authors: Anna Stockstad, Extension forestry educator; Matthew Russell and Jeffrey Hahn, former Extension educators

Reviewed in 2024

Page survey

© 2025 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.