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Spruce Budworm
Article #311 June 2020
By Bill Cook

It’s native.  It does what it’s supposed to do.  It can be an agent of forest regeneration.  But we don’t have to like it.

     Spruce budworm (Choristoneura fumiferana) is a cyclical native needle-eater of mostly balsam fir and white spruce.  Most of the damage is done in June.  Trees that have been heavily eaten may appear brown and possibly dead.  However, don’t cut that tree down just yet!  Re-evaluate in the fall, after most of the current year remains have washed off the tree and the older, green needles can be better seen. 
     Spruce budworm feeds on new growth and prefers mature fir and spruce.  The older needles usually remain alive but might be masked by the brown debris of current year needle feeding by the budworm.  When budworm populations explode, which happens from time to time, the larvae may also consume the older needles.  This can kill trees.  So can more than one year of defoliation. 
     Budworm activity can be identified by observing the tips of the branches.  Bare twigs, often spring green in color, are loosely wrapped in sparse webbing, needle debris, and budworm poop.  One tree might be heavily infested while the next tree might be free of the budworm. 
     The larvae are colored a dark army green above and tan underneath, with two rows of light spots, and a black head.  The full-grown larvae run about an inch long.  Sometimes, they can be seen descending from trees on nearly invisible threads.  They’ll feed through most of June, pupate, then emerge as dull-colored, inconspicuous moths by mid-July.  The moths mate, lay eggs, and are gone by mid-August. 
     Successive spruce budworm populations may remain in an area until much of their food source is exhausted.  This means new needles are eaten for several years.  Without needle replacement, the trees eventually die. 
     Weather conditions, parasites, and predators can knock-down budworm populations before widescale tree mortality occurs.  When natural controls fail to contain a budworm population, entire stands can be killed.  Massive historic spruce budworm outbreaks have killed millions of trees in New England and the eastern Provinces.  However, in the Lake States, the pattern has been localized flare-ups moving through the forests.  In Michigan, most of these outbreaks have been in the Upper Peninsula.    
       Landscape level forest management can reduce the impact of spruce budworm by breaking-up age classes of fir and spruce stands.  Huge areas of same-aged, mature stands provide the backdrop for some of the largest defoliations on the continent.  Extensive mortality can be followed-up by increased wildfire risk, as those dead trees provide accumulating amounts of dry fuel. 
     Forestowners that have mature stands of fir and spruce should keep an eye out for spruce budworm.  If stand mortality has been rising across the area, harvesting the mature trees prior to death by defoliation should be considered.  Working with a local consulting forester can help assess the risk and alternatives.  Many of Michigan’s Conservation Districts host a public service forester. 
          The Michigan DNR publishes a very nice annual forest pest report (Forest Health Highlights) which can be found in the forest health portion of their website.  The report contains updates on spruce budworm and other native and exotic pests. 
     Forest management has modified this historical scenario of budworm outbreaks, tree mortality, large wildfires, and forest regeneration.  Researchers and managers have figured-out how to regenerate stands before outbreaks occur and without wildfire.  Spruce budworm remains a significant agent of change and regeneration but its aftermath threat to humans has abated. 
     Insecticides are seldom used in forest situations, for several reasons.  However, they can protect individual trees around homes and other residential areas or parks. 
     The jack pine budworm (Choristoneura pinus) is a close cousin of the spruce budworm.  Its behavior with jack pine is similar to that of spruce budworm with fir and spruce.  About twenty years ago, jack pine budworm outbreaks swept across much of the Upper Peninsula, killing many mature stands of jack pine.  Fortunately, no large wildfires occurred in the aftermath. 
     Monitoring forests for pest issues is more important than ever.  With new exotic invasive species arriving more and more often, it’s good to know we have a few native pests that can wreak major damage.  Well, sort of.

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 - Spruce budworm larva suspended from a silk thread.

 - Current-year twig with the new needles eaten. 

 - Spruce budworm “nest” at the end of a branch. 

TRAILER- This website was created by a consortium of forestry groups to help streamline information about forestry and coordinate forestry activities designed to benefit the family forest owner and various publics that make up our Michigan citizenry.  This website is maintained by Bill Cook, Retired Michigan State Extension Forester/Biologist. Direct comments to cookwi@msu.edu.

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