Western Spruce Budworm Information

Why are there so many "dead" trees in the forest?

Western Spruce Budworm

First of all, the majority of the trees you may be seeing that appear brown and dead have been defoliated (the needles removed) by an insect  -- the Western Spruce Budworm. In many areas of the forest there has been a large cyclical outbreak of this insect in recent years. Though a lot of trees are killed by these insects, many trees will also appear dead for a season or two and then new growth will appear and the trees recover (see photos below). Larger trees are usually not killed by the budworm, even after repeated years of defoliation. There is not much we can do on a large scale to control or stop these outbreaks.

Western spruce budworms are finished eating for this year.  Right now (late July) they are pupae, that stage when the caterpillar (about 1" long) transforms into the moth.  In a week or two the air may seem full of moths in some areas.

Though small trees may be killed by repeated defoliation, the larger ones generally survive.  The two photos below are of a tree on the forest that was defoliated in 2009 and 2010.  This year (2011) it refoliated beautifully.  It's an example of how large trees can recover from defoliation.

Tree defoliated by insects - 2010

Above: Defoliated Douglas fir tree, photographed in 2010
Below: The same tree (from a different angle), refoliated in 2011

Large tree recovered from insect defoliation - 2011


More Information

The scientific name of western spruce budworm is Choristoneura occidentalis.  It is a native insect which has periodic outbreaks.

Western spruce budworm is a moth.  Like all moths, it has four life stages:  egg, larva (caterpillar), pupa, adult.   

The caterpillar stage is the only stage that eats anything.  The moths only live long enough to mate and lay eggs.

Western spruce budworm caterpillars mine buds and eat needles of Douglas-firs, grand firs, Engelmann spruces and western larches.  Caterpillars begin mining buds in May, and finish eating for the season in early to mid July. 

Caterpillars only eat needles produced in the current year, doing little or no damage to older needles.  Trees can tolerate three to four years of heavy defoliation before they are killed. 

Budworm outbreaks can last for seven years or more.  Management responses to defoliator outbreaks have included no action (allowing the outbreak to run its course), direct control with pesticide, or indirect control with silviculture.  

Historically, budworm-caused tree mortality has been light even after several years of repeated defoliation.  On the Malheur National Forest in northeast Oregon, an outbreak that lasted from 1980 to 1986 killed 11% of the host trees.  Following an outbreak in British Columbia that lasted from 1970 to 1974, 39% of the host trees were killed.  In both cases, most of the trees killed were less than 10 inches in diameter.  An outbreak on the Okanogan and Wenatchee National Forests that lasted from 1970 to 1979 killed about 4% of the host trees larger than five inches in diameter.

For more information: a document on Western Spruce Budworm