Western Spruce Budworm on Douglas-fir, Fir and Spruce in Montana

What is western spruce budworm?

It is the caterpillar of a small native moth. Montana DNRC's Forest Pest Management Program calls it a native moth that causes major defoliation in Montana's forests, and the Forest Service's Northern Region management guide says it evolved alongside Douglas-fir, spruce and true fir forests.

Being native matters. DNRC notes that budworm is a major seasonal food source for birds, bats and small mammals, that parasitoids and pathogens help regulate its numbers, and that outbreaks create snags and canopy openings that add habitat. The Forest Service guide's own management summary includes the line: accept some defoliation as a normal stand-development process.

Which trees does it feed on?

DNRC lists the hosts as Douglas-fir, grand fir, subalpine fir, spruce and larch, and says high populations may spill over onto neighboring pines. The Forest Service guide lists Douglas-fir, all true firs, spruce and western larch, with pines and western hemlock fed on during epidemics.

In practice the damage in western Montana centers on Douglas-fir and the true firs. Both agencies advise favoring pines and western larch in the forest, because outbreaks usually develop in Douglas-fir, grand fir and subalpine fir stands. In a yard, that puts Douglas-fir and spruce on the watch list. Our guides to Douglas-fir problems and blue spruce problems cover what else goes wrong with those two.

What does the damage look like?

DNRC's identification list is a good field guide. Look for chewed needles bound together with silk on the branch tips and the leader. Look for smooth, olive-brown caterpillars with pairs of cream-colored dots along the back, and later for bronze pupal cases that stay on the foliage after the moths leave. From a distance, crowns look thinned, scorched or stripped. MSU Extension adds insect droppings and webbing on the needles.

In August the small copper-colored moths themselves are noticeable. DNRC says they often gather around lights or fly in clusters near branch tips. Because the caterpillars feed mainly on new growth, the browning is concentrated on the outer tips of the branches and the top of the tree while the older, inner needles stay green.

That is a different pattern from the yellowing of old inner needles in fall, which is normal shedding, and from a whole tree fading at once. Our guide to pine needles turning brown helps tell those apart.

What is the life cycle, and when is the feeding?

There is one generation a year. According to DNRC, moths emerge in late July to early August and lay light green eggs in overlapping masses on the underside of needles. The Forest Service guide says a mass can hold up to 130 eggs. The tiny caterpillars that hatch do not feed. They spin silk shelters under bark scales and spend the winter there.

The following spring, from April to June according to the Forest Service, they come out and mine buds and old needles until the new buds open, then web the fresh needles together and feed inside that shelter. They pupate in midsummer and the moths appear again by August. The feeding that does the damage is packed into late spring and early summer, which is why a tree can look fine in May and scorched by July.

Will my tree recover?

Usually, if it is established and the defoliation does not go on for years. The Forest Service guide says foliage recovery can be dramatic and that within a few years tree crowns can appear healthy, though growth and live crown may be reduced. It also says that death of larger trees from budworm seldom occurs in Montana and Idaho.

The exceptions are worth knowing. DNRC says repeated, heavy defoliation can cause branch dieback and top kill, and that severely defoliated understory trees can be killed within a single year. The Forest Service notes that mortality is usually concentrated in smaller, suppressed pole and sapling sized trees, and that mature trees can be killed by repeated severe defoliation. The larger risk for a big Douglas-fir is what comes next: the guide warns that trees weakened by repeated defoliation may be attacked and killed by Douglas-fir beetle.

How often do outbreaks come to the Missoula and Bitterroot valleys?

More often than in much of western Montana. DNRC says populations west of the Continental Divide are cyclical, with outbreaks roughly every 10 to 15 years, while forests east of the Divide carry chronic populations. The Forest Service guide adds a local note: west-side forests usually have long periods between outbreaks, with the exception of the Bitterroot and Lolo National Forests.

Outbreaks are most severe, DNRC says, on warm, dry sites with dense, multi-layered canopies of host trees, and the Forest Service reports the heaviest defoliation and tree killing in Douglas-fir stands on south-facing slopes. Both agencies tie the trend to forests that have grown denser, with more Douglas-fir and grand fir, after past harvest practices and fire exclusion. We have not verified the current year's outbreak level, so for that contact your DNRC service forester, whose Forest Pest Management Program is based in Missoula.

What can I do for a yard tree?

First keep it vigorous, since vigor is what lets a defoliated tree rebound. That mostly means deep watering in dry summers, as described in our guide to watering trees through a Montana drought, and not adding other stresses such as root damage or soil compaction under the tree.

Spraying is an option for high-value trees, with limits. The Forest Service guide says individual trees can be sprayed from the ground for short-term protection, that insecticides registered for budworm include carbaryl and the microbial insecticide Bacillus thuringiensis variety kurstaki, often called Bt, and that Bt affects only caterpillars feeding on the foliage at the time. Timing is critical: the guide says to apply in early summer when most larvae are about three quarters of an inch long and most new shoots are about an inch long, before the heaviest feeding. MSU Extension likewise ties any treatment to monitoring for the caterpillars.

The guide is equally clear that this is temporary. Protection usually lasts only two to three years before populations resurge, because moths fly back in from surrounding stands, and repeat applications may be needed through an outbreak. DNRC's advice is to follow the pesticide label, use proper protective equipment and consider a licensed pesticide applicator. Reaching the top of a tall fir takes equipment a homeowner does not have, and the label is the law.

What are the options for forested acreage?

Here both agencies point away from spraying and toward the stand itself. DNRC's recommendations are to thin to reduce competition and promote individual tree vigor, to work toward a single canopy layer so caterpillars dropping from the overstory do not land on a second crop of host trees below, and to promote non-host species such as ponderosa pine, lodgepole pine and western larch. The Forest Service guide states that cultural manipulation of stands offers the greatest hope for preventing outbreaks or reducing impacts.

DNRC is candid that thinned stands may still be defoliated, but the trees are more likely to rebound once the outbreak subsides. On a wooded home site this work overlaps heavily with fire mitigation and defensible space, since crowded, layered Douglas-fir is also a ladder-fuel problem. A DNRC service forester can advise on larger parcels.

When should I call an arborist?

Call when a tree near the house has lost its top, when defoliation has repeated for several years, or when a weakened Douglas-fir shows signs of bark beetles on the trunk. A dead top above a roof or driveway is a hazard in its own right and can be taken out with proper pruning cuts. Topping the tree is not the answer.

We assess defoliated conifers and remove the ones that have become hazards as part of our tree services. More guides to the conifers of the Missoula and Bitterroot valleys are in the Montana Tree Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a tree assessment and an ISA Certified Arborist will look at the tree with you.

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