The Forest Has Caught a Bug

Several bugs, actually. And like you or I catching a bug – a cold, the flu – the forest in many places is looking and feeling sick, many trees with brown needles, or branches devoid of needles altogether. The photo above was taken August 4, 2019 in the Payette National Forest, looking west through and at some of the brown, damaged trees common in this section of the forest.

The specific bugs making the forest near me look sad and sick are defoliators known as the western spruce budworm, the Douglas-fir tussock moth, and to a lesser extent, balsam woolly adelgids (wingless insects). The needles of the trees they munch and thrive on turn brown, making the entire tree look dead. The trees, though, usually survive, although if 90% or more of their needles are destroyed, they may not make it.

Brown, eaten needles among the few remaining green ones on a tree nearly devoid of needles.

Of course, there are other notorious bugs in the forest that are causing simultaneous tree damage. The spruce beetle, western balsam bark beetle, Douglas-fir beetle, western pine beetle, fir-engraver, and mountain pine beetle to name a few of the better known. Most have decreased in number in the Payette since drought conditions in 2017 but they are still present and cause significant damage.

It makes me sad to see these infected trees covered in brown needles, their trunks surrounded by their dropped needles like a brown carpet. Or the trees still standing but completely dead after a beetle infestation destroyed their insides, robbing them of their ability to transport their life nutrients. The extent of the damage across the forest is most noticeable from a distance or from above, where you see an entire stand of trees that should be all green filled with significant swaths of brown.

It pains me to think how the trees themselves must feel, infested with these bugs. Don’t believe the trees know what’s happening to them? Or that they can’t communicate their distress? Then I invite you to watch this fascinating video from a TED talk about tree communication.

While clearly the tussock moth and western spruce budworm have been at work here for a few years, the results are most striking this summer as their infestations are peaking. Because some of the stands infected are close to home, places I regularly walk and run with my dogs, the damage feels very in-my-face. I find myself searching for photo ops that don’t include brown-needled trees, but that’s getting harder as the summer goes on.

I know that everything – every plant, animal, insect, fungus, virus and bacterium – has its place and role to play in the ecosystem, although sometimes the mix and balance is disrupted by invasive species. There are cycles that play out, and wildfires also play a significant role in overall forest health. The ecosystem usually repairs itself in time. It’s just hard to watch this play out when the visual impacts are so jarring. It’s also impossible to ignore the role climate change has in amplifying the impact, drought and hotter summers stressing trees and making them more susceptible to damage while also bringing changes in the types and numbers of natural predators to the various bugs causing the damage.

Idaho Forests and the Bugs that Impact Them

The following is probably more than you’ll ever want to know about these bugs and what they do to the trees in Idaho’s forests, but since I did the research to answer my own questions, I’ll share some of what I learned.

First, some general background about Idaho forests, the types of trees found, and what’s ailing them, courtesy of the US Forest Service’s document, Forest Health Highlights 2017.

graph of tree damage by agent

Western Spruce Budworm

According to Wikipedia, Choristoneura freemani, the western spruce budworm, is a species of moth of the Tortricidae family, and is the most destructive defoliator of coniferous forests in western North America.

budworm on tree needles
Western spruce budworm. Photo: Wikipedia

Quoting Widipedia: “It is now widely distributed throughout the Rocky and Coast Mountains. The first recorded outbreak was in 1909 on the southeastern part of Vancouver Island in British Columbia, Canada. Since that year, infestations have frequently been reported in western Canada.

“The budworm was first recorded in 1914 in the United States, in Oregon. However, it was not initially recognized as a serious threat to coniferous forests in the western U.S. Aerial spraying apparently terminated some smaller epidemics in the southern and central Rockies; others subsided naturally. The insect then appeared to be dormant in US forests until 1922, when two outbreaks were reported near Priest Lake in northern Idaho. Since then, significant outbreaks in the Rockies and in the Pacific Northwest have caused top-killing and serious economic losses in tree growth. Tree mortality from budworm can occur in regeneration, sapling, and pole-sized trees. Trees in mature stands severely defoliated by the western spruce budworm may become susceptible to bark beetles, which kill mature trees.

“In early May to late June, larvae leave their hibernacula to search for food. They first mine or tunnel into year-old needles, closed buds, or newly developing vegetative or reproductive buds.

“New foliage, which is normally the preferred food, is usually entirely consumed or destroyed before larvae will feed on older needles. Larvae become full grown usually in early July about 30 to 40 days after leaving their overwintering sites.”

budworm moth
Western spruce budworm moth. Photo: Wikipedia

It’s Not All Bad. The following paragraphs are from a Forest Service document about the western spruce budworm and puts a slight positive spin on their role in the forest:

Key Wildlife Value: The western spruce budworm contributes to the creation of snags and down wood by severely defoliating true fir and Douglas-fir trees and interacting with other disturbance agents to cause the death of all sizes of host trees. By causing topkill in all sizes of trees, C. freemani contributes to the formation of unique limb structures and facilitates the colonization of living tree boles by stem decay fungi. During outbreaks, high numbers of larvae, pupae, and moths provide abundant forage for many species of birds, bats, rodents, and invertebrates. Some of the mortality associated with western spruce budworm defoliation contributes to the formation of canopy gaps, increasing structural diversity.

Hosts: Primarily grand fir, white fir, and Douglas-fir, but also other true firs, Engelmann spruce and western larch. During outbreaks, larvae also may feed on understory non-host tree species, such as pines or hemlocks.

Important Habitats and Outbreak Dynamics: Most western spruce budworm outbreaks occur east of the Cascade Mountains crest in mixed conifer stands. Large, contiguous expanses of dense, multilayered forests dominated by white or grand firs and Douglas-fir provide ideal habitat for this insect, especially when forests are growing in areas that historically were dominated by fire-climax pine stands. Outbreaks occur sporadically, and appear associated with as-yet-undefined interactions between climatic trends and host abundance and quality. Western spruce budworm populations are always present at very low levels between outbreaks. Parasites and predators such as birds, ants, and small mammals help to regulate budworm populations when they are at low levels, but do not prevent or reduce population increases when forest stand and climatic conditions are favorable. When favorable conditions exist, budworm populations can increase rapidly, appearing to rise nearly simultaneously over relatively large areas, with moth dispersal playing a contributing, though not causal, role.

“[…] Typically occurring for many years over extensive areas, budworm outbreaks influence stand structure by causing varying levels of decreased tree growth, tree deformity, top kill, and host tree mortality across the landscape. In general, dominant and co-dominant trees suffer less mortality than those that are intermediate or suppressed. Tree mortality usually begins to occur only after four to five or more consecutive years of heavy defoliation. Defoliation commonly interacts with other disturbance agents present in host stands, especially Douglas-fir beetle, fir engraver, and root diseases (particularly armillaria root disease and black stain root disease), to cause more severe effects than might be caused by defoliation alone. Tree density and cover may be significantly reduced in heavily defoliated stands. Effects following outbreaks may range from minor amounts of topkill and understory mortality to patches of 100 percent mortality up to 4 ha in size.

Western spruce budworm infested acres decreased in 2017 to approximately 258,000 acres compared to 730,000 acres in 2016 and 1.1 million acres in 2015. In 2011 the total was over 1.8 million acres. The reason for the decrease in unclear.”

Tussock Moth

From Wikipedia: “Many tussock moth caterpillars (lymantriinae) have urticating hairs (often hidden among longer, softer hairs), which can cause painful reactions if they come into contact with skin.

tussock moth larva
Tussock moth caterpillar. Photo: Ladd Livingston, Idaho Department of Lands

Lymantria means “defiler” and several species are important defoliators of forest trees, including the gypsy moth Lymantria dispar, the douglas-fir tussock moth Orgyia pseudotsugata, and the nun moth Lymantria monacha. They tend to have broader host plant ranges than most Lepidoptera. Most feed on trees and shrubs, but some are known from vines, herbs, grasses, and lichens.

“The Douglas-fir tussock moth (Orgyia pseudotsugata) is a moth of the subfamily Lymantriinae found in western North America. Its population periodically irrupts in cyclical outbreaks. The caterpillars feed on the needles of Douglas fir, true fir, and spruce in summer, and moths are on the wing from July or August to November.

tussock moth on tree
Tussock moth on tree. Photo: USFS

“The Douglas-fir tussock moth is native to forests of western North America and outbreaks have been identified in British Columbia, Idaho, Washington, Oregon, Nevada, California, Arizona, and New Mexico. Outbreaks occur in cycles around eight to 12 years and usually last up to four years, sometimes longer. Reports from Idaho and Washington indicate 2011 had a major outbreak. The larvae feed on Pseudotsuga and Abies species, especially Douglas fir, grand fir, white fir, and subalpine fir.

 “Outbreaks subside on their own, but silvicultural techniques for managing affected timber can be employed….”

And finally, according to Wikipedia, balsam woolly adelgids (Adelges piceae) are small wingless insects that are an invasive species from Europe, arriving in the United States around 1900. They also infest and kill firs, especially balsam fir and Fraser fir.

Life’s Dance

Enjoying a healthy section of forest near home, May 2019.

The forest will always be the place I go to recharge, to reflect, to learn and grow. I can hate what these bugs are doing to the trees I love and the impact they’re having on my visual appreciation of the forest while simultaneously admitting they exist for some reason, part of an ever-changing ecological dance of good and bad I can only begin to grasp. I take solace in the hope that the majority of the trees sporting brown, dead needles this summer will eventually rebound, showing off healthy green needles in the near future. I’ll be there to welcome their restoration to health and vibrancy.

2 thoughts on “The Forest Has Caught a Bug”

  1. Thank you for a good article! Living in the forest we are learning and trying to be good stewards in the way we manage our acreage. It is good to reflect on the cycles and the balance of nature in all things that you bring up. Thank you once again for another good read! Appreciate!


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