It may be only about half an inch long, but the spruce budworm (Choristoneura occidentalis) is the most destructive foliage insect in Colorado and the western areas of the U.S. and Canada.
Colorado’s State Forest Service has an interactive map of the state’s areas of budworm and beetle damage in areas of native forest from 2020 (2021 isn’t available yet). Although the survey showed intense defoliation from Western spruce budworm only in south-central Colorado, don’t let that fool you. We saw considerable damage in the Evergreen area last year and, as temperatures continue to warm, the spruce budworm is moving steadily northward. Each year is worse than the last.
The best way to protect your valuable evergreen trees is to know:
- How to identify signs of spruce budworm damage
- When you’re most likely to notice budworm damage
- Ways to treat infestations of Western spruce budworm
- The right time to spray this destructive pest
- How to prevent
What the Spruce Budworm Eats
Spruce budworms eat – a lot!
You’ll find spruce budworm infestations on Colorado’s familiar spruce trees, including:
- Englelmann spruce (Piceaengelmannii) and
- Blue Spruce (Piceapungens)
But, despite its name, the spruce budworm really likes to eat foliage from our other treasured conifers, mainly:
- White fir (Abies concolor)
- Douglas fir (Pseudotsugamenziesii)
Depending on the level of the infestation, spruce budworms can eat ALL a tree’s new annual growth. During years of high spruce budworm infestations, this can be devastating.
Most trees can recover from the damage of two or three spruce budworm attacks, but not much more.
When Spruce Budworms Attack Trees
Spruce budworms have a brief, one-year lifecycle. It’s during the caterpillar (or budworm) stage that the insect damages your trees.
Spruce budworm larvae hibernate overwinter. They hatch in spring, just when our evergreen trees are putting out new leaves and leaf buds. The caterpillars head for this tender new growth to feed and continue devouring foliage during the spring and summer. It then turns into an adult moth, mates, and dies.
Spruce Budworm Populations Have Cycles
Spruce budworm outbreaks vary in severity, depending on environmental factors, including:
- Temperature drops in spring and summer – Freezing temperatures can kill larvae and kill the new leaf buds they feed on.
- Stormy weather– Windstorms can knock the small larvae from tree branches, and cold, wet weather slows larval activity.
- The length of the outbreak– Spruce budworm larvae can starve if they’ve already defoliated trees. When the larvae run out of food before they mature, there are fewer adults to mate and lay eggs.
Why Infestations Can Be Fatal for Your Trees
After several consecutive years of severe damage and defoliation from spruce budworms, spruce, fir, and Douglas fir tree branches can be totally bare. These defoliated trees will either die or struggle to grow.
When your coniferous trees lose their needles, they lose the source of their food energy. After using up their stored energy reserves, trees stop sending food and water to branches, and they die back. This stress also leaves them vulnerable to attack from diseases and other insect pests.
How To Identify Spruce Budworm Damage
If you see damage and die back on the uppermost branches and branch tips of your fir and spruce trees, you might have an infestation of spruce budworm. You can see damage to your evergreen trees from this pest year-round, as conifer needles tend to hang on the tree.
However, it’s certain that you won’t be able to see the insect itself in your tall Douglas firs when you’re standing on the ground. The budworm larvae are only about one inch long, dark brown, and blend in with the twigs and needles they destroy.
Foliage that’s damaged or killed by the spruce budworm will turn a reddish-brown. This damage may be partial, meaning that the larvae don’t eat all the needles on a branch, and not all the needles fall off.
You can tell the difference between budworm infestations and other damage, such as drought stress, by:
- The protective webbing that budworm larvae spin over themselves while they eat
- The concentration of foliage damage and dieback only in the uppermost branches
By comparison, severe drought in the same tree species will often show up as total foliage discoloration. You’ll also see large areas of dieback and needle drop from all areas of the tree.
PRO TIP: Don’t climb up into your trees to look for spruce budworms! Climbing trees is one of the most dangerous activities there is, and each year homeowners are severely injured and killed from underestimating the danger.
Since spruce budworm larvae are at the uppermost branch tips and not on the main trunk of your tree, it’s especially risky for even the most diehard DIY’er to climb in search of them.
How To Treat Spruce Budworm Infestations
First, verify that what you’re seeing in your trees is the Western spruce budworm. Misidentification of tree pests and diseases means treatment will be ineffective. Plus, misusing pesticides can harm your trees, along with other important parts of our ecosystems, such as honeybees.
There are several spray treatments for spruce budworm. They include:
- Spinosad, made from a toxic soil bacterium
- The natural bacterium Bacillus thuringiensis (Bt), and
- Concentrated, less-toxic sprays made from plants with naturally insecticidal properties, such as the Neem tree.
Before spraying, check your local codes to see if you need a pesticide applicator license.
Treatment Timing is Critical
Timing is critical when spraying for spruce budworm (and other insect pests) as it’s only at certain points during the insect’s lifecycle that sprays are effective. We keep track of the weather and the lifecycles of insects year-round to pinpoint the most successful time for spraying.
Spruce budworm has a VERY SHORT time in which to treat it.
The time to spray for spruce budworm on Douglas fir trees is right AFTER the bud – or flower – has dropped and when new growth begins to appear. This is when the budworm can be reached by the insecticide spray.
Spraying is not effective when the flowers appear or after the new growth has already been eaten by the spruce budworm.
This leaves only about a two-week period during which treatment is effective. Any later and the spray not only won’t reach the budworm, but it may harm beneficial insects that are a natural predator of the budworm.
PRO TIP: Remember that predators such as birds and spiders naturally lower populations of many insect pests by eating them. Always use insecticide sprays with care.
How to Prevent Spruce Budworm Damage
As with other insect pests, good tree care can help reduce the severity of spruce budworm damage. Homeowners should:
- Have large trees regularly inspected by a certified arborist
- Keep their trees well pruned and well-spaced, as crowded trees struggle to grow and allow insects to easily spread
- Check irrigation systems each spring to fix leaks and broken heads and ensure trees are getting enough water
- Remove diseased or hazard trees
The good news is that these preventive measures also help protect Colorado’s evergreen trees that are threatened by other insect pests, such as:
Hire a Professional to Treat Spruce Budworm Infestations
If you have big trees, call the Certified Arborists at LAM Tree Service to examine your trees and positively identify an infestation. You’ll get a professional diagnosis, advice, and application of the best treatment, and recommendations to keep your trees healthy.
Homeowners may be able to successfully treat plants and small trees with pesticides and herbicides. However, no one can successfully spray the entire crown of a large tree from the ground with DIY or homeowner-grade equipment. You’ll need professional equipment and specialized training to tackle the larger trees.
As part of our Plant Health Care program, we offer scheduled appointments for pest and disease treatments, as well as tree inspections and evaluations. Your garden and your trees will be healthier if problems are detected early and treated quickly by trained professionals. Keeping spruce budworm populations down on your property helps slow their spread, and our treasured forests will thank you.