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Spruce budworm (Choristoneura fumiferana) larvae develop in six stages, or instars.

Pest of the Month: Spruce Budworm

By Fuzz Martin


What does it look like?

Spruce budworm (Choristoneura fumiferana) larvae develop in six stages, or instars. In the first instar, the larvae are approximately 2 millimeters long and yellowish-green with a light-brown head. In the second instar, the spruce budworm larvae are yellow with a dark-brown or black head. The larvae grow and change from yellow to dark brown through the next four instars. Full-grown spruce budworm larvae are approximately 3 centimeters long and dark brown with light-colored spots along their backs.

Once fully developed, spruce budworm larvae transform to pupae. The pupae are typically located at the base of the needles and are reddish-brown with dark streaks and spots.

Adult spruce budworm moths are roughly 1.5 centimeters long with grayish wings that are streaked with dark-brown markings.

The egg masses of the spruce budworm are light green, and typically found on the flat underside of balsam fir or spruce needles.


Host and material range

The spruce budworm feasts mainly upon balsam fir, as well as red, black and white spruce. Tamarack, pine and hemlock are occasionally susceptible to spruce budworm infestations as well.

The emergence of the spruce budworm began in Maine in the early 19th century. Since then, the spruce budworm has spread across the northern United States, east of Montana, and into Canada. However, the budworm is generally found wherever host species grow.


Current threat

The spruce budworm is regarded as one of the most devastating insects native to northern fir and spruce forests. Outbreaks of budworm infestations have occurred in waves since the early 20th century, and have destroyed thousands of acres of host trees.

When the budworm larvae hatch, the larvae bore into new needles and expanding buds. During times of heavy infestation, the larvae will feed upon the previous year’s needles as well. Defoliation occurs when the bored larvae grow large enough to sever the needles and buds from their bases.

After a year or more of heavy defoliation, individual trees may succumb to defoliation and die. The length of time that a tree can endure defoliation by the spruce budworm depends on the vitality of the individual tree.


Prevention tips

Although there are no infallible methods of preventing spruce budworm infestations, there are measures that can be taken to minimize damage caused by the pests. The most effective prevention methods are to ensure the vigor of host trees and pre-treat host trees with a passive microinjected insecticide, containing azadirachtin or abamectin. The best time to pre-treat host trees is in the early spring, before the larvae emerge from hibernation and begin feeding.

Young balsam fir trees are weaker and less likely to survive defoliation than mature balsam firs. Planting young balsam fir trees next to a species of trees that is not susceptible to infestations, such as white pine, can prevent infestations and promote healthy growth of the young balsam firs.


Treatment tips

Since spruce budworm larvae bore into the needles and buds of host trees, the use of a spray insecticide is not 100 percent effective in killing the larvae. Passive microinjection treatments of insecticides containing azadirachtin or abamectin allow the insecticide to permeate the tree from within, and effectively kill the spruce budworm larvae as they feed.

It is important to treat host trees early in the development of the spruce budworm larvae to minimize defoliation and growth of the larvae. The best time to use a passive microinjected insecticide is in the early spring before the buds of the trees begin to expand and prior to the emergence of the larvae from hibernation.


What can you do?

Make certain that your balsam fir and spruce trees are healthy. Take steps to avoid injuring the roots, altering the drainage patterns or severely compacting the soil. Fertilize and treat susceptible trees with insecticide in early spring to promote healthy growth and vigor.


Fuzz Martin is the senior writer at EPIC Creative, Wis. Article provided by Mauget, a leader in passive microinjection and micro-infusion tree care. Contact Mauget or visit www.Mauget.com to learn more about spruce budworm.

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