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Eastern spruce budworm population is up

This year is high in the generational population of spruce budworm. Many in Minnesota have seen an emergence of spruce budworm among their tree populations.

The eastern spruce budworm is a native forest insect in Minnesota that is responsible for the defoliation and killing of extensive acreages of balsam fir and spruce trees. Budworm outbreaks have been known to occur cyclically every 25-40 years. Fortunately, with effective forest management, spruce budworm damage can be prevented.

Identifying Eastern spruce budworm

Two tan caterpillars with brown markings and cream dots entwined around a spruce stem.
Eastern spruce budworm larvae

The spruce budworm’s life cycle starts with moths laying up to 10 egg masses on spruce fir needles in July. The larvae hatch from the eggs and after overwintering, young larvae emerge before balsam fir budbreak in the spring. Budworm larvae feed on new foliage growth through May and June then pupate and emerge as moths by mid-July.

Budworm feeding damage is most notable on outer branch shorts in the upper crowns of spruce and fir trees. Partially eaten needles are webbed onto the branch tips and turn a reddish-brown color. The long-term damage of budworm defoliation can result in top kill in 2 to 3 years for balsam fir or 3 to 5 years in white spruce. Additional years of feeding could result in tree mortality.

Controlling damage

Defoliated spruce branch against blue sky.
Eastern spruce budworm damage on white spruce

If trees are showing signs of spruce budworm damage, there are a few measures that can be taken. Commercial thinning strategies can be used to promote healthy trees that can be less susceptible to damage from spruce budworms.

Yard trees can be sprayed to protect foliage and kill spruce budworm. However, it is important to note that the pesticides must be applied in early June prior to budworm needle feeding. These applications must occur yearly and should be applied topically.

During application, be aware of bees and pollinator gardens nearby for drift of pesticides.

One organic pesticide option to treat spruce budworm is Bacillus thuringiensis (Bt). Other pesticides that include acephate, bifenthrin, carbaryl, chlorpyrifos, cyfluthrin, flubendiamide, methoxyfenozide, spinosad, or tebufenozide as their main ingredient can be used for treating spruce budworm.

Mention of pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using.

Read more about Eastern spruce budworm.

Emma Severns, agriculture and horticulture intern, St. Louis County Extension

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