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Understanding spruce budworm in Minnesota’s forests

Anna Stockstad and Sarah Waddle

April 15th, 2024

Grand Marais, MN

Image credit: UMN Extension

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Today’s plan

30-minute presentation from UMN Extension
15-minute presentation from USFS Superior National Forest
15-minute presentation from North Shore Forest Collaborative
30 minutes Q&A/discussion

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What is eastern spruce budworm?

Native caterpillar/moth (Choristoneura fumiferana)
Preferred host species: Balsam fir and white spruce

Can also feed on black spruce, tamarack, and pine when population is high

Image credit: USDA Forest Service

Image credit: K.B. Jamieson, Canadian Forest service, Bugwood.org

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Identification

Image credit: UMN Extension, MN DNR

Scorched tips

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Host species

Balsam fir is highly susceptible!

Image credit: WI DNR

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Forest susceptibility

Low

High

Dense stands
Mainly balsam fir
Few non-host species

Healthy, vigorous trees
More non-host species

Image credit: Eli Sagor, UMN Extension

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Native range

Credit: Eric Otto, 2023; Rauchfuss & Ziegler, 2011; MN DNR 2023

2023 range

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History of spruce budworm in MN

First record of observation in 1912, continuing population since 1954
Outbreaks occur in same area every 25 - 40 years
Feeds in area for 6 - 10 years and then population shifts

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The ecology of spruce budworm

Figure: Eveleigh et al. 2007

It’s complicated!

Spruce budworm is just one part of the complex ecology of northern forest ecosystems.

Image credit: Judy Gallagher, USDA Forest Service

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Current outbreak

2023 total defoliation/mortality: 664,800 acres

Highest number of acres impacted since 1961

Since 2019: 417,000 acres per year

Source: MN DNR Forest Health Report 2023, MN DNR Drought Situation Report 2006

Flash drought of 2006

2021 drought and Greenwood Fire

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Life cycle

Image credit: Maryland Biodiversity Project

Adult moths can:

Fly long distances*
Lay eggs at multiple sites
Produce one generation of eggs per year

*Moths will not disperse when temperature is below 57℉ or above 86℉

*Royama, 1984.

<1”

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Life cycle

Image credit: USDA Forest Service

Up to 10 egg masses, 1 brood per year

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Life cycle

Image credit: MN DNR

Source: Ives, 1874.

Larvae overwinter in hibernaculum (bundles on needles/twigs)
Harsher winters associated with population decline - can’t complete life cycle
Impacts of warmer winters?
Larvae prefer hot, dry summers

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Life cycle

Image Credit: Province of British Columbia

Pupa

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Why should you care?

(But you probably care already, that’s why you’re here!)

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The impacts on MN trees and forests

Spruce budworm is the most damaging insect of spruce-fir forests in eastern North America

Larvae eat new buds each year → tree tries to repair itself → constant stress on trees → defoliation and mortality
Impacts are compounded by other stressors such as drought, suppression, more frequent freeze/thaw events
Widespread losses of balsam fir and white spruce

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Let’s start on the tree level and expand to more of a forest level. The spruce budworm eats the buds of the tree which are required for each growth new year. So each year the budworm is preventing new growth. Trees have a priority list of what they put their energy towards. When they are fighting off something like spruce budworm, they’re going to put a lot of energy towards repairing themself and producing new growth. This means, however, that they are under constant stress and they can’t put energy towards growing and accessing the resources that they need. When you compound this stress from spruce budworm with other stressors like drought or suppression, this can be too much for the tree to deal with and it can lead to mortality. So as a result of spruce budworm, we’re seeing widespread mortality and defoliation of balsam fir and spruce.

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Wider scale impacts of spruce budworm

Fire suppression → too much balsam → SBW + increased fire risk
Loss of resilience in spruce-fir stands
Economic impacts from loss of merchantable timber

Image credit: Chad Davis

Greenwood Fire 2021

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Let’s think a bit more broadly. We have a lot more dead balsam on the landscape due to spruce budworm. Balsam fir is perfect fuel for large wildfire. Fire is a natural part of our landscape here in NE MN, but we still want to avoid catastrophic fires like the Greenwood Fire in 2021. When these large fires occur, we lose a lot of carbon back to the atmosphere. And we want to keep as much carbon in our forests and forest products as possible.

For spruce-fir stands where there is a lot of mortality, you’re losing a lot of resilience. There’s plenty of diversity to maintain ecosystem functioning if one species is lost. This is not the case in spruce-fir stands impacted by spruce budworm.

And of course there are economic impacts because we’re losing merchantable timber when spruce and balsam fir is lost to spruce budworm. This has negative impacts on our wood supply and markets here in NE MN. MENTION BUILDUP OF FIRE

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Image credit: Estuary News Magazine

Image credit: Cook County News Herald, US Forest Service

Image credit: Marc Roberts, US Forest Service

Image credit: Chris Henkel, MN DNR

Disturbance is a natural part of forest ecosystems.

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How do you manage woodlands and trees impacted by spruce budworm?

Image credit: Ryan Pennesi

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Connect with a local professional

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Resilient forests better withstand disturbance

Harvest to open growing space 🡪 improves vigor of trees, making them more resilient to impacts of disturbance
Promote a mix of species

Credit: Emily Dombeck, UMN Extension

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When you work with a forester, you can come up with a plan for your woodland. Thinking ahead into the future, with spruce budworm and climate change and other concerns, the biggest thing to consider is increasing resilience. Resilient forests are better able to withstand the impacts of forests. A disturbance can occur but there is a good diversity of species where if one or two species are lost, such as balsam and spruce to SBW, there are other species that can fill in that ecological niche. To increase resilience in your woodland, you can thin and harvest in order to open up growing space, which improves the vigor of trees and releases them from suppression, making them less stressed and more able to withstand disturbances. Promoting a mix of species through your planting and management provides that species diversity that is necessary for an ecosystem to respond and adapt to disturbances.

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Managing woodlands

Start by asking two questions:

How much balsam fir?
What are the conditions?

Age?
Stocking (density)?
Vigor?

Image credit: UMN UFOR Lab

We can use forest management to minimize losses from budworm.

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Managing woodlands

Start by asking two questions:

How much balsam fir?
What are the conditions?

Age?
Stocking (density)?
Vigor?

Dense, mainly balsam fir? High rates of infestation and defoliation?

Trees are of poor health and vigor (low live crown ratio?)

A clearcut as soon as possible would be best - but know that markets for balsam are currently poor.

Best option for dead/dying balsam: cut and pile

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When NOT to harvest?

When NOT to thin spruce stands

During periods of excessive drought
During outbreaks
In older, dense stands that have never been thinned

“Thinning shock” - trees may not respond to thinning due to stress from drought or outbreak

Old trees have very slow growth rates and are unlikely to be released from suppression.

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Some things to know about harvesting

Logging makes most economic sense for at least 15-20 acres of spruce that are relatively large and healthy

Hedstrom’s: Spruce must be able to be cut into 16’ lengths with 6” diameter

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Managing impacted yard trees

Give your tree it’s best chance of survival

Thin
Mulch
Water
Protect soil
Avoid herbicides

Image credit: Colorado Noxious Weeds Control and Tree Health Care Services

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Managing impacted yard trees

Factors to consider in your management choices:

Economic
Ecological
Safety
Personal values and goals

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Chemical treatment

Safety is key- the label is the law
Product + Timing + Coverage
Bacillus thuringiensis kurstaki (Btk)

How does it work?
Pros/cons?

Hiring professional help

North Shore Pest- Acelepryn

Weighing costs and benefits

Image credit: Healthy Forest Partnership

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Sarah -

Must choose the product that will be effective, use it at the right time, and in the right place

For budworm- most sources recommend using Btk, in late may or when budworms are emerging and feeding on needles, and covering the whole tree (esp top) every year of the outbreak. Not necessary after the outbreak is over.

Btk- soil bacteria, must be ingested- activates in digestive system, only affects lepidotera (butterflies and moths). Degrades quickly- within days of application. Does not harm humans.

North Shore Pest management is offering a spraying service for people with 1-30 or 40 trees they want to protect around homes, cabins, driveways etc.

Using a power sprayer to apply Acelepryn with a sticking agent that helps it resist washing away. Residual life of 30ish days.

Charging per gallon, costs less to do more trees, ballpark $150-$700. Would reapply every year for the duration of the outbreak- as many as 6-7 more years.

Can spray up to 50-55 foot trees

Could impact other insects- though unlikely that many non target species will be on trees you are spraying.

Cost of spraying vs tree removal should be considered.

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Key takeaways

SBW is a native insect that feeds mainly on balsam fir and spruce in NE MN
Forest impacts include widespread defoliation and mortality
Management strategies differ depending on size of woodland, tree density, composition of balsam fir vs spruce
Chemical treatment is one option available for yard trees

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Local resources

Cook County SWCD

www.cookswcd.org

218-387-3647

Lake County SWCD

www.co.lake.mn.us/soil-and-water-conservation-district

218-834-8370

The University of Minnesota is an equal opportunity educator and employer. This PowerPoint is available in alternative formats upon request at 612-624-1222.

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Please reach out!

Anna Stockstad

Regional Extension Educator, Forest Ecosystem Health

stock523@umn.edu

218-726-6466

Sarah Waddle

Local Extension Educator, Cook County

swaddle@umn.edu

218-387-3015

The University of Minnesota is an equal opportunity educator and employer. This PowerPoint is available in alternative formats upon request at 612-624-1222.

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We value your feedback!

z.umn.edu/SBWsurvey

The University of Minnesota is an equal opportunity educator and employer. This PowerPoint is available in alternative formats upon request at 612-624-1222.

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