Forest Insect and Disease Newsletter - July 2020

Red pine shoot moth outbreak

By Brian Schwingle, Central Region forest health specialist

red pine with moth shootMunch, munch, munch. That must have been the sound over much of Minnesota, high up in red pine crowns in June. We became aware of the munching in mid-July when we saw a lot of red pine shoots—about the last 2 inches of growth— dying.

At fault was the red pine shoot moth, Dioryctria resinosella, infesting just about every red pine from at least as far west as Wadena to north of Bemidji, east to Remer and Willow River, and south of the Twin Cities. Anywhere from 25 to more than 80 percent of new shoots were infested on some pines. Red pine shoot moth damage is most common in trees more than 20 years old and in those along plantation edges or open-grown stands.

Though widespread, the outbreak isn't serious. First, the caterpillars can only damage this year's shoot growth; they can't kill branches. Second, attacked red pine can easily tolerate this sort of damage for a number of years. Last, we found several diseased and parasitized caterpillars, suggesting natural enemies may end this outbreak relatively quickly.

By the time this newsletter is published, most red pine shoot moths will be in the adult life-stage, laying eggs. The eggs hatch, and newly hatched caterpillars make a silken shelter in which to overwinter in a branch in the crown of the tree. Forestland and ornamental pine owners need not take any action this year or next year to help their pines, as they will tolerate another year of infestation.

The last time Minnesota pines experienced a large outbreak of the red pine shoot moth was in the late 1990s.

Some Minnesota trees now dwelling in drought

By Rachael Nicoll, Northwest Region forest health specialist

drought map of minnestoaCurrent drought status in Minnesota (updated July 21, 2020). Image from United States Drought Monitor. Author: Richard Heim, NOAA/NCEI

Nearly half the state is abnormally dry or in drought, according to the U.S. Drought Monitor. A diagonal swath running from west central to northeast Minnesota is affected, and much of this area has been at least abnormally dry since May 19, 2020. Portions of Lake, St. Louis, Carlton, Itasca, Aitkin, Cass, and Crow Wing counties are in a severe drought, resulting in an extended wildfire season.

This level of drought intensity stresses trees and can result in early fall color and leaf drop as well as other symptoms of water stress (however, many oaks may have maintained their reddish spring color due to spring weather conditions). In hardwoods, symptoms include wilting, drooping, and curled leaves with browned edges. Conifer needles may become yellow and brown starting at the tips, shoots can droop, and there can be an abundant cone crop. Continued drought can also result in dieback and susceptibility to secondary insect and disease issues.

The recent high temperatures heighten the impacts of the drought, but supplemental watering and properly mulching can prevent issues and help trees recover.

Take a close look at newly planted trees, as they may need extra care this summer to get through dry periods. We covered the issue of transplant shock in the previous edition of our newsletter, and inadequate precipitation can trigger or make this condition worse. Recently planted trees are still establishing their root systems (remember, this process can take 3-5 years), so they are more susceptible to the stresses of dry weather than more mature trees. As a rule, water your trees weekly while the root system is establishing if it has rained less than 1 inch per week to prevent water stress from occurring. Parts of northwest and much of southeast Minnesota have had above average rainfall this summer, but local rainfall patterns may make watering necessary.

Effects of past flooding common on trees this year

By Brian Schwingle, Central Region forest health specialist

It's not easy to diagnose the cause of general dieback of a tree's canopy. It can indicate an insect or localized disease is killing branches. It could mean the tree previously produced an abundance of seeds. Commonly though, it means roots have died, and lately, there is a good chance that those roots died because they were flooded for too long in a recent growing season.

It may not seem obvious, but roots need oxygen. That is why standing water kills roots, which causes dieback from the top and edges of the canopy.

We have seen a lot of flooding damage to trees in central and southern Minnesota in the last two years. The cause can be obvious if trees are still standing in high water, or if you clearly remember recent flooding around a lake or along a river. In the woods, though, it isn't always so apparent. If a dead tree or a tree with dieback is in a muddy depression, or there are smaller, water-loving plants growing around the tree (cattails, willows, red rosier dogwoods, bulrushes, and many more), there is a decent chance the tree died from past flooding.

Dieback on broadleaf deciduous trees typically develops a year or more after floodwaters disappear. Often, the damage is irreversible. Some of the most commonly killed species we see in forests of central Minnesota are black ash, green ash, bur oak, and quaking aspen.

In central or northern Minnesota in the areas not considered generally infested with emerald ash borer, if you see ash trees dying from the top, the dieback probably isn't due to the borer. Emerald ash borer, an ash tree killer, causes canopy decline over a few years. Extended flooding, severe drought, root damage from construction, or native bark beetle infestation causes more rapid dieback.

aerial view of dead treesHigh water resulted in rings of dead trees around a central Minnesota wetland.
oak stand with a few oaks dieingDieback on oaks in Mille Lacs County from abundant precipitation and a rising water table.
ash tree showing signs of subtle diebackSubtle dieback on ash from flooding in southern Pine County.

Update on forest tent caterpillar and spruce budworm

By Eric Otto, Northeast Region forest health specialist

Forest tent caterpillar

forest tent caterpillar on the roadWe have collected reports of forest tent caterpillar from foresters and landowners throughout northeast Minnesota. More caterpillars appear to be present this year than last year, but spotting them in 2019 was difficult. Caterpillars have been found in Aitkin, Itasca, Lake, and St. Louis counties, and are most likely present in other counties.

Forest tent caterpillars prefer to feed on aspen and birch, but can feed on other deciduous trees such as oak and basswood. Despite their activity this year, very little to no defoliation has been associated with the caterpillars.

In Minnesota, outbreaks usually occur every 10 to 16 years. The last outbreak was in 2013, but the majority of defoliation associated with the caterpillars was light that year. Since the peak defoliation was in 2013, we expect the next outbreak to be anytime from 2023 to 2029.

Spruce budworm

spruce budworm web on spruce treeSimilar to last year, spruce budworm has been very noticeable across northeast Minnesota. The main area of infestation is in St. Louis and Lake Counties, but smaller populations have also been reported in Itasca County. The main population appears to be heading eastward into Cook County and closer to the North Shore. In both 2018 and 2019, there were about 200,000 acres of recorded damage. We expect a similar amount of damage this year.

There have been multiple reports from foresters and landowners of dying balsam fir and white spruce trees. Spruce budworm starts to feed on other trees such as black spruce and tamarack when the budworm population is high and the amount of their preferred fir and spruce are not available. Initial feeding starts in the tops of trees and produces a scorched appearance. This stems from budworms partially eating the needles, which then catch in the budworms' webbing. Eventually, extensive defoliation from consecutive years of feeding will result in the trees' mortality. As the infestation moves through an area, the trees will have a gray appearance as the result of defoliation and mortality.

Due to the COVID-19 pandemic, no forest health aerial surveys will be conducted in 2020. Annual forest health surveys provide us with valuable information on forest canopy health across Minnesota. Getting a grasp on the landscape's impact from forest health issues this year has been from field observations and reports from foresters and landowners.

Stop oak wilt in Cass and Crow Wing

screen shot of websiteBy Rachael Nicoll, Northwest Region forest health specialist

Do you live in Cass or Crow Wing County? Oak wilt is an invasive tree disease close to entering these counties for the first time, and DNR forest managers are interested in learning about and stopping its spread. Look for and report oak wilt in red oak trees on your property or in your neighborhood. You can find more information on identifying and reporting oak wilt at a new website created by the DNR and UMN-Extension as a part of an invasive species citizen science project.

Herbicide damage to trees

By Megan O'Neil, Northwest Region forest health specialist

Herbicide damage on trees is common across the state, even in locations where farming and use of herbicide is not widespread. If herbicide comes in contact with trees, it can cause damage regardless of species. This is different from insect and disease damage, which usually affects only one species of tree. Types of herbicide damage you may see includes:

twisted or curled leaves on a spruce treeDistorted growth – twisted or curled leaves and curled branch tips especially on new growth. This sort of damage is common on trees in and around lawns, as the type of herbicides that cause the damage are frequently found in turf herbicide products, including "weed 'n feeds." Mites, leafhoppers, and certain aphids can also cause distorted growth. Look for signs of feeding caused by insects to rule out herbicide damage.

Leaf chlorosis – leaf blades turn yellow but leaf veins remain green. This damage can be caused by some classes of herbicide that affect photosynthesis. Leaf death will result if exposure continues. Leaf chlorosis also results from a lack of certain nutrients such as iron and zinc. Yellowing leaves may also stem from flood damage, root disease, or excess salt in the soil (commonly seen along sidewalks and roadsides).

Brown or black leaves – Leaves become slightly off-color and leaf margins quickly turn brown, then black, and die. Similar symptoms can be caused by drought or fungal leaf diseases.

There are two common ways that herbicide can get into your trees: root uptake and drift (i.e., contact with leaves, growing shoots, wounds on trunks, natural openings in bark). Root uptake can happen when herbicide gets into the soil, intentionally or not. You may see impacts of herbicide damage through root uptake along roadsides when it's used to control weeds. Trees can easily take up products used on lawns to control undesirable plants such as purslane.

Drift occurs when certain types of herbicides move through the air from the site of application and contact plant parts. It makes sense that drift can occur if herbicides are sprayed on windy days (10 mph or more). However, many people mistakenly think tree trunks cannot take in herbicide, so they spray weeds at the base of the trunk. If there are wounds on trunks, if the trunks have natural openings (common on birch, black ash, and cherries), or if the trunks have thin bark (common on young trees), they will absorb the herbicide.

A different kind of drift is volatilization, or vapor drift. Conditions that favor volatilization include calm mornings, fog, or temperatures above 85°F. Chemicals applied to vegetation are susceptible to vapor drift for a couple of days after spraying. The direction and distance of vapor drift is often unpredictable and seems random. Drift can be very damaging to emerging growth in the spring.

Trees can recover from moderate herbicide damage. Help chemically-damaged trees by keeping them healthy all year round. If they are in a yard, be sure to properly mulch around the root zone of the tree.  Water when drought conditions persist. Do not fertilize damaged trees and cease chemical spraying near them.

Lastly, consider avoiding herbicides for weed control whenever possible. We frequently see chemical damage on trees. Not only are trees taking in those chemicals, but people and animals are breathing them in, too. If you use herbicides to control undesirable plant species, be sure to read the entire label carefully. Keep an eye on the weather and avoid applying herbicide when new, sensitive tree growth is developing.

To report herbicide drift, take pictures, videos, and notes. The Minnesota Department of Agriculture is the lead agency for dealing with herbicide drift problems. Make formal complaints by visiting the Minnesota Department of Agriculture pesticide drift complaint process and timeline webpage.


By Eric Otto, Rachael Nicoll, and Brian Schwingle, regional forest health specialists

Sawflies can be common pests on small pine and spruce. Larvae feed on the needles, and if feeding is heavy it can be damaging. Sawflies can easily be mistaken for caterpillars, but they are actually in the insect order Hymenoptera, which also includes wasps, bees, and ants. The key difference is in the number of prolegs (leg-like projections): caterpillars have two to five pairs of prolegs on the abdomen, while sawflies have six or more pairs.

several yellowheaded spruce sawflies feeding on young black spriceYellowheaded spruce sawflies feeding in early June.

Yellowheaded spruce sawfly

Yellowheaded spruce sawfly attacks young, open-grown white, black, and blue spruce, such as those growing in yards, shelterbelts, and roadsides. We have noticed feeding damage in Koochiching and St. Louis counties this year. Larvae emerge from eggs laid in the needles and feed in groups from late May through June. New needles are preferred, but once they become scarce, larvae move onto older needles. The bases of needles are not consumed, so brown needle stubs remain and are diagnostic of sawfly feeding. Trees can die when there is complete defoliation or three to four years of consecutive feeding.

Pine sawfly

Four species of sawfly commonly feed on pine trees in Minnesota. This June we saw an increase in abundance of European pine sawfly feeding on jack pine in Morrison and Pine counties. Feeding was not heavy, but we'll keep our eyes out for heavier feeding next year. This sawfly is usually not too damaging since it consumes last year's needles and only has one generation per year. European pine sawfly also feeds on several other pines.

young spruce tree heavy damage from sawfliesHeavy sawfly damage on spruce.

For additional information on sawflies on pines, see the University of Minnesota's Sawflies webpage.


It's too late now to treat sawflies since feeding has mostly ended. With larvae close to fully-grown, the damage is done and pesticides will not be effective. (An exception to this is the introduced pine sawfly, which has two generations per year and prefers white pine.) Since sawflies feed on the same trees year after year, scout for sawflies in late May (and mid-summer for introduced pine sawfly). If the infestation is small, sawfly larvae can be picked off by hand and disposed of. If the population is substantial and on multiple trees, try using an insecticidal soap. As sawflies are not caterpillars, the natural microbial insecticide Bacillus thuringiensis variety kurstaki (Btk) will not be effective. University of Minnesota Extension has more information on sawfly management.

Maple velvet erineum galls

By Eric Otto, Northeast Region forest health specialist

close up pf maple leaves showing colorful velvet like patchesMaple velvet erineum gall.

Observations of colorful velvet-like patches on the upper and lower leaf surfaces of silver and red maples and other species of deciduous trees have been made in Itasca and St. Louis Counties. A tiny mite creates these silver, green, yellow, red, or crimson growths.

The growths can be quite noticeable, but they are no cause for worry. Female gall mites begin feeding on lower leaf surfaces and stimulate leaf cells to form the colorful patches, which is the plant's method of walling off the attack from the mites. The patches consist of abnormal, felty growths of hair that arise from the surface of the leaf. A patch is known as an "erineum gall." The galls provide protection for the mites as they feed and reproduce. It's easy to find the galls alarming, but they are primarily aesthetic with very little damage to the tree, and control is not necessary.

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