Forest Insect and Disease Newsletter - July 2021

Oak wilt confirmed for the first time in Crow Wing County

By Rachael Dube, Northwest Region forest health specialist

map of known range of oak wilt

The yellow shows the known
range of oak wilt in Minnesota
as of June 2021. List of affect counties.

When a resident of the Brainerd Lakes Area noticed a group of their beautiful pin oak trees abruptly losing leaves last summer, they became worried. After first leafing out the following spring, the trees started to die. Afraid the problem would spread to other trees, the homeowner submitted samples for disease testing. Unfortunately, their trees tested positive, and the news was not good: for the first time, oak wilt had reached Crow Wing County.

Oak wilt is a deadly, non-native disease that kills all species of oak in Minnesota. Now located in 40 Minnesota counties, oak wilt has slowly spread north since the 1940s, and DNR forest health specialists have been keeping a watchful eye on the spread. The new find is outside of Brainerd, 20 miles farther north than the previous location of oak wilt in this region. The pocket of oak wilt will be treated this autumn, and we will continue to closely monitor the surrounding area for new infections. Oak trees within a 20-mile radius of any known location of oak wilt are at high risk of developing the disease, so oaks in much of Crow Wing County and southern Cass County are now threatened.

red oak leaves littering the lawn.

Leaves littering a lawn in July may
indicate oak wilt.

The best thing anyone can do to prevent the spread of this deadly disease is to not prune oaks from April through July—the highest risk period for oak wilt transmission. Pruning during this time can attract sap beetles that carry oak wilt spores from infected trees to fresh cuts, promoting spread of the disease. It's also important not to move firewood. Moving oak firewood can spread oak wilt over long distances. Use only locally sourced firewood or firewood with the Minnesota Department of Agriculture certified seal.

A good indicator of oak wilt is a carpet of fallen leaves under an oak in mid-summer when leaves should still be on trees. Residents on the northern edge of the high-risk zone should especially be on the lookout for this symptom and report possible oak wilt to the EDDMapS website or by contacting their local DNR Forestry office. Because oak wilt and other oak health issues have similar symptoms, sharing pictures of the tree really helps with diagnosis.

For more details on oak wilt identification, prevention, and how best to deal with infected trees and wood, see the DNR's oak wilt webpage. The Oak Wilt Guide for Minnesota contains detailed information on methods for managing oak wilt.

Dry weather affecting the entire state

By Rachael Dube, Northwest Region forest health specialist

map of minnesota showing 60 percent severe drought in southern and eastern parts and 40 percent abnormally dry in central and northern parts of the state.

Drought conditions in Minnesota on July 13, 2021.
Credit: Adam Hartman, NOAA/NWS/NCEP/CPC

According to the U. S. Drought Monitor, at least 40 percent of the state is in Severe Drought conditions, 53 percent of the state is in Moderate Drought conditions and 7 percent of the state is in Abnormally Dry conditions. The last time this high a percentage of the state covered by Severe Drought conditions was in May 2015. There will likely be some damage to crops and pastures as well as low water levels. Rainfall totals have been low across the state, while temperatures have reached record highs in recent weeks.

Trees need a lot of water to survive. Healthy, mature trees can typically withstand short-term water shortages, but prolonged drought can impact their growth and survival. Trees in some areas are already showing signs of water stress, including small, off-color foliage, leaves with scorched margins, wilted leaves, and early leaf drop. Trees can also produce abundant seed crops during droughts. Unfortunately, trees affected by drought are also more susceptible to opportunistic insects and diseases.

Seedlings and saplings planted within the last few years are especially susceptible to drought conditions as they work to establish a new root system. We are seeing transplant shock in some newly planted seedlings. As a general rule, it is a good idea to water your trees if your area receives less than 1 inch of rainfall in a week. Properly mulching can also help protect a tree during dry conditions. Make sure to check local watering restrictions and do not fertilize water-stressed trees.

Extensive frost damage in northern Minnesota

By Eric Otto, Northeast Region forest health specialist

shriveled brown oak leaves

Frost damage on bur oak leaves.

Our recent complaints of hot, arid weather make us forget that in late May, most northern Minnesotans experienced nighttime temperatures in the 20s. Memorial Day weekend was more likely spent huddled under blankets than roasting marshmallows on a campfire.

That late spring frost wasn't enjoyable for some trees in the area. The cold temperatures caused extensive damage to bur oak and ash in counties in northwest and northeast Minnesota. The newly emerged leaves of oak and ash trees can be more susceptible to damage than those of other broadleaf trees, such as basswood and maple. Minimal damage was observed on those trees.

Understory bur and northern pin oak also suffered from frost damage in northern Pine and central Sherburne to southern Mille Lacs County.

If you suspect your trees suffered frost damage, look for some typical symptoms such as shriveling and browning or blackening of damaged leaf tissue. These damaged or dead leaves can also fall off the tree. Smaller trees and those located on the perimeter of a woods rather than those in the middle are typically susceptible to more damage.

Symptoms of frost damage can be quite alarming. However, there is little need to worry, as trees can leaf out again after sustaining this damage. Assuming the impacted trees are otherwise healthy, they will leaf out within a few weeks. For recently planted trees that might be more susceptible to damage, we recommend watering during dry periods, mulch, and prune if necessary.

Small pest making big impact on basswoods in central Minnesota

By Brian Schwingle, Central Region forest health specialist

In late May, a retired DNR employee in central Minnesota expressed concern for his sickly basswoods. They were missing a lot of leaves, and the remaining leaves were curled and partially brown. As is often the case, this single report from an observant citizen signaled a widespread pest issue in our forests.

From Lake Maria State Park (northern Wright County) west to Sibley State Park (northern Kandiyohi County) north to Birch Lakes State Forest (northern Stearns County), quarter-acre patches of basswoods were defoliated 25 to 80 percent, with only the outer rim of leaves remaining in the canopy. Nearly all mature basswoods at Birch Lakes State Forest were heavily defoliated. The culprit is a species of thrips, and the larvae that were scuttling around sickly leaves were only 1 millimeter long.

overall view showing basswood tree without leaves next to tree with leaves

Heavily damaged lower canopy on a basswood from introduced basswood thrips.

close up showing tiny white thrips

Tiny light-green thrips feeding on a basswood leaf, causing curling and brown spots.

basswood branch with leaves that are curled up.

Light feeding patches on a basswood leaf.

Since thrips are tiny, they are not easy to identify. We are leaning strongly towards a species called "introduced basswood thrips" as the cause of the damage. We last documented problems with this non-native critter in Minnesota in 1995, and before then, from 1982 to 1988. They feed on leaves as they elongate in the spring, causing leaf-drop, curling, and small yellowish or brownish spots.

Introduced basswood thrips can affect basswoods in the same area for many years in a row. If this happens, their crowns eventually become stunted, and their lifespans are likely cut short. However, short-term impacts from thrips are not that problematic. If a basswood loses a lot of leaves early in the growing season, it simply grows a second set of leaves later in the summer. Our early warm-up this spring and unusually dry conditions possibly favored this insect.

For affected basswoods in yards, do not fertilize them. Fertilizers can burden already stressed trees even more. If the area is in drought, watering a mature tree at its canopy drip-line (the canopy's edge) with a very slow trickle of water once every two weeks for several hours can go a long way in combating the stress of drought and pests. For basswoods in forests, there is nothing we can do. Our only hope is that the outbreak ends as it has before, due to predacious insects and weather conditions not favorable to thrips.

Editor's note: The word thrips comes from the Greek word for this insect, and like names for some animals such as deer and moose, the word thrips is both singular and plural; there may be many thrips or a single thrips.

Learn how to identify forest tent caterpillars

By Rachael Dube, Northwest Region forest health specialist

Forest tent caterpillars congregating on a tree trunk.

No tents here! Forest tent caterpillars congregating on a tree trunk.

Forest tent caterpillar made its annual appearance across Minnesota this spring, munching leaves on a variety of hardwoods. The forest health team will have a better understanding of the severity of forest tent caterpillar's landscape-level impact once we've completed aerial surveys later this summer. So far, we've heard reports of localized high damage in some areas and other areas with low to moderate damage.

Other reports came in sharing observations of webbed tents and caterpillars that look very similar to forest tent caterpillar. The tents are made by a close relative of forest tent caterpillars, and what's confusing is that forest tent caterpillars actually don't make tents! They only produce a small webbing mat when they molt.

How can you tell a forest tent caterpillar from its look-alikes? There are two other species in Minnesota that look similar to forest tent caterpillar and are also active at this time of year.

When trying to identify these caterpillars, look for the pattern on the upper surface of the body (see table below). Forest tent caterpillars have a pattern of white keyhole or footprint-shaped spots. They have blue stripes along the sides of their dark bodies and grow to be 2 to 2½ inches. As caterpillars mature into new life stages, there are other key differences to look for. Select the links in the table for more information on identifying each species.

SpeciesPattern on upper surface of caterpillarsTentPreferred hosts

Forest tent caterpillar

Keyhole- or footprint-shaped markings


Aspen, birch, basswood, oak, others

Eastern tent caterpillar

Solid white line

Large, often at tree branch forks

Cherries, apple, crabapple, plum

Gypsy moth (invasive, nonnative)

Five pairs of blue dots followed by six pairs of red dots


Oak, aspen, birch; more than 300 species of trees and shrubs

Caterpillar comparison (left to right): forest tent caterpillar, eastern tent caterpillar and gypsy moth.

Caterpillar comparison (left to right): forest tent caterpillar, eastern tent caterpillar and gypsy moth. Photo credits: eastern tent caterpillar (Robert F. Bassett, USDA Forest Service, and gypsy moth (Jon Yuschock,

Forest tent and eastern tent caterpillars are commonly seen in the spring, but you are not likely to see gypsy moth caterpillars anytime soon in most places in Minnesota. The gypsy moth detection program led by Minnesota Department of Agriculture finds isolated populations of gypsy moths and treats them before they grow into large infestations.

Pine leaf adelgid

By Eric Otto, Northeast Region forest health specialist

small purplish-brown colored insect on pine needles.

Winged adults of pine leaf adelgid facing the
base of a needle.

A tiny Minnesota native that has not been around much in the past 30 to 40 years is damaging white pines. That native is the pine leaf adelgid, a small, sap-sucking insect. It prefers feeding on white pines, particularly those near black spruce trees. Foresters and landowners in Cook, Koochiching, Lake, and St. Louis counties have reported seeing this insect and the damage it causes. The reason for its reemergence is still a mystery to our team.

Recently, a landowner who noticed limited damage three years ago reported that now the damage has become widespread. The reported area is in northeastern Koochiching and northwestern St. Louis counties. More specifically, the landowner noted damage along hiking trails on Lake Kabetogama in Voyageurs National Park. The impact is primarily on small white pine trees, but damage was also on larger tree branches.

Pine leaf adelgids are flat, scale-like, and dark purplish-brown. On white pine, adelgids insert their straw-like mouthparts into new shoots, injecting saliva that damages the shoots as they feed. They can kill young white pines. Heavily infested white pines can have an abundance of dead twigs and shoots, brown needles, and twisted, distorted growth. Winged adults characteristically form a row on pine needles with their heads facing the base of the needles.

The pine leaf adelgid has an interesting life cycle that requires black spruce as an alternate host. Galls form on black spruce but fortunately cause little damage. Damage to white pine is more common in areas where black spruce and white pine grow in close proximity.

Management for pine leaf adelgid is somewhat limited. Pruning infested white pine can impact the insect's life stage and reduce the amount of adelgids moving to spruce to complete their life cycles. Removing spruce or planting white pine in areas where spruce is not present could also help reduce the impact from pine leaf adelgid. For trees of higher value, a horticultural oil may be considered.

It will be important for the forest health team to monitor this insect and the damage it causes so we can determine how widespread this insect can become in northern Minnesota

Scale insects observed on broadleaf trees

By Eric Otto, Northeast Region forest health specialist

white colored mold worts on small ash tree trunk.

Scale insects on a small ash tree.

Foresters and landowners in Itasca and St. Louis counties, and most likely others, have observed an abundance of scales on broadleaf trees, particularly small ash trees. The scale insects are most likely lecanium scale, a native insect that feeds on sap. The scales appear as warts or reddish to dark brown "turtle shells" especially as the female scales enlarge. Scales excrete honeydew that is colonized by sooty mold, a black fungus that can cover leaves, whole branches, and other plant parts. The dark threadlike growth of the fungus resembles a layer of soot. The fungus does not infect plants and is generally not a concern for tree health.

Mature female scales are most noticeable in May and June when they become hardened and round. At this point they lay eggs, which hatch into "crawlers" that move to the leaves where they will feed. The crawlers return to branches to overwinter. Interestingly, male lecanium scales are only occasionally produced to mate with females. Female scales mainly reproduce via a process called parthenogenesis, in which eggs develop and hatch without fertilization.

High population levels can result in twig and branch dieback, but natural enemies of the scales can help reduce the population. Natural enemies include parasitic wasps, predatory mites, and lady beetles. If high populations remain, further control methods might be warranted.

Whitespotted sawyer or Asian longhorned beetle?

By Megan O'Neil, Northwest Region forest health specialist

comparison between male and female Asian Longhorned beetle and Whitespotted Pine Sawyer.The whitespotted sawyer is a longhorned beetle native to Minnesota that is regularly seen in the summer. We often receive calls from the public about this beetle because of its striking resemblance to the invasive Asian longhorned beetle. Once you know what to look for, the difference is easy to spot, spot being the key word.

The whitespotted sawyer and the Asian longhorned beetle are both relatively large beetles with long antennae (hence the name longhorned). They have dark bodies dotted with whitish spots. While the pattern of spots is random and different from beetle to beetle, the male whitespotted sawyer is solid black and has one white spot just behind its head, at the top of the body right between its "shoulders." From the picture below you can see the distinct spot that will help you distinguish the whitespotted sawyer from its doppelganger.

The whitespotted sawyer is an impressive beetle by Minnesota standards. Adult beetles are 0.75-1.25 inches long and have brownish-black wing covers. Females have spots scattered across their wing covers. The male's antennae are longer than their bodies. Sometimes they can be heard inside a tree as they eat their way out of dead and dying conifers, leaving large, round exit holes. The adults feed on needles and twig bark while the legless larvae eat the tree phloem just under the bark.

Asian longhorned beetles are roughly the same size as whitespotted sawyers. They have smooth, glossy black wing covers with white spots (giving them another common name, the starry sky beetle), but do not have the distinct white spot behind the head. Their legs are blue and their long antennae are banded black and white. The Asian longhorned beetle attacks healthy deciduous trees and prefers maple. The robust larvae burrow throughout the tree creating large tunnels below the bark, and adults chew pencil-sized holes through the bark when they emerge. Not only does this kill the tree, it creates structural issues that can cause limbs to break in trees that look healthy.

If you see the white spot behind the head there is no need to report native whitespotted sawyers to the DNR. It is important to keep the Asian longhorned beetle out of Minnesota, and you can do your part in keeping it out by not moving firewood.

Butterfly madness

By Val Cervenka, forest health program coordinator

Hackberry emperor butterfly on the ground

Hackberry emperor butterfly.

Entomologists like me get excited when there are reports of large numbers of insects about. I was lucky this year to get in on the tail end of a massive emergence of native hackberry emperor butterflies (Asterocampa celtis). A homeowner in a rural town south of St. Paul reportedly had thousands of these butterflies in his yard, and worried that they might be a non-native species. That's when an assistant area wildlife manager contacted me, and I felt the call of duty to investigate.

Doing some research before I made the short trip to the property less than 30 miles away, I found out that hackberry emperor butterflies had been emerging in large numbers all over Iowa and southern Minnesota. This was not a singular event, but this year's emergence seemed noteworthy to people posting to an insect listserve in Iowa. The Minnesota homeowner that contacted the DNR said there were more butterflies in his yard than he had seen in years past.

Hackberry emperor butterfly sitting with wings closed

Hackberry emperor butterfly
sitting with wings closed.

Hackberry emperor caterpillars feed in groups on their only host, hackberry. In winter, late-stage caterpillars take refuge in rolled-up leaves, emerging in the spring to continue feeding. Adult butterflies emerge from the chrysalis in early summer and live for about two weeks. Adults get moisture from an odd assortment of sources, including hackberry flower nectar, feces, and rotting carcasses…they're not picky! When they feed on hackberry flowers, they don't land, so are not considered efficient pollinators.

When I arrived at the property, I saw dozens of butterflies flitting around, which is amazing enough if you're not in the tropics. The homeowner said that only a few days before, the air was so thick with butterflies that he couldn't go outside without breathing one in. The hackberries surrounding his home, tucked in a wooded lot surrounded by corn and soybeans, were only lightly defoliated, and new leaves were already growing at the branch tips.

Next year I'll be ready, and head south a week earlier to perhaps be lucky enough to breathe in a butterfly.

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