Forest Insect and Disease Newsletter - June 2020

Forest tent caterpillar outlook for 2020

By Eric Otto, Northeast Region forest health specialist

group of ftc caterpillar on a tree truckWill forest tent caterpillars and the defoliation they cause be more abundant this year? That can be difficult to predict, but data from past outbreaks can help answer this question.

In 2019, the DNR's annual aerial survey mapped only 1,295 acres of forest tent caterpillar defoliation, the lowest amount recorded since 2006. In Minnesota, outbreaks typically occur every 10 to 16 years and can last two to seven years.

The last peak outbreak year was in 2013 when slightly more than 1 million acres were defoliated. However, the majority of this defoliation was light. Was it a true outbreak? Compare this to the significant outbreak of 2001 and 2002, when defoliation peaked at more than 7 million acres. Assuming 2013 was really the last peak outbreak year, the next outbreak should peak anywhere from 2023 to 2029, and we can expect a gradual increase in forest tent caterpillars this year and the years leading up to the next peak window.

The obvious sign of an increasing population is caterpillar sightings. Full-grown caterpillars are hairy and mostly velvety-black with blue stripes on their sides. A characteristic pattern of a yellow keyhole-shaped marking runs along the top of their backs. Caterpillars can be active from mid-May to the end of June. If caterpillars are abundant, defoliation of hardwood trees including aspen, birch, basswood, and others will increase. Bottom line: if more caterpillars are visibly out and about this year than last year, expect that the population is on the rise.

More information on the identification and management of forest tent caterpillar can be found on the DNR forest tent caterpillar webpage.

Preventing forest invasives based on how people play

By Forest Eidbo, Forestry terrestrial invasive species prevention specialist

With millions of acres open to hiking, foraging, hunting, off-highway vehicle riding, gravel biking, and more, Minnesotans love to visit their state forests. However, with every ride, hunting trip, and hike, there is a risk of people damaging forest resources by spreading invasive plants, insects, and diseases. This risk is why the Division of Forestry started creating state forest-specific tools for reducing invasive species risk. We develop these tools in terrestrial invasive species prevention plans.

The first step in developing a prevention plan is to investigate the state forest's current recreational use and which invasive species are present. Based on these factors, we create goals of how we want the forest to look in 10 years. To meet those goals we employ strategies aimed at specific visitors of a state forest. Central to all our management tools is the visitor – that is why learning the type, concentration, and location of recreationists is so important. We gather this visitor-use data using trail counters and monitoring recreation hotspots identified by local DNR staff. By knowing where people are recreating and what they are doing, we can tailor our strategies to the needs of a given forest.

We completed the first prevention plan last year at Sand Dunes State Forest near Big Lake, Minnesota. Sand Dunes State Forest is a popular destination, with a beach, two campgrounds, summer camp, and miles of hiking and equestrian trails. Some of the management strategies that the Division of Forestry developed, in collaboration with other DNR divisions, for the Sand Dunes State Forest are:

  • increasing invasive species signage and boot-brush kiosks at visitor hotspots
  • developing a partnership with the onsite summer camp to teach students how to prevent invasive species
  • creating a manure management plan to mitigate invasive plant spread through horse manure

Once we implement the management tools, we will continue to monitor the forest to ensure the plan's success. If monitoring indicators show that we are not on track to reach our 10-year desired conditions, we can re-evaluate strategies and correct our course.

By the end of this year, we plan to complete prevention plans for the Chengwatana, Paul Bunyan, and Finland State Forests. While these collaborative plans take months to write and an ongoing commitment to implement and evaluate, it takes no time at all for you to prevent the spread of invasive species. By simply cleaning mud and seeds from your footwear and gear before and after recreating, you take an important step towards protecting Minnesota's forest resources.

Woman cleaning her boots at a boot-brush kiosk

Cleaning mud off your boots helps protect the Paul Bunyan State Forest

What you need to know about invasive giant hornets

DNR forest health staff have been getting questions about what some call "murder hornets," so named because of their ability to kill honey bee colonies. In order to help dispel rumors and fears about the hornets coming to Minnesota, we're sharing information about them posted recently on the University of Minnesota Extension Yard & Garden News.

Frost damage to oaks

by Brian Schwingle, Central Region forest health specialist

young oak leaves showing frost damageLate spring freezes frequently kill oak leaves—especially red oak leaves —emerging in early May. The damage doesn't seem to hurt trees over the long-term since it happens frequently, and individual trees reliably re-leaf one to two weeks after the freezing temperatures. This year, damaging temperatures of 28°F and below were reported on May 12 and May 13 in many locations, killing new oak leaves. One weather station in the city of Mabel in Fillmore County (near the Iowa border) recorded 21°F on May 12. The last time May frost damage seemed this widespread in central and southern Minnesota was in 2016, although we did get some reports in 2019. Frost damage was particularly severe in southeast Minnesota on the lower parts of hills.

Damaged oaks had already started growing tiny leaves around May 20 in Goodhue County. Since cold air sinks, oaks in low-lying areas and leaves in the lower canopies are more severely damaged from frost. No action is needed to help frost-damaged trees; they'll grow new leaves.

landscape view showing oak tree frost on hillside


Beware pruning off dead branches

by Brian Schwingle, Central Region forest health specialist

In general, it's best to prune living tree branches in winter. Doing so avoids fresh cuts that attract certain insect pests, some of which carry disease, most notably oak wilt. Decay fungi also grow into pruning cuts, and pruning in late winter avoids some of these decay organisms and allows those pruning cuts to heal as quickly as possible.

But what about dead branches?

Technically, dead branches can be cut off any time of year without concerns of disease or decay. However, judging what is dead branch tissue and what is living is challenging, as I experienced in May on my crabapple.

close up showing pruning damageThe bottom circle in the photograph shows where I started to prune the branch that looked completely dead. The subtly green bark tissue exposed in my first two cuts showed that some cells in the tissue were still living, which could attract some unwanted problems. The upper circle shows where I accidentally damaged the living tissue growing over an old pruning cut. The tree will now have to use extra energy to heal that tiny wound.

After this experience, I realized my mistake in pruning branches that appeared dead. Next time, I'll prune them in the winter.


Growing pains: how to recognize and treat transplant shock

by Rachael Nicoll, Northwest Region forest health specialist

Imagine that you've recently planted your future forest or landscape trees. You envision a future in which these mere twigs have transformed into majestic trees. However, this image dissolves when the seedlings or saplings you've planted suddenly appear to be dying. What happened?!

Trees and shrubs experience a lot of stress when they are moved from the nursery to the planting site. They may lose parts of their root systems, their roots may dry out, or the trees become damaged during lifting and transportation. Poor growing sites, low-quality seedlings, improper planting techniques, and poor care after planting can further stress sensitive seedlings. Transplant shock often happens when trees do not grow new roots or fail to establish a new root system in their new environment. When transplant shock occurs, trees are unable to recover from stress, decline in health, and can eventually die if the landowner doesn't intervene. Transplant shock is often the root cause of poor health and death of recently planted trees and shrubs.

How do you recognize if your trees are suffering from transplant shock? Symptoms vary and look similar to disease and other stressors, but commonly include general decline, thinning foliage, branch dieback, leaf scorch and brown tips, poor leaf color, stunted growth, delayed leaf emergence in spring, premature fall color, and secondary insect and disease issues.

close up of pine showing signs of shockFortunately, transplant shock can be avoided through proper planting techniques and follow-up care of plantings. If transplant shock occurs, trees can often quickly recover with some proper TLC. Trees suffering from transplant shock can appear to be dead, so you can do a "scratch test" to see if they are still alive. Lightly scrape the bark on a small spot on one of the twigs with your fingernail or a knife to see if the tissue inside is bright green and moist. If so, your tree is alive! If the tissue is dried out and brown? Sadly, your tree is dead.

Prevention is crucial for minimizing transplant shock. Supplemental watering during periods of low rain is the most important prevention measure you can take. It has been dry in many parts of Minnesota this spring, so this step is especially important now. If you have the time and materials, properly mulching your trees can help them retain moisture, among other benefits. Also, do not fertilize in the first year. Establishing root systems cannot support the overly lush foliage.

Trees and shrubs can take three to five years to establish after planting, so controlling transplant shock isn't a one-time thing. We'll follow-up with more transplant shock and planting information in a future edition of our newsletter. Stay tuned!

How to report large and healthy American elms

by Brian Schwingle, Central Region forest health specialist

Dutch elm disease was first discovered in Minnesota in 1961 and between 1977 and 2014 killed more than 95 percent of Minnesota's big American elms (at least 21 inches in trunk diameter). Dutch elm disease still is going strong today and frequently kills elms before they grow to more than 20 inches in trunk diameter.

full image of an American elmAmerican elm is Minnesota's most common elm and grows well in wet forests. Ash trees also grow in wet forests, but are under severe threat from emerald ash borer. Once emerald ash borer kills most ash in a wet forest, the forest is at risk of converting to a non-forest setting such as a wet meadow or a buckthorn-choked swamp. One strategy to prepare for emerald ash borer is to diversify the forest before the borer's arrival. A very promising tree to grow in these ash-dominated forests is American elm, particularly Dutch elm disease-resistant elms.

There are only a handful of resistant elms, though, and researchers are looking for more, especially from plant hardiness zone 4a or colder (central and northern Minnesota). You can report large, healthy American elms that you come across in your woods to the survivor elm website. You need only record the geographical coordinates, the trunk diameter, and answer some basic questions about the elm. To be considered, American elms need to be:

  • 24 inches or larger in trunk diameter (4.5 feet above the ground)
  • within 1 mile of elms that died of Dutch elm disease
  • disease-free
  • naturally growing in forests or fields (city elms can be reported, but we recommend reporting only elms growing in natural habitats)

Maybe one day an elm you report will help protect a wet forest threatened by emerald ash borer.

Pine tortoise scale

By Megan O'Neil, Northwest Region forest health specialist

Pine tortoise scale on jack pine branch.If your jack pine trees look like they are covered in black soot or mold, you may want to take a closer look. That black, sooty mold could be caused by a small insect called the pine tortoise scale, which can be hard to recognize unless you know what you are looking for.

Even if you're looking right at them, scale insects can be difficult to see.  They can vary widely in shape and size and don't look like what you might think of as a typical insect. The immature insects are known as crawlers because they are the only mobile stage. As adults, their legs are no longer functional, and once they settle into a spot, they don't move. The adults attach themselves to tree branches and insert their sucking mouthparts into the bark to feed on sap. The black, sooty mold grows on excretions from their feeding.

Pine tortoise scale is named for its resemblance to a tortoise shell pattern. Scales are small, about a quarter of an inch, and can be found in large numbers on jack and Scots pine. The adults overwinter on twigs, lay eggs in the spring, and the crawlers hatch in late June to early July. Crawlers can spread to new trees by moving across branches that are touching or being blown by the wind. In small numbers, pine tortoise scale is not a problem, but when populations grow, they can kill trees.

Treatment is usually not necessary for pine tortoise scale. If you notice a small infestation, prune and destroy affected branches. Native predators such as lady beetles and small wasps can also help keep populations in check. A registered insecticide can be used to control the crawlers, but timing is critical, as the adults are not affected by insecticide.

If trees are already heavily infested and showing signs of a major decline, tree removal is a reasonable option. If you are removing trees while the crawlers are active, do not drag branches out of the location, as this could spread crawlers into un-infested areas. Next time you are in a jack pine stand, be sure to take a minute to look closely at the branches to see if you can spot any of these interesting insects.

Pine bark adelgid

By Eric Otto, Northeast Region forest health specialist

pine bark adelgid was aphid-like insectPine bark adelgid has been found in some Minnesota counties, including Beltrami, Itasca, and McLeod. Pine bark adelgid is found mostly on eastern white pine. Minor damage from light infestations is common, and trees can be stunted, discolored, and weakened. Heavy infestations can result in reduced tree vigor and mortality.

The pine bark adelgid is an aphid-like insect that sucks sap from a tree, disrupting the flow of nutrients. The distinctive sign of the adelgid is small, conspicuous patches of white, woolly wax on the main stem and branches of a tree or at the base of needles. From a distance, the main stem and branches appear flecked with white paint due to the abundant white, woolly wax the adelgid produces. The yellow-to-purple adelgids live under the wax, but a hand lens is necessary to see the tiny insects.

Pine bark adelgids overwinter mostly as immature females. When spring and warmer temperatures arrive, females mature and produce the distinctive woolly substance. Females lay eggs that produce winged or wingless immatures, called nymphs. The wingless nymphs remain on the tree, feed, and reproduce. Several generations are possible in a year.


A number of natural enemies attack the nymphs and keep the population from causing damage and in low numbers. Some common natural enemies include lady beetles, lacewings, and hover flies. Control is seldom needed, but if high-value home landscape trees are heavily infested, treatment could be considered. Dormant oil can be sprayed on trees in early spring before bud-break when the tree is still dormant. Insecticidal soap can be used in late spring when the insects are active.

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