Forest Insect and Disease Newsletter - Summer 2018

Statewide Forest Health Issues

Dry and windy April weather killed scattered evergreens

By Brian Schwingle, DNR forest health specialist

Two gypsy moth caterpillars feeding on white oak leaves An arborvitae that was killed by winter drying in Fillmore County.

The DNR forest health team received several reports in the first half of May that black hills spruce, white cedar (arborvitae), and white pine had died suddenly in Anoka, Fillmore, Freeborn, Goodhue, Hennepin, Houston, Rice, and Winona counties. Most of the affected trees were relatively young. A private nursery in Redwood County was still getting calls in early June about this problem on black hills spruce. Some cases of death from winter drying on spruce and arborvitae were also reported in St. Louis County, and less severe cases were reported on red and white pine in the Arrowhead region.

The widespread, scattered evergreen death in southern Minnesota is a severe example of winter drying, which can affect many types of evergreens. In extreme cases, any portion of the evergreen above what would have been the snow line died within a few days. This happened because in late April we had warm, very windy days with low relative humidity, causing needles to lose moisture. Frozen soil in the root zones of these trees prevented water from moving back into the needles to replace the lost moisture. Branches buried under the snow on those days remained alive because they were not exposed to the warm, windy, and dry environment.

If your conifer currently has more than 50 percent of its canopy killed by winter drying, we suggest removing it soon to avoid complications with bark beetles. On June 1, cedar bark beetles were starting to attack arborvitae in Winona County that were severely damaged by winter drying. For more details on winter drying, see the Winter Burn article by University of Wisconsin-Extension.

Central Region

By Brian Schwingle, DNR forest health specialist

Some bur and white oaks barely leafed out in central Minnesota, Part I

A white oak near Chisago City that struggled to leaf out fully this spring.

Homeowners and tree managers in central and east-central Minnesota became concerned in late May when they saw bur and white oaks barely leafing out. We haven’t yet solved this puzzle, but we are closing in on an answer, and we’ll write an updated report in the next newsletter.

The affected trees were scattered from Brainerd and Sauk Rapids to Hinckley and Stacy; we even received reports of some in Chanhassen, and last week we confirmed one heavily infested bur oak in Rice County. The hardest hit oaks appeared dead in late May, while others had widely scattered tufts of leaves in their canopies. These oaks were in open-grown situations in yards or along woodlot edges. Upon closer inspection, we found lots of tiny holes just over 0.5 millimeter in diameter on dead twigs, and pupating wasps, 2 millimeters long, buried in twigs.

Given the serious impact to mature bur and white oaks, we are working hard to get to the bottom of what is causing this severe dieback. We have captured some adult wasps from the affected branches, and a wasp identification specialist identified one of the two wasps we captured as a parasite of a cynipid wasp. Cynipid wasps can form inconspicuous galls within twigs. There are more than 700 species of cynipid wasps that attack oaks.

We anticipate that by the time our next newsletter comes out we will have more information to share. Until then, we recommend that homeowners leave affected oaks in place and allow them the opportunity to recover. Many of the affected oaks are already recovering and will survive.

Intense seed crop on maple and Siberian elm

A Siberian elm in Stearns County with missing leaves because of heavy seed production.

Some citizens were concerned this spring that maples were dying in central Minnesota. Fortunately, they were not dying; they were just making a bumper crop of seeds. Maples and Siberian elms across a large swath of Minnesota decided to produce ample seeds this year. Every now and then this happens across a large area, and it usually does not hurt the tree. In some instances, outer branches on maples that produce an intense seed crop can die, but these trees usually recover over time. 



Wet growing seasons and spruce windbreaks: a bad combination

Black hills spruce with Stigmina needle cast in Washington County.

In a way, you can think of a row of spruce like a daycare: it is a germ bath.
A row of spruce provides year-round privacy, protection from the wind, cover for animals, and a pleasing view. They are so common that there is a good chance you have a row of spruce in your yard. Unfortunately for you (and me too), it seems only a matter of time before needles in the lower part of the canopy will start to die. A common reason for this needle loss is a needle cast disease caused by a fungus. That fungus often is a Stigmina species. I found Stigmina in all the diseased spruce needle samples I looked at this spring. Eighty percent of the spruce needle samples given to the University of Minnesota Plant Disease Clinic yielded a Stigmina needle cast diagnosis in 2017.
Needle disease in the lower canopy of spruce in people’s yards is a frequent tree health concern that DNR personnel receive. The abundant precipitation coming down on central and southern Minnesota over the last decade, plus moist conditions within rows of spruce, promotes needle disease. We have also observed that low-lying areas where cool air and moisture sink favor needle casts.
Spruce growing all by themselves in the sun and away from other trees often do not get problematic needle casts, even when their branches contact the ground. The air moves freely around these trees, and along with sunshine, dries out needles, so needle cast pathogens can’t produce spores as much. Also, those solo spruce trees are not stressed by growing in shade, and are not growing next to fellow spruce trees that can pass on diseases to them.
Fungicides can protect new spruce shoots from being infected by needle casts, but they won’t get rid of current infections on older needles, and the number of treatments needed to prevent disease every year is not usually worth the cost and time invested. Fungicides also don’t address the environmental conditions that favor needle casts.
Here are options we recommend to minimize spruce needle cast diseases:

  • Plant spruce in full sunlight as individual trees and not as parts of rows.
  • If you desire a privacy or windbreak of trees, do not plant spruce next to each other. Alternate spruce with a different tree, such as an arborvitae, pine, fir, or a deciduous tree. Understand that these trees will eventually need much more space to grow, so make sure that spruce branches will not encroach on other spruce branches.
  • If you currently have a row of younger spruce trees, remove every other spruce once their branches contact other spruce trees’ branches.
  • Do not allow irrigation water to contact spruce needles or spray up through the canopy.
  • Pruning away lower canopy branches may delay progress of needle cast in the tree’s canopy.

Remember, diversifying any tree row or forest, regardless of species, can avoid many disease problems.

map of minnesota showing northeast regionNortheast Region

By Jess Hartshorn, DNR forest health specialist


A fond farewell

Two gypsy moth caterpillars feeding on white oak leaves

It is with a heavy heart that I share with you my decision to leave the Minnesota DNR. This was a very difficult decision to make, and I will remember my time here fondly.

It’s hard to believe that I have been with the DNR for two years as your Northeast Region Forest Health Specialist. During my time here I have learned a lot—like don’t trust the integrity of rural road shoulders no matter how dry they look—and met a lot of interesting and fun people who have shown me a lot of blurry phone pictures of dead trees. I want to take this time to thank everyone for teaching and motivating me, and allowing me to do the same for you.

At the beginning of July I will head to South Carolina to explore my next chapter as Assistant Professor of Forest Health at Clemson University. No doubt, the knowledge and experience I have gained with the DNR will help me immensely on my journey. I am excited about my future and also excited about all the great things my friends and colleagues will continue to accomplish here in Minnesota.

In the words of the famed philosopher, Michael Scott, “Catch you on the flippity flip."

map of minnesota showing central region


Is your white cedar looking yellow?

This spring, many northern white cedars (Thuja occidentalis) across northern Minnesota looked yellow. There were two main causes of this yellowing: winter injury and arborvitae leafminer. Because prevention and control are very different between the two, it’s important to accurately determine the cause.

With the help of the University of Minnesota Plant Disease Clinic, Forest Health Specialist Jess Hartshorn determined that a regional outbreak of arborvitae leafminer caused some of the yellowed tips (Figure 1). Arborvitae leafminers are small, silvery moths that lay eggs on arborvitae (Thuja) foliage in June and July. When the eggs hatch, larvae tunnel into the foliage and feed into the fall, then overwinter in the tree. Feeding resumes the following spring, and larvae pupate by late May. Adults emerge a few weeks later and chew their way out of the foliage, creating very small exit holes (Figure 2). These yellow tips tend to be scattered throughout the canopy. Because the larvae feed in the fall and spring, damage often is not noticeable until January or February and easily can be mistaken for winter injury.

Winter injury, while it can look similar to leafminer injury, does have a few distinct differences. It tends to occur on the south and southwest sides of the tree, due to increased sun exposure (Figure 3). It also tends to happen on foliage above the snowline, leaving a distinct line between damaged and healthy tissue. Additionally, if trees other than arborvitae are present, winter injury symptoms may be visible on those, too.

If you’re wondering whether yellow tips are due to leafminers or winter injury, remove and inspect the brown tips. Frass (insect feces) in hollow, brown tips is a characteristic of insect activity. If brown tips are not hollow or filled with frass, the cause may be winter injury.

three image showing various states for winter brooms 1. Northern white cedar near Hill City showing yellowing shoots from arborvitae leafminer infestation. 2. Black arrow pointing to an exit hole created by the arborvitae leafminer (magnified 40x).3. Red pine showing symptoms of winter burn on the south side of the tree.

map of minnesota showing northwest region regionNorthwest Region

By Mike Parisio, DNR forest health specialist

Jack pine budworm population continues to fall in Bemidji area

Two gypsy moth caterpillars feeding on white oak leavesJack pine budworm larvae in June 2018, captured for educational purposes.

In 2016 and 2017, the jack pines along the Fosston Trail to the northwest of Bemidji experienced quite a bit of defoliation from jack pine budworm (JPBW). On Friday June 8, I took a trip to get a feel for how abundant JPBW might be during 2018.

When I arrived at the spot, the first thing I noticed was the obvious lack of pollen cones on the jack pines. In fact, I had to travel quite a way to find some trees that had accessible pollen cones in the lower branches. Upon inspection, I noticed feeding damage on most of the pollen cones, but I was hard-pressed to find any JPBW larvae hiding inside. I noticed that the JPBW larvae had been feeding on needles in new shoots as well, but again, I had a difficult time finding any larvae despite the numerous shoots affected.

In the grand scheme of things, this is a good sign. The lack of pollen cones means the trees are no longer providing a highly nutritious food source to keep the current outbreak going, forcing the larvae to feed on the less nutritious needles.  I also suspect that the larvae are being forced to move between pollen cones and new shoots, which causes them to exert extra energy and exposes them to natural enemies. These factors should reduce their success this year as the outbreak continues to drop off.

Since I could not find many larvae during my visual search of individual pollen cones and shoots, I tried a whole branch approach. By shaking a branch vigorously, larvae detect a disturbance that’s greater than something like a natural breeze. In their panic, they reveal themselves by rappelling on a silken thread and hanging below the branch until the danger has passed. Bright sun helped illuminate larvae while they hung in the air, so I collected all those hanging after each shake of a branch. After about the third shake, I found very few to no new larvae. All in all, I collected only 11 larvae from three separate branches. Though obviously not a rigorous survey, it gives me a general idea that the JPBW numbers will be much lower than in previous years, where a larva could be found hiding in almost every pollen cone in certain spots.

I have yet to survey some of the jack pine-dominated areas down in far southern Cass County, which also were defoliated over the last several years. However, I hope when I do I’ll have similar results indicating JPBW numbers have dropped and are returning to normal background population levels in that area.