Winter 2018 Articles

DNR forest health program staffing changes

2018 was a year of transition for the forest health program. We started out fully-staffed, then in June lost the northeast region Forest Health Specialist, Jess Hartshorn, to a position at Clemson University, promptly followed in July by northwest region Forest Health Specialist Mike Parisio, who took a position with the Vermont Department of Forests, Parks, and Recreation. That left Brian Schwingle, the central region Forest Health Specialist, to cover the work of three, and Val Cervenka, Forest Health Program Consultant, to encourage and support Brian! Our perseverance and patience paid off with the addition of a new forest health specialist position in Brainerd, which we hope to fill soon along with the two vacant positions in Bemidji and Grand Rapids. The Forest Insect & Disease Newsletter will be back on track in 2019!

Puzzle of bur oak dieback narrowed down


In the summer edition of this newsletter, we described how some widely-scattered bur oaks across central Minnesota did not leaf out at the same time as their neighboring bur oaks and had branch tips that died. With close inspection, we discovered that a tiny twig-boring wasp was responsible for the twig dieback. We sought experts at the University of Minnesota and at the USDA Systematic Entomology Lab in Washington, D.C. to help us put a name on the wasps. The experts identified the wasps as species in the family Cynipidae. Cynipid wasps are known for causing abnormal growth on leaves and twigs, and there is a surprising—and often confusing—variety of wasps and the galls they cause on oaks in particular.

In the material we sent to the labs for identification, the taxonomists also discovered a few wasps that parasitize cynipid wasps, which is great news, because it reminds us that native tree pests have natural enemies evolved to keep them under control. The taxonomists also identified cynipid wasps that live within the galls created by other wasps—adding to the intrigue of this puzzle.

Time and monitoring will tell if the twig-damaging cynipid wasps make an appearance in 2019. We plan to capture additional wasps in 2019 to identify exactly which species is causing the damage.

Emerald ash borer confirmed in two new locations

Emerald ash borer (EAB) was found in several infested ash in early September around the junction of Interstate 94 and State Highway 24 in Clearwater, Wright County. This new find in Wright County is isolated about 35 miles from the closest confirmed EAB infestations in Anoka and Hennepin counties. In early December, EAB was confirmed near Orono and is about eight miles from the nearest confirmed EAB infestation in Plymouth. We recommend that homeowners and communities in these areas scout intensively for woodpecker-damaged ash trees, which are good indicators of EAB infestations. If you suspect an ash is infested, take pictures and report it to MDA’s Arrest the Pest website. One option to consider is planting other tree species near ash trees in your yard to take the place of trees that succumb to EAB. More management recommendations can be found on DNR’s EAB website.

Leaf mining on northern white cedar

Damage to white cedar needles from arborvitae leafminer.Damage to white cedar needles from arborvitae leafminer. The hole was made by an emerging adult moth.

Thanks to several Minnesota DNR foresters in northeastern Minnesota, we were able to confirm that an arborvitae leafminer (Argyresthia species) infested thousands of acres of northern white cedar for the second year in a row. This organism emerges from infested foliage as a tiny moth in late spring or early summer, and the caterpillar stage feeds in late summer, fall, and the following spring. While eastern locations in Maine and Quebec have reported significant damage from arborvitae leafminers, Minnesota, Ontario, and Manitoba have not.

We first became aware of this infestation during our 2017 aerial survey, but because of the insect’s lifecycle and difficulty in accessing cedar swamps, we were not confident in the cause until recently. Fortunately, the leaf feeding on the trees seems minor. We will be monitoring this pest’s activity in the coming years.

Arborvitae leafminer feeding caused concern in a 2017 aerial survey.Arborvitae leafminer feeding caused concern in a 2017 aerial survey.

Blood-red aspen wood

Two gypsy moth caterpillars feeding on white oak leaves Red-stained aspen wood on a fresh beaver wound from oxidized defensive chemicals. Photo by Jane Harms.

The biggest tree mystery of the autumn was blood-red bigtooth aspen wood that a citizen photographed near Inger in Itasca County. The photographer noticed the bright-red staining on one-day-old beaver wounds in mid-October. The wood looked like it had been painted. The redness faded after a few days. The photographs were passed among several DNR staff before arriving on University of Minnesota Plant Pathologist Bob Blanchette’s computer screen. Bob explained that he had seen this phenomenon before, and said that some aspen and cottonwoods release defensive compounds that oxidize to a pink or red color. Usually this occurs for a very short time and disappears, and it is rarely seen.


Limited bur oak blight in 2018

Bur oak blight (BOB) is a fungal leaf disease that causes leaf browning and leaf loss on bur oaks in late summer and early fall. Vigorous trees affected by BOB can recover and look normal again the following spring.

Bur oak blight has caused noticeable leaf browning and leaf loss on bur oak in central Minnesota for several consecutive years, but 2018 was different. Bur oak blight was almost non-existent in central Minnesota, and according to our surveys, Houston County in southern Minnesota was the only county where BOB made a significant impact. It also happened to be our wettest county in May. This demonstrates that BOB can essentially disappear from a susceptible bur oak if the weather during leaf expansion is dry.

Northern Minnesota’s two biggest forest pests remain the same as in 2017

Eastern larch beetle continued to infest large tracts of tamarack in northern Minnesota, and spruce budworm continued to hammer balsam fir mostly in Lake County.

We mapped about 180,800 acres of forest that were infested with eastern larch beetle in 2018. This was a relatively small decrease in area impacted from 2017, so it’s too early to get our hopes up for a downward trend. It is the second largest number of affected acres mapped since the beginning of the outbreak in 2001. Eastern larch beetle has impacted 535,000 acres of forest since the beginning of the outbreak, and University of Minnesota research indicates a changing climate is at least partly to blame.

Accumulated acreage affected by Eastern larch beetle

Spruce budworm defoliated balsam fir and spruce on about 196,450 acres in 2018. This was almost a three-fold increase in affected acreage from 2017, though the dip in affected acres in 2017 seemed to be an exception. An area southeast of Ely sustained the most severe spruce budworm impact, and that area has been impacted since 2013. Most of the heavily damaged area is U.S. Forest Service land. Spruce budworm will likely stay active in that area until 2020, and many older balsam fir are expected to die. We mapped new spruce budworm infestations west of Two Harbors in southeast St. Louis County.

Repeated feeding from spruce budworm kills balsam firs.Photo by Marc Roberts, U.S. Forest Service.Repeated feeding from spruce budworm kills balsam firs. Balsam firs with lightly brown needles will continue to live the next year. Photo by Marc Roberts, U.S. Forest Service.