Forest health


Forest Insect and Disease Newsletter - Fall 2017

Statewide Forest Health Issues

Emerald ash borer moves to Martin County

By Val Cervenka, DNR forest health program coordinator

Purple prism EAB trap near Welcome. Purple prism EAB trap near Welcome. Photo by the Minnesota Dept. of Agriculture

In August, four emerald ash borers (EAB) were captured on a purple prism EAB trap near Welcome, in southern Minnesota. This discovery is important, since it is at least 60 miles from previous finds in Fillmore and Dodge counties, and Martin County does not border other counties quarantined for EAB. The traps were placed and monitored as part of the national survey conducted by the US Department of Agriculture. According to the Minnesota Department of Agriculture, capturing several beetles on a trap indicates there is a significant infestation in that location.

The Minnesota Department of Agriculture has declared an emergency quarantine on the movement of regulated articles out of Martin County. Regulated articles include:

General information can be found on the Minnesota Department of Agriculture emerald ash borer website.

Residents of Martin County are invited to attend a public meeting to learn more about the discovery and to hear options for dealing with EAB, how to limit its spread, and to provide input on the adoption of a formal quarantine:

Emerald Ash Borer Informational Meeting
Wednesday, September 20, 2017
5:30-7:30 p.m.
Martin County Courthouse, Room 103
201 Lake Avenue
Fairmont, MN 56031


map of minnesota showing central region

Central Region

By Brian Schwingle, DNR forest health specialist


Hail damage on red pines in northern Chisago County

Red pines with Diplodia shoot blight from a July hail stormRed pines with Diplodia shoot blight from a July hail storm.

The effect of the mid-July hail storm in northern Chisago County is still highly visible on red pines. Even though all tree species had physical damage along their branches, only the red pines appear truly damaged from a distance, because physical damage from the hail allowed Diplodia shoot blight to take off. The more blighted shoots a tree has, the more likely it will die. A reasonable management approach for landowners is to cut down red pines that have lost over 50 percent of their crowns this fall and winter. Logs should be debarked or processed into lumber or firewood before April 2018 to avoid a buildup of bark beetles if pines are felled after December. Unless there is severe drought in 2018, many of the remaining red pines will recover their green crowns in the coming years.

Decrease the spread of oak wilt with two simple steps

Oak wood infected with oak wilt is covered to block oak wilt spores from reaching nearby oaks. Photo courtesy of Mark Miller, Minnesota DNR.Oak wood infected with oak wilt is covered to block oak wilt spores from reaching nearby oaks.

Recent aerial surveys followed by confirmation on the ground were a depressing reminder of the huge, negative impact oak wilt has had in Sherburne, Isanti, Anoka, Chisago, and Washington counties. Oak wilt can be controlled on individual properties by breaking root grafts with the installation of primary barrier lines (typically done with vibratory plowing) followed by either injecting fungicide into or removing root-grafted oaks that are still healthy, then cutting and removing all infected oaks.

However, these control techniques are often not financially possible for many homeowners. If a homeowner or landowner cannot afford to control oak wilt by vibratory plowing and removing grafted oaks, there are still two fundamental practices that any homeowner can do to slow the progress of the disease across their property:

Branch tip flagging on oaks due to oak bullet galls

A red oak in Pine County with branch flagging from oak bullet galls.A red oak in Pine County with branch flagging from oak bullet galls.

Scattered dead shoots in red oak canopies are a common site across much of central and southern Minnesota this year. In most cases, the damage is not severe. The most common cause of the damaged shoots are hard, round galls congregated on the ends of branches called oak bullet galls, made by tiny cynipid gall wasps. There are numerous species of gall wasp in Minnesota, and many of them make galls on oak leaves and stems. Control of gall-forming insects on oaks is not necessary since their impact on the landscape is slight. For heavily-infested yard trees, proper care and adequate watering is usually all that's needed to maintain tree health. More information may be found at the University of Minnesota Extension insect and mite galls website.

Damaged oak shoot with a round, hard gall on the dead portion.Damaged oak shoot with a round, hard gall on the dead portion.

In some cases on both red and white oaks, branch tip flagging may be caused by a fungal pathogen called Botryosphaeria. This is the second year in a row that Botryosphaeria shoot blight has been relatively common on oaks; we wrote about this pathogen on bur oak in our August 2016 newsletter. Oaks with Botryosphaeria shoot blight may have been stressed, and in most circumstances, shoot blight will fade over time and the tree will recover. However, if older oaks are sufficiently stressed, they may succumb to critters other than Botryosphaeria.

Don't give up on trees defoliated by Japanese beetles


Brown linden trees in the background were heavily defoliated by Japanese beetles.
Brown linden trees in the background were heavily defoliated by Japanese beetles.

Like last year, Japanese beetles fed heavily this summer on linden, paper birch, cherry, wild grape, and many other species in the urban areas of central and southern Minnesota. Since Japanese beetles tend to defoliate trees in the latter half of the summer, trees do not produce a second set of leaves to feed on—which could be viewed as the silver lining, if you seek a silver lining in the Japanese beetle story.

Homeowners should not cut down their defoliated trees this year, since those trees should leaf out just fine next year. Deciduous trees can lose their leaves in late summer for several consecutive summers without showing signs of stress.

Controlling Japanese beetles on large trees is difficult, since the beetles can fly very well and their numbers are very high in certain areas of the state. Several reliable websites suggest, and we concur, that Japanese beetle traps are not effective at controlling these beetles. For an array of Japanese-beetle-fighting tactics, read the recent University of Minnesota Extension Yard & Garden News.


map of minnesota showing northeast regionNortheast Region

By Jess Hartshorn, DNR forest health specialist


Dutch elm disease

American elm in a residential neighborhood showing classic DED symptoms in July 2017.American elm in a residential neighborhood showing classic DED symptoms in July 2017.

Dutch elm disease (DED) had a banner year in northern Minnesota. This massive outbreak most likely was a result of diseased trees left standing combined with mild winters. Residents reported elms dying in a matter of weeks, and many have called the current outbreak reminiscent of the early 1980s when DED ravaged boulevard trees in northern Minnesota cities.

When that outbreak subsided in the early 1990s, Grand Rapids arborists were only removing a few trees each year. With DED seemingly under control, city regulations eased, and homeowners were no longer required to remove diseased trees. That changed in 2016 when Grand Rapids arborists removed 15 diseased elms, and intensified in August 2017 with the removal of more than 12 diseased elms daily for about a month. Unfortunately, treating DED is only possible in trees with very minor infection, and most infected trees will need to be destroyed. Homeowners with healthy, uninfected trees can have elms treated next spring to help prevent new infections.

Effects from past drought and wind damage

Red pine pocket mortality with dead and dying trees radiating out from an infection center.Red pine pocket mortality with dead and dying trees radiating out from an infection center.

In 2011 and 2012, much of Minnesota suffered significant drought and heavy windstorms, but some portions of the state are just now starting to see the effects of these events. This summer, several stands in northeastern Minnesota saw a sharp decline in the health of pine stands. These stands had several issues, including pine bark beetles, Armillaria root rot, and red pine pocket mortality, which is due to various weevils (beetles) that feed on and kill roots and introduce a fungal root disease called Leptographium. Symptoms of red pine pocket mortality often go unnoticed for several years, after a number of trees have died and surrounding pines start declining in health. It's important to closely monitor stand health, not just during, but after an event such as drought, hail, or windstorm.


map of minnesota showing northwest region regionNorthwest Region

By Mike Parisio, DNR forest health specialist


What on earth is that? Maple blight aphids abundant in 2017

Maple blight aphibs on maple leaf. Maple blight aphids on maple leaf. Photo by Mississippi State University Extension

The northwest region received a flurry of calls beginning in mid-July regarding a strange white fuzz growing on the undersides of silver maple leaves, causing many people concern that they were observing some bizarre fungus attacking their trees. A closer look revealed that aphids, known either as maple blight aphids or woolly alder aphids, were living beneath this white fuzz. Although conspicuous as a group, these large masses of white fluff provide safety and help to disguise individual aphids, protecting them from predator and parasitic wasp attack (and from confused humans).

The reason for the aphids' two names is that, like other types of woolly aphids, the maple blight aphid has a complicated life cycle involving multiple host plants. Eggs are laid on silver maple in the fall and hatch the following year, becoming large colonies of little cotton balls by mid-summer. Later in the summer, winged adults produced in these colonies migrate to alder, where they form new colonies on twigs and branches rather than the leaves. Some adults eventually leave the alder once more and return to silver maples to lay eggs, while some immature aphids remain on alder to overwinter.

I observed these fluff masses myself on alders lining the Paul Bunyan Trail on the east side of Lake Bemidji last summer, but I do not recall receiving any calls from the public concerning woolly aphids in 2016. For whatever reason, maple blight aphids seemed to be more abundant or maybe just more noticeable in 2017. Fortunately, aphids like these attacking trees of substantial size generally have minimal impact on overall tree health.

Acorn weevil grubs during fall seed collection

Acorn weevil grubs in acornAcorn weevil grubs in acorn. Photo by Steven Katovich, USDA Forest Service, Bugwood.org

If you've ever collected acorns for the DNR or have been on the receiving end of bushels of acorns brought to you by the public, you've probably noticed quite a few acorns with small holes and piles of acorn weevil grubs on the bottom of collection containers. Adult acorn weevils lay eggs in developing acorns while still attached to oak trees, which serve as both a food source and protective nursery for developing grubs. Once acorns drop to the ground, larvae chew an exit hole and leave acorns to make their way into the soil to complete development (or become trapped in the bottom of a bucket in this case). Adult weevils, with their conspicuous, long snouts, emerge from the soil in one to two years and repeat the cycle. Though these weevils do no harm to the parent tree, acorns consumed by acorn weevils grubs are unusable and will float when placed in water. Healthy, viable acorns will sink and these are the ones used by the state forest nursery to grow new oak seedlings.

In addition to collecting acorns, fall is also a great time for collecting conifer cones. If you're looking for a new hobby and want to help the DNR reforest Minnesota, visit our seed and cone collection websites for more information.