Forest Insect and Disease Newsletter May 2014

Top News Stories

Gypsy moth quarantine delayed

Egg masses from Gypsy moth. Photo from Forestry Images

Gypsy moth egg masses

The Minnesota Department of Agriculture has delayed the start date of the gypsy moth quarantine previously scheduled to start May 1, 2014. The new start date will likely be in mid-summer. The area to be quarantined includes Cook and Lake Counties on the north shore. More information on the quarantine This link leads to an external site.

Gypsy moth population numbers are very low, so visitors are unlikely to see gypsy moth caterpillars this summer. However, it is important for businesses, vacationers, and residents to avoid accidentally moving caterpillars to another site. Before leaving an infested area be sure to inspect your outdoor belongings. Look for fuzzy, buff-colored egg masses roughly the size of a quarter on anything that sat outside for more than a few days. Because the female moths can't fly to lay their eggs, they look for sheltered spots within crawling distance of the tree on which they fed as caterpillars. Wheel wells, tents, and the undersides of picnic tables and lawn furniture are all fair game. If you find an egg mass, contact to report it.


Forest tent caterpillars: what's in store for 2014?

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Walnut Twig Beetle Survey map

Forest tent caterpillar defoliation, 2013

Minnesota's native forest tent caterpillar (FTC) populations go into outbreak mode every ten to sixteen years. Last year, the population increased from 2012 and resulted in the defoliation of 1.3 million acres. Based on historic trends, we should be expecting two to four million acres of defoliation this year, but on 38 of 39 egg mass survey plots this spring, we found only one or two egg masses or none at all (although on one plot near Tower in St. Louis County, there were enough egg masses to defoliate the plot trees six times over).

From these results, it's difficult to predict what will happen with FTC populations and the impacts they will have this summer. Time will tell.

For information about FTC identification, impacts, management or making local predictions, go to DNR's forest tent caterpillar web page.

Winter-burn on Pines

Winter Burn on pine needlesAs early spring temperatures rise, symptoms of winter-burn begin to show on many evergreens, especially on the south and southwest sides of trees. Red needles are more noticeable on pines planted along highways, and on juniper and arbor vitae in yards or next to buildings. Winter-burn damages needles, but does not typically kill trees. Evergreen trees in Minnesota had a long, hard winter with plenty of opportunities for winter injury, but evergreens are adapted for Minnesota's tough weather. Wait to prune or remove damaged trees—chances are the trees are alive and healthy. New buds are generally protected during the winter and will grow once spring arrives, if winter damage isn't severe.

Winter burn can be caused by de-icing salt spray from passing traffic landing on pines growing next to highways. The absorbed salt kills some needles, turning them brown or red.

Moisture lost by needles from drying winter winds, days of intense winter sunshine, and low humidity can also cause winter-burn. Water in the tree and needles cannot be replaced by roots in frozen ground, so needles may turn red or brown and die off. Parts of trees protected by snow, shade, or less wind remain green.

To help prevent winter injury, keep evergreens properly watered throughout the growing season until the ground freezes. Choose tree species that are adapted to local growing and winter conditions. Avoid planting white and red pines, balsam fir and white spruce within 150 feet of a roadway to prevent salt damage.

Consider planting yew and arbor vitae on the north and northeast sides of buildings, out of exposure to sun and wind. Evergreens susceptible to winter burn can be protected by wrapping in burlap or other materials.

Oak wilt is moving north and west out of the Anoka Sand Plains

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Walnut Twig Beetle Survey map

Current oak wilt distribution in east-
central Minnesota

For the past several years, oak wilt has moved out of the pin oak stands that grow on sandy soils, such as in the Anoka Sand Plains, into the red oak stands on loamier soils. Eastern Stearns, southern Benton, Mille Lacs, Kanabec and Pine counties have had a slow but steady movement of oak wilt into their oak woodlots and forests.

Long-distance spread of oak wilt is caused by people moving infected red oak firewood or logs, so it stands to reason that if people stopped moving infected firewood and logs, we could slow the expansion of oak wilt out of the Anoka Sand Plain counties.

Spring is a popular time for people to move firewood to vacation sites. It is vital to not move firewood cut from oak wilt-killed trees because the disease is easily spread, especially during the spring and early summer. Beetles known as sap beetles are attracted to the oak wilt fungus and are able to carry fungal spores to freshly wounded oaks and infect them, and with that, a new oak wilt pocket has been formed.

If you don't currently have oak wilt in your area, there are two approaches to minimize the likelihood of its establishment on your property:

  • Avoid wounding oak trees from April through July. Don't prune, thin, harvest, build trails near or wound the roots of oak trees in back yards, woodlots and vacation sites. If wounding cannot be avoided, carry a can of latex spray paint and immediately paint each wound created, including those on exposed roots. Even fifteen minutes can mean the difference between prevention and disease.
  • Do not accept oak firewood or oak logs from unknown sources or from those that have oak wilt.

Outdoors section

Check for yellowheaded spruce sawflies in early June

Damage to white spruce casued by yellowheaded spruce sawfly

Damage to white spruce caused by yellowheaded spruce sawfly

Yellowheaded sprice sawfly feeding damage to white spruce

Yellowheaded spruce sawfly feeding damage to white spruce

Yellowheaded spruce sawfly larvae feedin on white spruce

Yellowheaded spruce sawfly larvae feeding on white spruce

Yellowheaded sawfly larva feeding on white spruce needle

Yellowheaded sawfly larva feeding on white spruce needle

Yellowheaded spruce sawfly is a native pest of all species of spruce in Minnesota. It belongs to the same large group of insects as bees, wasps, and ants. The larvae feed on the needles, defoliating the tree. Defoliation affects the appearance of the trees, and repeated defoliation over a few years may kill saplings and young trees. Damage is most severe on trees less than ten feet in height that are open-grown in yards, windbreaks, and plantations, since the sawfly larvae prefer sunlight to shade.

Look for sawfly larvae feeding on new growth in late May or early June. Larvae may be mechanically controlled by handpicking or chemically with an insecticide labeled for sawfly control on trees or on spruce. If there were sawflies on trees in your area last year, check those trees first since sawflies will usually return the next growing season.

Yellowheaded spruce sawfly pupae overwinter in the ground beneath trees fed on during the summer. Adult sawflies look like small wasps, and emerge about the time lilacs are in full bloom in late May or early June. The female flies to a nearby spruce, cuts a slit at the base of a new needle with her egg-laying structure called an ovipositor, and deposits a single egg before moving on to another needle.

Larvae hatch in seven to 14 days and are only about ⅛ inch long, making them very easy to miss at this stage. They resemble butterfly or moth caterpillars, but have six or more pairs of legs along the abdomen, while caterpillars have fewer than six pairs. This becomes important to know when deciding on a treatment, since chemicals used against moth caterpillars may not be effective when used on sawfly larvae.

The larvae may be active on trees from late May to late June. Young larvae feed on the edge of new needles, but older larvae will consume the entire needle, leaving only a short brown stub. Once all the new needles on a branch have been eaten, the larvae will also feed on older needles. Full-grown larvae are about ¾ inch long, have a yellow to reddish-brown head and an olive-green body with gray-green longitudinal stripes. When larvae complete feeding, they drop to the ground to pupate, overwintering in cocoons.

Complete defoliation of a spruce tree may be fatal in a single season, but spruce trees are not typically killed in a single year. Partial defoliation usually occurs over a number of years before trees die.

Controlling yellowheaded spruce sawfly is not difficult as long as you check for them early and often and get rid of them before they have caused significant defoliation. People generally don't notice the larvae until a lot of needles are missing, but by then some larvae will have already completed feeding and dropped to the ground to overwinter. Then, even if you kill all the larvae remaining, there will still be an overwintering population beneath the trees that will attack them again next year. Also by this time the appearance of the tree is diminished because of the missing needles. Mark your calendar to help you remember to check for sawfly larvae by early June.

Check for Rhizosphaera needlecast of spruce in early June

White spruce with Rhizophaera needlcast

White spruce with Rhizophaera needlcast

Infected spruce needle. Rows of white spots are healthy stomates; black dots are fungal fruiting bodies.

Infected spruce needle. Rows of white spots are healthy stomates; black dots are fungal fruiting bodies.

A very common disease on the needles of ornamental spruce trees is Rhizosphaera needlecast. This fungus is most common on Colorado blue spruce but also occurs on white spruce; Norway spruce is considered relatively resistant. In mid- to late summer, the fungus causes older needles to become mottled yellow to brown on white spruce, and purplish-brown on blue spruce. Infected needles will drop later in the summer. New needles produced this year will stay green through the summer, but if infected this year will turn color next spring. Damage usually appears first on the lower branches and spreads upward in successive years. Branches are killed if they lose needles for three or more years.

In general, Rhizosphaera needlecast isn't noticeable until the lower branches die. To check for Rhizosphaera needlecast, examine needles two years old or older. Check needles that are turning color as well as some that are green. A healthy needle will have rows of pin-prick-size white stomata, the pore-like openings where gases are exchanged. On a diseased needle some stomata will have black fruiting bodies growing out of them. Each of these fruiting bodies is less than 0.1 mm, so a hand lens is helpful in seeing them.

Spores from the fruiting bodies are spread by rain splash and wind and can be blown from needles on infected spruce trees to uninfected trees. The spores infect wet needles, so when watering, avoid spraying water onto the needles as this will increase the chances of the tree becoming infected.

If a tree is infected with Rhizosphaera, it may be treated with a fungicide. Make sure the fungicide is labeled for Rhizosphaera and trees or spruce, and follow all label directions carefully. Apply the fungicide with a garden sprayer when new needles and shoots are half-grown in late May or early June, and again when the needles reach their full length two to three weeks later. Two years of treatment is usually needed to control this disease. The fungicide will not cure infected needles; it will only prevent healthy needles from becoming infected. New needles will not grow back on the bare branches where diseased needles were shed. However, if you can control the disease, new shoots should hide the bare branches inside the tree after a few years.

Oak wilt in woodlots and forests

Oak Wilt patchOak wilt is a non-native, fatal disease of oaks likely introduced into Minnesota in the 1940s. Infected red and pin oaks die suddenly in July and drop their leaves all at once. White and bur oaks often die over a period of years, often a branch at a time.

In the forest setting, oak wilt causes pockets of dead trees that slowly enlarge over a period of years, with the dying trees on the outermost edge. Oak wilt infection is correlated with wounds associated with storm damage, and often spreads rapidly after spring and summer storms. Oak wilt is spread three ways: roots from infected oaks graft with roots from healthy oaks, sap beetles spread the fungal spores from infected oaks to healthy oaks, and humans may transport diseased firewood or logs.

Over the years, management techniques for controlling oak wilt have been refined in urban and suburban settings, primarily by disrupting root grafts with vibratory plows, limiting management activities to the winter months, utilizing red oak logs before April 1 and seasoning red oak firewood on site. More information.

Both overland spread by sap beetles and underground spread by root grafting must be addressed in order to effectively stop oak wilt from spreading short distances. To minimize spread by beetles, do not wound oak trees from April to mid-July, when sap beetles are active and most likely to spread the oak wilt fungus to wounded trees. Sap beetles are attracted to the fungus, which has a fruity smell if you have a sensitive nose.

If a suspected oak wilt pocket is found, verify the disease by sending a sample to the University of Minnesota Plant Disease Clinic This link leads to an external site. or by finding a fungal fruiting structure (pressure pad) on red oaks that were killed the year before. Diagnostic information is available on DNR's Oak wilt website.

The next step is to delineate the oak wilt pocket, the area that contains dead and dying oaks, by marking those trees in late summer (since even healthy trees appear dead in winter). Then include a buffer around the marked oaks to account for underground spread.

During late fall and winter, the fungus can be contained inside the buffered area by disrupting root grafts with vibratory plows or trenchers that can reach down five feet. If the pocket is new and small, the stump-pulling technique can be used to disrupt roots, as described in Michigan's Forest Resource bulletin. This is a PDF file. You will need Adobe Acrobat Reader to download it. This link leads to an external site.In addition, logs from living red and pin oaks inside the buffer and those that died years before need to be removed but do not require special treatment. White and bur oak trees may be left standing, since the fungus does not produce pressure pads on these species. Similarly, logs and firewood from white oaks and bur oaks do not need special treatment.

Oak wilt fungus pressure pads under the bark of red oaks. Photo from Sinclair and Lyon.

Oak wilt fungus pressure pads under the bark of red oaks. Photo from Sinclair and Lyon.

Other control methods used in suburban settings such as fungicides and biocides (a chemical or microorganism that can control any harmful organism by chemical or biological means) can be used, but this would be very costly to do in a woodlot.

Long distance spread of oak wilt by humans can be managed by proper handling of infected red oak logs and firewood. The trunks of red and pin oaks that died during the summer are likely to produce spores next spring, so these trees are marked during the summer and specially handled. After felling, you may burn or bury the logs, use infected logs by sawing them into lumber to be kiln dried, or chip them for biofuel. Cover cut and split firewood with a tarp, burying the edges with soil to prevent beetles from accessing the infected wood. Tops and limbs can be left on site because they do not pose a threat.

Finally, monitor the site for at least three years and be prepared to treat new pockets or re-treat the borders of the old pocket.

Delay thinning oaks until after a forest tent caterpillar outbreak ends

Oak mortality in a mixed hardwood stand thinned during a forest tent caterpillar outbreak.During a forest tent caterpillar outbreak, the Forest Health unit recommends waiting one or two growing seasons after the end of the outbreak before doing any thinning in an oak stand. This allows the trees to regain their vigor and be able to withstand attacks by two-lined chestnut borers. We observed up to 100 percent mortality of the residual oaks that were thinned during the last outbreak of forest tent caterpillar.

Head to the great outdoors on Saturday, June 14, and enjoy Minnesota's wonderful parks, trails and recreation sites!

image: PlayCleanGo brand markPlayCleanGo: Stop Invasive Species In Your Tracks is holding its second annual PlayCleanGo Day on Sat., June 14, from 11 a.m. to 3 p.m. In collaboration with the Department of Natural Resources and the Three Rivers Park District, PlayCleanGo encourages individuals and families to take to the outdoors, while supporting efforts to prevent the spread of terrestrial (i.e., land-based) invasive species. On Sat., June 14, entrance fees are waived at all Minnesota state parks. PlayCleanGo volunteers will be on hand at 10 state park locations and three regional park locations to provide information and education materials on terrestrial invasive species. Visitors can learn what to look for and the simple action steps they can take to help prevent the spread of terrestrial invasive species on our treasured lands. Some parks will have short tours of areas infested with terrestrial invasive species. For a full list of park locations participating in PlayCleanGo Day, visit

Business section

Mill levy for ash trees

In the face of emerald ash borer (EAB), the city of St. Paul and the Minneapolis Park Board have begun the systematic removal and replacement of all ash trees on city lands, including boulevard trees, in addition to the removal and replacement of any ash trees known to be infested. Infested trees need to be removed as soon as possible to prevent the spread of EAB and to protect the public from trees that quickly become hazardous once they die.

The purpose of the systematic approach is two-fold. First, it will help diversify the urban forest to increase its resiliency. Second, it will allow better management of city budgets by spreading the cost of ash tree removal and replacement over a number of years. Even so, the cost of removing and replacing so many trees is a burden. To help cover the cost, Minneapolis has established a new tax levy which is expected to raise $9 million for the project. More information This link leads to an external site.

No new counties quarantined for emerald ash borer

Don't v firewoodHennepin, Ramsey, Houston and Winona remain the only Minnesota counties known to be infested with emerald ash borer (EAB). A new infestation was found in the city of Winona,This link leads to an external site. and while the new find wasn't a major surprise, it was disappointing news because EAB had previously been confined to a small corner of Winona County. However, even with the new find, infested areas in southeast Minnesota are relatively small, and because of that, managing the infestation in the city of Winona will help buy time for cities elsewhere in the area.

That being said, the EAB population is beginning to build in areas that have been infested the longest. Even though EAB numbers are still relatively low in those locations, dying ash trees are beginning to appear here and there.

As more and more ash trees die, the volume of infested wood will go up, and with it, the risk of emerald ash borer spread. Even though many people know about emerald ash borer, it is easy to forget when you have a lot of wood on your hands. While it is legal to move infested wood within a quarantined area, it is not in the best interest of local businesses and residents to move it. Emerald ash borer infestations currently occupy only a small portion of the infested counties, but your actions could change that. Remember to buy firewood close to your destination and burn your excess on site. Don't share leftover firewood or bring it home. Help keep emerald ash borer populations small.

Hazard Tree Inspection Reminder

Now that winter has ended and signs of spring evident, it's time to think about a particular type of safety inspection, the spring clean-up of DNR-administrative sites and inspecting trees for serious defects.

Information on tree defects and how to evaluate them. This link leads to an external site.

DNR staff is obligated to do these inspections and evaluations under Operational Order #97, which states, "It is the DNR's policy to try to preserve the natural beauty of its recreation sites while providing for reasonable public and employee safety on intensively-used recreation sites and administrative sites by detecting and correcting situations involving hazard trees insofar as it is feasible within the constraints of the DNR's resources. All operating units are responsible for implementing this policy on DNR-managed land and facilities. At co-located facilities the participating units will handle policy implementation cooperatively under the applicable facilities management agreements or arrangements."

Serious horzontial cracks and cankered area

Serious cracks and cankered area

Top tree with epicormic branch waiting to fail

Epicormic branch waiting to fail

Inrolled crack and extensive decay

Inrolled crack and extensive decay

Saturated spring soil brings toppling trees

Water-logged soil from spring rains may lead to trees toppling if they don't have healthy root systems to anchor them. Trees planted incorrectly or in narrow spaces along boulevards, sidewalks, and driveways can be especially susceptible if they have lost part of their root systems to compacted soil from construction equipment, grading, or as a result of restriction by asphalt or concrete.

Up to 40 percent of a tree's root system can be damaged before its anchoring ability is impaired, but the damage may not be obvious above-ground.

Symptoms of failing roots include crown dieback, broken, missing, or decayed roots, the presence of fungal fruiting bodies at the root collar, or leaning trees. Trees are in imminent danger of falling especially if they have developed a new lean, resulting in a soil mound on the side of the tree away from the lean. When soil becomes waterlogged, it loses its ability to hold the root system in place, and with the next good wind the leaning tree becomes a serious safety hazard.

Food section

Garlic mustard: Eat it to beat it

Garlic mustard:From Pest to Pesto, A Culinary Guide book coverSpring has sprung, and one of the first things out of the ground is garlic mustard. It's already out in the southern half of Minnesota and is clearly visible along trails and in the woods. Spring and early summer are good times to survey for garlic mustard and treat those early rosettes. It's also a good time to harvest the young leaves for your dinner table. Here are some ideas and recipes to spice up your meals and help get rid of this nasty weed.

Garlic mustard:From Pest to Pesto, A Culinary Guide This is a PDF file. You will need Adobe Acrobat Reader to download it. This link leads to an external site.