Statewide Forest Health

Protect oaks from oak wilt by waiting to prune

By Don Kissinger, urban forester, 715-348-5746, don.kissinger@wisconsin.gov and Paul Cigan, forest health specialist, 715-416-4920, paul.cigan@wisconsin.gov

To protect oak trees from the often-fatal oak wilt disease, don’t prune, cut or injure oak trees from April through July. 

Pruning and cutting oaks in spring and early summer leaves them vulnerable to oak wilt, which rapidly kills trees in the red oak group and weakens those in the white oak group. Any damage during this time, including broken branches caused by storms, exposes living tree tissue beneath the bark and provides an opportunity for the oak wilt fungus to infect the tree.

Sap-feeding beetles introduce the disease by carrying oak wilt spores from infected trees or firewood to fresh wounds. Healthy oaks can become infected in as little as 15 minutes after the creation of a wound. 

Sap-feeding beetle on diseased oak tree in Sawyer County.

Sap-feeding beetle on diseased oak tree in Sawyer County.

The trees most likely to die from oak wilt infection are in the red oak group, including northern pin oak, northern red oak, red oak and black oak. The white oak group is more likely to survive infection and includes bur oak, swamp white oak, white oak and English oak.

Tree paint or wound dressing is not normally recommended on pruned or wounded surfaces, but for damaged oaks an immediate light application of these products may be the only defense against oak wilt infection from April through July.

Pruning in spring can be damaging to any deciduous tree because their energy reserves are low as they produce new buds and leaves following the winter months. In general, the best time to prune is in winter when trees are dormant. 

As of January 31, oak wilt has been found in all Wisconsin counties except Ashland, Iron, Forest, Taylor, Door, Kewaunee, Calumet and Manitowoc counties. Several of these counties contain the highest abundance of healthy and productive oak forests in the state. Taking recommended precautions will help keep them that way for years to come.

Oak wilt and other diseases move easily on or in firewood logs year-round, so keeping firewood local, or purchasing Wisconsin-certified firewood, is another important component of protecting trees and keeping forests healthy. Visit the DNR firewood page for more information and a directory of certified firewood vendors.

More information, including a recently released oak wilt video, is available at the DNR oak wilt page. Additional information about proper pruning techniques is available from community foresters or through DNR resources such as this tree pruning poster

Treat your valuable ash trees against EAB

By Bill McNee, DNR forest health specialist, Oshkosh, bill.mcnee@wisconsin.gov, 920-360-0942

Spring is the best time to evaluate valuable ash trees and determine if they should be treated to protect them from emerald ash borer (EAB). Emerald ash borer is currently the most damaging threat to trees in the state, killing more than 99 percent of the ash trees it infests.

Insecticide treatments to prevent EAB infestation are usually applied between mid-April and mid-May, so it is important to start planning now. The first thing homeowners should do is check their ash trees for signs of infestation. Woodpecker damage is easy to see this time of year and is often the earliest visible sign of EAB. Photos of other signs and symptoms can be found on the DNR EAB website.

Ash trees with woodpecker "flecking" indicate EAB infestation.

Signs of EAB infestation include woodpecker damage where the birds pick away ash tree bark to feed on larvae.

Emerald ash borer has become so widespread that homeowners should consider treating valuable ash trees no matter where they are in Wisconsin. The highest risk of EAB infestation is within 15 miles of a known infestation, but it is widely believed that there are additional, undetected EAB infestations throughout the state. To see a map of known EAB infestations, visit the Wisconsin EAB website.

While the best time to treat ash trees is before they are infested, treatments of infested trees can still be successful if done while EAB populations within the tree are low or moderate. Some ash trees may be too heavily infested to save or they may have other problems that make them poor candidates for treatment.

Trunk injection treatment for EAB. Credit: Matthew Karst.

Trunk injection treatment to protect against EAB. Credit: Matthew Karst.

You should consider several factors when deciding whether to treat your ash trees. Insecticide treatments can be costly, but the investment may be worthwhile if you consider the many benefits that healthy yard trees provide, including higher property values, better air quality, shading and cooling for homes and more. Removing and replacing your ash trees is another option and may be the best choice for heavily infested and lower value trees. For trees that you decide are worth saving, however, the cost to treat may still be less than removing and replacing them with other species. This factsheet from UW-Extension can help you decide whether a tree is worth treating.

If you decide to treat, or if you want to discuss treatment options with a professional, call a certified arborist or search online and in phone books for other businesses. Check the credentials and insecticide applicator certification of any business before hiring them to treat your ash trees.

Treatments are not economically practical for ash found in woodlots. Any questions about woodlot management should be directed to a professional forester.

Despite the cold winter temperatures in late January and early February, don’t postpone treatment of ash trees. Weather data and collection of overwintering EAB larvae at two sites in Waukesha County predict high insect survival rates in most of the state. Female beetles can lay up to 200 eggs, so EAB populations will quickly rebound from any mortality that occurred due to cold weather.

More information about EAB and management options can be found through the Wisconsin EAB website.

Prepare for the return of gypsy moths

By Bill McNee, DNR forest health specialist, Oshkosh, bill.mcnee@wisconsin.gov, 920-360-0942

Gypsy moth eggs should start hatching in a few weeks as temperatures continue to warm. Homeowners should survey their trees for egg masses and either treat or remove the eggs now to help protect trees from defoliation and reduce future caterpillar populations.

Property owners should start by looking for the egg masses. They can be found on nearly any outdoor surface, including tree trunks, undersides of branches, buildings, firewood piles and other outdoor objects. They are tan-colored lumps about the size of a nickel or quarter. Each egg mass can contain 500 to 1,000 eggs that will soon hatch into hungry caterpillars. While statewide gypsy moth numbers are currently low, isolated trees and locations may have high populations.

Gypsy moth larvae hatch from egg masses on outdoor bowl.

Gypsy moth larvae hatch from egg masses on outdoor bowl.

If egg masses are found, there are two options to help reduce pest numbers. Available at many garden centers and retail stores, horticultural oils that suffocate the eggs can be directly applied to the masses. These are most effective when temperatures are above 40 degrees and a return to freezing is not imminent.  Alternatively, egg masses within reach can be scraped into a can of soapy water and left to soak for a few days before being discarded in the trash.

Insecticide treatments may be appropriate for larger trees that have many egg masses. Some types of treatment are done before eggs hatch, and some are done while the caterpillars are small. Property owners looking to hire a business to treat large yard trees this spring should contact them soon. A list of certified arborists is available on the Wisconsin Arborist Association website. Additional businesses offering insecticide treatments may be found online or in a phone book. Questions about woodlot management should be directed to a professional forester.

Additional information about management options for homeowners, including sticky barrier bands and burlap collection bands, is available at the Wisconsin gypsy moth website.

Don’t change EAB plans due to cold weather

By Bill McNee, forest health specialist, Oshkosh, bill.mcnee@wisconsin.gov, 920-360-0942

The recent frigid temperatures in late January and early February are likely to kill many overwintering emerald ash borer (EAB) larvae, but the tree-killing pest isn’t going away. The insulating properties of tree bark keep the larvae warmer than the outdoor air temperature, and wind chills do not affect the larvae because they are sheltered. The pest is also adapted to subzero temperatures based on its native range in eastern Asia where cold winters are common.

Prepupal larva seen during post-polar vortex sampling to assess larval mortality.

Overwintering prepupal larva, from February 2019 in Brookfield, WI.

On the morning of January 31, many parts of Wisconsin had low air temperatures between -30 and -35 degrees F. In places where it was this cold, scientific studies predict that most EAB larvae will be killed. Where low temperatures were between -20 and -30, larval mortality is likely to be lighter. Overwintering larvae were examined about 1 week after the cold snap in Brookfield (Waukesha County), where the low was -26, and few of the larvae were observed to have physical signs of freezing damage. Many of the larvae began moving within three days of being brought indoors. However, physical damage to the larvae may not yet be apparent, and normal-looking larvae may ultimately die from the cold temperatures.

Populations of the pest are likely to rebound, since each female beetle that emerges this summer will lay as many as 200 eggs. It is not currently known if larval mortality will significantly delay ash tree decline and mortality, although this may occur in areas that experienced the lowest temperatures. A cold spell in early 2014, that was not quite as cold, had no noticeable impacts on EAB spread or ash decline/mortality in areas known to be heavily infested.

The introduced biological controls of EAB, commonly known as “EAB wasps,” are at least as cold-tolerant as EAB. It is expected that these wasps will persist in and around release sites and continue parasitizing EAB eggs and larvae.

Forestry experts do not recommend changing EAB management plans solely due to the cold weather.

  • Continue to look for EAB in ash trees. Woodpecker damage is a good sign that an ash tree is infested with EAB or other pests.
  • Insecticide treatment of high-value ash trees near known infestations should be continued this spring.
  • Don’t delay tree removals or timber harvests that are already scheduled. Giving non-ash tree species more time to grow means that the future impacts of EAB will be reduced.
  • Continue planting non-ash tree species.
  • To help slow the spread of EAB, review firewood rules in Wisconsin. Buy firewood in the local area where you plan to burn it or buy Wisconsin-certified firewood that has been treated to eliminate pests. Certified firewood is often available at DNR-managed properties or you can buy from an independent certified firewood dealer.

Additional information about emerald ash borer, insecticide treatments and forest management can be found online at www.emeraldashborer.wi.gov.

Ice, snow and deep frost: winter creates problems for trees

By forest health specialists Linda Williams, Woodruff, Linda.Williams@wisconsin.gov, 920-360-0665; Mike Hillstrom, Fitchburg, Michael.Hillstrom@wisconsin.gov, 608-513-7690; Alex Feltmeyer, Plover, Alexandra.Feltmeyer@wisconsin.gov, 715-340-3810

Heavy ice and snow loads can cause significant problems for trees. Branch tips and whole branches may break under heavy loads, and in windy conditions entire tree tops can come crashing down.

Freezing rain on February 4, 2019 covered trees around the state with a ¼” thick layer of ice. The weather then turned bitterly cold, leaving the ice stuck to the branches, weighing them down and straining the trees. Following that event, a snow storm affecting most of the state led to more ice, snow and damage to branches. 

Tree branches covered in 1 inch of ice.

These extraordinary ice formations may look light and frosty, but they add a lot of weight to tree branches and can cause branch breakage.

Some trees are more susceptible to winter damage. Small trees can be particularly prone to injuries caused by heavy ice and snow. They can bend so far over from the weight that their tops are near or even touching the ground. In some cases, their stems may break at ground level causing the entire tree to fall over. Other vulnerable trees include those with ingrown bark at branch unions. This type of branch union is weak and more likely to split under heavy loads.

White pine saplings bent over from weight of ice on branches.

These young white pines were coated in ice, causing them to bend over. They may not rebound well after the ice and snow melt.

White pine sapling weighed down with snow, causing significant branch droop.

The heavy snow on the branches of this young white pine have caused the branches to droop and the top to bend over significantly. Heavy snow loads can cause branches to break.

While forest trees are just as prone to winter damage, yard trees present the most safety hazards and should be seen by an arborist as soon as damage becomes evident. Storm-damaged trees may have dangerous features such as hanging branches, branches under tension, spring poles and cracks in the main stem. Depending on the damage, some yard trees may also require pruning to correct their form and protect them from future injuries. Due to the dangers associated with storm-damaged trees, consider having a professional arborist or logger do your tree pruning or removal.

Tree roots can be damaged in cold weather too. In areas of southern Wisconsin, a lack of snow during some of this winter’s cold snaps has forced the frost deep into the ground. Some of you may recall the winter of 2002-2003 when we had very little snow and frost went 5’ deep in many parts of the state. Not only did it freeze septic systems and water lines that winter, but it also damaged tree root systems by killing some of the fine roots and root tips that grow deep below the surface and prefer not to freeze. Trees that lost a significant number of roots experienced crown dieback and stress that led to insects and diseases attacks, including two-lined chestnut borer on oak and Armillaria root rot on conifers. Deep freeze damage to tree roots is not often the single cause of tree stress or death but is more often one of several stress agents that leave trees more susceptible to insects and diseases. Fortunately, the deep frost can be pulled out of the ground, and some of the damages avoided, if there is enough snow cover for the remainder of the winter.

For more information on how snow and ice can impact trees, check out these resources from Wisconsin [PDF] and Minnesota. Contact a certified arborist or DNR forester if you want to discuss management options for storm-damaged trees.

Revised emerald ash borer guidelines now available

By Bill McNee, forest health specialist, Oshkosh, Bill.McNee@wisconsin.gov, 920-360-0942.

The Wisconsin Department of Natural Resources has released updated guidelines to aid woodland owners and managers in managing ash stands in light of the invasive emerald ash borer (EAB). The guidelines that became effective on January 1, 2019 reflect new research, increased pest detections and the anticipated transition to a statewide EAB quarantine (which occurred in March 2018).

Forestry professional and landowner standing in hardwood stand assessing conditions.

Landowners should work with professional foresters to make management decisions related to EAB on their land.

The silvicultural guidelines are intended to help make informed stand-level decisions regarding management of stands that are not yet infested by EAB and stands that are already impacted by the tree-killing beetle. They should be used along with other materials, including best management practices and other guidance documents, to develop appropriate management plans. 

The revised guidelines include recommendations to:

  • Implement management plans as soon as practical, across all of Wisconsin, to reduce a stand’s ash component to no more than 20%. More management options are likely to be available by taking immediate action
  • Review existing management plans to determine if they need revision due to changes in EAB distribution, stand condition, market prices, etc.
  • Diversify woodlands with site-appropriate tree species
  • Utilize merchantable ash before trees are infested or killed. It may be appropriate to retain some live ash on site for ecological benefits, species diversity, wildlife habitat, temporary cover or seed production

The guidelines also place a greater emphasis on assessing stand and site conditions before making management decisions. The document offers stand management alternatives for both upland and lowland stands, while recognizing that conversion of lowland ash stands to other forest types will be impractical in many cases.

If you would like more information about emerald ash borer or these guidelines, visit the DNR EAB webpage or talk to your regional forest health specialist.

Revised Heterobasidion root disease guidelines now available

The Wisconsin Department of Natural Resources has released updated guidelines to reduce the risk of introduction and spread of Heterobasidion root disease (HRD) in Wisconsin through preventive stump treatments. The revised guidelines became effective on January 1, 2019, and a professionally designed version will be available soon. Woodland owners are strongly encouraged to work with a professional forester to make management decisions about HRD and preventive stump treatments.

Pocket of HRD-caused thinning and mortality in red pine plantation, Grant County.

Pocket of HRD-caused thinning and mortality in red pine plantation, Grant County.

Heterobasidion root disease is one of the most destructive diseases affecting conifers in the Northern Hemisphere. In Wisconsin, HRD is most commonly found in pine and spruce plantations. Infection by the wood-decaying Heterobasidion irregulare fungus kills living tissues and leads to growth loss and tree mortality. Spores landing on a fresh cut pine or spruce stump will infect that stump and root system and spread via root contact to neighboring trees, killing them as it invades their root systems. This pattern of spread creates pockets of dead and dying trees that expand outward. Growth reduction of trees leads to economic losses for plantation owners, and mortality of trees, including seedlings and saplings, has long-term implications for future stand composition and management.

The HRD stump treatment guidelines are designed to help make decisions about the use of preventive stump treatments when harvesting a stand. These treatments limit disease introduction by preventing fungal spores from developing on the exposed surfaces of newly cut stumps.

Cutting bar on logging equipment spraying blue chemical used in HRD stump treatments.

Pesticides used in stump treatments can be applied using logging equipment (blue mist exiting through nozzles in saw bar) or backpack sprayer.

Stump treatments are typically recommended between April 1 and November 30 if a stand is within 25 miles of a known HRD infection site and the stand is more than 50% pine and/or spruce. However, there are other factors to consider when deciding whether to use the preventive stump treatments. Certain situations when you may not need to treat stumps, referred to as “Exceptions” and “Modifications” in the guidelines, consider variables such as economic feasibility, landowner risk tolerance, unexpected weather patterns and future desired stand composition. Highlights of the revised stump treatment guidelines include:

  • Timing of treatments and general distance recommendations did not change from older guidance
  • Addition of spruce as a species recommended for stump treatment
  • Additional Exceptions and Modifications where treatment may not be necessary
  • For private landowners with a higher risk tolerance, a 6-mile radius from known infection sites can be considered when evaluating whether to apply treatments
  • Even if a stand is considered at low risk for HRD infection, woodland owners should carefully consider the potential impacts to their stand if preventive treatments are not used and HRD becomes established

Along with the guidelines, a web-based HRD map viewer has been launched. This interactive map displays confirmed HRD locations and 25-mile and 6-mile radius buffers around HRD locations. The purpose of this map is to help users determine whether a stand is within either the 25 or 6-mile buffers where stump treatments are recommended. You can enter an address or GPS location, turn map layers on and off, and zoom in and out to an area of interest.

If you would like more information about HRD or these guidelines, visit the DNR HRD webpage or talk to your regional forest health specialist.

2018 Forest Health Annual Report now available

The Forest Health Annual Report summarizes notable impacts pests, diseases and weather on the health of Wisconsin’s forests. The report is a collaborative product written by DNR forest health specialists from around the state. It outlines the damage and spread of both native and invasive pests and diseases during that year and puts these into context of observations from previous years. Management programs and their results are also described. Highlights from the 2018 annual report include:

  • American beech with resistance to disease-causing scale insect entered into a breeding program for species conservation and reintroduction
  • Confirmation of jumping worms in northern counties
  • Biological control of emerald ash borer
  • Closure of the state gypsy moth suppression program
  • Innovation in survey of the invasive plant lesser celandine
  • Dieback and mortality in balsam fir caused by freezing weather in April followed by unseasonably hot temperatures in early May

The 2018 report and others dating back to 2003 are available on the forest health publications page. Historical reports dating back to 1951 are archived and available upon request. Contact Marguerite Rapp at marguerite.rapp@wisconsin.gov for more information on the archived reports.

Dispose of wreaths properly to avoid spreading invasive insect

image of elongate hemlock scale The small brown blotches on the underside of these needles are an invasive species found on some holiday decorations purchased from chain stores in 2018. Help prevent the spread of elongate hemlock scale by properly disposing of wreaths, swags and other potentially infested materials. (Photo credit: WI Dept. of Ag, Trade & Consumer Protection)

Please remember to properly dispose of wreaths, trees and other holiday decorations from chain stores that may be infested with an invasive insect.

If you purchased any holiday wreaths, swags, boughs and other arrangements from chain stores, please dispose of them by burning or bagging them and putting them in the trash as they may be infested with an aggressive invasive insect that can harm Wisconsin’s native forests, Christmas tree farms, and even ornamental conifers in your yard.

During this recent holiday season, Department of Agriculture, Trade and Consumer Protection (DATCP) inspectors found an invasive insect pest from Asia called elongate hemlock scale (EHS) on holiday wreaths, swags and boughs, and in arrangements of evergreen boughs in hanging baskets, porch pots, mugs and sleighs.

These items came from suppliers in states where EHS is already established. This insect poses a risk to Christmas tree fields as well as native and ornamental coniferous trees in Wisconsin. To prevent the introduction of EHS to Wisconsin, DATCP officials are asking those who purchased the listed decorative items from chain stores in 2018 to properly dispose of them.

Trapping project found no non-native beetles

By Mike Hillstrom, forest health specialist, Fitchburg. Michael.Hillstrom@wisconsin.gov; 608-513-7690

The Wisconsin DNR’s forest health team received funding to trap for non-native beetles in 2018. This project was funded by the US Forest Service through an Early Detection and Rapid Response (EDRR) grant. The project is complete, and we happily share that we did not find any non-native beetles!

Forest health staff member Scott Schumacher is hanging a Lindgren funnel trap (12 funnels stacked to look like a tree trunk) from a tree branch to survey for non-native beetles.

Forest health staff member Scott Schumacher hangs a Lindgren funnel trap to survey for non-native beetles.

The Forest Service periodically provides funding to states to trap for non-native bark and ambrosia beetles. The goal of trapping is to detect, delimit and monitor newly introduced exotic beetles and to quickly assess and respond to newly detected infestations.

We placed traps at 12 high-risk sites in Jefferson, Waukesha, Milwaukee, Racine, Manitowoc, and Brown counties. Sites were selected in collaboration with the Wisconsin Department of Agriculture, Trade and Consumer Protection and were based on proximity to large commercial port entries on Lake Michigan or recycling facilities for pallets and other waste packaging. Three Lindgren funnel traps (12 funnels stacked to look like a tree trunk) and lures were assembled at each site and checked every two weeks between early May and early August. Specimens collected from the traps were sent to a Forest Service taxonomist for identification.