Mix of forest species protects against insects

Ray Mueller


The widespread infestation by the emerald ash borer in Wisconsin and elsewhere makes it more important than ever to have multiple tree species in forests, northeast Wisconsin woodland owners were reminded at a recent forestry field day at two adjacent properties covering 200 acres in southwest Manitowoc County.

Issuing that reminder was Bill McNee, a Department of Natural Resources forest health specialist who is stationed in Plymouth in neighboring Sheboygan County. He noted that the forest, which is owned by the nonprofit Ice Age Trail Alliance (based in Cross Plains), is fortunate to have six to seven tree species throughout much of its expanse along Rockville Road.

Bill McNee

Emerald ash borer

A major concern for the health and survival of ash trees is the spread of the emerald ash borer, which has been officially identified in dozens of Wisconsin counties and because of which quarantines on the movement of ash as firewood are in place in a majority of the state's counties. Manitowoc County has not had a confirmed EAB infestation, but it is on the quarantine list because it is adjacent to other counties with a confirmation.

Given the large number of ash trees in Manitowoc County, McNee is somewhat surprised that the county does not yet have an EAB confirmation. Mountain ash trees are not threatened by the EAB.

McNee noted that ash trees accounted for only 10 percent of the stand in the forest visited on the field day but that some forests on the flat land with heavy soils closer to Lake Michigan in the county have 90 to 95 percent ash trees in some cases.

Even without a confirmed presence of the EAB in the county, McNee urged woodland owners who have forests with an abundance of a single tree species to establish other species because infested ash trees have a 99.7 percent mortality rate. Noting the merits of stand diversity, he wishes many woodland owners would have introduced new species “yesterday.”

Detecting EAB

In many situations, woodpeckers offer the first tangible evidence of an EAB population in ash trees. He explained that the birds detect the EAB larvae under the tree's bark and remove the bark to obtain a meal, resulting in white or tan spots where they dug into the bark.

Other signs of an EAB infestation in ash trees are small leaves; a thin and pale canopy; and eventually branches that are dead — all because the S-shaped tunnels that the larvae have carved beneath the bark stop the upward flow of water within the tree, McNee said. Removing the bark to see the tunnels and seeing the D-shape hole in the bark through which a mature beetle emerged are other telltale signs.

When ash trees are infested, they will undertake natural survival tactics such as setting seeds or sending out new sprouts. He said this defense is attempted most often by the green ash variety but much less so by white ash.

All woodland owners who notice any of those signs are advised to contact the closest DNR regional office so a followup check can be conducted, McNee said. He reminded them not to confuse the emerald green EAB beetle, which has a length of one-half inch, with other forest insects, most of which are larger.

Control measures

The EAB, which probably arrived from China and which was first identified in southeast Michigan in the early 1990s, does have a natural parasite wasp that is also a native of Asia, McNee noted. Since it was introduced in Michigan in 2007, it has provided 20 to 40 percent control of eggs or larvae in some situations, he indicated.

In Wisconsin, that wasp has been placed at a few sites, McNee reported. But this is done only where there is an established EAB population and where the forest has at least 25 percent ash trees on a tract of at least 40 acres, he explained.

The best thing that woodland owners and the public can do to limit the spread of the EAB is to avoid moving firewood that is obtained from ash trees and to observe the quarantine regulations. Specific information on that is available at www.emeraldashborer.wi.gov.

Removing the canarygrass

Another threat to ash trees, which can tolerate standing water, comes from competition by Reed canarygrass at lowland sites. McNee said this is most common in forests on lowland with heavy soils in Manitowoc County and elsewhere.

The remedy for this involves destroying the canarygrass and planting fast growing species such as willows in those areas to reduce the competition.

With the DNR since 2001, McNee has also worked in the gypsy moth control program. In addition, he advises woodland owners on threats from diseases and native pests.