Forest health


Forest Insect and Disease Newsletter August 2015

Brain Schwingle, Forest Health Specialist, Central Region


White-spotted Sawyers, Oh My!

White-spotted Sawyer

A white-spotted sawyer adult. Note the white spot between its "shoulder blades." (They technically aren't called shoulder blades, but you see what I mean.) The location of this spot separates the white-spotted sawyer from the dreaded Asian longhorned beetle, not yet found in Minnesota.

(photo by Whitney Cranshaw, CO State University, Bugwood.org)

A plague not by biblical standards, but perhaps by longhorned beetle standards, is happening in far northwestern Minnesota and adjacent areas in Ontario. The white-spotted sawyer—that one-inch-long, mostly black beetle with antennae longer than its body—is really irritating people in those areas. They are busy landing on everything. They are also laying their eggs on dying and freshly cut conifers. There are other longhorned beetles that may be out and about, but from pictures and descriptions, the primary species being reported is the white-spotted sawyer.

The larvae of the white-spotted sawyer are roundheaded borers that feed on dying and dead conifer wood. They go from egg to adult in 1 or 2 years, so something in northwestern Minnesota likely happened in 2013 to make a bunch of dead conifers. Eastern larch beetle is certainly helping add to the dead tamarack total up there and could be aiding the longhorned beetle population. Our Canadian friends put the blame on the branch-busting snows they received in April 2013. We also had ample snows that damaged conifers in April 2013.

The adult white-spotted sawyers do some feeding on branches, but most trees will be able to withstand this minor irritation. The adults also can bite you if they land on you: again, a minor irritation relative to other human maladies. People should expect to hear the larvae chewing in newly-killed and dying conifers in 2016. Other than freshly-cut coniferous logs and dying trees, these beetles are not a concern for the health of our forests.

 

Jack Pine Defoliation from Brainerd Northwards

2014 Gypsy moth survey results
Click to enlarge.

Feeding damage from jack pine budworm became apparent to the masses from Brainerd to Park Rapids in early July. It might have even solicited an, "Oh my!" from a few citizens. Our aerial damage surveys probably underestimated the amount of impacted acres due to an early flight in that area, but our preliminary data still show about 4500 acres defoliated by jack pine budworm. This is a 1300 percent increase over 2014 data, and suggests the beginning of a 3-4 year outbreak in the west-central counties.

Foresters should expect noticeable defoliation for a couple of years for a given stand. Mortality during and after a jack pine budworm outbreak is variable, but in general, a significant number of co-dominant and dominant trees will start to die after a couple of years of noticeable defoliation. If a stand is near rotation age and has undergone defoliation, it would be wise to regenerate that stand to minimize losses.

White-spotted Sawyer

Jack pines looked orange in early July after jack pine budworm feeding, west and northwest of Brainerd.

As for unmanaged older jack pine, those trees will undergo significant damage from jack pine budworm. At some point, they'll die from excessive needle feeding or because they'll be attacked by bark beetles. Ornamental jack pines can be protected from defoliation with an approved insecticide. It's too late to use any insecticide in 2015: the damage has been done and the budworms are gone until next year. For people interested in protecting their ornamental jack pines, we recommend using the biological insecticide Bacillus thuringiensis variety kurstaki (Btk) shortly after shoots start to elongate in 2016 (i.e. when needles are ½ - ¾ inch long). Full-canopy insecticide coverage is crucial since budworms prefer feeding in the tops of trees.

 

Spruce Budworm Defoliation Continues in the Northeast

2014 Gypsy moth survey results
Click to enlarge.

According to our preliminary survey data, there was an uptick in acres defoliated by spruce budworm this year in Lake and St. Louis counties. The general area currently being impacted is in its second year of defoliation (see map). If historical patterns remain true, noticeable spruce budworm defoliation will occur on fir and spruce in this area until sometime between 2022 and 2026. Balsam fir will tolerate 4-6 years of defoliation, so foresters should consider establishing timber sales in impacted fir and spruce stands in this area.

For ornamental spruce and fir trees, there are insecticides labeled for use against spruce budworms. Everything I wrote about insecticides and jack pine budworm in this issue generally applies to spruce budworm. However, since fir and spruce can tolerate more defoliation than jack pine, you could start applying insecticide after the second or third year of spruce budworm defoliation to protect ornamental trees.


Oak Problems

First Confirmation of Oak Wilt in Morrison County

Oak wilt was confirmed for the first time in Morrison County in July 2015. It has been there for a few years already. The forest health team's aerial survey detected the mortality center near Little Falls. The closest known oak wilt infection is 19 miles southeast in Benton County. This disease jump illustrates the difficulty of detecting non-native tree diseases when they are not abundant. It also suggests that forest managers and ornamental oak owners in the Brainerd area should be looking for oak wilt and taking preventative precautions. An updated map of oak wilt is posted on the DNR's oak wilt homepage. If you are sick of bad news from the DNR's Forest Health team, think of this news positively: the earlier we know about oak wilt, the better. We can slow the growth curve of oak wilt simply by not wounding oaks from April through July. Several analyses in Minnesota showed that not wounding oaks from April through July significantly reduced new oak wilt infection centers, even when taking into account naturally-occurring infections.

 

Unique and Widespread White Oak Sickness

White-spotted Sawyer

White oak twig dieback in May 2015 caused by anthracnose infection from 2014. (photo credit Jill Pokorny, US Forest Service)

In southern Minnesota you may have noticed white oaks with a lot of dead leaves in their canopies. According to Jill Pokorny, USDA Forest Service Plant Pathologist, this is due at least in part to leaf anthracnose from 2014. Those previous-year leaf and petiole infections have progressed to form cankers in some twigs. Pokorny says that dead leaves at the tips of white oak branches could be from the fungus Botryosphaeria, Botryosphaeria with anthracnose, or just anthracnose. One or two years of such twig death won't be a problem for larger white oaks. We will continue to monitor this disease over the next several years.

 

Bur Oak Blight: the New Norm?

White-spotted Sawyer

Bur oak blight symptoms are dead sections of leaf veins, dead wedge-shaped sections of leaves, shed leaves, and retained dead leaves with swollen petiole bases.

Bur oak blight is severely defoliating scattered bur oaks around the state. A heavily infected bur oak can lose a large percentage of its leaves due to this fungal disease, making it difficult to differentiate it from oak wilt. One key to identifying bur oak blight is the very outer leaves in the canopy will be the healthiest, in great contrast to oak wilt, where the very outer leaves of some or all branches are sick first.

Bur oak blight is a native disease that has become more prominent over the last decade. Its presence is associated with consecutive, overly wet springs. Though early spring 2015 was relatively dry, the amount of infected dead leaves still in the canopy in 2015 coupled with soaking rains in May and July (in many parts of the state) kicked the disease into high gear.

Since bur oak blight is a late-season defoliator, trees can tolerate years of heavy leaf loss. At some point, though, a stressed bur oak will become even more stressed from bur oak blight and will become susceptible to twolined chestnut borer and/or Armillaria root disease. For ornamental trees, appropriate watering during drought is one way to delay susceptibility to these problems. Luckily, not all bur oaks are highly susceptible to bur oak blight.

 

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