Forest Insect and Disease Newsletter April 2016

Brain Schwingle, DNR Forest Health Specialist

Tamarack Regeneration with Eastern Larch Beetle Present: A Case Study

Click to enlarge.

2014 Gypsy moth survey results

Red areas show where eastern larch beetle was actively killing tamarack in 2015.

How to appropriately time harvests in the face of eastern larch beetle infestation and what regeneration technique is most effective are challenging questions for Minnesota foresters. An outbreak of this native bark beetle has been going on in Minnesota since 2000. Recently, my Wisconsin DNR counterpart described the status of an infested seed-tree regeneration harvest in a tamarack/white cedar stand in Wisconsin. The Wisconsin forester and tree health specialist shared the following situation with me:

  • Pre-harvest: A tamarack stand with a tamarack site index of 50 was heavily infested with eastern larch beetle. The stand also had a component of northern white cedar.
  • Harvest: A seed-tree, logger-select harvest took place in February 2015, leaving eight to ten tamaracks and cedars per acre.
  • Harvest volume: About 23 tamarack cords/acre and seven white cedar cords/acre were harvested.
  • One year post-harvest: As of March 2016, 80-85 percent of the seed trees remained alive and apparently un-infested by eastern larch beetle; other scattered seed trees were infested.

According to the USDA's Silvics of North America, tamarack produces a good seed crop every three to six years, so if many seed trees can survive until September 2017, chances are good this regeneration technique will succeed at this site.


Updated Pine Thinning Recommendations to Minimize Bark Beetle Problems

In October 2015, the DNR Forest Health and Timber Programs worked together to update the timber sale specification that aims to minimize bark beetle problems in pines on state lands.

White-spotted Sawyer
Red pine firewood showing bluestain in the sapwood.

This guidance was updated with Minnesota's climate, logging operation limitations, and bark beetle biology in mind, so it is broad and subject to change. We recommend all forest managers utilize the updated pine thinning specifications to minimize bark beetle problems. The following specifications refer to cut material over three inches in diameter:

  • December 1–May 31: pine cut in this period should be hauled or destroyed by June 1.
  • June 1–August 31: pine cut in this period should be hauled or destroyed within three weeks of cutting.
  • August 31–November 30: there are no special restrictions for pine cut in this period.

The June 1 deadline is to prevent the first generation of pine engravers (Ips pini) from laying eggs in residual pines that were left from a late-winter or early-spring thinning or blowdown. The three-week window of removal between June 1 and August 31 not only protects residual pines, but it helps loggers' revenue by minimizing bluestain development in pine logs. Bluestain is introduced to pines by pine engravers, and could develop quickly—within three weeks in the heat of summer. The three-inch diameter guideline deals with maximizing removal of pine logs that are merchantable and could contribute to a localized pine engraver outbreak. The larger the diameter, the more food available to contribute to the local pine engraver population, so as pine material increases in size, it becomes increasingly important to adhere to the above specifications.

An additional consideration for smaller private plantations, which have more flexibility with loggers, is not to thin within a year of a severe drought, such as the drought of 1988, for example.

Minnesota DNR foresters should use the updated specification found on the Timber Program's Appraisal Guidance tab on the DNR Intranet. Please do not use the old specification, which is still in the Timber Sale Module system.

Sooty Mold on Sugar Maple Bark

White-spotted Sawyer
The black bark below the tapholes on these sugar maples is from sooty mold.

Ever wonder why some sugar maples have areas of black bark? It's sooty mold, as beautifully shown in the photo, and is not a concern for the health of the tree. Sap from the tap holes runs down the trunk, and sooty mold grows on it. If you see black bark on sugar maples, it means there is or was some sort of injury: tap holes, sapsucker feeding holes, wind damage, frost cracks, squirrel feeding, and pruning cuts are common sources of sap on sugar maple trunks.

Sawflies on Spruce

White-spotted Sawyer
Young larvae of yellowheaded spruce sawflies in early June.

Recently I received four reports of yellowheaded spruce sawfly damage in 2015 on spruce from the Virginia (Minnesota) and Hinckley areas. It's common to find this pest in a given area in any given year, but this sawfly species has had years where populations in Minnesota were large.

Yellowheaded spruce sawflies can seriously damage spruce seedlings and saplings, so I recommend controlling them if they caused problems the previous year. On ornamental spruce in your yard, spray young larvae less than 1/3-inch long off trees with a strong jet of water (and laugh evilly as you're doing so). Do this daily to keep them off the trees, starving them. In young plantations, spray young sawfly larvae with an insecticidal soap or horticultural oil if they are common. Young larvae are difficult to see (see image above), so begin scouting for them from late May to early June.

Sawflies on White Pine

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Introduced pine sawfly severely defoliated these white pines in Morrison County.

I recently inspected a 25-acre white pine plantation in northern Morrison County that was partially defoliated in 2015. Many of the white pines throughout the plantation were severely defoliated, as you can see in the image. The culprit was the introduced pine sawfly. Its pupal cases were commonly found on branches, along with some dried-up larval carcasses.

Thanks to Forest Service entomologist Steve Katovich for helping me confirm the identity of this sawfly.

Populations of introduced pine sawfly occasionally erupt and pester various pine species in Minnesota. Morrison County is no stranger to this sawfly. Usually no chemical control is necessary to sustain the life of pines, but this sawfly will kill pines at times.

If you are managing a severely-infested white pine plantation, contact your forester or regional forest health specialist. For ornamentals, my recommendation is similar to that in the previous article about yellowheaded spruce sawflies: knock them off needles with a strong jet of water (laughing evilly), or spray larvae when they're less than 1/3-inch long with an insecticidal soap or horticultural oil. Scout for them starting in late May and throughout June.

Oak Wilt Frequency on Bur and White Oak

White-spotted Sawyer
Red oak leaves with symptoms of oak wilt. photo by M. Grabowski

If the insects that vector oak wilt aren't flying around carrying oak wilt spores yet, they will be soon. A lot of information in cyberspace and written in hardcopy claims that bur oaks and white oaks aren't as susceptible to oak wilt as red oaks, but this is misleading. For bur oaks, it is highly misleading.

Bur oaks get oak wilt relatively frequently, and whole groups of bur oaks can be killed. For example, last year I inspected 10 clustered, large bur oaks that had all been killed by oak wilt. Oak wilt also infects white oaks and contributes to their death.

For the last two years, the University of Minnesota's Plant Disease Clinic has confirmed oak wilt in 49 percent (38 positive) of bur oak samples, 23 percent (5 positive) of white oak samples, 78 percent (45 positive) of samples in the red oak group, and 33 percent (2 positive) of samples in the unspecified white oak group.

It's possible to have false negative test results from a submitted sample. This is due mostly to sampling at the wrong time or from the incorrect location on the tree. The oak wilt pathogen is not present in all roots or all parts of the trunk in an infected oak, so for very tall oaks with no lower, reachable branches, I recommend waiting to sample the lower trunk until September, taking small wedges out of different sides of the trunk to have the best chance of a correct diagnosis.