Forest Insect and Disease Newsletter - Spring 2017

Diplodia observational study underway

By DNR Forest Health Unit staff

Diplodia in young spruce seedlings

The forest health unit investigated a resurgence of latent Diplodia infections in three-year-old red pine seedlings (3–0) at the Badoura State Forest Nursery in 2016. A sample of laboratory-tested seedlings revealed rates of latent infection higher than the nursery's current threshold of 10 percent, resulting in the decision to cull a half million bare-root red pine seedlings.

Latent infection means that although the Diplodia pathogen is present, it is not actively causing disease symptoms and seedlings appear healthy. A red pine seedling with a latent Diplodia infection will not always die. If a seedling undergoes severe drought during or shortly after planting however, a latent infection can show up as Diplodia collar rot and kill the tree. This was well-documented in neighboring Wisconsin during the 1990s, when drought was followed by several plantation failures. What was not documented during the drought in Wisconsin was the percentage of Diplodia-free red pines that died from drought alone or the percentage of seedlings with latent infections that still survived the drought.

The forest health unit is interested in understanding how latent Diplodia infection translates to actual mortality during the first five years after planting. In late April, we established a 2.5 acre observational plot at General Andrews tree-improvement nursery, and seven DNR staff helped plant more than 600 bare-root seedlings from Badoura to study long-term survival. We hope this sample of the culled 3-0 seedlings will provide valuable information about what would have happened had any infected but currently healthy-looking seedlings been used for reforestation. As a control, we also planted more than 600 containerized seedlings donated by PRT, a seedling vendor from Canada. A sample of 200 of the containerized PRT seedlings was submitted to the University of Minnesota Plant Disease Clinic to be tested for latent Diplodia infection. We will learn lab results later this summer.

We plan to monitor the planted seedlings over the next five years. We plan to monitor the planted seedlings over the next five years. Whenever a seedling dies, it will be sent to the Plant Disease Clinic to determine the cause of death so we know how many succumb to latent?Diplodia?infection over time. Because we already know the baseline infection level (15 percent) for the field from which the test seedlings came, we should be able to see how numbers compare to observed mortality rates in the observational plot.

Using this information, we hope to have a better understanding of how latent Diplodia infection affects seedling mortality during the first five years after transplanting and be able to evaluate the 10 percent threshold infection level based on local Minnesota data. Although we will still have to deal with issues like improper seedling handling at planting time and inevitable heavy losses to deer browse, we should have a better understanding of at least one more piece of the puzzle that dictates planting and reforestation success.

The forest health unit would like to thank Badoura State Forest Nursery for lifting and donating seedlings, General Andrews Nursery for providing use and prep of nursery grounds, and PRT Ontario for donating containerized seedlings. Additionally, we thank Andrew Arends, Doug Hecker, and Dave Schuller who helped plant the seedlings. Through these generous contributions, this project should be relatively cost-free.

Gypsy moth treatments scheduled this spring

By Jess Hartshorn and Brian Schwingle, DNR regional forest health specialists

Female gyspy moth
Female gyspy moth. Photo by:Minnesota Department of Agriculture

The Minnesota Department of Agriculture (MDA) is scheduled to treat isolated gypsy moth populations in three locations in Minnesota this spring: in the metro city of Richfield, near Hinckley in Pine County, and in eastern Winona County. The treatments, applied aerially, are conducted when relatively high concentrations of gypsy moths are discovered through the use of detection traps. Information about the timing and location of scheduled treatment sites can be found at MDA's website on gypsy moth treatments.

Gyspy moth caterpillar. Photo by: Minnesota Department of Agriculture
Gyspy moth caterpillar. Photo by: Minnesota Department of Agriculture

TTwo of the sites will be treated with the naturally-occurring soil bacterium Bacillus thuringiensis var. kurstaki (Btk). Btk is a biological insecticide used against common caterpillar pests such as forest test caterpillar, spruce budworm, and gypsy moth. When eaten by caterpillars, Btk produces a toxin that is lethal to the caterpillar, but not to humans, birds, bees, fish, or Fido. Btk breaks down very quickly in the environment, and is preferred because of its high degree of host selectivity and environmental safety.

The Winona County site will be treated in June using gypsy moth mating disruption, which prevents male moths from finding mates. This is accomplished with an aerial application of female gypsy moth pheromone, a scent produced by the females to attract males. Because of the abundance of pheromone in the air, the males are unable to find female moths, and die without mating.
See MDA's gypsy moth website for more details and general information about gypsy moth.

Budworms in northern Minnesota

By Mike Parisio and Jess Hartshorn, DNR regional forest health specialists

Budworms web on spruce
Photo by: Joseph O'Brien, USDA Forest Service,

Spruce budworm has been steadily moving across northern Minnesota for decades, defoliating and killing balsam fir and white spruce. More than 130,000 acres were effected in the Arrowhead region in 2016, a 24 percent increase compared to 2015, and we expect affected acreage to be steady or increase in 2017. You will notice defoliation in mid- to late-June. Areas with multiple years of severe defoliation will see dieback and mortality, first of balsam fir and then of white spruce. Control in forests during outbreak periods is not feasible, and outbreaks must run their course. One forest management recommendation is to regenerate affected forests prior to most mature trees dying. In yards, spruce and fir that have lost 75 percent or more of their needles will not be able to recover. They should be replaced before they die and become hazardous. For more information, see our 2016 annual report .

Photo by: Connecticut Agricultural Experiment Station,

On the jack pine budworm (JPBW) front, we should be witnessing the wrap-up of an outbreak spanning the last couple of years from northern Morrison County to the Bemidji area. That being said, we will still have to keep an eye out for any lingering pockets of higher populations and monitor jack pine stands for any post-outbreak top-kill or mortality that will occur this upcoming season. Depending on what we see, there may or may not be some opportunities for salvage-type harvest operations.

We'll also be shifting our monitoring efforts northward, as the northwest counties can expect to see an increase in JPBW activity soon. Our aerial surveyors will be on the lookout, but those working on the ground in the Beltrami Island State Forest should pay close attention as well. Abundant jack pine stands in this area will provide good habitat and a substantial food source for JPBW during the early stages of outbreak. Stands over 40 years old with undesirable stocking (basal area less than 70 ft² or greater than 110 ft²) are of particular concern, as these are the stands that reliably get hit the hardest.

Forest tent caterpillar prediction for 2017

By Jess Hartshorn, DNR northeast region forest health specialist

Forest tent caterpillar
Figure 1. Cluster of forest tent caterpillars on quaking aspen near Grand Rapids

While acres affected by forest tent caterpillar (FTC) declined in 2016, defoliation was likely underestimated due to weather conditions that postponed surveys. An early-season defoliator, FTC is typically finished feeding in June. Many trees are able to produce a second flush of leaves by early July, making it difficult to accurately assess defoliation during late-summer flights. For example, homeowners in Crow Wing and Aitkin counties reported patchy areas of significant defoliation that were not seen during aerial surveys.

Because 2016 flights did not adequately detect all FTC defoliation, it's difficult to predict activity for 2017. Egg mass surveys in two north-central Wisconsin counties (Vilas and Oneida) suggest low FTC populations with isolated pockets of damage (WI DNR Newsletter ). Defoliation was spotty across northern Minnesota and those spots that saw significant defoliation in 2016 are likely to see some level of defoliation in 2017. Two areas in particular near Tower in northern St. Louis County have had severe defoliation for several years, and these stands will likely start to see branch dieback if they haven't already.

Larvae recently began feeding and will continue into June. Larvae cluster on the trunks of aspens, oaks, basswood, and other hardwood hosts (Figure 1). If you find these clusters you can destroy them by knocking them into a bucket of soapy water or spraying the leaves with a non-harmful insecticide like Bacillus thuringiensis var. kurstaki (Btk).

2016 forest health aerial survey data available at Minnesota Geospatial Commons

Every year the Minnesota DNR forest health team aerially surveys a large percentage of our forests. The survey data from 2016 is now available at the Minnesota Geospatial Commons website (search "forest health"). Natural resource managers, forest owners, and forest ecology researchers will find the data most useful.
The purpose of these aerial surveys is to indicate where significant and highly noticeable tree defoliation and mortality was occurring at the time of the flight. Other objectives of the surveys are to detect rare tree threats (e.g., oak wilt in northern Minnesota) as early as possible and to document long-term health condition changes in forest canopy.