Forest Insect and Disease Newsletter - September 2020

Fall webworm made many webs this summer

By Brian Schwingle, Central Region forest health specialist

close-up view of a large web covering a tree branch with caterpillars insideFall webworms close-up in a web. number of web covering a tree
Webs in a cherry tree in southeast Minnesota made by fall webworm.

If you observed a disheveled web the size of a bowling ball or larger in your tree's outer canopy late this summer, it almost certainly was spun by the fall webworm. Fall webworm seems to be a routine annual web-maker in walnuts in southeast Minnesota. Their population appears to have increased in the last two years, at least in the Twin Cities.

Besides walnuts in the woods, fall webworm likes to make webs in cherries and alders. In urban areas, we've also noted it in ash, lindens, birches, and crabapples. I took a camping trip to the Pacific Northwest this year and saw fall webworm nests all the way to the ocean!

This pest is not a problem for tree health, because in most trees, it only removes a tiny portion of leaves. It seems to hit cherries the hardest, but even in the worst cases, it eats leaves in late summer, which isn't as damaging to a tree as when they lose their leaves in spring. In most cases, the webbed branches will grow new leaves next spring.

In the woods, just ignore these pests. In your yard, if you don't like the webs, just take a rake, pull the nests down, and squash the caterpillars underfoot. If you don't like to squash soft-bodied insects, just toss in the compost or leave on the ground for a bird to feast on.

Many types of caterpillars make webs in trees, but the timing of web-making and the shape of the web allow for easy identification. Of course, the characteristics of the caterpillars are unique, too. Webs you may see formed by other caterpillars in mid- or late summer: ugly nest caterpillar, cherry scallop shell moth, oak webworm, striped oak webworm. Striped oak webworm was only abundant this year in Chisago County.

Oaks on the decline from variability in growing season precipitation

By Rachael Nicoll and Brian Schwingle, Regional forest health specialists

2020 has been a tough year for all of us, including our oak trees. DNR foresters and the forest health team have fielded many calls this summer from concerned landowners in northwestern, central, and southern Minnesota reporting dieback and mortality of their oaks.

This is not new, as we have been investigating areas experiencing abundant mortality or dieback of bur and red oak for several years. This year, some of the reported dieback has happened rapidly. On most sites, though, the decline in the health of these trees started before 2020. We refer to all of this death as "decline."

Declining bur oak in Kandiyohi County from soil compaction, drought, excess rainfall, and old age.

What is oak decline, exactly? We use the phrase to describe progressive dieback and eventual death of oak trees from multiple factors over several years. Decline occurs when multiple factors such as severe drought, excess rainfall, soil compaction, tree age, and pest and disease issues interact to damage trees. Older trees growing on dry sites with shallow soils or south-facing slopes, or wet sites such as edges of wetlands or depressions with poor drainage, are more easily weakened by drought, flooding, or defoliators. They also become more susceptible to pests and diseases. Twolined chestnut borer and Armillaria root disease usually deal the final blow to these stressed oaks, but the preceding year(s) of damage set the stage.

To illustrate, we documented large areas of bur and red oak death in central and south-central Minnesota starting around 2015. Much of this was due to the following:

  1. drought late in the 2011 growing season, one of the top 10 driest on record for central and south-central Minnesota,
  2. flooding early in the 2012 growing season, one of the top 10 wettest on record for east-central and central Minnesota,
  3. drought late in the 2012 growing season, one of the top 10 driest on record for east-central, central, north-central, and south-central Minnesota, and
  4. Armillaria and twolined chestnut borer attacks on extremely stressed oaks.
Tall Red oaks with bare canopies.Harvest after severe flooding and drought in 2012 created additional stress, causing decline in this northern red oak stand in northern Pine County.

Drastic swings in precipitation damage roots, which can kill an oak outright or make it susceptible to diseases and pests over the following decade while it attempts to regrow its roots. Determining the cause of death in landscape-level decline is difficult, since there are several factors involved and symptoms don't appear immediately. Oak mortality in 2020 in north-central Minnesota is likely a result of the extremely dry growing season in early 2017, the seventh driest on record. In contrast, mortality in 2020 in south-central Minnesota is partly due to extremely wet growing seasons since 2016, three of the wettest six on record from May through October.

Decline usually first appears as stunted leaves or dieback in the outer canopy. Epicormic sprouts grow out of the trunk or upper limbs. Decline is often slow, and the dieback or epicormics often go unnoticed. When Armillaria starts causing root disease or twolined chestnut borer infests the tree, death can happen quickly, as we have seen in some areas this summer.

Unfortunately, the damage is difficult to reverse once decline has progressed. Further, decline is difficult to prevent as extreme weather events are unpredictable and vary by location. If you see declining oaks in your yard, the best course of action is to water them only during droughts and properly mulch them. Otherwise, leave them alone: avoid watering, fertilizing, compacting the soil, and applying herbicides under their canopies.

In forested settings, avoid harvesting for several years after a drought. During the growing season on wetter sites, operate heavy equipment only during frozen ground conditions to avoid stressing healthy trees, and remove oaks with dieback and epicormic sprouts during thinning operations.

White pine cone beetle busy this year

By Eric Otto, Northeast Region forest health specialist

single white pine cone showing showing holesEvidence of infestation on a white pine cone.

The white pine cone beetle, Conophthorus coniperda, has been noticeable this year, infesting a number of white pine cones. There is currently a heavy white pine cone crop, providing the beetle with an ample food source. Damage was noted in Itasca County, but it is likely present in additional counties.

This native beetle occurs throughout the range of eastern white pine. Are you noticing more cones than usual on the ground near white pine trees? There is a good chance it is the result of the beetle's damage. Small and large cones found on the ground can be brown, dry, hardened, and shriveled.

The female beetle bores into a cone at its junction with a twig in early spring to late summer, girdling the connective tissue and killing the cone. The cone eventually falls to the ground with an accumulation of pitch where the initial attack took place at its base. The female beetle creates an egg gallery within the cone and lays eggs along the sides of the gallery. The eggs hatch and larvae feed on the cone tissue. The larvae pupate and emerge as adult beetles and remain in the cone as they overwinter.

The white pine cone beetle can be a problematic pest to seed production in natural stands and seed orchards. However, if seed production is not a priority, infestation from the beetle is of small concern. If managing for seed production, reduce the beetle population by collecting cones and burning them, either during the fall or before mid-April beetle emergence.

Aspen mystery

By Megan O'Neil, Northwest Region forest health specialist

As forest health specialists, we get calls and emails every day that usually start with, "Something is wrong with my tree." Some of the problems are easy to diagnose and others are more complicated.

Aspen tree showing browning leaves at the covering the bottom and middle of the tree.For example, last week I received a call about a dozen or so aspen trees whose leaves were turning brown from the bottom of the tree and moving up. These trees were in a very small location, less than a quarter-mile along a bike trail. Other tree species in the area were not displaying the same pattern of decline. Consulting with the other DNR forest health specialists, we suspect this is bronze leaf disease.

Bronze leaf disease is a fungus that infects trees in the poplar family. In late August or September, the leaves turn brown or bronze, as the name implies. Affected leaves usually first appear in the lower canopy, but eventually the entire tree will show symptoms. The brown leaves can stay attached to the branches throughout the winter. Minnesota's native big-tooth and trembling aspen are both susceptible, but the disease is worse on hybrid aspen. This disease cannot be controlled by chemicals, so early detection is important if you want to save your trees. Pruning out diseased branches is an option if only the lower branches are infected.  Some trees die after three to five years of repeated infection.

As we move from summer to fall, if you see something wrong with your tree, be sure to check the Minnesota DNR's forest health webpage and the University of Minnesota Extension's What's wrong with my plant? webpage. Both of these are great resources to help answer the question of "What's wrong with my tree?"

Browning willow

By Eric Otto, Northeast Region forest health specialist

small brown beetle
Unidentified leaf beetle species. brown leaves on a willow branch
Feeding damage and browning on willow leaves associated with a leaf beetle.

For the past two years, foresters have reported leaf beetle damage on lowland and upland willow near Hibbing and in northern St. Louis County. Infestations of beetles were heavy in July and August this year. The beetle associated with the feeding is a species of leaf beetle, but the exact species isn't known. There are many different leaf beetles that feed on willow, so identification can be difficult.

These leaf beetles feed on leaf tissue between the veins, resulting in a skeletonized appearance. Over time, skeletonized areas turn brown, causing willows to look scorched or dead from a distance. Total browning of leaves in one season is rarely severely damaging. However, if feeding continues into the next year or two, willow health can deteriorate and in rare circumstances can result in mortality.


Jack pine budworm outbreak

By Megan O'Neil, Northwest Region forest health specialist

close up of a Jack pine budwormJack pine budworm

Populations of jack pine budworm are typically cyclical, and we expect to see the next large outbreak between 2023 and 2025. But the Bemidji area is getting a taste of what is to come. Multiple foresters have reported jack pine stands showing clear signs of an outbreak. This could be the beginning of a population on the rise.

Jack pine budworm is a native Minnesota insect that primarily feeds on jack pine, but this pest won't hesitate to feast on other conifer species, such as white or red pine, if they are present in a jack pine stand.

In May, budworm caterpillars emerge and feed on male pollen cones. As the cones wilt, caterpillars move to young foliage to continue feeding for about six weeks. Pupation occurs in July or early August and moths emerge after about a week to mate and lay eggs. Caterpillars develop in two weeks and do not feed, but spend the winter in this stage and begin feeding the next May.

The first noticeable sign of attack is the browning of jack pine needles in early July. Caterpillars web new needles together to make a feeding shelter. They feed on old needles after new needles are gone, and what's left will turn reddish in a few days. Budworms defoliate shoots and buds, which leads to twig mortality. Light budworm outbreaks reduce tree growth and top-kill is common after an outbreak.

Be sure to keep your eye on your jack pine, and if you are seeing signs of budworm, plan to manage accordingly.

Don't plant ponderosa pines in Minnesota

By Brian Schwingle, Central Region forest health specialist

ponderosa pine on leftside of image look brown vs red pine on the rightside of image.Figure 1. Ponderosa pines (left row) in Winona County with brown canopies compared to healthy red pines (on the right). ponderosa needles and shoots have brown tips.Figure 2. Blighted ponderosa needles and shoots. ponderosa pine in foreground has large number of brown needles vs the whtie pine in the background.
Figure 3. A ponderosa pine in Carver County with blighted shoots and needles, in contrast to the healthy white pine in the background.

For many years, it was common for landowners to plant ponderosa pines in central and southern Minnesota. Two reasons for planting ponderosas must have been that Minnesota's native white, red, and jack pines did not originally grow in the prairie and savanna regions of our state, and scientists were predicting a drier climate. Ponderosa pine is rated as having high drought tolerance.

Over the last couple of decades, the climate in central and southern Minnesota has not gotten drier in the growing season (roughly April to October), but wetter — a lot wetter. Precipitation and humidity favor fungal tree diseases. Two types of fungal diseases that affect ponderosa and red pine are needle blights and shoot blights. In one setting in southeast Minnesota, we were recently able to compare how ponderosa and red pine were handling these diseases.

As you can see in the background in Figure 1, ponderosa pines had brown lower canopies, while red pines in the foreground did not. The brown canopies in the ponderosa were due to blighted needles and shoots (Figure 2). Dothistroma, or brown spot needle blight, was affecting the needles, and Diplodia shoot blight was killing entire shoots. The ponderosa were full of these diseases, while impacts on red pine were minimal.

Around the Twin Cities, we have also seen ponderosa pines severely blighted by Diplodia and needle blight fungi (Figure 3).

Red pine seems to have fewer disease problems than ponderosa pine in many Minnesota settings, but white pine is the most disease-free of all our pines. The only significant tree-killing pest or disease you have to worry about with white pine is white pine blister rust, a disease that can be managed. We recommend planting white pine, red pine, and jack pine over ponderosa pine anywhere in Minnesota.

Back to top