Forest Insect and Disease Newsletter July 2014

Question of the month

Q: I hear a lot about invasive insects like emerald ash borer and gypsy moths, but what are some of the good bugs that help keep our forests healthy?

A: There are many more beneficial insects than insect pests; we just don't hear about them as often. One beneficial insect, the friendly fly, is named for its habit of repeatedly landing on people without biting. The larvae of this fly feed on forest tent caterpillars inside their cocoons. During the third or fourth year of a caterpillar outbreak in Minnesota, the friendly fly can kill nearly 90 percent of the cocoons.

Parasitic wasps also keep insect populations in check. Most parasitic wasps are extremely small, which is why we don't usually see them at work. There are thousands of different species, none of which sting people, but nearly every insect species is attacked by parasitic wasps during at least one life stage. Beneficial wasps feed on pests such as spruce budworm, gypsy moth, fall webworm and emerald ash borer.

Many types of beetles, the largest and most diverse group of insects, are predacious and feed on aphids, scale insects, caterpillars, other beetles and more. The larvae of one beetle family called checkered beetles feed on harmful wood-boring insects such as bark beetles, potentially preventing an outbreak.

Val Cervenka, DNR forest health program coordinator

Feature Article: Eastern Spruce Budworm in Minnesota

Eastern spruce budworm (Choristoneura fumiferana) is a native pest that evolved with boreal forest conifers. Contrary to its name, it prefers to feed on balsam fir, although it will also feed on spruce needles, especially those of white spruce. Periodic outbreaks of spruce budworm are part of the natural cycle of events associated with maturing balsam fir. Northeastern Minnesota is apparently spruce budworm's preferred location in our state; measurable defoliation has occurred there continuously since 1954, as shown by the chart below.

Acres of spruce budworm defoliation bar chart 1954 to present

Life Cycle

The female moth lays five to ten egg masses, each containing 2 to 60 eggs that overlap each other like fish scales. Egg masses are laid on spruce and fir needles toward the outer branch shoots and in the upper crown of the trees. Eggs hatch 10 to 14 days after being laid, and the tiny larvae spin down on a silk thread and are blown considerable distances by the wind. Many drop onto small trees in the stand. These young larvae do no feeding damage to the tree, but seek sheltered locations under bark scales or lichens where they spin a cocoon-like shelter and overwinter.

Just before balsam buds expand in the spring, overwintering larvae emerge, drop onto understory trees nearby, and begin to feed. These larvae may also be dispersed by the wind a considerable distance away. Young larvae prefer to feed on newly opened male flowers. If these are not available, the larvae will bore into expanding buds or needles. Larvae feed on new needles first but will move onto old needles after new needles have been consumed. Needles chewed off at the base and webbed to the shoot turn brown, giving the tree a rusty color. Most feeding is done in June, with damage becoming obvious in mid to late June. In late June to early July the larvae are 1-1¼ inches long, and stop feeding to form a pupa.

Adult moths emerge from pupal cases in early to mid-July, mate, lay eggs, and die. Clouds of moths can be seen in late evenings, whirling around the treetops. They disperse over long distances carried by prevailing winds, and storms may transport masses of moths 100 miles or more.


In balsam fir, top-kill usually occurs after two to three years of heavy defoliation, and tree mortality begins after three to four years. Mortality within stands can vary from zero to 100 percent of both mature and immature fir trees, although in northeastern Minnesota, 100 percent mortality is rare except in patches within stands. White spruce survives spruce budworm attack much better than balsam fir. Mortality in white spruce generally does not occur until after trees have suffered heavy defoliation for five to seven years and even then the percent of trees killed is much less than in balsam fir.

Spruce budworm does not eliminate balsam fir or white spruce from the landscape. Forests and spruce budworm have evolved together and budworm is part of the natural cycle of events associated with maturing balsam fir. When an outbreak ends, budworms typically will not return to an area for 25 to 40 years. As an example, a spruce budworm outbreak occurred in the Cloquet Valley State Forest from 1974 to 1986, killing a large number of balsam fir. Budworm has returned to this area, and in 2014, an outbreak became apparent, feeding on the balsam fir and white spruce that survived the outbreak of 1974-1986.

Click to enlargeSpruce Budworm defoliation map

The black areas on the map below represent spruce budworm defoliation of balsam fir and white spruce in 2013. Defoliation occurred on 38,000 acres were defoliated, and of these there was mortality on approximately 34,000 acres. The area outlined in blue is an estimate of the area where a new outbreak of spruce budworm was observed earlier this summer including the eastern half of the Cloquet Valley State Forest. A more accurate map of this year's defoliation will be produced once the aerial survey is completed later this summer.

When spruce budworm moves back into an area where it has not occurred for decades, it typically will remain in that area, defoliating trees for the next eight to 10 years. After three to four years, balsam fir trees will start to die and mortality will continue for as long as budworm continues to defoliate the trees. Individual stands may have some areas where all of the balsam fir die and other areas where few or no trees are killed. Much of this difference depends on where winds disperse the moths and larvae, but also on the density, vigor, and age of the fir.

Work by Harold Batzer at the U. S. Forest Service found that stand composition greatly influences vulnerability to spruce budworm. Generally speaking, the more balsam fir in the stand, the greater the potential mortality and the more species diversity in the stand, the less damage there is to the balsam fir. Site index and stand age made little difference in his study although the health and vigor of the tree does play a role. Below is a table Batzer developed showing the potential for dead balsam fir in square feet of basal area per acre. The table does not include an estimate for dead white spruce because Batzer found that although balsam fir may be severely defoliated they are usually not killed by spruce budworm.

Chart showing potential dead balsam fir basal area in square feet per acre

From Batzer, H. O. and A. R. Hastings (1980). How to Rate Spruce-Fir Vulnerability to Budworm in Minnesota. U.S. Forest Service North Central Forest Experiment Station, St. Paul, Minnesota.


Forest management during a spruce budworm outbreak

When a spruce budworm outbreak begins in a new area, logging of the most vulnerable stands with the highest merchantable volume should begin immediately. A pre-salvage effort is much better than trying to salvage dead trees, as it is usually difficult to find a market for dead fir trees. Studies in Minnesota have shown that dead balsam fir start to deteriorate and break apart after being dead for a year or more.
In general, high mortality is more likely in stands with the following characteristics:

  1. Fifty percent or more of the stand is composed of balsam fir and white spruce
  2. Mature fir stands 50 or more years old
  3. High basal area of balsam fir or white spruce
  4. Small percentage of non-host species
  5. Stands in which spiked tops of host species protrude above the forest canopy
  6. Stands on poorly drained soils that are abnormally dry or wet
  7. Extensive stands of mature spruce-fir types


Spraying budworm has not been done by the DNR Division of Forestry, counties, or the U.S. Forest Service in Minnesota for a long time, because the demand for balsam fir is not high enough to justify the high cost of spraying. In most of North America, large spruce budworm outbreaks occur every 40 to 50 years, causing mortality over extensive areas, resulting in a boom and bust cycle in the availability of merchantable balsam fir. There has been spruce budworm defoliation in Minnesota every year since 1954, killing fir in part of one or two counties before moving on to an area it has not fed on for the past 30 to 40 years. This pattern does not cause a drastic change in the availability of merchantable fir in Minnesota. The result is that fir is always available in sufficient quantity to meet demand.

Another problem with spraying for spruce budworm is that it does not affect the budworm population or end an outbreak. For the most part, spraying of forests in the past has been done to protect the needles on trees so they remain alive until they can be harvested. There is nothing to prevent stands of trees sprayed in one year from having high budworm populations and needing to be sprayed the next year and the next. Because the moths are dispersed by wind and seek trees with lots of needles, stands sprayed in May will likely have moths laying eggs on trees in July, establishing a population for next year.

Yard tree management during a spruce budworm outbreak

As a native pest, budworm has many natural enemies that help keep their populations in check. Birds and many insects feed on and kill all life stages of spruce budworm. Healthy, vigorous fir and spruce will be able to withstand more defoliation than stressed trees, so keep trees healthy by watering during dry periods. Mulching around the trees will reduce competition from grass and retain more moisture in the soil.

Crowded trees grow more slowly, produce fewer needles, and will be killed by budworm more quickly than trees with more growing space. If your trees are crowded, thin by removing some to give the others more light and growing room.However, thinning during a spruce budworm outbreak is unlikely to have much benefit. If budworms are defoliating trees, they are already stressed and will not be able to respond to the thinning with increased growth and new needles.

Yard trees can be sprayed to protect their needles and kill spruce budworm.

The trees will likely have to be sprayed every year as long as the outbreak continues. Moths from surrounding areas can lay eggs in July on trees that were sprayed in the spring. Most people do not have the equipment required to provide good spray coverage of tall trees and will have to hire a licensed pesticide applicator. Licensed aerial applicators may be hired to spray larger acreages of trees. The Minnesota Department of Agriculture has a list of licensed pesticide applicators. If you choose to use pesticides to control budworm they must be applied early in the year before much needle feeding damage has occurred.

Removal of dead trees

Dead balsam fir and white spruce trees around homes and buildings are a fire hazard. Develop and maintain a defensible space around your home and buildings in the event of a fire. More information

Balsam fir needle rust

According to a Cornell University fact sheet, This is a PDF file. You will need Adobe Acrobat Reader to download it. "Infection of fir needles requires cool, moist conditions during shoot elongation." That explains why 2014 has been a banner year for needle rust on balsam fir in northeastern St. Louis and Itasca counties. Scattered trees had nearly 100 percent infection of this year's needles, and by mid-July, infected needles had white spores growing on them. The needles themselves were discolored by the rust infection, from pale green to yellow to white.

Infected needles die and are shed in early autumn. The fungus overwinters on the fern host for most of the fir-fern rust diseases, so the prevalence of needle rust on balsam fir each summer is determined by the amount and duration of cool, moist weather that occurred in the spring and in proximity to infected ferns.

There are eleven species of rust fungi that cause fir-fern rust and all are in two genera, Milesina and Uredinopsis. On ferns, rust infections cause pale green to yellow spots with brown edges surrounded by leaf veins. In our region, the following ferns can be hosts to fir-fern rusts: wood, polypody, sensitive, bladder, lady, oak, bracken, and ostrich. Unfortunately, one can't determine which fungus it is just by looking at the rust on balsam fir. Diagnosis is based on rust fruiting and overwintering structures on the ferns and is no small task.

White rust pustules on balsam fir needles.? Bruce Watt, University of Maine,

White rust pustules on balsam fir needles. Bruce Watt, University of Maine,

Infected needles showing discoloration and fruiting bodies. Bruce Watt, University of Maine,

Infected needles showing discoloration and fruiting bodies. Bruce Watt, University of Maine,

Brown, shriveled needles in late July infected by fir-fern rust. DNR photo


Eriophyid gall mites on maple

Three of the most common types of maple leaf galls caused by mites are spindle galls, bladder galls, and erineum galls. Erineum galls look like shiny, colorful patches that are actually made of hundreds or thousands of hair-like galls packed tightly together. Early spring foliage is most affected, particularly leaves growing near mite overwintering sites on the trunk and large branches.

Spindle gall mite

Spindle gall mite. DNR photo

Bladder gall mite.  Photo from

Bladder gall mite. Photo from

Erineum mite.  DNR photo

Erineum mite. DNR photo

Bladder gall mite. Photo from

Bladder gall mite. Photo from

Gall type

Maple species

Mite species

Other information



Vasates aceriscrumena

On upper leaf surface:
Green then red


Red, Silver

V. quadripedes

On upper leaf surface:
Green then red, later maybe black


Most maples

Eriophyes elongatus
E. modestus
Aculops maculatus

On upper or lower leaf surfaces:
Red erineum
Green erineum
Red erineum

The brilliant red galls are very conspicuous on green maple leaves, and cause much concern to tree owners unaware that the presence of galls is rarely detrimental to tree health. Gall mites only infest expanding leaves, so mite activity decreases as the growing season progresses. Sufficient foliage is produced during a growing season so that the tree sustains itself without harm. On very young or newly-planted maples, control may be warranted. On these trees it is easiest just to remove leaves with galls by hand, as the affected leaves are most common on the lower branches. A systemic pesticide may be applied in spring or early summer that will kill the mites inside the gall, but the galls themselves will remain on the leaves.

Gall mites are elongate arthropods related to ticks. They are very small, about 0.0078in long. Their mouthparts are adapted to piercing individual leaf cells and sucking out the contents. Eriophyid mites are host-specific. They induce their host plants to produce abnormal growths in which the mites feed, live and reproduce. They are disseminated by crawling, wind, attaching to other insects, and by shipment of plant materials.

Eriophyes mite on leaf surface. Photo from

Eriophyes mite on leaf surface. Photo from

A bladder gall showing connection to the leaf blade.  Photo by M. Frankum.

A bladder gall showing connection to the leaf blade. Photo by M. Frankum

Although there are some variations in the life cycles of these mites, a generalized life cycle is as follows. Adults overwinter under bark scales on branches and trunks and are able to withstand severe weather conditions. Soon after buds begin to expand, the adults move onto the leaves and feed there. Mites causing spindle or bladder galls feed on the under surface of the leaves, while erineum mites feed on the upper surface. In response to secretions from their saliva, the leaf creates a pouch or gall where the females are enclosed and continue to feed. Eggs are laid inside these protected places. The larvae feed and reach the adult stage in a matter of weeks. Adults leave the gall through an existing opening in the leaf tissue. They may initiate other galls as long as developing leaf tissue is present. By July, mite activity on foliage has finished.


What happened to forest tent caterpillar?

Female FTC moth. DNR photo.

Female FTC moth. DNR photo

The forest tent caterpillar outbreak seems to have fizzled out, contrary to earlier estimations. We're not sure why the populations didn't build to the numbers we thought we would see. Results from this year's aerial monitoring survey will tell us more about where the caterpillars were and how much damage they caused.

We also monitor the timing of forest tent caterpillar moth flight during their outbreaks, using two methods. First we collect a few dozen cocoons, bring them indoors and watch for the first day that moths emerge from them. This year, the first moths emerged from collections on July 12, which is at least three weeks later than average according to our records. Second, we monitor the arrival of forest tent caterpillar moths to lights in the Grand Rapids area. The actual start of the moth flight occurred on July 19 in Itasca County, about one month later than usual.


Spruce needle rust

The weather this past spring was perfect for both the dissemination and infection of fungal spores that cause spruce needle rust. We are just starting to see the consequences of a cool, humid, and prolonged spring as the discoloration of needles and the production of fungal fruiting bodies begin. In August we will see infected needles turning yellow, fruiting bodies protruding from needles and shedding spores, and later, infected needles shedding. In the meantime, watch white, Colorado blue, and black spruce, as whole trees take on a tan or yellow to pinkish cast. More information This link leads to an external site.

Spruce needle rust 
DNR photo

Spruce needle rust. DNR photo

Needle discoloration due to spruce needle rust    DNR photo

Needle discoloration due to spruce needle rust. DNR photo

Spruce needle rust aecia on needles    DNR photo

Spruce needle rust aecia on needles. DNR photo

Yellow-headed spruce sawfly

After a twenty-year hiatus, damage from yellow-headed spruce sawfly (YHSS), Pikonema alaskensis, has become more prevalent over the last few growing seasons in northern Minnesota. Because open-grown white spruce is the preferred host, young spruce in ornamental, shelterbelt, and roadside plantings can suffer defoliation, topkill, and death. These sawflies tend to infest the same tree or groups of trees for several consecutive years. White spruce growing under dense shade or in dense stands tends to remain free from attack.

During the entire month of June, sawfly larvae emerge from eggs laid in needles to feed on the foliage of terminal and upper branches. Since the larvae do not consume the base of the needles, brown needle stubs remain giving branch tips a ragged appearance. When the newest foliage is consumed, the larvae move to the older foliage. Feeding is usually completed by July. Completely defoliated trees die; those with some foliage remaining on the lower branches may live, but usually lose their leaders and some upper branches.

If you find sawfly-damaged white spruce this year and anticipate taking control actions next year, you'll want to scout the young plantings two or three times next June since sawflies can emerge during the entire month. If there are only a few trees, you could hand pick the larvae off the tree and then kill them. If there are more trees, a low population, and the larvae are still small, the use of insecticidal soaps is the best technique. If the population is high, conventional insecticides (acephate, bifenthrin, carbaryl, malathion, or permethrin) are suggested. Additionally, you may have to treat spruce more than once if scouting later in June shows that another population of sawflies is defoliating them. Please note the any product containing Bt will not work on sawflies.

More information This link leads to an external site.

Defoliation caused by YHSS. DNR photo

Defoliation caused by YHSS. DNR photo

Needle discoloration due to spruce needle rust    DNR photo

Feeding damage and young YHSS larvae. Note needle stubs. DNR Photo

Mature larvae of YHSS

Mature larvae of YHSS. DNR photo

From the archives

This article by Mike Albers was first published in the June 2001 issue.

Forest tent caterpillar season

Another caterpillar season is quickly coming to an end. Although many people reached their daily and possession limit long ago, local forest health specialists are still busy supplying expert guide service. Remember that although catch and release is practiced by some people, it is neither necessary nor recommended.

There was a lot of action on many area forests in June and July with caterpillars hitting on just about every kind of leaf thrown at them. Lunkers were being taken in all kinds of habitat from weed beds near lake shores to the deep forest. Many were even congregating on gravel bars often referred to by professionals as roads.

Some good runs are still expected along Lake Superior the last week of July, but action has already started to drop off in most other parts of the state as the caterpillars stop feeding in preparation for spawning. It's still too early to tell, but natural reproduction is expected to be excellent in many parts of northern Minnesota, so at this time no stocking of area forests is planned by the DNR. Studies later this year will give experts a better idea of reproduction success and will be used to set next year's season as well as daily and possession limits.

Due to this year's population, a special fly season has been added to the end of regular season. No special license is required for the fly season.

With caterpillar season drawing to a close, moth flight season can't be far behind. Be sure to read next month's issue for "Tips on Wing Shooting."


New Forest Health Specialist for Southern Minnesota

Hello! My name is Brian Schwingle, and I'm the newest part of Minnesota's forest health team. I started on June 2, and I cover central and southern Minnesota (Todd County east to Pine County south to Houston County). I'm here to help managers diagnose what's killing or ailing their trees and make management recommendations to help control, minimize, or prevent damage to forests. I will emphasize state lands and landscape scale forest damage. Do not hesitate to contact me if you need help with a pest that's negatively impacting the landscape. If you are noticing a critter affecting trees in a relatively large area, please don't assume I'm aware of it. I welcome forest health reports.


Control of Oak Wilt in Bluff Country

One great thing about oak wilt is that it can be controlled through several options if control efforts are made in the early stages of disease development. This is not common when it comes to non-native insects and diseases. In steep terrain where erosion and equipment accessibility are concerns, there are two control tactics available to land managers that may be good options. They involve using herbicides to kill healthy oaks that are grafted to diseased oaks. These tactics are still considered experimental, but in some stands in the Lake States, they have achieved control equivalent to physically cutting root grafts with vibratory plows.


Bur Oak with Oak Wilt

Hey, stop looking at the scenery and look at the trees! Note the small bur oak showing symptoms of oak wilt immediately adjacent to the dying larger bur oak. It is highly likely the smaller oak was infected through grafted roots with the larger bur oak. DNR Photo

Classic oak wilt leaf symptoms on bur oak

Classic oak wilt leaf symptoms on bur oak are leaf margin browning and leaf drop. DNR Photo

Close of bark showing bug

Where's the buggy? Where's the buggy? Click on photo see. DNR Photo

Bur oak showing epicormic sprouts

Bur oaks with completely dead crowns may not be completely dead. Clusters of epicormics sprouts are not uncommon on bur oak infested with two-lined chestnut borer. DNR Photo

If you are struggling with controlling oak wilt and are unaware of your options, consider contacting your regional forest health specialist for details. The order of activities and timing of them is crucial for successful control of the disease. Two excellent publications, Oak Wilt Management—What Are the OptionsThis link leads to an external site.and How To Identify, Prevent, and Control Oak Wilt This link leads to an external site.outlining some options (though not the herbicide options). Note that the range map of oak wilt in these publications is not up-to-date. Currently the best source for understanding the range of oak wilt is found at Alien Forest Pest Explorer interactive map.This link leads to an external site.


Scattered Bur Oak Mortality in Southern Minnesota

Female FTC moth. DNR photo.

A woodlot in southern Minnesota where environmental stress (drought), two-lined chestnut borer, and Armillaria are killing bur oak. Photo by Mallory Macal

Besides clusters of dead and dying elms due to Dutch elm disease, the most visible landscape damage in southern Minnesota forests is scattered dead and dying bur oaks. Recent field investigation revealed that native two-lined chestnut borer (TLCB) and Armillaria root disease are tag-teaming to undermine these trees. Recent drought conditions likely increased their susceptibility to the two pests and contributed to their demise. Various sections of the southeast quarter of Minnesota have had drought in some months in 2004, 2006, 2007, 2009, 2011, and 2013, according to the Palmer Modified Drought Index.

Of course, the relatively newly-described leaf disease, bur oak blight (BOB This link leads to an external site.), may have weakened some southern Minnesota bur oaks along with drought, and made them even more susceptible to TLCB and Armillaria. A complicating factor in diagnosing this stress-induced decline is the presence of oak wilt. Please take time to understand the different symptoms oak wilt causes from drought, TLCB, BOB, and Armillaria. The differences are sometimes subtle.

Unfortunately, in a forest, options are extremely limited when dealing with high, damaging populations of two-lined chestnut borer and spreading Armillaria. If damage is not significant and stands are not near maturity, forest managers must beware of temporarily stressing any TLCB-infested stand by thinning it. Wait until symptoms subside and normal precipitation returns before thinning an oak stand.

For stressed ornamental bur oaks, watering them properly during a drought may stave off TLCB and Armillaria.


photo: Adult EAB bugEmerald Ash Borer Update in Houston County

Foresters in Houston County found at least five new spots in the county infested with emerald ash borer (EAB) in the month of July, including the city of Caledonia and a state forest stand. Emerald ash borer has now spread over 10 miles from the original Houston County find, which was first confirmed in 2010.

Report suspect EAB infestations to the Minnesota Department of Agriculture (MDA). Encourage callers to first go to the EAB website This link leads to an external site.and review the document "Does my tree have EAB?" Request that callers e-mail a photograph of the insect, galleries, or larva for confirmation to , and if confirmed, MDA officials will update the range map.This link leads to an external site. Having an updated range map is a very helpful tool for communities and property owners to plan for the arrival of EAB.

Lastly, please utilize wood from infested ash trees locally, or follow appropriate compliance agreement protocols if selling ash logs. Though it is legal to move ash material within quarantines, the spread of EAB will be drastically slower, even within quarantine counties,This link leads to an external site. if people leave infested trees on the property on which they grew.


Ash Tree seed crop

This ash has many bare branches due to a heavy 2013 seed crop. DNR Photo

Ash Trees with Thin Crowns

Scattered ash trees at least in central and southern Minnesota and neighboring Wisconsin have thin crowns. Why? Ash produced an immense seed crop in 2013, and leaves never sprouted in 2014 where the ash seeds grew. Green ash trees produce seeds every year, but not to the level produced in 2013. Recent drought conditions may be the cause of this seed proliferation. Most of these ash trees are expected to recover, but care should be taken in 2014 and 2015 not to stress them. For ornamental ash trees, do not fertilize them in 2014 or 2015 and water during drought.