Forest Disease and Insect Newsletter - June 5, 2001

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More in May and JuneJune 5, 2001

Winter injury among conifers

The most common form of damage visible at this time is excessive winter injury on all types of conifers (although not all trees) in the Metro Region, east central and northeast counties. On the conifers, the damage appears as brown to red foliage on various parts of the tree. Depending on the tree's exposure, the damage can be on one side of the tree, near the top, on isolated branches scattered through out the crown or it can affect the entire tree. If the damage is near the bottom of the crown or faces a road, suspect salt damage instead, as the symptoms are nearly identical. Minor to moderate damage may slow growth some, but should be eventually hidden as the new shoots expand. Severely damaged trees will likely show a marked reduction in growth and may be more susceptible to bark beetles and other secondary pests. See "Are bark beetles in your future" below.

Winter injury among hardwoods

Many maple and some ash are considerably behind schedule in bud break and leaf expansion. Unlike the conifers that are responding to winter drying, the hardwoods are responding to the rather dramatic change from an unusually warm dry Indian summer to cold wet weather in early December. The slow leaf expansion has left the trees looking sparse because the leaves are smaller than usual for this time of year. Trees go into fall dormancy slowly, and accumulate tolerance to cold slowly. In a normal year, fall has cool nights and warm days that bring great fall colors until leaf fall and dormancy begins. As winter progressively gets colder, the trees become more tolerant of cold until spring breaks the cycle. When the weather is radically different , like this last year, the trees have a hard time coping.

Other signs of weather related stress

Besides being slow to leaf out, ash, maple and elm trees have produced especially heavy seed crops this year in the Metro Region. In many cases, the seed crop outweighs leaf biomass, giving the trees a browned, wilted appearance now that the seeds are maturing. The appearance will improve as the seeds fall from the trees. However, that means lots and lots of sprouts where you don't want them.

Branch die back is common this year among oak and spruce. Secondary fungi are involved in some cases and in the case of the oaks, a twig borer is involved as well. In spruce, the symptoms are identical to Cytospora canker with dead branches scattered through out the crown. However, the typical resin soaked lesions are not always present indicating the fungus is not the cause of the die-back, but may be taking advantage of tissue weakened by other causes. The warm dry winter last year as well as the drought seen late last summer put the trees under stress going into this winter. The prolonged winter this year, without much in the way of extreme cold spells allowed secondary fungi to remain active longer than usual, taking advantage of the weakened trees. Pruning and extra TLC are called for to address both the heavy seed crops and the branch die-back.


The early stages of ash anthracnose cause affected leaflets to drop early in the season well before the typical blotch symptoms are noticeable. Metro Region has had several reports already of leaf drop. The unusually wet spring we've had so far combined with the cold spell in May will favor foliar diseases like anthracnoses, needlecasts and the rust diseases.

Anthracnose is a common name for several leaf diseases that are found on a large number of hardwood species but are most severe on ash, oak, and maple. Anthracnose leaf diseases are caused by a few species of fungi which overwinter on the buds, bud scales, and branches of host trees. Growth of these fungi begins early in the spring. Sufficient spores are produced for leaf infection and disease development during any period of warm, wet weather in the spring and summer. The severity varies greatly from year to year. In late spring and early summer this is the most common leaf disease on green ash. Watch for leaves falling following rain events during the first half of June.

Symptoms of anthracnose on infected leaves range from tiny, dead spots to large, dead blotches. Many leaves become curled and distorted due to the infection. Infected and dead leaves are shed. The symptoms are more severe in the lower crown. While control of foliar diseases is not usually needed, treatment is recommended this year, particularly on trees under other forms of stress, ie new transplants, recently disturbed trees and those showing the stress symptoms described above. Trees forced to refoliated because of FTC defoliation are particularly at risk of long term problems related to foliar diseases.

Oak tip blight and dieback

Last spring and summer, many of the oaks across central and southeastern MN showed signs of tip blight. Small twigs and branch segments two to ten inches long wilted and browned as the weather warmed. This year, some of those same trees are showing extensive dieback. In a few cases, the affected branches have died back to the main trunk, with last year's leaves still hanging onto the original segment killed last summer. Two organisms seem to be involved, one a fungus and the other a small wood borer in the buprestid family. A third likely factor is weather-related stress.

The Fungus

The fungus, Botryosphaeria quercuum, produces symptoms which are indistinguishable from oak twig pruners and cicada injury. However, close inspection shows distinct lesions and pycnidia, the asexual fruiting structures of the fungus, on the bark of affected twigs. The wood beneath the bark is discolored dark brown to black, with streaks that extend beyond the edges of the canker and/or infected twig. Development of pycnidia and wood discoloration begins as soon as the leaves wilt, often before they themselves have become brown.

oak tip blight

B. quercuum frequently produces an annual canker. Affected trees generally wall off the infection and produce new shoots at the base of the damaged portion. Repeated infection can lead to clusters of shoots at the branch tip. In weakened trees, the fungus can become more aggressive and produce a perennial canker that can kill larger portions of the tree. That is what we are seeing among some trees this year. Branch segments 1-6' in length have died back and in some cases, whole branches have been killed.

The Borer

Several of the samples collected last year had signs of a small borer. In a recent collection, evidence of a tiny buprestid at or near the margin of the damaged tissue was found in 12 of 16 samples (75%). Buprestid larvae are characterized by cream-colored elongated bodies with wide flatten heads. The endmost segment has two non-jointed circi or spine-like structures. These particular grubs, as yet unidentified, are 0.5 mm or less in width and 2-3 mm long. Spent egg cases were noted on the face of several leaf scars. Tunnels began at a leaf scar or at the base of a small twig, circled the twig and then moved up toward the distal portion of the stem. Their habit seems to parallel that of the twig pruner Elaphidionoides villosus, a round rather than flat-headed borer that attacks small twigs tunneling first around the stem and then along the twig toward the end. The presence of the larvae now and late last summer indicates the beetle overwinters in a late instar or prepupal stage like the two-lined chestnut borer, Agrilus bilineatus, another buprestid with a similar life-cycle. Unlike the chestnut borer, the twigs these borers are attacking are small branches (1/2 inch or less).

The wood of affected twigs is discolored around each tunnel. In a few cases, multiple tunnels have been found at the base of lateral twigs on the same stem. In those cases, stem tissue between the affected laterals was healthy and green, while the lateral twigs themselves were discolored with fungal fruiting structures on the bark. Although discolored streaks typically associated with the fungus extended in the wood beyond the tunnels, the healthy tissue in-between suggests the tunnels predated fungal infection. The entrance holes created by the borer are apparently allowing the fungus easy entry.

Healthy trees generally are able to fend off borer attack. Healthy trees are also generally able to wall off infection by B. quercuum, limiting the damage to the tips only. The fact that we are now seeing extensive dieback indicates the two organisms worked together to take advantage of weakened trees unable to fend them off.


Once started, there is little one can do to control fungal infections. Prune out affected portions, being sure to clean your tools between cuts. Since many of the oaks are slow to leaf out this spring (and to avoid spreading oak wilt, where it occurs), you should wait until July to prune your trees to be sure you do not remove portions that are still alive. If possible, sucker sprouts should be pruned out. If the trees are suckering badly, you may need to weigh the threat of branch failure that these sprouts will pose in the future if allowed to develop into large branches, against the loss in the tree's aesthetic value if you prune the suckers out now. Sucker sprouts are never as strong as the original branches and are much more prone to breakage later on.

Chemical treatment of the beetles is not advisable because the timing is critical and nearly impossible to pinpoint. Therefore, prevention is the best course of action for both the beetle and the fungus. Avoid wounding and maintain overall tree health through proper tree care. To help damaged trees recover, minimize additional stresses so they can devote starch reserves to replacing lost tissue. Water during excessively dry spells and control other pest outbreaks (like FTC).

Birch leaf miner

Birch trees can be stressed by defoliation and one of the most common insect defoliators is the birch leaf miner. As you may have guessed from its name, the tiny larvae feed between the upper and lower surfaces of the leaf blades. A single treatment of acephate applied early in the summer when the first evidence of leaf mining appears usually gives good control throughout the season. The first sign of infestation is a yellow blotch on the leaf, where the egg was deposited and the larva has just begun to mine a small area. The trees should be watched and if a second generation begins to develop, a second treatment can be made.

june beetle (Phyllophaga rugosa)

June bugs

A few oaks and ashes in Long Lake township of Crow County were heavily defoliated in May by the June beetle, Phyllophaga rugosa. Beetles congregate during the evening to feed and mate on trees. The adults are reddish brown, shiny, from 3/4 to l inch long and are barrel-shaped. Their eggs are laid in masses in the soil at depths of three to seven inches.

Newly hatched larvae, called white grubs, feed on organic matter, then move to tender roots of tree seedling, grasses, and other plants. Occasionally, root feeding causes heavy losses to conifer seedlings and other young trees. Grubs reach one inch at maturity, are milky white, strongly curved, and have brownish heads. After feeding on roots for two to four years they change into adults. Control involves use of labeled insecticides that are spread on and washed into the soil.

Larch casebearer

larch needles

During the week of May 21st, new larch needles in southeastern Itasca County were partially mined and browned by larvae of the larch casebearer. This insect caused extensive browning and withering of larch needles last year across northern Minnesota.

Larch casebearer has a few different survival strategies than most of the other forest insects.

  1. It carries its home on its back. The larva creates a portable shelter ( its "case") during late summer out of a hollowed-out larch needle.
  2. They overwinter as larvae. And, they overwinter inside their cases which are attached to larch twigs. A cold spot indeed.
  3. They use the case as shelter for pupation.

In the spring, the overwintering larvae feed on new needles and soon pupate. Adults emerge from May to August and lay eggs. Eggs hatch and each young larva mines a single needle. Once the needle is hollow, the larva clips both ends and lines it with silk. Voila, a home. These cases are created in mid- to late-summer, where the larvae finish feeding and then prepare to overwinter.

Larch beetles

Pheromone traps were placed in Carlton, St Louis and Itasca Counties in cooperation with Dr Steve Seybold, U of MN. This trapping study is looking at the attractiveness of different baits in the traps as well as the timing of the adult flight periods. The traps were placed on May 4th and immediately attracted beetles. The spring flight of overwintering adults apparently started at an earlier date. There was still ice in the ground at the St Louis County site on May 4th.

The larch beetles overwinter mainly as adults under the bark of the stems and large branches of tamarack. In studies in Newfoundland, adults and larvae were found under the bark attempting to overwinter. In those studies the larvae apparently froze and only the adults were able to survive the winter. In Minnesota, both adults and larvae were found overwintering under the bark and they both were able to survive. Either last winter was milder than the winters in Newfoundland or our larch beetle larvae are just tougher than Newfie larvae. Last winter was pretty mild in terms of cold temperatures. There was also lots of snow which may have allowed larvae to survive, especially below the snow line. Most of the overwintering larvae were pupating as of May 24th.

Are bark beetles in your near future?

Despite the rains we had in May, soils are dry in the counties just north of the Twin Cities. And they have been dry for several months. Pines have already indicated their level of stress in the form of winter injury. Suspicious pockets of dead and dying pines can be seen from the air. Can pine bark beetles be far behind? Here are some suggestions for preventing or limiting a local outbreak of bark beetles using the trap tree technique.

The trap tree technique is used to reduce or prevent attacks of living trees which are growing near an active bark beetle infestation. This option utilizes recently cut, living trees in order to draw the attack of bark beetles to this breeding material rather than to the remaining stand. Trap trees are collected and destroyed once the beetles have started their brood and before they emerge, thus reducing the potential beetle population. Low value stems (crooked, forked, etc.) are excellent choices for trap trees since their removal also improves the quality of the stand.

The success of the trap tree technique relies on three principles:

  1. Overwintering adults which emerge in the spring prefer to attack nearby slash and logs on the ground.
  2. Bark beetles will aggregate their attack on a few cut logs or highly stressed trees in preference to healthy trees.
  3. Timing is critical. While bark beetle larvae are still developing inside these logs, trap trees are destroyed or debarked. This limits reproduction and directly reduces the population numbers. IMPORTANT: If trap trees are not removed or destroyed before the new beetles emerge, the landowner has accentuated the problem by increasing the beetle population in his stand.
a trap tree program in action

The operation of the trap tree technique is labor intensive. It is cost efficient where the landowner has access to cheap labor and where the cash needed for other techniques is not available. Trap trees may not be a viable option in urban situations where the logs might pose a safety hazard or where timely log and slash removal and disposal is difficult. Unless the bark is removed, using trap logs for firewood is not a disposal method since the larvae can complete their life cycle in the wood pile.

Procedures for implementing a trap tree program: (Note: parentheses indicate summer time frame.)

  1. About April 1st (or anytime thereafter, until the end of summer), cut live pines and lay them in the bark beetle pocket or on the edge of the pocket. Cut 4-5 trees per acre of bark beetle infestation with a minimum of 3 trees per pocket. It is preferable to leave the trees entire so that some drying takes place. This will make the downed trees more "stressed", thus more attractive to bark beetles. Keep the logs in the shade. Bark beetles will avoid sunny areas as temperatures in the sunny areas may become too high. Flag or otherwise mark the log locations because they become difficult to relocate once the foliage and vegetation reach their peak.
  2. In mid - to - late May ( or 3 weeks after cutting the trap tree logs during the summer), begin inspecting the inner bark of trap trees for the presence of advanced stages of beetle development (large bark beetles larvae and pupae). If either are found, the log should be treated as in #4 below. The presence of exit holes in conjunction with galleries necessities immediate action. Destroy this material at once. If neither are found, continue to monitor the logs at 3-4 day intervals.
  3. Trap logs should be removed or treated to destroy habitat in late May ( or 4 weeks after cutting the trap tree logs during the summer), but this will vary with location and weather. To destroy bark beetle habitat, all the bark must be removed or the slash and logs should be chipped, burned, buried, submerged or piled and wrapped airtight with a plastic tarp. For any of the treatments, branches <2" in diameter can be left untreated. If the logs are buried, a pit should be dug and the whole bole and branches >2" in diameter should be buried under at least 6" of soil. DO NOT CUT AND PILE TRAP LOGS FOR USE AS FIREWOOD unless the bark is removed and destroyed.
  4. Evaluate each pocket to determine if the trap logs were effective in preventing attack on nearby trees. Check all the edge trees for signs of active infestation . If there are no new signs of infestation, the trap logs worked in one cycle. In this case, only monitoring should be continued for the remainder of the growing season. If nearby trees were still attacked, two things should be done. First, remove or destroy the newly infested, living trees. Second, continue the trap log procedure as outlined above, but contact your local forester or regional specialist before starting a second trap tree cycle.

Remember to DESTROY the trap log habitat by any of the following methods.

Debark the log and destroy the bark (particularly if adult beetles have begun to form).

Burn the log (Cook the bark, it is not necessary to consume the log to ash).

Chip the log (This is very hard on the beetles).

Cut the logs into short lengths, stack, spray with water, and then wrap tightly in plastic (this encourages fungi that will kill the beetles, but leave them wrapped for 4-6 weeks).

Bury the logs under 6" or more of soil.

Submerge the logs under water.

Be creative and show some originality in the destruction of the beetle's habitat, but DO NOT CUT AND STACK the infested logs behind your house for firewood unless the beetles and their habitat have been destroyed.


Look for these sawfly larvae in the late spring and early summer. Defoliation of seedling and sapling conifers can lead to dieback.

yellow-headed spruce sawfly
Yellow-headed spruce sawfly
Hosts: white, blue, black, and Norway Spruce.

Adults (that look like small wasps) emerge in the spring at the time lilacs are in full bloom. The female deposits eggs at the base of the new needles. The larvae hatch in 7 to 14 days and are present in late May through June. The young larvae feed on the edge of new needles. The leaders and tops of seedlings and saplings will appear thin and will become brown. Older larvae will consume the entire needle often leaving only a short brown stub. Full grown larvae are 3/4 inch long, have a yellow to reddish-brown head and an olive green body with grey longitudinal stripes. When larvae complete their development and feeding they drop to the ground, form a dark brown papery cocoon in the soil and overwinter.

European pine sawfly
European pine sawfly
Hosts: Scotch, Austrian and red pine.

Eggs overwinter and hatching begins in early May. Young larvae feed in groups on the outer edges of old needles which produces tufts of brown needles below the developing new shoot. Older larvae eat entire needles and leave only the needle sheaths. One larval colony of 80 to 100 larvae can eat all the old foliage off a 2-ft-tall tree; 15 to 20 colonies can completely defoliate a 6-ft-tall tree. When full grown, larvae drop to the ground, spin cocoons, and pupate. Adult females lay their eggs in clusters in the needles in September and October.

Cercospora blight of junipers

cercospora blight

This needle disease, caused by Pseudocercospora juniperi, occurs throughout most of the US and Canada. Rocky Mountain juniper is the most severely affected host in the Great Plains region but eastern red cedar can also be severely damaged. This disease is suspected causing defoliation in red cedar trees that are growing in wildlife plantings in Cottonwood County. Symptoms are initially seen in the lower inner branches and progress upward and outward as the disease spreads through the tree, leaving it devoid of foliage. Cercospora blight can be very severe in older windbreaks and wildlife plantings.

The fungus overwinters in infected leaves on the tree. Spores are formed and disseminated during periods of warm wet weather from April to October. The spores are dispersed short distances by rain splash. Long distance spread is apparently rare. Spores germinate and penetrate host leaves through stomata or directly through the cuticle. Free moisture must be present for spore germination and infection to occur. Treatment is possible. Bordeaux mixture is labeled for control of Cercospora blight. Control is achieved when two applications are made; one in mid June and one in early July.

Rhizosphaera needlecast of spruce

Rhizosphaera needlecast is a common needle disease of Colorado blue and white spruce trees. It causes the older needles to turn purplish-brown and fall prematurely. Needles on the current shoots will be green and healthy. So the tree will look sparse and discolored while the current needles are beautifully green.

Signs of the disease on an infected needles can be seen by using a 10x hand lens. The normally flat, white rows of stomata (pore-like openings) will be black and tufted when infected by Rhizosphaera. Those black tufts are the tiny fruiting bodies of the fungus that causes the disease. In the spring and summer, spores from infected needles are rain splashed onto current needles. With consecutive years of infection, branches become bare and, eventually, the branches die. When this happens, the spruces loose their value in the landscape.

Genetics and microclimate play a large role in disease occurrence and severity. Hopefully, some of your individual trees will not be as severely affected because they are genetically tolerant or resistant to the disease. Microclimate can be manipulated to discourage disease development. Maintaining adequate spacing and airflow will help to limit the amount and severity of disease. Remove branches close to the ground and mow underneath the branches to increase air flow. Fungicides can be used also. Make two applications of either chlorothalonil or Bordeaux mixture. The first application is made when the shoots are ? elongated and the second 3 weeks later. Thereafter, spray 2 or 3 additional times at 3 week intervals. At least two years of treatment will be necessary.

Pine needle rust

During May, outbreaks of pine needle rust can be observed on the needles of young pines. Infected needles have yellow spots or bands with an orangish color in the center. The hosts are mainly jack, red, and Scots pine.

The most recognized feature of this disease caused by the fungus, Coleosporium asterum, is the white blisters formed on the pine needles. Orange spores are usually evident beneath the white covering. Later in the season, the affected area on the needle turns brown. This leaf disease has an alternate host, the aster or goldenrod. Infection of the alternate host is characterized by the development of orange, cushion-like masses on the undersides of affected leaves early in the summer.

Because the needles produced in the current year are not infected until late in the growing season, control is not necessary to prevent mortality. Destruction of alternate host plants within 300 meters from planting sites by mowing or chemical means provides some control.

Noteworthy observations:

In Region 1 during May, eastern tent caterpillars were abundant on cherry trees.

On May 11th, the first red turpentine beetle was trapped in a pheromone trap in Crow Wing County north of Crosby.

May 15th was peak spore production for pine-oak gall rust on jack pine in Wadena County.

In mid-May in the Metro Region:spruce needle miners, carpenter bees and spring loopers (primarily on ash).

As of May 24th, spruce budworm caterpillars were predominantly in the third instar (some still in second) in spruce plantations south of Jacobson in Aitkin County.