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HeadlinersJune 29, 2001

Caterpillar season

Another caterpillar season is quickly coming to an end. Although many people reached their daily and possession limits long ago, local Forest Health Specialists are still busy supplying expert guide service. 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 the second and third week of June 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 were still expected along Lake Superior during the last week of June but action had 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 years season as well as daily and possession limits.

Don't forget, due to this year's huge caterpillar populations, a special fly season has been added to the end of regular season. No special license is required for fly season. Unlike caterpillars, catch and release of these flies is strongly recommended.

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

Forest tent caterpillar, really

Anyone who has been outdoors in northern Minnesota knows that populations of forest tent caterpillar were very high this summer. Trees and shrubs on millions of acres were completely defoliated.

The biomass of caterpillars and amount of defoliation observed this year in northwestern counties was unprecedented in recent time. Defoliation of tamarack, white spruce and, even, red maples were reported. The heaviest defoliation was observed along and north of State Highway # 1. Scattered light to moderate pockets of defoliation were observed south of the highway. FTC defoliation was completed for 200l as most caterpillars entered the pupal stage during the week of June 18th.

In Grand Rapids, the first caterpillars started spinning cocoons and pupating on June 13th. By June 22nd, approximately 50 to 75% of the caterpillars had formed cocoons with the remainder still feeding and wandering around. By June 26th, 95% of the caterpillars had formed cocoons

For most of northern Minnesota FTC finished forming cocoons during the last week of June. On June 27th, the first moths were observed in Nashwauk and June 28th in Grand Rapids, both in Itasca County. The major moth flights are likely to occur near July 4th.

It is colder by the Lake and right along the shore of Lake Superior no cocoons were found as of June 19th. The lilacs were just in full bloom, so it was early, phenologically speaking. Caterpillars along the Shore were still very active; feeding, wandering around, crossing roads and crawling up buildings, etc. Just inland, however, caterpillars were forming cocoons.

The rains we have had this spring and early summer should help reduce stress on the trees and most should produce a pretty good second flush of leaves in July. Sarcophagid flies ( friendly flies) that parasitize the pupae in the cocoons are present in many locations especially in areas that have now had two or more years of defoliation. The numbers don't appear to be high enough in most locations to significantly reduce next years populations.

Field observations of the forest tent caterpillars in central Minnesota during June 21st and 22nd revealed the following information:

  1. Defoliation is noticeably less than last year around Mille Lacs Lake and in southern Todd County as detected by aerial mapping and ground surveys.
  2. Many caterpillars died and are stuck on leaves and twigs. Similarly, many live caterpillars appear to be parasitized or diseased (limp bodies, not feeding).
  3. Tent-making and cocooning started June 2lst, although a few cocoons were found earlier in two locations.
  4. Trees are refoliating and the new leaves are untouched by caterpillars.
  5. More parasitic flies were observed this year than last, but not enough to indicate that there will be fewer tent caterpillars defoliating our trees next year.

During the last week in June, several locations in Regions 2 and 3 will be sampled to determine the causes of and level of parasitism that occurred this year to late instar caterpillars and pupae.


Oak anthracnose

Severe anthracnose on white and bur oaks is very common in the north central and central counties. In Region 1, there were about as many inquiries about this disease as there have been for FTC. The frequent rains and cooler temperatures that occurred during June created ideal conditions for the spread of anthracnose. The word "anthracnose" was coined in France and taken from the Greek "anthrax" and "-nosis," which mean carbuncle and disease, respectively. It was first applied to a grape disease but eventually it was given to many plant diseases characterized by distinctive and limited lesions on leaves, twigs and fruits. The lesions usually have a raised border around a more or less depressed center. It is often accompanied by twig dieback.

oak leaf affected by anthracnose

Several different fungi cause anthracnose on various plants, but on our oaks there are two fungi, Gnomonia quercina and Gleosporium quericinum, that overwinter on leaves, buds and twigs, then spread to uninfected parts via rain-splashed or wind blown spores the following spring or summer. Young leaves develop brown spots or blotches, especially near veins that also turn brown or black. Spore-producing structures develop on the diseased veins and leaf blades. If the infection occurs after leaves approach full size, only small necrotic sports may develop because mature leaves are more resistant. Infection often first becomes prominent on lower branches, then spreads upward. Since individual oak trees vary in susceptibility to these fungi, one oak can be severely infected and a nearby oak may be totally free of anthracnose. Control involves collection and disposal of fallen leaves and twigs and removing a few branches to increase air movement through the crown to allow rapid drying of leaves.

Hail to the leaf

Hail damage can be quite injurious to trees and shrubs depending on the size of the hail stones, their velocities and angles of impact. The leaves of deciduous trees are especially vulnerable, with damage varying from holes punched through the leaves to complete shredding of the foliage. Trees with often will releaf, if not too late in the season.

Branches with horizontal arrangement are especially exposed to this assault. Larger hail can damage to the point of severing them at the point of impact, at the node between previous years growth (this is quite common with white oak), or at union with the main trunk or branch. Often the scars from hail damage can be seen years later.

Main stems can also be damaged, as high winds associated with these storms bend saplings and expose them to more direct impacts with hail stones. The single terminal leaders of conifers may be damaged to the point that their value as timber products is reduced.

The greatest damage from hail is not from impact wounds but rather from increased physiological stress which allows opportunistic insects and disease organisms to invade the tree.

Hail damage commonly affects small geographic areas and unless it occurs in inhabited areas, it often goes unnoticed until the secondary damage from insects and disease becomes quite apparent.

Utilization of diseased/infested wood

As we move into July, trees struggling with various forms of disease become noticeable as their leaves wilt, yellow and/or drop. Afflictions range from the wilt diseases, slow declines (usually a combination of age, stress, and disturbance), mechanical injury such as construction damage or bark beetle infestations (often associated with weather-related stress). Removing and disposing of these trees can be an expensive proposition. Since taking the wood to the landfill wastes a valuable resource and is prohibited by most landfill operations, many homeowners struggle to find an effective solution to the problem.

If you have large quantities of wood, your first alternative is to contact your local DNR forester to obtain a list of the wood suppliers in your area. Besides taking the wood off your hands, some will remove the trees for you (this depends on the volume and quality of the wood). This is particularly true if you have whole stands that are being cut or thinned for new construction or major renovation. If that doesn't describe your situation, there are still alternatives. Here are some of them.

Insects and Disease Concerns

The primary concern is whether or not you are spreading potentially dangerous insects and diseases by using and/or moving wood products. The primary answer is YES YOU CAN, so you need to be careful. However, there are things you can do to prevent the accidental movement of key pests (see below, especially if you have a bark beetle infestation).

Burn the wood prior to April first. Spring is the season most insects and disease organisms initially infest trees. They emerge from their overwintering grounds and move to new locations. Often their travels involve moving into or out of freshly cut wood. If you burn the wood prior to their emergence, you eliminate the possibility of them getting out to infect new trees.

Remove the bark. Bark and turpentine beetles (including pine and elm bark beetles) need intact bark over moist wood in order to reproduce successfully. Moving fresh cut wood with the bark still attached risks moving any insects inside to the new site. There they can introduce wilt diseases or infest healthy trees leading to additional tree loss. You can prevent moving these insects by removing the bark. The wood will dry quicker and any bark beetles inside will die before they have a chance to infest new trees.

Removing the bark prevents the oak wilt fungus from fruiting. It too must have intact bark over moist wood in order to reproduce.

Removing the bark also prevents moving hitchhikers, like the gypsy moth that like to hide eggs and pupae in bark cracks, from one place to another.

Chip the wood. Once dry, none of the wilt disease fungi, and few other tree pests, insect or fungal, can survive long in chipped wood. While there is a very small chance that some of the canker causing fungi could survive long enough to release spores, the chance is slim indeed. If you are worried about canker diseases, you can spread the chips out to speed drying or cover the chips temporarily with black plastic to 'bake' them dry. Wood chips can be used in the landscape as mulch or sold for other wood products. Beware of fresh poplar or willow chips - they might sprout on you!!!

Cover the wood pile for 1 year. Fresh firewood can be stored for longer periods of time by covering the wood for the first year with plastic and sealing the edges so no insects can get into or out of the pile. The best way to 'seal' the pile is to bury the edges of the plastic in dirt and place wood or rocks over the edges to keep them in place. While decay fungi can survive under the plastic, insects and most disease organisms cannot survive for longer than a year. The oak wilt fungus only fruits during the first year after the tree's death. Elm bark beetles utilize the wood for only one year after the tree's death. Once the wood has aged, it can be uncovered and used as desired.

Special bark beetle concerns. If your trees died because of a bark beetle infestation, it is particularly important that you get the trees down and that the wood is handled properly. When the wood is cut and how it's handled can help clean up an infestation or promote its expansion. If possible, cut several live trees during the winter or early spring and leave them on the ground to serve as 'trap trees'. As bark beetles emerge from their overwintering sites, they will be attracted to the cut trees and will attack them in large numbers. Once the beetles are inside the cut logs (around mid-May), the logs must be removed. All cutting and tree removal should be done by June 1st to prevent the second generation of beetles from expanding the population. The wood must be debarked, chipped or buried to keep the second generation of bark beetles from attacking the remaining trees. The slash (smaller branches and twigs from the tree tops) should be burned, or cut into small pieces and scattered where it will dry quickly. For specific details on managing a beetle infestation, see the DNR publication "How to Identify and Manage Pine Bark Beetles" or contact your local area forester.