FIAD Header

Shade Tree and Forest HealthJune 5, 2001

Exotic tree pests: What can you do?

Exotic species are those that are not native to our area. Not all exotic species are detrimental, (for instance oats and wheat) but many are. A combination of factors can lead to unchecked population expansion. They include:

  • lack of controlling natural enemies (purple loosestrife and fire ants)
  • development of new associations between alien parasite and native hosts (AIDS virus in humans and the gypsy moth in US oaks)
  • aliens becoming more effective as predators in the new ecosystem (brown tree snake and feral cats)
  • artificial or disturbed habitats providing favorable ecosystems (weeds in crop and lawn habitats)
  • invasion by some highly adaptable and successful species (zebra mussels, water hyacinth and gypsy moths)

When an exotic species becomes established they can result in:

  • destruction of native habitats and/or a threat to native species (White pine blister rust, chestnut blight, feral pigs and the brown tree snake which alone has eliminated a dozen species of birds and lizards)
  • reduced crop and animal production (many weed species)
  • loses costing millions in management expenses (gypsy moth and Dutch elm disease)
  • a threat to human health and quality of life (AIDS, influenza, and all of the above)
  • A study by Pimentel et al estimates that exotic species in the United States cause losses valued at more than $138 billion per year, in addition to being responsible for 42% of the species currently on the federal Threatened and Endangered Species list.

    What about tree pests; how are they getting here?

    The majority of exotic tree pests are either insects or disease organisms. How they enter the United States depends on their life cycle, reproductive behavior and habitat needs. The Asian longhorned beetle, which has resulted in the clear cutting of a number of neighborhoods in Chicago and New York City, entered as larvae or pupae (the resting stage before becoming an adult) inside wooden packing material used to ship products around the world. This is a common means of transportation for wood borers, because they tunnel into the wood and emerge some time later as adult beetles. Once inside a tree, they can be very difficult to detect, so they can be inadvertently moved in raw wood products such as fire wood, rough pallets or packing material, or logs headed for processing. Not including inspection costs, tree removal and replanting in those two cities cost taxpayers on average $500 per tree times the thousands of trees already removed. Since neither infestation has been controlled as yet, those costs still continue to mount.

    The female gypsy moth doesn't fly. So to protect her eggs, she crawls into hidden or shaded spots on anything she can find outdoors. This includes rough firewood, nursery stock, lawn furniture, playground equipment, vehicles, equipment or supplies stored outdoors and so on. Once moved, the larvae can hatch and move to nearby trees to start another infestation. Although the original infestation here in the U.S. began as a deliberate introduction into Massachusetts, subsequent spread by inadvertent movement by humans began the infestation in Michigan that is now moving across Wisconsin and into Minnesota. The insect now infests millions of acres of woodlands, threaten native species and cost taxpayers more than $11 million per year in management expenses.

    Dutch elm disease, white pine blister rust and chestnut blight were all likely moved on infected seedlings or wood, possibly fire wood or raw logs. The fungi involved reproduce on the wood where their spores are carried to nearby trees by either wind, birds or insects. Dutch elm disease control across the U.S. costs about $100 million per year. Before chestnut blight was introduced approximately 25% of our eastern forests consisted of American chestnut. Now they have almost disappeared from our forests. Even though white pine blister rust lead to the creation of the Conservation corp (CCC) that put much of the U.S. back to work during the great depression, it has lead to the decline of much of our white pine resource.

    The hemlock and balsam woolly adelgids are relative new comers to the U.S. But they are quickly spreading across native habitats, killing trees as they go. These insects are related to aphids, which reproduce quickly and are spread by infested wood products, nursery stock and birds. The threat of forest loss (based on the pattern seen in other states) is so high that one or two states have banned the movement of all nursery stock of those species, greatly impacting industry.

    So what can you do?

    Inspect your belongings when you move from one area to another. Gypsy moths in particular, move on household belongings. Because egg masses can contain up to 1000 eggs, the chance of reproduction and establishment is good.

    Buy from reputable nurseries and ask your nurseryman if products not grown locally have been certified pest free.

    Avoid moving wood or plant products without first checking and if need be, treating them. If possible, remove the bark of firewood to be transported. If that's not possible tightly cover the wood piles with heavy plastic (burying the edges in the dirt to prevent insect escapes) for at least one full season.

    Inspect your trees and shrubs on an annual basis for signs of injury. This is a good idea anyway, just to protect your plants. But early detection of a new pest is the best way to control it effectively at minimal cost.

    If you find a suspected exotic pest, collect a sample and put it in a container in the freezer or in alcohol, so it can be effectively identified later on. Then contact your local state authority. Extension agents, state foresters, nursery and agricultural inspectors are among those that can help you get the advice you need as well as take care of any infestation. Even though exotic pests are regulated, don't worry about prosecution. Only those that intentionally move exotic pests for their own benefit are likely to be prosecuted. So let someone know before the infestation gets out of control.

    Voles are the villains

    Meadow voles also known as meadow or field mice have been busy this past winter. With an early snowfall, deep snow depth and a long period of snow coverage, girdling of seedlings and small trees has been quite extensive in some areas. Fruit trees and ornamentals have been especially hard hit. Cover crops and grasses (including lawns and golf courses) can also be injured by the runway and tunnel systems that voles build.

    Since girdling can be caused by rabbits as well, the lack of uniformity in gnaw marks and the angle in which they are inflicted can provide evidence of voles. The gnaw marks are rather small, about 1/8 of and inch and usually occur in irregular patches. The presence of feces, small piles of cut vegetation in conjunction with runways is another telltale sign.

    Girdling was quite visible after snow melt, with the light colored wood exposed. Deciduous tress and shrubs at the present time are showing a lack of vigor, slowness to leaf out, and wilting leaves. Conifers often show a discoloration of needles, with some already having dead branches covered with red needles. If the stem is completely girdled the plant will die.

    Control of vole damage is best accomplished by practices that eliminate tall vegetation and litter around the trees, but this isn't usually practical except under agricultural or urban settings. Grass should be mowed as short as possible before snow fall occurs with the removal of vegetation and mulch from the base of the tree.

    Repellants containing Thiram or capsaicin (hot sauce) may be effective if applied to the stem from the ground level to a height of about two feet in the fall and especially before the first snow fall. The use of mouse traps (snap traps or live traps) can be effective on the small scale but is labor prohibitive with large areas. The use of toxic baits containing zinc phosphide is often the most effective and common means of vole control within large areas that can't be managed by the previous methods.

    Remember that fall is the time to prevent winter injury from voles.

    Deer repellents and bud capping

    Deer browse on young trees can cause leader death or damage that results in undesirable bushy form rather than straight and single-trunk trees. Such browsed trees are usually unacceptable for timber or landscaping purposes. To protect trees from deer browse, a number of chemical taste or odor repellents have been marketed. Bud capping (stapling a white paper over the terminal leader and buds) has provided physical protection of young conifers. Bud caps are labor intensive.

    To test selected chemical repellents and bud capping, three experiments were set up on state-owned plantations last fall and the results were read in April and May of this year. The chemical repellents included Ropel (bitter taste), Treeguard (bitter taste) and Plantskydd (blood odor). The locations of the three plantations, the experimental treatments, and the results are described below.

    1. At the Crow Wing county plantation groups of 30 to 33 jack and white pine seedlings (l0 to 24 inches tall) were treated as described and distinguished by colored tapes, and the following observations were recorded May 3.
    2. At the Pine County plantation, l6 miles east of Sandstone, oak seedlings (l to 4 feet tall) were treated as described and the following observations were recorded April l3th.
    3. At the St. Louis County plantation, approximately l5 miles north of Orr, white pine seedlings (l0 to 24 inches tall) were treated as described, and the following observations were recorded April 27th.
    Percent of seedlings protected
    Treatment Crow Wing County Pine County St. Louis County
    White and jack pines Red oaks White pines
    Bud capping, white paper 0   97
    Bud capping, newspaper 6    
    Ropel 0 21 78
    Ropel and BC 3   91
    Plantskydd 31 44 97
    Plantskydd and BC 6   100
    Treeguard 0   93
    Treeguard and BC 3   100
    Ropel and Plantskydd   25 89
    Treeguard and Plantskydd     97
    None 0 No control tmt. 85

    Variables at the Three Plantations:

    1. Crow Wing County. Rainfall after experiment was set up: Oct. l4 & l5--.71inches; Oct. 25 to 27--.97 inches; Nov. 1,2 & 5--2.1 inches. Heavy mouse girdling. Deer per square mile (simulated from modeling): 18.
    2. Pine County. Rainfall after experiment was set up: Nov.--.72 inches; Nov. total--4.48 inches. No mouse girdling. Deer per square mile (simulated from modeling): 22.
    3. St. Louis County. Rainfall after experiment was set up: Nov. l,2,3--.84 inches; Nov. 6,7--.74 inches. No mouse girdling. Deer per square mile (simulated from modeling): 12.


    Where number of deer per square mile is 18 or more, bud capping and/or the three tested repellents provided very little browse protection.

    Where number of deer per square mile is 12 or less, bud capping and /or the three tested repellents may provide good browse protection.

    Where rainfall after bud capping and /or application of the three tested repellents is less than l.6 inches before snowfall, good browse protection may occur. Heavier rainfall may cause removal of chemical repellents. Of the three tested repellents, Plantskydd provided the best protection.

    Thanks to the local DNR foresters for help in setting up, treating or reading this study: Doug Hecker, Dave Sapocie, Molly McGlip, Bob Bachman and Keith Simar.

    Burlap, rope and wire around tree roots

    Many roots are cut when a mechanized spade removes large trees from the ground where they have grown for several years. To ease handling, each ball of soil and roots may then be wrapped with burlap and rope, and perhaps further secured with strong wire caging. Today's burlap has a tighter weave than that of years ago, restricting diameter growth on roots lucky enough to grow beyond the burlap. Rope is usually wound around the stem of the tree where it starts girdling the tree as soon as the tree starts growing. Wire caging has openings that roots grow through. It may take a number of years for the roots to enlarge, but the roots will eventually be strangled by the wire. The wire just doesn't disintegrate/ rust quickly enough.

    To allow good root growth, burlap, rope and wire caging should be removed or trimmed away once the plant is seated in its hole. Without their removal, root growth can be restricted and root death is likely. Above ground symptoms are branch dieback or tree death that can occur within two years after transplanting but sometimes as long as fifteen years after transplanting.