Tree Care: Boring Insects
|By Phillip Meeks|
Imagine a most-hated list for insects that impact landscape trees. Even if a different list was compiled for each region of the United States, a borer or two would show up on every one. Flat-headed wood borers, eucalyptus longhorned borers, western clearwinged moths, bronze birch borers, ambrosia beetles, dogwood borers and Asian longhorned beetles are examples of the pests a landscape contractor might encounter. "Borers typically start as eggs laid in the ground that hatch in almost microscopic worms," explains Doug Gerber, president of Landscape Plant Diagnostics in Overland Park, Kansas. "These bore into the tree bark, where they spend the season eating the cambium layer just under the bark and then lay low for the winter, after which they emerge as moths and mate, lay eggs, etc."
That thin, moist layer of tissue between a tree?s outer bark and the more solid wood of the trunk is filled with xylem and phloem cells which transport water, minerals and sugars throughout the tree. If the flow of these materials is interrupted, the tree will die.
"Different species of borers cause different types of damage," explains James M. Scarlata, president of Tree Doctors, Inc., in Manistee, Michigan. "some species bore directly into sapwood, causing damage to xylem tissue, while others feed on cambial tissue and many girdle the tree.
Occasionally, a borer may be a carrier for disease. Gerber underscores the fact that one of the worst cases of tree loss in U.S. history occurred when a borer, the elm bark beetle, vectored Dutch elm disease and nearly obliterated all the American elms in North America.
Keeping trees strong
Bruce Hagen, an urban forester with the California Department of Forestry, says, "Many urban trees are stressed due to drought, over watering, soil compaction, root loss, fill soil, excessive pruning, poor soil aeration or limited soil volume." In other words, a tree planted in an urban or suburban situation has its mountains to climb. Add to this the potential damage from mowers, trimmers, children and vandals, and you?ve got yourself a recipe for poor tree health. Tree stress is a neon sign to borers that says, "Come on in."
Therefore, it?s to the contractors? advantage--and the borers? disadvantage--to keep tees in a favorable condition. "The best treatment for borers," says Hagen, "is to identify and mitigate stressful conditions. Maintain stable growing conditions through the life of the tree. Older trees gradually become more susceptible to secondary pests." Hagen also explains that the best treatment to encourage trees? natural defense against borers is proper watering, according to the particular tree?s requirements. He adds that eucalyptus longhorned borers and red turpentine beetles are greatly reduced by irrigation.
Robert D. Childs, an extension specialist entomologist with the University of Massachusetts, recommends bark mulch around the base of a tree as one borer management strategy. This suppresses weed growth, thus eliminating the need for close-proximity trimming. One mistake with a trimmer can wound a tree trunk and open door wide for borer infestation.
Helping trees defend
Thomas Pramuk, an arborist in Napa, California, has dealt with Pacific flathead borers, western clearing moths, bronze birch borers, sycamore borers, carpenterworms, ambrosia beetles, western oak bark beetles, sequoia pitch moths, red turpentine beetles and pine engravers, and reports success with Astro (synthetic pyrethroid) as a preventative borer treatment.
Especially where exotic pests are concerned, attacks may not be limited to weakened hosts, since the tree hasn?t had time to develop its own natural immunity to non-native pests. Such is the case with Asian longhorned beetle problem in New York and Chicago.
"[This beetle] supposedly was imported accidentally in wood used to create a shipment from China," says Gerber. "So far, the only means of eradication has been the removal of its food source, and thousands of trees have been cut down around the Chicago area."
Daniel Parry, a public affairs specialist with the USDA?s Animal and Plant Health Inspection Service (APHIS), quotes figures that, as of January 2002, 1500 trees in Illinois and 5700 in New York have been removed as a result of Asian longhorned beetle infestation. The quarantined areas for these two states now stand at 30 square miles and 120 square miles, respectively.
In the year 2000, 15,000 trees in Chicago, Illinois, were treated with Imicide manufactured by Mauget Company. In 2001, the USDA treated 59,230 trees in New York and Chicago.
The USDA will be continuing Asian longhorned beetle treatments this spring, with microinjections of Imicide. They are projecting to treat 150,000 trees for the year 2002. A small number of soil injections will also be conducted in Chicago, in places where the situation makes it feasible.
Close to a quarter of a million trees will be treated just in the Chicago and New York areas alone. Imagine the potential opportunity for the landscape contractor to serve his client, and make it profitable as well.
Furthermore, Parry reports that a close relative of the Asian longhorned beetle, the citrus longhorned beetle, has shown up in Washington State. This pest brought into the U.S. via bonsai stock--has a longer host list than its cousin, he says, so it?s impact could prove to be even more devastating if efforts to bring it under control fall short.
Post arrival options
If the chemical route is chosen instead, the particular situation will determine the best approach. "Chemical treatments," says Scarlata, "when necessary, may include spray applications to bark surfaces, soil injection and injection into trunk/rootflare of trees. Chemical treatments available will vary by species, time of year and regional label restrictions."
Says Gerber, "Treatment for borers has been limited since the removal of a chemical called lindane. Actually, it was legislated out of the commercial trade and made illegal to produce anymore." In its stead, Gerber recommends horticultural oils such as Sunspray Ultra-Fine, and systemic insecticides. Imidacloprid, chlorpyrifos and permethrin remain among the popular materials for borer control.
Microinjections work by transporting chemicals via a trunk?s xylem and phloem tissue, and therefore, the benefit of this type of treatments will be directly proportional to how advanced the infestation is at the time of treatment. Scarlata says, "If a tree already has some damage, spray application is the most reliable as the chemical can be distributed evenly over the entire tree. Translocation of injected chemicals will be inhibited by the damage to the xylem and cambial tissue. Another Advantage to spray applications is that the insect is exposed to the insecticide of the outside surface of the tree, before boring or oviposition occurs, while injected chemicals are ingested by the insects after they start feeding."
"Whatever the treatment," cautions Gerber, "it?s important to note that using the proper amount, at the proper time, is the best way to control insects that have become pests to plants and trees."
If infestation is too advanced and removal must be done, handle this in such a way as to lessen the spread of the pest. Chipping may be effective, for instance, and the contractor should be cautious of carrying felled wood to other locations. Also be aware that in the case of an insect such as the Asian longhorned beetle, it may be unlawful for anyone outside the USDA to handle the removal of an infested tree.With the exception of non-native insects running amok, borers aren?t necessarily the most threatening insects to healthy trees. However, conditions common in a landscaping situation can weaken tree to the point that borers can easily deliver the death blow. Keeping the tree healthy enough to fend for itself is the contractor?s best modus operandi in most situations, but if invasion has already occurred, he may be able to make a difference with parasitic nematodes, horticultural oils or the application of chemicals through spraying, soil injections or trunk injections. The treatment of choice, of course, will depend upon such factors as the client?s preferences, insects, hosts and local limitations.
Keep in mind that once the borer moves in, it may be an uphill battle to bring the tree back. Therefore, it will pay the contractor to deal his strongest, most effective hand up front. As far as the welfare of the affected tree is concerned there might not be a second chance.