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The chemical barrier is established on a line midway between the diseased tree and the adjacent healthy tree and should extend as far as necessary to disrupt all potential root grafts. A series of holes, 15- 18 inches deep, 3/4-1 inch in diameter, and 6-12 inches apart, are made along this line. One part of Va pam is mixed with 3 parts water, and 2-8 fluid ounces of the diluted chemical are placed in each hole. Vapam should be applied 2 weeks before the tree is removed. Va pam should not be applied within 8-10 feet of a healthy tree since injury may occur because of root loss and chemical uptake. Soil temperatures below 50oF. and waterlogged soils reduce effectiveness of treatment. Regrowth of roots across the control barrier is not a problem since the diseased tree and its root system die and grafting can no longer take place. Root grafts can occur under sidewalks and driveways; therefore, it is advisable to angle the holes beneath asphan or concrete to disrupt root grafts. All root grafts must be disrupted if this means of spread is to be stopped. Pruning Diseased Trees Early beetle infections can be removed from elm trees. A minimum of 8-10 feet of disease-free wood (no discoloration beneath the bark) below obviously infected branches must be removed. The entire circumference of the branch must be examined to be certain that the fungus has been removed. If elms are pruned when beetles are active, tree wound dressings should be applied to all pruned surfaces more than two inches in diameter. These trees should be examined regularly for any further development ofthe disease so that they do not become a source of the fungus and beetie. it is very unlikely that the Dutch elm disease fungus can be spread on pruning equipment, but it is advisable to wipe equipment clean when moving from diseased to heanhy trees. Insecticides Although a few insecticides are labeled for application to the crowns of healthy trees to prevent or reduce insect feeding, they add only a small amount of protection and are not nearly as effective in reducing bark beetie numbers as proper wood disposal. Therefore, this type of application is generally not recommended. The insecticide chlorpyrifos (Dursban) can be applied to the base of heailhy trees to prevent orreduce overwintering by native elm bark beetle aduits. It is recommended as a supplement to sanitation in areas where the native beetle is a significant factor in the spread of Dutch elm disease. Dursban must be applied on a community-wide basis to be effective. It is not recommended for individual homeowners. Beforespraying is initiated, sampling techniquesshouid be used to determine the presence of the native bark beetle in the area. Details on sampling and application can be found in Minnesota Tree Line 27, Native Elm Bark Beefle Control. Trunl<spraying does not affect the European elm bark beetle because it overwinters as a grub in dead or dying elm wood and not as an adult at the base of healthy trees. The best method for controlling elm bark beetles that overwinter as larvae (grubs) is sanitation - promptly removing and disposing of brood wood. Fungicides Systemic fungicides, when properly administered, will protect healthy elms from infection and will cure trees in an early stage of beetle infection. Benomyl (Lignasan BLP or Corex) and thiabendazole (Arbotect 20- S) have been approved for eim tree injection. Of the two chemicals, Arbotect 20-S is the most effective because it may provide protection for two and possibly three growing seasons and can save diseased elms not infected via root grafts, provided the infection is in an early stage, the chemical distribution is complete, and the dosage is adequate. Complete distribution ofthe chemical can be achieved with a root flare injection below grade with two injection sites per inch of trunk diameter (hole diameter: 3/16 to 5/16 inch). Elms can be injected when they have leafed outfully but should not be treated before June 1. Best long-term protection resuils from treatments administered after July 1. Effective use of either chemical is expensive and injurious to the tree. Injection is recommended only for highly valued trees, and then only if the tree is in imminent danger of becoming infected. Therapeutic treatment oftrees with minor infections, less than 5 percent of the crown, may be the most cosl- effective. Step-by-step instructions for injecting fungicides are provided in Extension Folder 504, Howto Inject Elms W1th Systemic Fungicides. Ailhough promising and useful in certain situations, systemic fungicidesmust 8