Scouting and Control of Common Pests of Fraser Fir  



 
Rust Mites
 
Rust mites are also eriophyid mites, but they live on the surface of the needle instead of inside the bud. The hemlock rust mite, Nalepella tsugifoliae, is a frequent springtime problem on hemlocks grown in the foothills of western North Carolina. Mite numbers buildup quickly in the spring, increasing from February through May depending on the weather. However, white pine and Fraser fir Christmas tree growers also have occasional problems with this pest at higher elevations.
    Symptoms. Rust mites suck juices on conifer needles. When many mites are present their damage gives the needle a dusty, rust-colored appearance. To distinguish rust mite damage from a nutrient deficiency, look at damaged needles with a hand lens. Tiny marks or scratches running parallel to the midrib of the needle can be seen on rust mite-damaged needles as if they had been rubbed with sandpaper. These marks won't be apparent if the rust-colored appearance is due to a nutrient problem. Damaged needles often drop off in the spring or prematurely in the fall.
    Pest description. Rust mites are worm-like eriophyid mites similar to the rosette bud mite. Rust mites have a wedge-shaped body with only four legs on one end. Rust mites are very small, about the size of pollen, and require a hand lens (7X to 15X) or dissecting microscope (30X) in order to be seen clearly. They can be found on both the upper and lower surface of the needle. They can be clear, tan, cream, or orange-colored and if viewed long enough with a hand lens, they can be seen to walk or rear up on their abdomens. With a dissecting microscope, clusters of tiny round eggs can sometimes be seen on the underside of the needles.

    Similar appearing problems. Rust mite damage is often mistaken for nutrient deficiencies or winter injury. Sooty mold caused by balsam twig aphid or Cinara aphid activity, as well as spruce spider mite damage, can also be mistaken for rust mite damage. Be sure to check damaged shoots with a hand lens to distinguish between these problems and to find the rust mites themselves. Even with a hand lens, pollen on needles can be mistaken for rust mites. Be sure to look carefully.
    When to scout. Rust mites are almost always a springtime pest, but they can also reappear in the fall. They are not a problem in Fraser fir Christmas trees in western North Carolina every year. Below normal rainfall during extended springtime temperatures encourages rapid increases in rust mite populations. Rust mite activity in the mountains can often be predicted based on early activity in hemlocks in the foothills.
    The first scouting survey should be done between mid-March and mid-April, depending on temperature. It is valuable to determine rust mite presence in a field before the balsam twig aphid treatment window. If rust mites are present and need to be controlled at this time, it can affect the choice of pesticide for twig aphid control.
    Scouting method. In hemlocks, rust mites can often be evaluated using a hot spot or random walk scouting method. However, rust mites in Fraser fir are often not as evenly distributed across a field. Therefore, it is best to use the rigid block scouting method until patterns in the field can be determined and hot spot scouting can be used. When walking through the field, look for trees with bronzing foliage, particularly near the base of the tree. Much like when scouting for the spruce spider mite, if no damage is observed, randomly pick a tree every 10 to 15 trees.
    To examine trees for rust mites, take some shoots of the most current growth from the upper portions of the tree, and some from the lower. If the buds have recently broken, take both the new growth and previous year's growth, noting if the mites have moved onto the new growth.
    When scouting for the rosette bud mite and spruce spider mite, it is only necessary to estimate the percentage of trees that have the mite. Whether a shoot that is examined has a single spider mite egg or 20 mites doesn't matter. When scouting for rust mites, the number of mites on a needle does become important. As each shoot is examined for rust mites, keep track of the percentage of shoots that have mites, as well as the greatest number of mites on an individual needle, adding the number on both the upper and lower surface of the needle.
    Treatment threshold. To determine if a pesticide is necessary, both of the following criteria must be met:

1. At least 80 percent of the shoots have mites on them. In most cases, it is not necessary to treat until the majority of the trees have at least a few mites on them. This percent incidence is determined by dividing the number of shoots with at least a single mite somewhere on the shoot by the total number of shoots examined.

2. There are at least 8 mites on a single needle on one shoot. Only one needle on one shoot has to meet this criteria to reach treatment threshold if 80 percent incidence has been reached. Count mites both on the front side and back side of the needles to reach this sum.

    Scouting frequency. If rust mites are found in trees in the spring, numbers can quickly increase if the weather is cool and dry. Trees may need to be scouted every week or every other week during critical periods. Hot spot scouting may allow you to follow how the weather is affecting mites. As cool, spring-like temperatures give way to summer heat, the rust mite populations usually begin to fall off. Sometimes this may not occur until July. Keep scouting until you witness a decline. If rust mites were a problem in the spring, you may need to check for them again in the fall.
    Control. Successfully treating for rust mites requires using a miticide effective against eriophyid mites, such as Sevin, Joust, or dimethoate. Some of the newer miticides on the market state that they only control spider mites or tetranycid mites. These may not control eriophyid mites which are biologically different.
    Scouting after control. It is necessary to scout trees 1 to 2 weeks after treating trees to determine if rust mites were killed. Trees may need to be scouted a second time even after a successful chemical treatment if cool temperatures persist for several weeks.
 
 

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