Scouting and Control of Common Pests of Fraser Fir  

Rosette bud mites (Trisetacus fraseri), (RBM), are currently found in only a few counties in western North Carolina, and throughout high elevation (greater than 4,000 foot) fields. However, these mites continue to spread throughout the region. Contact your local county extension agent to determine if rosette buds are a problem in your area.
    The RBM damages the tree by entering newly developing growth and spending their entire life inside the new buds. Mites feeding on the interior of the bud cause it to become distorted. The shoot for next year is destroyed in the process. The effect of heavy infestations on a tree is dramatic, creating stunted, uneven growth. It can reduce tree quality and grade considerably.
    Symptoms. Rosette buds are large, rounded buds on the terminals of Fraser fir branches that either fail to break in the spring, or else break to form multiple, small shoots.
    Pest description. A rosette bud may be colonized with as many as 3,000 microscopic mites, called eriophyid mites. An eriophyid mite is entirely different from the spruce spider mite. Eriophyid mites are worm-like, having a wedge-shaped body with only four legs on one end. Other types of mites, such as spider mites, have eight legs at maturity. Eriophyid mites are very small, requiring magnification such as a hand lens (7X to 15X) or dissecting microscope (30X) in order to be seen clearly.
    Similar appearing problems. The swollen buds and stems, sometimes called gouting, that are caused by the balsam woolly adelgid can be mistaken for rosette buds. To distinguish between the two, look at the surface of the bud for balsam woolly adelgid nymphs or adults, and cut open the bud to look for rosette bud mites.
    When to scout. The RBM can be found in and damage even small trees. Start scouting trees from the second year in the field in areas where rosette buds are common. The presence of rosette buds in a block can be identified while scouting for other pests. Only scout specifically for rosette buds if they are present in the block, and the need for chemical treatment the following year is being evaluated. Trees only a year from harvest do not need to be treated. Rosette buds cannot be distinguished from healthy buds until after August.
    Scouting method. Look for terminal shoot buds with swollen, flattened, and usually a darker or redder brown color.
    If found, remove the buds from the tree and slice the bud open with a sharp knife from the tip of the bud down to the stem. If it is a true rosette bud, it will have a cavity in the middle and will be missing the green triangular shoot for next year. Use a hand lens to examine the cavity for the presence of mites. Look inside several buds, as not every bud will have mites in it. Rosette buds on older growth will be cracked open, and will be desiccated and dead in appearance.
    To decide if trees need to be treated for RBM, the percentage of trees with rosette buds must be assessed. Walk every 6 to 10 rows back and forth through the block. Walk slowly, observing the sides of both trees between which you are walking. Only be concerned with the sides of the trees facing you. Count how many trees there are with at least one rosette bud. Also keep track of the total number of trees in the two rows on a counter. Divide the number of trees with rosette buds with the total number of trees. This will give an approximate field percentage of RBM.
    Treatment threshold. Due to the life cycle of this pest and the nature of the damage, the benefit from a RBM treatment won't be realized until the year following the application. Therefore, it is of no benefit to treat a field of trees that is within a year of harvest. In fields 2 or more years from harvest, a RBM incidence of 10 percent or more should be considered for treatment. In fields with partial harvest upcoming or fields with multiple aged trees, the decision and timing of treatment can be more complicated. Contact your county extension agent for help with the decision.
    Scouting frequency. Blocks of trees with identified rosette buds should be scouted once a year through the year before harvest.
    Effects of weather. The rosette bud mite requires humid, wet conditions in the spring at bud break to survive moving from the old bud to the new growth. During dry springs, or if late freezes damage new growth, mites are often killed before moving into the new growth, and rosette buds are often fewer than the year before.
    Control. Four cultural practices are essential for controlling rosette bud mites: not interplanting young trees with old; maintaining good fertility; shearing early and lightly to encourage good bud set; and harvesting heavily infested trees as early as possible to reduce the number of rosette bud mites in the field.
 
Fraser fir bud after dimethoate treatment

    If necessary, treat trees in early to mid-June with a single application of dimethoate to wet the current year's growth. Thorough coverage and proper timing are essential for control.
    Scouting after control. Dimethoate is systemic and kills the mites after they have already started to feed and to affect how the developing bud will grow. Because of this, rosette buds will appear on trees even if the mites have been successfully killed. However, live mites will not be present in these buds, and there will be a healthy shoot that will grow the following season. Examination of the buds under a microscope in the fall or winter is required to learn if mites are absent and the treatment was successful. Contact an extension agent with the local North Carolina Cooperative Extension Service for help in determining if treatments were successful.