Scouting for Rosette Bud Mites
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Scientific name: Trisetacus fraseri
Where from: Native to North America
Type of pest: Cosmetic damage
Pest description: The rosette bud mite look almost exactly like hemlock rust mites. They live by the hundreds in cavities in affected buds. They are tiny, wedge-shaped with four legs on the wider end.
Damage to tree: Rosette buds are larger than normal and flattened instead of pointed in Fraser fir. They do not break and grow because there is a cavity inside instead of a tiny shoot. The resulting loss of buds causes uneven density, gaps, holes and weak bottoms. Good quality trees can still be grown but it will take 1-2 years more to fill the trees out properly.
Where found in field:
- Rosette buds are more likely to be found in protected fields where humidity stays higher.
- Fields at higher elevations have traditionally had more problems with rosette bud mites. Avery County has the greatest incidence of rosette bud mites as well as some areas of Jackson County or other high elevation farm locations.
Where found in the tree: Rosette bud mites are inside affected buds of Fraser fir. Rosette buds are more common in the lower half of the tree.
- If rosette buds are found in your area, pay attention to bud development in the early part of the rotation to determine if blocks have rosette bud mite infested trees.
- To determine need for treatment: In trees more than two years from harvest, determine the percent incidence of trees having rosette buds. Do this by walking the entire length of two or more rows through a block to view at least 30 trees. Keep track of the total number of trees you walk past on one counter and the number of trees with rosette bud mites on another. It is only necessary to view trees from the direction you are walking. Divide the number of trees with rosette buds by the total number of trees examined to calculate the percent incidence.
How weather affects: The rosette bud mite moves from the old bud into the newly breaking shoots at bud break. They can survive this transition much easier when the weather is foggy or rainy. During dry springs, rosette bud mite incidence decreases.
How to develop treatment threshold: Treatment thresholds will vary depending on each grower’s needs for quality and cost of treatment. A good working threshold is to treat trees more than 2 years from market if more than 10% of the trees have rosette buds as determined by the scouting protocol described above.
What can be confused with this pest/damage:
- Some individual trees produce larger buds than normal that can be mistaken for rosette bud mites. To distinguish these from rosette buds, slice the bud in half to reveal the cavity with mites in them. Healthy buds will have a green, triangular bud that will break and grow in the spring as well as well-organized bud scales surrounding it.
- Trees damaged by the balsam woolly adelgid may have buds with swelling at the base (gouting) that can also be mistaken for rosette buds. However, woolly adelgid trees will still have a green growing tip inside the bud. In buds affected by rosette bud mites, there is only a cavity and no growing tip.
Important natural enemies: None known.
For more information on rosette bud mites including photographs and control recommendations see Rosette Bud Mites.
For the complete Fraser fir scouting manual for western North Carolina, see Scouting Manual.