Scouting for Spruce Spider Mites
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Scientific name: Oligonychus ununguis
Where from: Native to North America
Type of pest: Cosmetic
- Spider mites are small and red to almost black in color. They have eight legs at maturity.
- Immature mites are smaller versions of the adults though with only three pairs of legs. While they are molting, they do not move and may appear dead.
- The eggs are small and round with a single hair coming out of the center. Only eggs are found in the winter.
- Occasionally spider mites will produce thin strands of webbing like a spider.
Damage to tree: Spider mites are a pest of most conifers. The spruce spider mite feeds by sticking its feeding tube into the needles. This causes tiny yellow spots to form on an otherwise green needle. When the damage is light, the spots will be at the base of the needle. As feeding progresses and damage becomes worse, the entire needle will have spots. Use a magnifying lens and flatten needles to see damage clearly. When damage is heavy, the whole needle appears yellow from a distance, but on closer inspection, the spots are still evident. Needles with heavy damage are more likely to drop off in the fall or after the tree is harvested.
Where found in field:
- Spider mites are more of a problem at elevations lower than 3,000 feet in western North Carolina.
- Hotter and drier areas of the field such as south and southwestern exposures and ridge tops are more prone to spider mites.
- Spider mites are more of a problem along dusty roads as the dust that settles on needles kills predatory mites and not the more sedentary spider mite.
Where found in the tree: Look for spider mites on the most current growth. Spider mites are often found first on the bottom half of the tree.
- Thorough scouting method specifically for mites: Scout throughout the block, looking at a single shoot from 10 to 20 trees. Choose trees with yellow foliage as well as trees at random. Select a small shoot (2 to 4 inches) from the lower half of the tree and examine foliage both front and back for mites, eggs and damage. If the tree exhibits mite damage, choose this shoot from the damaged areas. Keep track of the number of shoots examined and shoots with either spider mite eggs or mites. At the same time, also scout for rust mites. Also take note if predatory mites or other predators are found.
- In conjunction with twig aphid scouting: While scouting for twig aphids by beating foliage over a plate, also check some trees for spider mites and rust mites by selecting small shoots and scanning them with a magnifying lens for mite activity.
- Hot spot scouting: Identify hot spots of mite activity early in the rotation and mark on map or with flagging. Check these areas in summer and fall during periods of hot, dry weather and low humidity.
How weather affects: Spider mites are more of a problem during warm, dry weather and low humidity. That is because wet weather slows mite egg hatch. Also, predatory mites prefer humid conditions.
How to develop treatment threshold:
- Spider mite damage is seldom an issue in trees more than a year from harvest.
- In the past, treatment thresholds of between 10% to 40% of trees infested were used with the lower thresholds for trees nearer to market.
- Many miticides can only be applied once a year due to the possibility of resistance developing. Therefore, it’s important not to overuse the same material. If mites are present but wetter weather is in the forecast or mite predators are present, delay treatment to allow natural controls a chance to work. Scout again in 1 to 2 weeks to determine if mite numbers are declining naturally.
What can be confused with this pest/damage: Yellow foliage can be caused by many things such as poor fertility and scale feeding. However, with closer inspection, spider mite feeding is the only thing that causes tiny, pinpoint size yellow spots on an otherwise green needle. Scale feeding causes yellow blotches in an otherwise green needle that resembles a camouflage pattern. Nutrient problems cause an overall yellowing of the needle.
Important natural enemies:
- Predatory mites feed on spider mites. They are about the same size as spider mites but move much faster – running along the shoot or on a beat sheet. There are many species of predatory mite, ranging in color from reddish orange to tan.
- Lacewing larvae
- Minute pirate bugs
- Predaceous thrips
- During wet periods, spider mites may succumb to fungal pathogens. Their dead bodies will be covered by black fungus.
For more information on spruce spider mites including photographs and control recommendations, see Spruce Spider Mite.
For the complete Fraser fir scouting manual for western North Carolina, see Scouting Manual.