CTN - 029
Jill R. Sidebottom,
Extension Forestry,1993
Updated November 1, 1995
If you have Fraser fir Christmas trees with off-color or chlorotic foliage, especially after a dry spell in the summer or fall, chances are you have damage from the spruce spider mite (Oligonychus ununguis) (SSM).
To determine if the SSM is really causing the problem can require careful looking with a magnifying lens. As a Fraser fir grower, it's important you learn the difference between SSM damage and a nutritional problem, and can find and identify the mite itself.
Figuring out how to prevent SSM damage from reoccurring requires you to evaluate everything from your growing site, to your pesticide use, to your groundcover management. Preventing SSM damage is important. Heavy damage reduces the quality and, therefore, the grade of go-to-market Fraser fir. Cut trees may have to be replaced if needles drop after shipping.
Mite damage may also delay harvest. Dark green trees tagged for harvest in July may not be marketable because of subsequent SSM damage. Harvest may be delayed as long as two years for the discolored, damaged foliage to be covered by good growth.
Is your damage caused by the spruce spider mite? Many things can cause Fraser fir needles to yellow. However, the SSM is the one that causes tiny, yellow spots to form in an otherwise green needle.
When damage is still light, spots are found at the base of the needle. To see light SSM damage well, you may need to spread the needles apart and even use a magnifying lens.
As mite numbers increase, the entire needle becomes spotted. The yellow spots merge and turn brown. Mite-damaged needles are more likely to drop off in the fall, or after the tree is cut. Occasionally, there may be webbing, similar to the web spiders make, among the needles. Hence the name, spider mite.
Damage is not uniform across a block of trees. Trees with heavy damage may be next to others with almost no damage at all. Spruce spider mite damage is also more common next to dusty roads, on windy ridges, southern exposures, and following several weeks of dry weather.
If needles are uniformly yellow-green, the problem is probably nutritional. If you find large brown spots in the center of an otherwise green needle, the problem may be fertilizer burn, or a needle cast disease.
Finding the spruce spider mite. If the damage appears to be caused by the SSM, look for the mites on the most current growth of Fraser fir on sunny days when the foliage is dry. Choose shoots to examine from the lower half of the tree. Scan both sides of the shoot with a 5 to 10X magnifying lens. All mites of this type have small, soft-bodies, with eight legs and piercing-sucking mouthparts. The mite feeding with the piercing mouthparts causes the yellow spotting.
The spruce spider mite ranges in color from green to red. In western North Carolina on Fraser fir, these mites appear dark red to almost black, with lighter-colored head and legs. The SSM egg is small and brilliant red with a single hair in the center. Mite eggs are often easiest to see near the buds.
Mites cannot always be found on damaged foliage. If the damage occurred several months before, the mites may have died out because of changes in the weather, or natural predators. It is important to check for the presence of mites before applying a miticide, since there is no need to apply a pesticide if there are no mites to kill.
Spruce spider mite lifecycle. The lifecycle of the SSM is simple. The mite overwinters on shoots as eggs. When temperatures are warm enough, the eggs hatch. Immature mites are similar to adults only smaller, with three pairs of legs instead of four. Immatures molt three times before becoming adults. During molting, the mites go into a resting stage where they don't move and may appear dead. At maturity, females lay eggs and the cycle begins again.
What makes the SSM such a dynamic pest, is how quickly this lifecycle can be completed. It can take as little as two weeks to go from egg to adult. Therefore, many generations occur each growing season. In addition, at any given time during the growing season, all stages from eggs to immatures to adults can be found. This can affect pesticide performance.
Natural predators help control mites. With the SSM's ability to quickly reproduce, Fraser fir fields would soon be overrun with spruce spider mites if it were not for the underlying level of protection provided by natural predators including syriphyid fly larvae, larvae and adult lady beetles, spiders, and predatory mites. Growers inadvertently affect predator populations through management practices.
Along dusty roads, SSM causes more damage because the dust on needles scratch the more active predators, causing them to desiccate and die.
The predatory mites need high humidity to survive. On windy ridges, SSM damage is more common because predatory mites cannot survive.
In fields surrounded by woods resulting in little air movement and higher humidity, the SSM almost never causes damage to Fraser fir.
If groundcover is absent, natural predators have lost an important habitat, and their numbers will be reduced. Predators can feed on insects, other mites, and pollen in the groundcover when there are no SSM in the fir.
The most important factor determining predator survival is pesticide use. Some pesticides kill beneficial insects and mites as well as pests, while other pesticides are more selective. Pesticides commonly used in Fraser fir that are not selective include lindane, Asana, and sevin. Lindane and Asana are often used to control the balsam woolly adelgid. Even the year after a summer application of these pesticides, SSM may increase. Pesticides such as Lorsban and Provado are more selective and will not impact predators as much.
The effect of weather on spruce spider mite damage. The SSM is more active and reproduces faster in warmer temperatures. Mite damage is more likely on south and west facing slopes, and at lower elevations (below 3,000 ft. in western North Carolina) where warmer, dryer conditions increase mite activity. Often, summertime SSM damage is more severe following a warm fall or spring which allowed an increase in egg production.
Rainfall keeps mite eggs from hatching, washes mites off of the tree, and allows a fungal parasite to kill both eggs and mites. In years with adequate rainfall throughout the growing season, the SSM seldom damages trees. Mite damage is also more common in locations in western North Carolina that receive less rainfall (Figure 1).
Controlling the spruce spider mite. Keeping the SSM from damaging your trees requires you do three things: manage your trees to reduce the likelihood of SSM becoming a problem, scouting for SSM to determine if a miticide needs to be applied, and treat promptly and thoroughly with a miticide if it is warranted.
Managing your trees to reduce the likelihood of SSM becoming a problem. Choose sites for Fraser fir production carefully. At lower elevations, the effect of aspect (the direction the slope faces) becomes more important. Growing trees in fields where air movement is reduced by windbreaks or shelter woods will greatly reduce SSM damage, even at lower elevations. Site selection also affects other Fraser fir pests such as Phytophthora root rot. (See CTN #022, CONTROL OF PHYTOPHTHORA ROOT ROT IN FRASER FIR CHRISTMAS TREES, for more information on the control of this disease).
Do not grow trees on bare ground. Maintain managed groundcovers around all Christmas trees. See your local County Extension Agent for more information on managing groundcovers in your area.
Never apply an insecticide unless needed. Never mix two insecticides together when one will control the pest.
Always use the least toxic pesticide available.
Avoid treating for the balsam woolly adelgid during the summer. If lindane or Asana will be used, apply them from November through March when predators are not present. See CTN #20, THE BALSAM WOOLLY ADELGID (APHID), for more information on the control of this pest.
Scouting for SSM. Many people depend on their pre-budbreak pesticide applications for the control of the balsam twig aphid to give season long SSM control. However, mite numbers can quickly increase with hot, dry weather even with a spring pesticide application. Mite numbers can also decrease quickly with rainfall or predator activity. By scouting, chemical controls can be properly timed before trees are damaged by the SSM.
Scout all Fraser fir fields from the year after planting through harvest to determine if mite numbers are great enough to damage trees. The number of times a field is scouted for mites depends on the size of the trees, the prevalence of SSM, and the weather through the growing season. For trees that do not receive a balsam twig aphid treatment, start scouting in mid-April. For trees that are treated for the balsam twig aphid, start scouting in early June. Continue scouting until the first hard freeze.
Scouting pattern. Use the rigid block scouting method for the SSM. Enter the block two to four rows from one corner. Record the row you start with on the scouting form so that on your next visit you will be able to start your survey one or two rows above or below this row. Walk the full length of the row, scanning from side to side up to five rows in each direction depending on the size of the trees. To detect mite problems, it is necessary to see a full side of most trees. In a field of 3-to-5 foot trees, you may see five rows in each direction. In a field of 6-to-8 foot trees, you may see only three rows well. When you see an off-color, problem tree, go to it to sample from, but return to your original row to continue through the field. When you reach the end of the row, step over six to ten rows as previously determined from tree size. Continue this pattern until you have covered the entire block.
Sampling procedure. To look for spruce spider mites, pull a single shoot of the most current growth from the suspect tree and examine it for the presence of mites, mite eggs, or mite damage with a magnifying lens. Take the majority of shoots from the bottom two feet of the tree, but check a few shoots near the top of the tree. Look at the small shoots from inside the tree canopy instead of the exposed outer shoots, since this is where mites are first found.
Examine the shoot with the hand lens for mites or mite eggs. If any mites or eggs are found at all, count that shoot as 1 sampled and 1 infested. There is no need to count the number of mites or eggs. If no mites or eggs are found, count it as 1 sampled and 0 infested. If the shoot has damage but no mites, make note of that too. Only look at a single shoot per tree. If you walk 50 feet without seeing any trees that have damage, pull a shoot from a tree at random. Whenever you find an infested shoot, walk at least another 30 feet before sampling another tree even if you see suspect trees. Continue sampling at least one tree every 50 to 60 feet. You should be examining at least 15 shoots per acre. You can keep track of the number of shoots sampled on a counter, and the number of shoots infested with mites in your head.
Economic threshold. Each time you scout for mites, you will make two decisions based on the percentage of trees examined that have mites. Depending on the size and value of the trees and on the weather conditions, decide first whether chemical treatment is necessary and second when it will be necessary to scout for mites again.
The need for chemical treatment. You will finish scouting the block with a ratio of shoots sampled to shoots infested. For instance, you may have found 5 shoots with mites or eggs out of 21 shoots examined. Divide the smaller number by the larger and multiply by 100 to calculate the percentage of shoots infested. In our example the number is about 24 percent.
The economic thresholds for the SSM are based on the percentage of trees that have mites. The economic threshold for treatment changes as the trees grow older and more valuable. The following thresholds are suggestions and can be modified depending on your cost of control and your markets.
| Size of the Tree | Economic threshold |
|---|---|
| 1. less than waist high | 40% |
| 2. waist high to year before sale | 20% |
| 3. year of sale | 10% |
1Percentage of infested shoots to sampled shoots.
Deciding when to scout again. If your SSM percentages have not reached your thresholds, you should schedule you next SSM survey based on the percentage of infested shoots and the weather.
Keep records of scouting results. By examining scouting records, patterns of SSM activity and SSM hot spots can be identified. This may help reduce the number of times trees need to be scouted.
Hot spot scouting. Hot spot scouting can be used during hot, dry periods to determine how the weather is affecting mite activity in areas where mite problems have already been identified. The 'hot spot' becomes a representation of what is going on in the rest of the block. Identify the 'hot spot' through other scouting methods and mark it with flagging and on a field map. If mite activity is increasing in hot spots, go back and use the rigid block scouting method over the entire block to determine if treatment is necessary.
A sample scouting form for all Fraser fir pests is available through local county Extension office. For more information on scouting for mites, see the video, DETECTION AND CONTROL OF THE SPRUCE SPIDER MITE.
Chemical control. When scouting results determine that the treatment threshold has been reached, treat promptly for the SSM. Make sure that mites are present when you treat. Many growers have treated mite-damaged trees where mites had already been killed by natural causes, thereby wasting time and money.
Do not use the same miticide year after year because the SSM might become resistant to it. Chemical control of the SSM is difficult because good coverage is essential to good control. When applying a foliar miticide make sure the spray wets all of the current growth. The foliage at the base of the trees should be especially targeted. Air-blast mist-blowers and back-pack sprayers often do not provide enough coverage to control mites well. Fog applications at pressures lower than 200 PSI with a high-pressure sprayer and a hand-held gun give the most thorough coverage and best control.
Chemical control is also made more difficult because miticides kill different stages of mites. Some do not kill the eggs and must be reapplied within 10 to 14 days to allow eggs to hatch. Others just kill the eggs and immatures. Contact your local County Extension Agent to learn which miticides are labeled, and the safe use of those miticides.
Recommendations for use of agricultural chemical brand names and any mention of commercial products or services does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned in this publication.