The spruce spider mite (SSM) (Oligonychus ununguis) is a common problem in Fraser fir Christmas trees and other conifers in western North Carolina. Considered a cool season mite in other parts of the country, temperatures in the mountains of North Carolina never get hot enough to slow this mite down. The SSM can therefore damage Christmas trees any time throughout the growing season.

Spider mites have piercing and sucking mouthparts. Their feeding causes yellow spots on needles. From several feet away needles may appear generally yellow but when viewed close-up it can be seen that there are tiny yellow spots on the foliage. When damage is light, spots are found at the base of the needles. As mite numbers increase, the entire needle becomes spotted. The yellow spots merge and turn brown. Mite-damaged needles are more likely to fall off the tree, especially after the tree is cut. Occasionally there may be webbing present among the needles when the mites are actively feeding, similar to spider webs, hence the name, spider mite.

Spider mite damage can cause serious economic loss, especially if the trees are nearing harvest. In some cases, trees tagged for market have had to be left to grow another year or more after the SSM has caused so much damage that the plant is unsaleable. Damaged needles don't recover. They can only be hidden from view by new, green, undamaged growth. That may take one or two growing seasons, depending on the density of the tree and bud set.

Spider mite damage is never uniform across a field. Trees with heavy damage may be right next to trees with almost no damage at all. That is because trees vary in their susceptibility to spider mites.

Spider mite damage can occur on Fraser fir Christmas trees any time from March through November. Damage to ornamental conifers such as dwarf Alberta spruce, hemlocks, and junipers grown in the foothills usually occurs in the spring. In the foothills, the spruce spider mite becomes less active in the hotter temperature of summer and other mites become a problem.

Rust mites cause similar damage on both Fraser fir and hemlocks. Rust mite damage in the foothills is the most common problem, whereas spider mite damage on Fraser fir is more common. Rust mites are a springtime pest occurring as early as January in the foothills on hemlock and in March and April in Fraser fir in the mountains. Rust mites cause bronzing instead of yellowing on the foliage. When seen through a handlens, rust mite feeding appears as scratches on the needles instead of the distinct yellow spots caused by spider mites.

The SSM can also survive on harvested trees and continue to feed and reproduce on the Christmas tree in the home. This occurs when the weather is mild and dry during harvest and the mites have not become dormant. Homeowners have complained of webbing on trees, needle shed, and the mites themselves moving onto ornaments, presents, and furnishings. However, this only rarly occurs.

It is important to be able to distinguish both spider mite damage and the mites themselves. Spider mite numbers can build rapidly and crash just as rapidly. If a field of trees isn't visited for a month or more, spider mites can easily build-up, cause damage, and then die out. Many growers have treated trees damaged by spider mites when no mites were left to control, wasting pesticide application without preventing damage.

Spruce Spider Mite Appearance

Spruce spider mites, like all mites of the tetranychid or spider mite group, have small, soft bodies with eight legs at maturity. These mites range in color from green to red, though in western North Carolina they appear dark red to almost black with lighter-colored heads and legs. The SSM egg is small and brilliant red with a single hair in the center. Eggs that have hatched appear clear.

Spruce Spider Mite Location on the Tree

Look for both mites and eggs on the most current growth of Fraser fir. The best time to look is on sunny days when the foliage is dry. Mites are usually more common on the lower half of the tree, and first appear on the smaller, interior shoots found partway into the canopy of the tree. Remember that mites cannot always be found on damaged foliage. If the damage occurred earlier in the year, the mites may have died out because of changes in the weather or natural predators. In the winter, only eggs are present unless the weather is mild.

The lifecycle of the SSM is simple. The mite overwinters as eggs on tree shoots. When temperatures are warm enough, the eggs hatch. The immature mite, or larva, is similar to the adult, only smaller, and has three pairs of legs instead of four. The larva molts to the nymph with has eight legs like the adult, but is slightly larger than the larva. Finally, the nymph molts to the adult. Both males and females are produced, females being larger than the males. During molting, the mites go through a resting stage where they don't move and may appear to be dead. At maturity, females mate with males, lay eggs, and the cycle begins again.

What makes the SMM such a dynamic pest is the speed with which this lifecycle can be completed. It can take as little as two weeks from egg to adult. Therefore, many generations occur each growing season, especially in the mountains where the mites can be active from early spring to late fall. In addition, at any given time during the growing season, all stages of the mite may be found. This affects pesticide performance as not all miticides control all stages. The egg state is particularly difficult to control.

With such high reproductive capabilities, spider mites would overwhelm all Christmas trees if there weren't factors keeping them in check. The most important factor reducing mite numbers are natural predators, especially predatory mites. Other predators include hover fly larvae, lacewing larvae, dusty wings, and lady beetles. Predatory mites require high humidity to survive. That is one reason that the spider mite numbers rise during hot, dry weather.

Moisture also affects the spider mite itself. 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 plenty of rainfall, spider mites are seldom a problem.

Temperature is the third important consideration. The spruce spider mite can live and function at low temperatures - even below freezing. However, the warmer the temperature, the faster it is going to be able to feed, mature and reproduce.

Field location and production practices influence mite activity. Growers can change their management practices to reduce problems with spider mites. Growers interested in producing organically grown Fraser fir for Christmas trees should be especially careful of field locations which can create more problems with mites. Important factors are listed below:

• Elevation. In the mountains of western North Carolina, spider mites are more of a problem at lower elevations. Fields below 3,000 feet will almost always have spider mites every year. The warmer temperatures at lower elevations allow the mites to reproduce more quickly.
• Aspect. Fields facing south and west have more problems with spider mites than those facing north and east. Aspect affects both the temperature and the humidity at the site.
• Rainfall. Rainfall averages in western North Carolina range from less than 50 inches a year to over 100 inches. Some counties such as Ashe and Alleghany tend to have more problems with spider mites than counties such as Avery or Transylvania that receive more rainfall. In some years, though, problems with SSM are region-wide because of drought. Keeping track of rainfall in a field with a rain gauge can help determine when mites are more likely to be a problem. Fields receiving an inch of rain a week should have few problems with spider mites unless some other factor is creating a problem.
• Air movement. Trees grown on windy ridges typically have more SSM damage than those surrounded by woods which reduce air movement and foliage drying. In these protected fields, the SSM is almost always present, but even during dry periods do not cause economic damage. That is because these habitats are best for predatory mites.
• Dust. Trees grown along dusty roads are most often the first to be damaged by the SSM. The dust on the needles scratches the more active predatory mites, causing them to desiccate and die which in turn allows the more sedentary spider mite to reproduce unchecked.
• Pesticide use. Some pesticides such as Asana are broad spectrum and last a long time in the environment. These will kill off natural predators, allowing spider mite numbers to build. Other pesticides such as Sevin and Imidacloprid appear to increase reproduction of mites. To avoid these problems, only use a pesticide against a pest when scouting results indicate that there is a problem. Use the least-toxic material available at the lowest effective rate.
• Ground cover management. Predators feed on insects and other mites, as well as pollen produced by flowers in the ground covers in the absence of pests. When ground covers are killed out, natural predators have lost an important habitat and they may not be present in high enough numbers to give control if spider mites should become active.

Even with a good location for Fraser fir production, judicious use of pesticides and proper groundcover management, spider mites can still be a problem. Many people depend on 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 effective spring controls. The only way to know if chemical controls are necessary and to avoid economic damage is by scouting.

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 throughout the growing season. For trees that do not receive a balsam twig aphid or balsam woolly adelgid treatment in the spring, start scouting in early April. For trees that are treated in the spring, scouting can be delayed until early June. Continue scouting until spider mites die out in the fall. This may take several hard freezes.

Target areas of the field where mites are more likely to develop first such as windy ridges, south-facing slopes, or near dusty roads. If mites aren't found in these areas, it's unlikely that they will be elsewhere in the field. Some trees are genetically more prone to mite activity and mite damage than others. Mark these trees if they are identified so they can be examined first. Checking these 'mother' trees is an easy way to assess how weather conditions are affecting mite numbers.

Scan trees while walking to find trees with SSM damage. From a distance, they may appear yellow or off-color. The earliest damage is usually found in the bottom third of the tree. The stem that the needles are attached to stands out on damaged shoots. That's because there are yellow spots on the base of the needles closest to the stems caused by mite feeding. Get your eye accustomed to looking for this damage by finding problem trees, then stepping back several feet to see what they look like. As mite feeding continues through the season, there will be more yellow spotting and the entire needle will appear yellow from a distance.

Growers that are colorblind may have a harder time detecting tree damage and even finding the red-colored spider mites. Find out who on your farm is best at scouting, and make sure they have the time to scout when needed.

Take a single shoot of the most current growth from symptomatic trees. If no damage is spotted, take a shoot at random from a tree about every 50 feet (every 10 trees). Whenever a sample is taken, walk at least another 30 feet (six trees) before sampling another tree, even if damaged trees are passed by. In this way the entire block is examined evenly.

It is only necessary to take a single shoot per tree, even if the tree has damage on it. Take the majority of the shoots from the bottom third of the tree, but also check a few shoots near the top of the tree. Look at the small shoots from inside the tree canopy rather than the larger exposed shoots, since mites are first found in the inner part of the tree.

Examine each shoot with a handlens for spider mites, spider mite eggs, and spider mite damage. There is no need to count the number of mites or mite eggs, only the presence or absence of mites and mite eggs.

Keep track of the number of shoots examined in a block on a counter. Keep track of the number of shoots infested with mites or mite eggs either in your head, or with a second counter.

Economic Threshold

The economic threshold is a way of describing pest numbers that have reached a point where pesticide intervention is necessary to prevent economic damage to the trees. Some slight amount of damage may occur even if the economic threshold isn't reached, but it will only affect a limited number of trees and shouldn't affect tree sales or tree quality.

The economic threshold for the spruce spider mite in Fraser fir Christmas trees is based on a percentage of trees infested in a field. The economic threshold changes with the size and value of the trees. Calculate the percentage by dividing the number of shoots with mites and/or mite eggs by the total shoots examined in a field and multiplying by 100. For instance, if 21 shoots are examined and 4 are found to have mites, the percentage of trees infested is estimated at 19%.

The economic threshold used by most growers is listed below. However, each grower has different needs for tree quality. These numbers should be adjusted up or down by each grower based on his or her own experience and needs.

Modifying the Treatment Decision

A grower may decide to delay treatment even if the economic threshold is reached. If the weather will be getting cooler or wetter, a pesticide treatment may not be necessary. Also, even if the economic threshold has just been met but there are a lot of predators present, delaying treatment may give the predators time to reduce spider mite numbers naturally. If treatment is delayed, be sure to scout again using the hot spot scouting method within a couple of weeks to see if spider mite numbers are in fact declining.

Deciding When to Scout Again

If the SSM percentages have not reached the economic threshold, decide when to scout again before leaving the field. Putting the next scouting trip on your calendar will help guarantee that the field will not be neglected during a critical time.

Mite numbers rise and fall. This is only apparent when the scouting results of one scouting trip are compared with another. Therefore it is important to keep accurate records. Record the date trees were scouted, the weather conditions during scouting, the number of shoots examined, and the percentage of trees infested. Make some notes as to the level and severity of damage found. Record whether mostly eggs, immature mites or adults were found. Also record if any natural predators were present. Have these notes in hand the next time the field is scouted so that changes can be evaluated.

Any time a grower treats for insects in Christmas trees, it is important to consider mite control as well. As already stated, many insecticides can cause mite flare-ups. When treating for hard to control pests such as the balsam woolly adelgid, consider adding a miticide if any mites are present, even if treatment thresholds have not been reached.

When scouting results determine that the treatment threshold has been reached, treat promptly for the SSM. Treat the entire block, even if only one area has mites. In all likelihood these unaffected areas will get mites and require treatment if left alone.

Chemical control of the SSM is difficult because good spray coverage is essential for good control. Chemical control is also made more difficult because miticides work in different ways. Most don't control all the stages of mites. Some do not kill the eggs and must be reapplied within 10 to 14 days to control the newly hatched immatures. Others control only the eggs and immature mites and not the adults. Also some miticides control only spider mites and not rust mites.

Most growers also try to control other pests when treating for spider mites, hoping to get the most out of the effort of applying a pesticide. Consider the time of year to determine if it is possible to control other pests as well.

The method of applying a pesticide will determine how well it works just as much as the material that is chosen. In Christmas trees, tractor-driven air-blast mistblowers are the fastest way to apply pesticides. However, the coverage is not always adequate to control spider mites. Having access roads every 12 to 15 rows and treating from opposite sides of the block will aid in getting good coverage. Be sure to scout after application and especially in the middle of the field where the pesticide may not have been applied heavily enough.

The best foliar applications are made with high-pressure sprayers and a hand-held gun. Don't treat trees as you would for the balsam woolly adelgid. Create a fog instead of a straight-stream using pressure lower than 200 pounds per square inch.

For spot treatments or for growers who do not have many trees, a back-pack mistblower is a good way to apply miticides.

There are many miticide choices available to the Christmas tree grower. These vary in the mode of action which will effect timing. They also greatly vary in price. The mode of action is listed for each material. Do not use the same miticide year after year as spider mites can become resistant. It is a good idea to vary materials from different chemical families as well. Your local N.C. Cooperative Extension agent can help in making this decision.

These miticides require good coverage to give good results. It is important to scout, using the hot spot scouting method, after pesticide application to determine if adequate control was achieved. Wait at least 1-to-2 weeks after application before scouting. Don't be fooled by immature mites that are molting. These may appear dead when they aren't. If there is any question, remove a branch with mites and look at it again the following day.