Start scouting conifers for mites
Found on many species of conifers, mites can degrade the quality of the trees by causing bronzing of the foliage and sometimes even loss of needles.
We are beginning to find mites active on several conifer species. Although the mites may be actively feeding by now, we often don’t see the damage until later in June or July. Spruce spider mites are often a problem on fir and spruce trees, but populations can build up on nearly all Christmas tree species. Spruce spider mites are considered a cool season mite and thrive when daytime temperatures are in the 60s and 70s. Newly hatched larvae are pinkish in color, but turn dark green or dark red after initial feeding. Christmas tree growers, landscape management professionals and homeowners should keep an eye out for the dark mites or their webbing, especially if you had damage from mites last year.
If you suspect mite problems perform a foliage check. Take a piece of white paper (eriophyid mites show up best on a black background), hold it under a branch suspected of having mites, and strike the branch hard against the paper. This should dislodge the mites, and even though they are very small, you should be able to see the dark, oval spider mites against the white background or the cream-colored eriophyid mites on the dark sheet.
Spruce spider mites are small, about the size of a period at the end of a sentence. Examine three to four places around the plant. If you dislodge 10 or more mites at each site, it would be advisable to apply a registered miticide according to label directions. One of the problems with this “shake and count” method is that other little critters may fall off too and be counted as mites. A good way to verify high levels of mites is to look for the number of eggs (either hatched or unhatched, see Photo1) and the degree of feeding injury on the needles using a good hand lens or stereo dissecting scope.
In landscapes, the tree should show some injury before triggering a spray in order to preserve natural enemies and give them chance to bring about control. The best way to insure conifers in Christmas tree plantations or landscapes, particularly spruce, fir and arborvitae, that will develop spruce spider problems is to apply a preventative or regularly scheduled insecticide treatment in early summer as this will kill off the predatory mites and allow the spider mite population to explode. Predatory mites are important biological control agents for plant-feeding mites like spruce spider mites and Admes mites. It is important to conserve and protect the populations of native predatory mites (Photo 2) that will be present. Broad spectrum insecticides may control plant-feeding mites, but are very harmful to populations of predatory mites. If a pesticide is needed to control damaging levels of mites, it is preferable to choose a miticide, Savey or Hexygon, that won’t harm predatory mites and predatory insects.
Mites we commonly find in Christmas tree plantations and on conifers in the landscape are spruce spider mites.
Spruce spider mites are the most common mite found on Michigan conifers. They have tiny mouthparts modified for piercing individual plant cells and removing the contents. This results in tiny yellow or bronze speckling on the needles. When many of these feeding spots occur near each other, the foliage takes on a yellow or bronzed cast. Once the foliage of a plant becomes bronzed, it often drops prematurely. Webbing is usually present in severe infestations. Scout the oldest foliage, near the stem of the tree – that’s where the mite populations build up first.
The mites overwinter as eggs placed at the base of needles or scales. Three or more generations are produced each year at intervals of two to three weeks. The spruce spider mite is primarily a problem in the spring and fall, especially during years where severe drought conditions persist. Providing the tree with plenty of water during prolonged dry periods in the summer months will help the tree cope with mite feeding activity. Spider mite numbers also tend to build up in trees that are being crowded by other trees. Spruce trees need enough space between trees to prevent branches of different trees from touching. Hot summer temperatures cause the mite to become inactive.
For more of spruce spider mite, see the Pennsylvania State University fact sheet.
Eriophyid mites, sometimes called rust mites, have been found on Colorado Blue and Black Hills spruce in Michigan. These mites are very tiny, carrot-shaped and cream colored. In order to see them, you need at least a 15x hand lens. Even then, the insects are barely noticeable. Eriophyid mites discolor and distort foliage by feeding on the buds and needles. With these mites we usually find the pale yellow to bronze needles on current season’s growth. Damage symptoms can be confused with other symptoms from drought, winter or herbicide injury, etc. Eriophyid mites are more of a problem in commercial Christmas trees and landscape conifers seldom have problems with eriophyid mites.
For more on eriophyid or rust mites on Christmas trees, read the North Carolina State University article Rust Mites on Christmas Trees CTN-034.
In past years, we have most often found Admes mites on white spruce or Black Hills spruce, but we will find them on Colorado blue spruce and Norway spruce as well.
Admes mites are larger than the spruce spider mite and have a dark red/brown body with light tan legs. Their feeding can cause discoloration of the foliage. If populations are reaching damaging levels, the treatment for this pest is similar to that of spruce spider mite.
A list of recommended pine needle scale insecticides for Christmas tree growers is provided at the MSU Christmas Tree Team Site.
If you see bronzing of the foliage or find mites when sampling, you may want to send a sample into MSU Diagnostic Services to confirm which type of mite you have.