SPRUCE SPIDER MITES ( Oligonychus ununguis )
Spruce spider mites, can be found on many species of conifers with the most severe damage most often found on spruce and true firs. These mites degrade Christmas trees by causing discoloration and sometimes even loss of needles. On rare ocassions, trees are killed by repeated mite infestations. Most new injury is caused in the spring and early summer and will not become apparent until mid-August or later. The mite damage is most severe during long, dry periods, although activity may subside during very hot, dry weather.
Fig. 1. A single adult mite is visible here, along with some eggs (red) and some empty egg shells (clear).
Concentrate scouting efforts on trees that are more likely to have spider mites based on visible symptoms of mottling, discoloration or needle loss. Examine trees growing in dry or sunny locations, or on the edge of a block of trees. Look for the mites themselves or other signs such as fine webbing between the needles, mottling of older needles, and eggs as well as egg shells on the shoots. The eggs are bright red and very tiny, and can usually be found in the spring, on the underside of the newer shoots.
Fig. 2. The pale green mottling on these needles is chlorosis caused by mite feeding.
To monitor a mite population, hold a sheet of white paper beneath a branch and rap the branch a few times with a pencil or ruler, etc. If there are some mites present, you should knock some off, and they can be seen moving on the paper. They are small - about the size of a period on this page. If you are having trouble identifying them, try squashing some of the tiny spots that may appear on the paper. A reddish smear will be from a mite and not just a tiny speck of dirt. Count the mites using a hand lens, which will also help you make a positive identification.
If only a few mites are found, they should not be of much concern, but keep an eye on them as the population can increase quite rapidly. If you find more than five mites per branch examined, you should consider using a registered miticide to protect the tree from further injury. There may be at least three generations per year, so there may be eggs hatching at virtually any time, and a new generation can occur every two to three weeks. Although natural predators may maintain control over low populations, spraying for other pests may reduce the mites' natural enemies.
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