By Charles Marr
Kansas State University
There are words that naturally seem to go together.
Salt immediately brings to mind pepper. Cream seems to go together with sugar. When someone mentions hot, dry weather, the words that immediately should come to mind are spider mites.
Spider mites and hot, dry weather seem to be naturally linked. Mites are tiny, insect-like creatures that feed primarily on the undersides of leaves--sucking juices from the plant. They don't eat holes in the leaves, but the first symptom is a pale, tiny spot on the leaf. With many mites feeding, the individual spots coalesce forming a white speckling on the leaf. Mites are held in check by several natural predators; however, in hot, dry weather, the mites seem to "get out ahead" of the predators very rapidly.
Sevin insecticide has no effect on spider mites, but kills predators, which causes the mite population to explode overnight.
Mites usually develop on the younger, newer leaves, at or near the top of the plant. However, mites sometimes are "spotty," in that they develop on plants here and there, but not necessarily evenly over a large area.
On needle-leaf plants, it is difficult to see the spotting, so the natural symptom is a pale discoloration of the entire needle--followed by a general browning.
Mites are not true insects and do not respond to many conventional insecticides that have been developed. Controlling spider mites also can be a difficult process, since they are small and located on the bottom sides of leaves. Using a wetting agent, such as Spreader-Sticker or liquid soap, makes the water spread out over the leaf improving control. Use a fine mist spray directed at the bottom side of the leaf, or completely cover the leaves, in the case of needle-leaved plants. Take your time and spray from several different angles to make sure you are covering the area well.
A "pump-up" sprayer works best, since you can generate a fine mist--almost a fog.
The standard miticide that is available, in many garden centers, is Kelthane. Malathion can work, but not as reliably as Kelthane, and has a wide label for many vegetable crops and a short waiting interval from application to harvest. Cygon works well in controlling mites, but has a limited label for garden crops and a longer waiting interval. However, Cygon (dimethoate) is a good choice for ornamental plants.
Insecticidal soaps work well in controlling mites, if they are applied frequently. Our observations have been that it takes several applications of insecticidal soap--every two to three days-- to get control of spider mite populations. If the mites are not controlled, you may see some "spiderweb" webbing appear near the mites. Leaves will turn a bronze-brown color and shrivel up. Many gardeners confuse this symptom with hot, dry weather.
The key to spider mite control is to recognize the symptom early, use a fine mist spray directed as the undersides of leaves and repeat several times.
So, if anyone mentions the hot, dry weather, you can immediately respond with "spider mites."