Spider Mites on Ornamentals

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Spider Mites on Ornamentals
“A Problem during Spring and Summer!!”

Dalton Dockery, PhD
Director, N.C. Cooperative Extension of Columbus County

Over the last couple of weeks several homeowners and landscapers have called about ornamental plants such as boxwoods leaves turning yellow or bronze. After visiting with these individuals it seems that we have an infestation of “Spider mites” which is not uncommon for this time of year.

Spider mites are widely distributed in the United States and feed on over 180 wild, ornamental, vegetable, and fruit plants species. Spider mites pierce the epidermis of the host plant leaf with their sharp, slender mouthparts. When they extract the sap, the mesophyll tissue of the leaf collapses in the area of the puncture. Soon a chlorotic spot forms at each feeding site. The accumulation of these tiny spots is called stippling. After heavy feeding, an entire plant may become yellow or bronze as the individual spots coalesce. Heavily damaged plants will drop damaged leaves and may die. Mites also spin silk webs that can cover leaf surfaces and accumulated feces, exoskeletons, and other debris.

Eggs are laid singly on the undersides of leaves. Six-legged larvae hatch from the eggs. They develop into eight-legged nymphs, which pass through two nymphal stages. After each larval and nymphal stage there is a resting stage. The adults mate soon after emerging from the last resting stage, and in warm weather the females soon lay eggs. Each female may lay over “100 eggs” in her life and up to 19 eggs per day. Development is most rapid during hot, dry weather. A single generation may require as many as 20 or as few as 5 days to reach adulthood and begin producing offspring. Typically spider mites overwinter in the soil or sheltered places as adults that reemerge in spring. However, with mild winter weather or protected areas like cold frames and greenhouses they may continue to feed and lay eggs at a reduced development rate.

Deciding if control is necessary is difficult because no reliable thresholds are available. In addition, populations are often suppressed by natural enemies such as predatory mites and minute pirate bugs. Therefore decisions must be made base on the presence of mites, whether a particular plant or site has had spider mite outbreaks in the past, and the value of the plants. A few mites in a cool landscape with complex vegetation may never grow into a damaging population. However, mites on plants in direct sun or surrounded by impervious surfaces or turf may outbreak quickly due to higher temperatures, greater plant stress, and fewer natural enemies.

Control Measures

Cultural control:
The best cultural control measure for spider mite management is to plant less susceptible plant species in areas that are hot, dusty, or stressful. Nitrogen fertilizer can induce mite outbreaks by making plants more nutritious to feed on. In addition, since spider mites feed on many weeds and wild plants like blackberries and violet, removing these plants from landscapes or from around susceptible plants may reduce infestations.

Biological control:
Spider mites have many predators that often keep them under control. These include predatory mites, minute pirate bugs (Orius spp.), lacewing larvae, lady beetles, and others. Disrupting natural enemies with insecticides is often a cause of spider mite outbreaks. Imidacloprid and other neonicotinoids can also lead to spider mite outbreaks.

Chemical control:
In many cases mite populations can be reduced with insecticide soap or horticultural oil. Spider mites are usually found on the underside of leaves. Thorough application of pesticides to the underside of the plant foliage is essential for good control. In hot weather, another application may be suggested 7 to 10 days later, to kill mites that were in the egg and resting stages during the first application.

Most insecticides do not control spider mites because mites are not insects. There are many miticides available that target specific aspects of mite physiology. These products are also safer for beneficial insects than broad-spectrum insecticides. This can prevent resurgence of mite populations or secondary pest outbreaks that occur when pests that were not killed by insecticides are released from predation because all the natural enemies were killed. Contact your local Extension office at 910-640-6605 for further assistance or email me at dalton_dockery@ncsu.edu.