Landscape and Nursery - More on Two-Spotted Spider Mites

Two-spotted spider mites are on the increase at this time with the hot and dry weather. The following is more information on this pest.

Two-spotted spider mite, Tetranychus urticae, is a general feeder that attacks a wide variety of plants including shade trees, shrubs, and flowers. It is extremely small and feeds on the underside of leaves, so it may be overlooked until the population is so large that the host plant is seriously injured.

This spider mite overwinters as small, orange, eight-legged, mature females under bark or ground cover around the base of the host plant (most other species of spider mites overwinter as eggs). It has two dark spots composed of an internal accumulation of waste food that shows through the transparent body wall. After mating, females lay eggs on the underside of leaves at the rate of 2-6 a day. Each female may lay as many as 70 during her lifetime. Depending on weather conditions, a complete generation of twospotted spider mite may be completed in 7-20 days. All life stages of this species may be found on most host plants during the summer months. Drought seems to favor the development of severe infestations. This species may complete as many as 10 generations in one year.

This key pest damages plants by sucking plant fluid from the foliage. Light infestations usually go unnoticed; heavy infestations cause the leaves to become yellow or bronzed. Infested foliage can be stippled or mottled in appearance. In some cases leaves may drop prematurely. Severely infested plants usually have a fine cobweb appearance on the leaves, particularly on the lower leaf surface. Since these mites are difficult to see with the naked eye, tap a branch sharply over a piece of white paper--if mites are present, they will show up on the paper as tiny, moving specks. This key pest can be managed by applying registered miticides such as Avid, Conserve, DeltaGard T & O, Floramite, Hexygon, horticultural oil, insecticidal soap, Mavrik Aquaflow, Scimitar, Tame, or Talstar during late June through July according to label directions. These products should be applied before serious plant damage occurs and repeated when indicated at 7-10 day intervals if the miticide specimen label permits.

Spider mites may develop resistance to any miticide used against them, so it is necessary to switch to another chemical class of miticide after every third application. Spider mite populations may increase following the use of certain insecticides because natural predators are killed. Therefore, it is a good idea to add a miticide when treating mite-susceptible plants with insecticides.

Information from, Gregory Hoover, Ornamental Extension Entomologist, UD