Garden Mastery Tips
|
August 2007 |
Spider Mites
Are you finding webbing in your flower beds? Most likely it is caused by spider mites, which are common pests in the urban/suburban landscape. They can inflict serious damage to trees, shrubs and flowers.
Spider mites are not insects; they are closely related to spiders and ticks. They are from the Tetranychidae family, and the Acari order. Technically, they are a type of arachnid. Unlike insects, which have six legs and three body parts, spider mites have eight legs and a one-part body. Spider mites also do not have wings, antennae, and compound eyes. Their colors range from red and brown to yellow and green, depending upon the species, of which there are many. They are almost microscopic individually, but can cause serious damage when they occur in large numbers. The mites are about the size of the period at the end of this sentence.
Most common species of spider mite over winter on trees and shrubs as tiny round eggs on leaves and bark. These eggs hatch in March or April. The first-stage larvas have only six legs, but after molting, they become eight-legged nymphs. Both the larva and the nymphs resemble the adult. Depending upon the weather and the type of spider mite, development from egg to adult varies from five to 21 days. Many generations occur each year. Most species feed heavily and reproduce rapidly in spring and fall. Some species, however, are most active during warm, dry periods.
Severe infestations can cause discolored leaves, producing a bronzed, bleached look to the plant. Leaves and needles may become yellowed or scorched and drop prematurely. The spider mites can cause the death of plants, or cause serious stress to them.
The most important spider mite is the twospotted spider mite, which attacks a wide range of garden plants, including many flowers, vegetables (e.g. beans and peas), fruits (e.g. raspberries, currants, and pear). The twospotted spider mite is also found on houseplants. It is a prolific producer of webbing.
In early spring and late fall, you may observe the damage caused by the spruce spider mite, which feeds on more than 40 species of conifers. This is one specie that prefers cool seasons, and they may cease development and produce dormant eggs in order to survive hot summer weather.
Dry conditions seem to favor all spider mites. They feed more during dry periods, as the lower humidity allows them to evaporate excess water they excrete. Most of their natural enemies require more humid conditions and are stressed by arid conditions. In addition, plants which are stressed by drought can produce changes in their chemistry which make them more nutritious to spider mites.
There are several ways to control spider mites:
- Hose mites from plants with a strong stream of water, making certain to get the underside of foliage.
- Use of insecticidal soap.
- Lady beetles, thrips and green lacewings aid in control of mite populations.
- Adequate watering during dry conditions can limit stress conditions which attract spider mites.
Resources
Spider Mites by W. S. Cranshaw and D. C. Sclar, retrieved June 8, 2007 http://www.ext.colostate.edu/PUBS/insect/05507.html.,
Sunset Western Garden Problem Solver, Sunset Books Inc., Menlo Park, CA 1998
Spider Mites, Authors: Eric Day, Director, Insect Identification Laboratory; Virginia Tech, retrieved June 7, 2007 http://www.ext.vt.edu/pubs/entomology/444-221/444-221.html.