How to Manage Pests
UC Pest Management Guidelines
Twospotted Spider Mite
Scientific Name: Tetranychus urticae
(Reviewed 12/09, updated 3/10, pesticides updated 6/15)
In this Guideline:
Description of the Pest
Twospotted spider mites infest the undersides of caneberry leaves, where they may form colonies and produce light webbing when abundant. Twospotted spider mites are very small (about 0.02 inch in length) and are barely visible to the naked eye. Nymphs, adult males, and reproductive females are green to a yellowish hue in color. Reproductively dormant females are bright orange and should not be confused with the predatory mite, Phytoseiulus persimilis, commonly found in mite colonies. Under a hand lens, one can see two dark blotches on either side of the adult twospotted spider mite's body and two red eyespots on the head.
In areas where temperatures are cold in winter, twospotted spider mites overwinter as dormant adult females at the base of the caneberries or weeds in and around the field. With the onset of warm weather, these mites migrate to the foliage of the plant and begin to lay eggs. In the mild winter coastal-growing regions of California, it is unusual for a large percentage of mites to become dormant; instead they continue to grow and lay eggs, although at a slower pace during the winter months than in summer. The twospotted spider mite undergoes one larval and two nymphal stages before becoming an adult. The life cycle, under ideal conditions of hot, dry weather, can take place in 10 days.
Spider mites feed by sucking juices from the plant and cause a gray stippling on the leaf surface. As the population grows and feeding progresses, leaves turn yellowish brown before drying up and falling off. Feeding by twospotted spider mites on fruiting floricanes reduces plant vigor and fruit yield and size. Mite feeding can also weaken primocanes, predisposing them to winter injury in areas of cold winters and reducing yield the following season.
Twospotted spider mites can be a problem in any caneberry planting if condition are conducive to their development, but they pose a special problem in plantings that use macrotunnels because of the hot, dry conditions that are created by the tunnels. The key to successful management of twospotted spider mites is to monitor populations and to initiate control measures in a timely manner. Once populations are large, much damage has been done, and the mite populations are difficult to control.
The most effective biological control agent of twospotted mite is the predatory mite Phytoseiulus persimilis, which is an introduced species. Phytoseiulus does best in temperatures of 60° to 80°F and will not do well above 100°F. Because the temperatures in macrotunnels are generally higher than outside, P. persimilis does not perform as well inside of the tunnels. This predatory mite has apparently established itself in some locations and provides some natural suppression in these areas. It may also be purchased and released in fields for additional control.
Normal pruning of primocanes and removal of dead floricanes in caneberries can be helpful in reducing the buildup of twospotted spider mite. Varieties with heavily pubescent leaves can make establishment difficult for twospotted spider mites and may be useful for those situations where twospotted mites are a significant problem. Because the warm, dry conditions within a macrotunnel are very conducive to the population growth of twospotted spider mites, limiting the use of tunnels or venting them to maintain a lower temperature will limit the numbers of this pest mite. Also, controlling dust by watering or oiling surrounding roads is an important factor in limiting twospotted spider mite populations.
Organically Acceptable Methods
Cultural and biological controls including the release of predatory mites, and narrow range oil sprays, such as Organic JMS Stylet oil, are organically acceptable methods.
Monitoring and Treatment Decisions
No precise treatment thresholds have been established for twospotted mites in caneberries. Monitor to keep track of increasing pest mite populations as well as predatory mite populations. A ratio of 1 predator to 10 twospotted mites is considered favorable for biological control. When using chemical controls, it is important to know that good coverage is essential. In many cases, especially with the spray oils, mites that escape contact with the control material will survive.
UC IPM Pest Management Guidelines: Caneberries
Insects and Mites
M. P. Bolda, UC Cooperative Extension, Santa Cruz County
Acknowledgment for contributions to Insects and Mites:E. Show, Driscoll Strawberry Associates Inc., Watsonville, CA
E. J. Perry, UC Cooperative Extension, Stanislaus County