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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|A.||PHYTOSEIULUS PERSIMILIS#||10,000-20,000–conventional fields||NA||NA|
|40,000+ –organic fields|
|COMMENTS: Release mites early in the season before foliage on the canes begins to close the space between canes.|
|(Acramite 50 WS)||0.75–1 lb||12||1|
|MODE-OF-ACTION GROUP NUMBER1: un|
|COMMENTS: Use permitted on bearing and non-bearing crops. PHI is for bearing canes. Use minimum of 50 gal water/acre. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|(Organic JMS Stylet Oil)||3–6 quarts/100 gal water||4||0|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|COMMENTS: Amount is for 100 gal/acre; may use up to 150 gal/acre water carrier. Spray with ground equipment for optimum coverage of leaf surfaces. Oil sprays need to be applied frequently to achieve acceptable control, however, frequent applications of oils can damage the plant and compromise fruit yield. Heed label warnings about compatibility with other materials.|
|(Savey 50 DF)||4–6 oz||12||3|
|MODE-OF-ACTION GROUP NUMBER1: 10A|
|COMMENTS: Do not make more than one application/year.|
|(Trilogy)||1–2 gal/100 gal water||4||0|
|MODE OF ACTION: Unknown. A botanical insecticide.|
|COMMENTS: Apply with sufficient water carrier to provide complete coverage. Most effective when applied before mites and eggs are present in large numbers. Repeat applications on 7- to 21-day intervals until mite pest pressure is over. Oil sprays need to be applied frequently to achieve acceptable control, however, frequent applications of oils can damage the plant and compromise fruit yield. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|MODE OF ACTION: A botanical miticide.|
|COMMENTS: Apply in 100–200 gal water/acre, apply every 10 days and check for phytotoxicity.|
|(GC-Mite)||1 gal/100 gal water||0||0|
|COMMENTS: Good coverage is essential for control; the use of a spreader/sticker may improve contact and efficacy of treatment. Oil sprays need to be applied frequently to achieve acceptable control, however, frequent applications of oils can damage the plant and compromise fruit yield. Apply no more than once in a 7-day period.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|#||Acceptable for use on organically grown produce.|
|1||Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers (“un”= unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).|