How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

The most common spider mites encountered on beans is the twospotted spider mite, but the strawberry spider mite and Pacific spider mite can also be found in this crop. Infestations may include a mixture of spider mite species. Adult mites are nearly microscopic, have four pairs of legs, are greenish to pink or cream colored, and have various sized black spots on the body. Under warm conditions spider mites move rapidly within the colony area. Spider mites have four stages of development: (1) the oval, somewhat translucent egg; (2) a six-legged translucent larval stage; (3) an eight-legged nymphal stage; and (4) the eight-legged adult stage. A resting or quiescent stage occurs at the end of the larval and nymphal stages. A generation may pass in as few as 5 to 7 days in mid-summer, or in a month during cool periods.

DAMAGE

All active stages of spider mites damage beans by piercing individual plant cells and removing the contents, sucking juices from infested leaves and pods. Damaged leaves become somewhat stippled on the upper surface and grayish because of webbing and feeding on the undersurface. Spider mites are most serious on lima beans and common dry beans, but can cause problems in blackeyes, especially on field edges near roads and after treatment for lygus bug.

MANAGEMENT

Cultural and Biological Control
Reduce spider mite problems by keeping fields, field margins, and irrigation ditches clean of weed hosts such as field bindweed and lambsquarter. Spider mite populations may also increase more rapidly in areas where dust deposits are heavy on bean plants. Thus, reducing dust may reduce the spider mite problem. Spider mites are usually less severe in sprinkler-irrigated fields than in furrow-irrigated fields. Spider mite populations do not develop on blackeyes as rapidly as they do on limas, kidneys, and small whites. Spider mite populations may be held at very low levels by a number of insect and mite predators, particularly early in the season. Sixspotted thrips are effective early season predators, feeding primarily on spider mite eggs. Spider mites provide an important food source for predators such as minute pirate and bigeyed bugs. Minimizing early season insecticide applications to help conserve beneficials will reduce spider mite outbreaks.

Organically Acceptable Methods
Cultural and biological control are organically acceptable.

Monitoring and Treatment Decisions
There is no precise survey technique for evaluating spider mite infestations. Infestations usually begin on the lower portions of the plants and move upward as mite numbers increase. Start inspecting plants for spider mite damage along with other pests and their damage during the vegetative growth stage. Continue looking for mites from the flower bud to bloom period and during the pod fill period. Evaluating spider mite infestations is most efficient if randomly selected, older, lower leaves are picked and inspected for stippling on the upper surface and webbing, mites, and feeding scars on the lower surface. If spider mites can be found easily on older leaves at early bloom before the first insecticide application for lygus bug control, it would be advisable to use an acaricide at the time of the first treatment for lygus bug control.

PUBLICATION

UC IPM Pest Management Guidelines: Dry Beans
UC ANR Publication 3446
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
R. F. Long, UC Cooperative Extension, Yolo County

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