Web-spinning spider mites are an annual problem for alfalfa seed producers in the low desert production areas of Southern California and Arizona.

Spider mites insert needle-like mouthparts into leaves, removing plant sap and causing chlorotic spot stippling on leaves. Severe feeding damage may turn leaves brown and desiccation causing defoliation. Damage usually starts in the lower plant canopy and moves upward as the mites move to new leaves. Severe feeding damage reduces alfalfa seed production. Several spider mite species are found in low deserts:

• Twospotted spider mite

• Carmine spider mite

• Strawberry mite

• Desert spider mite

A management plan should be developed prior to the start of alfalfa seed production. The plan should include the following components:

• Decision of returning to hay production, seed production or terminating the alfalfa after seed harvest (returning to hay production greatly limits choices of acaricides (miticides)

• Survey surrounding crops and weeds as potential sources of spider mites

• Dust mitigation

• Abatement of sources of spider mites

• Scouting plan for spider mites detection and treatment decisions

• Application of miticides when needed to prevent seed yield losses.

Crops such as melons and many weed species can harbor web-spinning spider mites and may become a source of infestation for an alfalfa seed crop. Also, spider mites may be harbored on the lower leaves of alfalfa plant throughout the year. Some insecticide applications, such as organophosphate or pyrethroids for lygus bug and stink bug control, can flare spider mite populations via destruction of predators or through hormoligosis (chemical stimulation of increased egg production). Many predators feed on spider mites including western flower thrips, minute pirate bugs, big eyed bugs and predaceous mites.

Dust from field roads drifting onto alfalfa plants favors web-spinning spider mite flare-ups. Water or treat field roads to minimize dust from vehicle traffic. Post speed limit signs (5 mph) on field roads.

Abatement of sources of spider mites is important to reduce the potential for migration into the alfalfa seed field.

Abatement should include weed control and if possible removal or treatments of spider mites in surrounding crops such as melons.

Alfalfa seed production fields should be scouted twice weekly for spider mites beginning early season and continuing until the crop is prepared for harvest. Fields should also be monitored for spider mite predators and for other pests in the field. Proper scouting will lead to accurate assessments of spider mite pressure versus the predator population levels that may result in reduced use of chemicals through improved timing of applications. It may be practical to treat only portions of a field.

When web-spinning spider mites are present in an alfalfa field prior to seed production, a miticide spray may be needed to prevent damage leading to reduction of seed production. Stressing the alfalfa for water can stimulate bloom, but also favors the build-up of spider mites.

Historical knowledge of spider mite problems influences whether a grower controls spider mite populations immediately, or delays treatment for a while. Treat fields before populations reach damaging levels to maximize the efficacy of available chemicals. When possible, spot or strip treat localized spider mite infestations. Use ground application equipment when possible (prior to bee placement) to improve coverage. To prevent spider mite problems consider including a miticide with the first insecticide application for lygus bugs.

There is historical research trial evidence indicating that an application with a highly efficacious miticide early in the season with the first lygus bug treatment can prevent damaging population levels of spider mites for the remainder of the seed production season. Miticides registered for alfalfa seed production work best when used against low populations and none can resolve a significant spider mite problem.