How to Manage Pests

UC Pest Management Guidelines


Spider Mites

Scientific Names: Tetranychus urticae, Tetranychus spp.

(Reviewed 1/17, updated 1/17)

In this Guideline:

Description of the Pests

Spider mites are small pests, with adults about the size of a small pinhead, variable in color (green or yellow) with dark pigmented spots. Adult spider mites have eight legs and are oblong to spherical in shape. The eggs of spider mite species found in alfalfa are very small, whitish, and spherical in shape. You will need a hand lens to see them.

Spider mites are usually found on the undersides of leaves, with colonies beginning on the lower (older) leaves and moving upward on the plant.


Spider mite feeding first appears as stippling (small yellow areas) on leaves. Severe damage desiccates leaves, and they may fall from the plants. Heavily infested plants may be stunted and have a yellowish appearance. Tonnage reduction of almost 0.2 tons of hay per acre has been documented in the low desert from severe spider mite infestations. Reductions are thought to be greatest when alfalfa is growing slower and infestations occur early in the cutting cycle.


Spider mite infestations may occur in any alfalfa-growing area, but damage and yield losses are most common in the low desert production areas of Imperial and Riverside counties. Spider mite infestations in the Central Valley are rare and can usually be managed by a timely irrigation. Infestations and losses are most closely associated with bedded alfalfa production.

In the low desert, spider mites have been most damaging from March through May. More than one cutting may be affected. On bedded alfalfa, spider mites build up on weeds during the early spring and as the weeds dry up spider mites move onto the alfalfa. This is generally not a problem on solid-planted alfalfa grown in the Central Valley or the Intermountain counties.

Control options for spider mites in alfalfa include:

  • weed management,
  • proper irrigation and fertilization to minimize plant stress,
  • timely harvest, and
  • chemical control.
Biological Control

Western flower thrips are often an effective predator of spider mites, migrating into alfalfa from surrounding crops as their host plants desiccate. Spider mites are also fed on by minute pirate bugs, big-eyed bugs, and lacewing larvae.

Cultural Control

In the low desert, an important component of mite management is to control weeds along field edges during the winter to eliminate potential host plants that can serve as overwintering sites and initial locations of spider mite infestations. Since water-stressed alfalfa is more prone to infestation than non-stressed alfalfa, a timely irrigation will often alleviate the problem. Minimizing plant stress through improved irrigation, fertilization, and cultural practices such as timely harvests is also beneficial.

Organically Acceptable Methods

Clarified extract of neem oil (Trilogy) can effectively control spider mites. Best results are noted when the alfalfa plant is short, allowing for more thorough spray coverage.

Monitoring and Treatment Decisions

Look for spider mites on the undersides of leaves. Treatment thresholds have not been established, but pesticide applications may be economically justified inalfalfa grown for hay when:

  • spider mites are present and cutting cycles are longer than 30 days (more defoliation of lower leaves and quality reduction) or
  • regrowth is being hindered and the field has been monitored to confirm spider mites are the cause of the slow growth.
Low desert areas

Reinfestations are usually not severe when temperatures are 108F (or higher) or when alfalfa is green-chopped and moved immediately from the field. If fields of susceptible crops (such as cotton, melons) are adjacent to spider mite-infested alfalfa, they may become infested when the alfalfa is harvested if spider mites migrate from the drying plants. In these situations it may be necessary to apply a pesticide to the adjacent crop to protect it from migrating mites; a pesticide application to the field's border may be adequate.

Common name Amount per acre** REI‡ PHI‡
(Example trade name)   (hours) (days)
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
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. Not all registered pesticides are listed. Always read the label of the product being used.
  (Onager) 10–24 oz 12 14
  COMMENTS: A growth regulator that is a contact toxin to eggs and juveniles. Adult mites are not directly affected, but it causes adult females to lay sterile eggs.
  (Trilogy) 32 oz 4 0
  MODE OF ACTION: Unknown. A botanical insecticide.
  COMMENTS: Can be applied at cutting. Thorough coverage is essential for good control.
** See label for dilution rates.
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 an organically grown crop.
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 the 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 are assigned by IRAC (Insecticide Resistance Action Committee).



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Insects and Mites

L. D. Godfrey, Entomology, UC Davis
E. T. Natwick, UC Cooperative Extension, Imperial County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County
V. M. Barlow, UC Cooperative Extension, Riverside County and UC IPM Program

Acknowledgment for contributions to Insects and Mites:
D. R. Haviland, UC Cooperative Extension, Kern County
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
C. G. Summers, Entomology, Kearney Agricultural Center

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