How to Manage Pests

UC Pest Management Guidelines


Spider Mites

Scientific Names: Tetranychus urticae, Tetranychus spp.

(Reviewed 11/06, updated 11/06)

In this Guideline:


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 mites 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/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/or when 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, populations 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 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 and/or attracted by alfalfa foliage following an irrigation. 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 that not stressed alfalfa a timely irrigation will most 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. Activity is slower than that of other miticides because of the insect growth regulator properties of this product, which cause it to take several days to control a population. 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 treatments 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),
  • treatments are applied early in the cycle (right after bales removed) and spider mites have been a problem before cutting, or
  • regrowth is being hindered and the field has been monitored to determine the cause of the slow growth is spider mites.

Low desert areas. There are primarily two treatment timings for mites on alfalfa: a) treating the stubble at cutting, or b) treating the foliage between cuttings. Treating the stubble at harvest is a two-step process in which the alfalfa is cut while the stubble is sprayed, and then the foliage is laid on top of the stubble. Initial tests of alfalfa stubble treatment as foliage is harvested has shown positive economic results, but the new alfalfa growth is unprotected after cutting and can be reinfested with spider mites from cut alfalfa as it dries. These reinfestations generally occur across the entire field and not just under the windrows.

Reinfestations are usually not severe when temperatures are 108°F (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 treat the adjacent crop to protect it from migrating mites; a treatment to the field's border may be adequate.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

Calculate impact of pesticide on air quality
When choosing a pesticide, consider impact on natural enemies and honey bees and environmental impact.
  (Trilogy) 32 oz 4 0
  MODE OF ACTION: A botanical (Group 26)1 miticide.
  COMMENTS: Can be applied at cutting. Thorough coverage is essential for good control.
**  See label for dilution rates.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until the field can be grazed or cut. In some cases the R.E.I. exceeds the P.H.I.; the longer of these two intervals is the minimum time that must elapse before harvest may take place.
# Acceptable for use on an organically grown crop.
1 Modes of action are important in preventing the development of resistance to pesticides. 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. 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 is assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at



[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

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

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