How to Manage Pests
UC Pest Management Guidelines
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.
Organically Acceptable Methods
Monitoring and Treatment Decisions
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.
UC IPM Pest Management Guidelines: Alfalfa