Spider Mites

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.

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.

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.

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.

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 108ºF (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.