Description of the Pest
Spider mite eggs are spherical, clear, and colorless when laid but become pearly white as hatch approaches. Nymphs, adult males, and reproductive adult females are oval-shaped and generally yellow or greenish in color. There are one or more dark spots on each side of their bodies, and the top of the abdomen is free of spots. Mating and egg laying typically occur year-round in most coastal growing regions.
Occasionally twospotted spider mite causes economic damage to artichokes. Because of their small size and hidden feeding habits, mites can feed on artichoke leaves and not be noticed until their populations are so large that large areas of the field are infested.
Mites feed by piercing the outer cells of leaves and sucking plant juices. The first sign of feeding is the appearance of whitish-yellow stippling on the upper side of the leaf. Stippling often appears first in long clusters along the large veins of leaflets. As the damage increases, the leaf area between the veins begins to yellow. Eventually the entire leaf yellows, turns brown, and dries up. Leaves often have a large amount of webbing present. When the infestation is serious, the photosynthetic capacity of the plant is reduced, and the plant appears weak with lanky leaves. Infestations can spread to buds; infested buds have white spots on the surface and are covered with webbing.
Spider mite populations are worst during hot, dry weather. On the central coast, the day temperatures are generally below 75°F for most of the year, only occasionally reaching higher temperatures for a few hours. Inland areas with warmer temperatures may be affected year after year by the spider mites and should be avoided for growing perennial artichokes. Spider mite infestations in annual artichokes grown in the desert can be avoided by planting only after fall temperatures have started falling. Spider mites are often serious when the crop is under stress, resulting from factors such as water, nutrition, or diseases. Reducing or eliminating crop stress can stop a small infestation from getting out of control.
Spider mites on artichokes have the tendency to move upward in the plant canopy, whereas the predatory mites stay on the lower leaves where it is cooler and humid. Leaving spent stalks on the plant too long after all the artichokes have been harvested allows the spider mites to climb higher into the plant where they multiply rapidly in the absence of their predators. From these stalks, the mites spread readily to other plants and other artichoke fields. Removing spent stalks promptly after harvest would confine the spider mites to the lower canopy closer to their predators and help keep the infestation under control.
Predatory mites such as Phytoseiulus persimilis, Galendromus (=Metaseiulus) occidentalis, and Amblyseius californicus feed on these mites. Other natural enemies include minute pirate bugs (Orius tristicolor), a small, black lady beetle (Stethorus spp.), a small, black rove beetle (Oligota oviformis), bigeyed bugs (Geocoris spp.), brown lacewing (Hemerobius spp.), green lacewing (Chrysopa spp.), sixspotted thrips (Scolothrips sexmaculatus), damsel bugs (Nabis spp.), and a cecidomyiid midge larva (Feltiella acarivora).
Organically Acceptable Methods
Cultural and biological controls are acceptable for use on organically certified crops.
Monitoring and Management Decisions
Miticides are not available for the control of spider mites on artichokes.