Sixspotted mite—Eotetranychus sexmaculatus
This spider mite (family Tetranychidae) is a sporadic pest of avocado and also damages plumeria in California. It is a minor pest of citrus in some growing areas and can occur on various other plants. Note that numerous species of mites can occur on plants.
To locate this mite and its damage, use a hand lens to examine along the lateral veins and midrib on the underside of interior canopy leaves. Look for brown to purplish discoloration, mite webbing, and the mites.
The oval body of sixspotted mite is lemon yellow, commonly with 2 to 6 dark spots on the abdomen. Some individuals have no distinct spots. Numerous fine bristles cover the body and legs. Adult females are 1/60 inch (0.42 mm) long and immatures are smaller. In comparison with adult females, males are smaller, thinner, and tapered toward the rear.
The tiny eggs are globular and pale greenish yellow, pearly white, or translucent. Eggs have a slender stalk (stipe) projecting from the top. Tiny threads are commonly attached from the tip of the stalk down to the leaf surface.
Early in the growing season the foliage damage of sixspotted mite can be confused with damage from persea mite and the mites have similar appearance. However, persea mite forms denser webbing and in individual circular patches, and its feeding discoloration occurs as scattered roundish spots. The silk of sixspotted mite is less dense and when abundant, discoloration damage can be nearly continuous along leaf veins.
Sixspotted mite develops through 5 life stages: egg, 6-legged larva, and 8-legged protonymph, deutonymph, and adult. Adult females lay about 25 to 40 eggs during their 10- to 20-day life span. Eggs hatch in 5 days to 3 weeks, the quicker at warmer temperatures. The abundance of sixspotted mite on avocado is highest during spring and early summer. When temperatures are warm, egg to adult development is about 10 days. There are multiple generations per year.
Sixspotted mite sucks and feeds on only the lower surface of avocado leaves. It causes irregular brown to purplish discoloration, mostly along the midrib and larger veins. Sixspotted mite produces webbing, but not the dense silk patches formed by persea mite.
Sixspotted spider mite feeding can severely stress trees at relatively low densities by causing premature leaf drop. However, mite abundance usually remains at two to three mites or fewer per leaf and at this low abundance sixspotted mite is not damaging and is easily overlooked.
Sixspotted mites are generally under good biological control in avocados and citrus in the interior growing areas (e.g., Riverside and San Diego counties) due to predators and warm weather. Locations near the coast, such as foggy areas in San Luis Obispo and Santa Barbara counties, are more likely to experience damage. Sixspotted mite can become a problem anywhere if the trees are covered with dust or drought stressed or if a broad-spectrum insecticide or miticide used to control other pests disrupts the natural biological control of spider mites.
Provide trees with the appropriate amount and frequency of irrigation during the dry season to reduce the likelihood of a mite outbreak and increase trees' tolerance to their feeding. Avoid or minimize fertilization of trees, especially with quick release nitrogen formulations, because excessive nitrogen in leaves can increase mite abundance.
Predaceous mites, especially Amblyseius (=Typhlodromalus) limonicus, Euseius hibisci, Galendromus helveolus, and Typhlodromus rickeri are the most important natural enemies of sixspotted mite. They commonly prevent this pest from becoming a problem. The spider mite destroyer (a tiny, black lady beetle), and sixspotted thrips are also important predators of sixspotted mite. Other natural enemies as discussed in the "Solutions" section of Persea Mite can also feed on sixspotted mite.
To improve the effectiveness of naturally occurring mite predators, control ants, minimize dustiness (e.g., periodically hose off small trees), and avoid the application of broad-spectrum, persistent insecticides and miticides for all pests. See Protecting Natural Enemies and Pollinators for more suggestions.
A low abundance of this mite can be tolerated. If sixspotted mite or other mites are unacceptably abundant, horticultural (narrow-range) oil or wettable sulfur can be sprayed on foliage. Thoroughly cover the lower surface of leaves where both persea mite and sixspotted mite occur. Do not apply sulfur and oil within 2 months of each other or foliage may be damaged by the pesticides. For more information, see Pest Notes: Spider Mites.
Adapted from Integrated Pest Management for Avocados and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).