Avocado Brown Mite

Spider mites (family Tetranychidae) and predatory mites (Phytoseiidae) are tiny eight-legged arthropods, except for first stage larvae, which have six legs. Persea mite is a key pest of California-grown avocados. Avocado brown mite and sixspotted mite are sporadic pests. Several beneficial mites are important predators of pest mites and certain insects. Natural enemies and certain management strategies vary among pest mites. Identify the pest and natural enemy species in your grove and learn their biology so you can manage these pests appropriately as needed. For details about sampling techniques, see MONITORING PERSEA AND SIXSPOTTED MITES.

(View photos to identify mites)

Avocado brown mite (family Tetranychidae) is dark brown, oval, and tiny (about 0.01 inch or 0.3 mm long). Its tiny amber-colored eggs have a short projecting stalk. At low numbers most eggs are laid singly along the midrib. Eggs are increasingly found throughout the upper leaf surface as mite numbers increase. In summer there may be two complete generations per month. Temperatures of 90° to 95°F or higher, and especially if it is dry, usually kill these mites and their eggs, as does the first cold weather in fall or early winter.

Avocado brown mite is a sporadic pest, mostly in coastal growing areas. Bronzing of leaves, mite cast skins, and partial defoliation of some trees by avocado brown mite is most noticeable from about July to September. Severe infestations tend to occur in border row trees along dirt roads, where road dust is detrimental to mite predators. Ash deposited on leaves from wildfires reportedly also causes brown mite outbreaks.

Avocado brown mite feeds almost entirely on upper leaf surfaces. Bronzing damage is not severe when mite numbers are low to moderate (about 10 to 20 adult females per leaf). If the spider mite destroyer lady beetle (Stethorus picipes) is present and reproducing well at this time, brown mite does not normally become a problem. Damage occurs if avocado brown mite averages about 80 to 100 adult females per leaf (about 200-300 motile stages, adults and nymphs combined). At these higher densities mites also colonize the lower leaf surface and sometimes fruit, and partial defoliation can occur. These higher numbers cause leaf bronzing along the midrib, then along smaller veins, and finally the entire leaf turns brown.

Persea mite damage is distinct from avocado brown mite damage, see section on PERSEA MITE.

Natural enemies and temperature (hot or cold weather) usually maintain this mite at innocuous levels. Maintain good biological control by conserving natural enemies. Control dust and avoid applying broad-spectrum pesticides for any pests. When treating any pests, including avocado brown mite during late summer or fall, spot treat individual trees where possible to allow natural enemies to persist and continue biological control.

(View photos to identify natural enemies of mites)

Naturally occurring populations of the spider mite destroyer (Stethorus picipes) provide the majority of brown mite biological control. Predaceous mites (especially Euseius hibisci and Galendromus helveolus) are also helpful, but predatory mites are primarily effective against sixspotted mite. Most other natural enemies listed as attacking persea mite also feed on avocado brown mite.

Controlling dust, which improves predator activity, is critical for maintaining biological control.

• Oil or pave main orchard roads to reduce dust drift onto trees.
• When it is necessary to use dirt roads, drive slowly.
• Use a water truck or trailer to wet unpaved roads and prevent airborne dust, especially during summer months when heat convection currents carry dust well up into tree canopies.

Use biological and cultural controls along with sulfur and some oil sprays in an organically certified crop.

Monitor for brown mite during summer through fall, together with monitoring for other pests such as caterpillars and persea mite. Look for bronzed leaves and brown mites, especially when monitoring in coastal groves. Consider monitoring specifically for brown mite in border rows along dirt roads during summer through fall where trees are dusty, were sprayed earlier in the season with a broad-spectrum insecticide, and after wildfires. To locate avocado brown mite and its webbing, use a hand lens (10X) to inspect along the midrib on the upper leaf surface.

Major outbreaks have occurred after spraying a broad-spectrum insecticide to control greenhouse thrips or omnivorous looper.

There is no suggested threshold for when treatment is warranted. Pesticide applications for avocado brown mite are rarely needed.

• Photos of mites and their damage
• Photos of natural enemies of mites