How to Manage Pests
UC Pest Management Guidelines
Scientific name: Oligonychus
In this Guideline:
Spider mites (family Tetranychidae) and predatory mites
(Phytoseiidae) are tiny 8-legged arthropods. 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.
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 populations most eggs are laid singly along the
midrib. Eggs are increasingly found throughout the upper leaf surface as populations
increase. In summer there may be two complete generations per month. Temperatures
of 90° to 95°F or higher 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.
It causes no significant damage when
population densities 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 become a problem. Damage occurs if avocado brown mite averages
about 50 to 70 adult females per leaf (about 100-200 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
populations cause leaf bronzing along the midrib, then along smaller veins, and
finally the entire leaf turns brown.
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.
Naturally occurring populations of the spider mite
destroyer (Stethorus picipes) provide the majority of brown mite
biocontrol. 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.
Organically Acceptable Methods
Biological and cultural controls along with sulfur
and some oil sprays are acceptable control methods in an organically certified
Monitoring and Treatment Decisions
Look for bronzed leaves and brown mites during summer through
fall monitoring for other pests such as caterpillars and persea mite, 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. Major outbreaks have occurred after spraying a
broad-spectrum insecticide to control greenhouse thrips or omnivorous looper.
To locate avocado brown mite and its webbing, use a hand lens (10X) to inspect along the midrib on the upper
leaf surface. There is no suggested threshold for when treatment is warranted.
Pesticide applications for avocado brown mite are rarely needed.
||Amount to use
|When choosing a pesticide, consider
information relating to the impact
on natural enemies and honey bees and environmental impact.
||NARROW RANGE OIL#
||MODE OF ACTION: Contact including smothering and barrier effects.
Requires good coverage to be effective. Check with certifier to determine which products are organically acceptable.
||MODE OF ACTION: Unknown. An inorganic miticide.
||COMMENTS: Do not
treat with sulfur when temperatures exceed 90°F to avoid leaf damage. Sulfur
sprays are often not effective in coastal areas where temperatures do not promote fuming action.
UC IPM Pest Management Guidelines: Avocado
UC ANR Publication 3436
B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
J. G. Morse, Entomology, UC Riverside
M. S. Hoddle, Entomology, UC Riverside
Acknowledgment for contributions to Invertebrates:
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County
M. Blua, Entomology, UC Riverside
P. Oevering, UC Cooperative Extension, Ventura County
D. Machlitt, Consulting Entomology Services, Moorpark, CA
T. Roberts, Integrated Consulting Entomology, Ventura, CA
B. B. Westerdahl, Nematology, UC Davis
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