Avocado brown mite—Oligonychus punicae
This spider mite (family Tetranychidae) is occasionally abundant on avocado trees growing in coastal areas. From about July to September it can cause noticeable bronzing (browning) of leaves and premature leaf drop. Note that numerous species of mites can occur on plants.
Identification
To locate avocado brown mite and its webbing, use a hand lens to inspect along the midrib on the upper surface of avocado leaves. The mites are mostly dark brown with an oval body and two red eyespots. The head and legs are a lighter color than the rest of the body.
Adult females are about 1/75 inch (0.34 mm) long; males and immatures are smaller. Adults and immatures have numerous bristles covering the body and legs. The globular eggs are pale brown and have a slender stalk projecting from the top.
Avocado brown mite feeds almost entirely on the upper surface of leaves and produces relatively little silk. This and its dark brown coloration distinguish avocado brown mite from persea mite and sixspotted mite, which are greenish to yellowish, produce noticeable silk, and feed mostly on the underside of avocado leaves.
Life cycle
Avocado brown mite develops through 5 life stages: egg, 6-legged larva, and 8-legged protonymph, deutonymph, and adult. When its abundance is low, adult females lay most eggs singly along the midrib. As abundance increases, eggs are increasingly found throughout the upper leaf surface. Temperatures of 90° to 95°F or higher kill many of these mites and their eggs, as does the first cold weather in fall or early winter. In summer there may be 2 complete generations per month.
Damage
Avocado brown mite feeds by sucking on avocado leaves. It causes no significant damage when its numbers are low to moderate, 20 adult females or less per leaf. Damage occurs if avocado brown mites average about 50 to 70 adult females per leaf or about 100 to 200 motile stages (adults and nymphs combined) or more. At these higher densities, mites also colonize the lower leaf surface and sometimes the fruit and can partially defoliate trees. These higher populations cause leaf bronzing along the midrib, then along smaller veins, and finally the entire leaf turns brown.
Avocado brown mite damage can occur in late summer and fall when leaf browning and the mite's whitish eggshells and cast skins can become obvious to the naked eye. Abundant mites can occur in dusty conditions because dust prevents the mite's predators from being effective. The application of a broad-spectrum, persistence insecticide to control other avocado pests (e.g., greenhouse thrips or omnivorous looper) kills the natural enemies of avocado mite and can cause the mite to quickly become very abundant and damaging.
Solutions
Temperature (hot or cold weather) and predators commonly keep populations of this mite at low, innocuous levels. The most important predator is the spider mite destroyer, a tiny, black lady beetle (ladybug). Predaceous mites, especially Euseius hibisci and Galendromus helveolus, are also helpful, but are primarily effective against sixspotted mite. Black hunter thrips, brown lacewings, dustywings, green lacewings , and sixspotted thrips also prey on avocado brown mite.
Provide trees with the appropriate amount and frequency of irrigation during the dry season to reduce the likelihood of mite outbreaks and increase tree tolerance to their feeding. Avoid fertilizing avocado, especially with quick release nitrogen formulations, because excessive nitrogen in leaves can increase mite abundance.
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.
If avocado brown mites are unacceptably abundant, horticultural (narrow-range) oil can be sprayed on foliage. Thoroughly cover the upper surface of leaves where this species occurs. Note that if persea mite or sixspotted mite are also abundant, lower leaf surfaces must also be well covered with spray because these mites feed mostly on the underside of leaves. 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). |
