How to Manage Pests

Persea mite—Oligonychus perseae

This spider mite (family Tetranychidae) is a common pest of avocado. It can occur on more than one dozen other plants, but generally is a pest only on avocado.

Identification

When persea mites are abundant, infestation can be recognized by the numerous brown-spotted, green leaves hanging from the tree and dropped prematurely on the ground. When viewed from a distance, the canopy of a heavily infested tree can appear lighter-colored overall than that of a healthy tree.

Persea mite feeds mostly beneath roundish patches of silk that resemble a silvery blotch on the underside of leaves. The mite colonies are small and can be very numerous. Their feeding causes discrete, circular, brown to yellowish spots that initially are visible only on the lower surface of leaves. These spots later become visible on the upper leaf surface.

Adult females are about 1/50 inch (0.5 mm) long and have an oval-shaped body that is slightly flattened and elongated. Females and immatures are greenish or yellowish with two or more dark blotches on the abdomen. Old females that have ceased egg laying turn darker green and become somewhat smaller and inactive. Males are smaller than reproductive females, somewhat pear-shaped, slightly flattened, and yellowish with or without dark spots. Eggs are round and pale yellow. They develop red eye spots as they approach hatching.

Persea mite damage early in the growing season can be confused with damage from sixspotted mite, Eotetranychus sexmaculatus. Both mites feed mostly or only on the underside of leaves. Sixspotted mite webbing is less dense and usually does not occur in small, circular patches. Sixspotted mite feeding on avocado causes brown to purplish, irregularly shaped blotches that are often relatively contiguous along the veins, in comparison to the roundish, mostly scattered spots created by persea mite. There are six dark spots on the yellowish body of sixspotted mite. Its round eggs are translucent to white. See Pest Notes: Sixspotted Spider Mite on Plumeria for more information on this pest.

Mite feeding on the upper leaf surface is generally caused by avocado brown mite, Oligonychus punicae. Avocado brown mite feeding causes the upper leaf surface to appear bronzed (brown) and the damage does not appear in discrete, circular spots as with persea mite. Avocado brown mite is brown to dark colored and unlike persea mite produces little obvious silk. Its tiny eggs are amber or honey colored with a short projecting stalk. Eggs occur mostly singly along the upper surface of the midrib vein.

Life cycle

Persea mite develops through 5 life stages: egg, 6-legged larva, and 8-legged protonymph, deutonymph, and adult. During her life each adult female lays about 2 to 4 dozen eggs beneath silk. When occupied, these silk-covered “nests” typically contain eggs, immatures (larvae and nymphs), and adults. Development time from the egg to a reproductive adult is about 2 to 3 weeks when temperatures average 70°F. There are multiple generations per year.

Heavy winter rains and wind can reduce persea mite abundance and subsequent damage. Cool winter temperatures slow persea mite population growth. Mite densities are lowest in March and gradually increase through spring while feeding on new leaf flush. Populations generally peak in July and August. Persea mite abundance is suppressed and may decline rapidly when humidity is low and the temperature reaches 100°F or higher on several consecutive days.

Damage

Persea mites' sucking and feeding on the underside of leaves causes circular, yellowish leaf spots that turn brown. Each mite colony produces dense webbing that resembles a silvery spot on the underside of the leaf. Feeding by abundant mites causes premature leaf drop, which can contribute to sunburn of fruit and during the next growing season reduced fruit yield. But in residential avocados, persea mites can commonly be tolerated.

Persea mite is most damaging to Hass and Gwen avocados and a few other varieties. Esther, Pinkerton, and Reed are of intermediate susceptibility. The Bacon, Fuerte, Lamb Hass, and Zutano varieties are much less susceptible. Persea mite attacks numerous other plant species, but is an uncommon pest on them. Alternative hosts include acacia, bamboo, carob, eucalyptus, grape, Prunus species, rose, and willow. Weed hosts include cheeseweeds (mallows), lambsquarters, milkweed, and sowthistle.

Solutions

Cultural controls, mite predators, and application of an acaricide (miticide) are the main controls for persea mite. Eliminate or reduce alternative host plants for persea mite that are growing near your avocado, such as weeds. Provide trees with appropriate irrigation and other good cultural care to maintain the flush of new growth and compensate for mite-induced leaf drop. Avoid fertilizing avocado where mites have been a problem. Excessive fertilization, especially with quick-release nitrogen formulations, can increase persea mite populations and their damage during late spring and summer.

Minimize tree stress to reduce the effect of persea mites feeding on trees. Appropriate irrigation frequency and amounts, good management of avocado root rot and other key pathogens, and early harvesting of fruit will reduce the adverse impacts of mite feeding. If abundant mites cause extensive leaf drop, protect bark and wood from sunburn by whitewashing (painting) major limbs and trunks with white, interior latex paint diluted with an equal amount of water.

Mites have numerous natural enemies, but they are not highly effective because of persea mites’ protective, webbed nests. However, conserving natural enemies can reduce persea mite abundance mostly when the mites are moving outside of their nests. Predators commonly provide good biological control of avocado brown mite and sixspotted mite.

Predaceous mites include Neoseiulus (=Amblyseius) californicus, Euseius hibisci, Galendromus annectens, and G. helveolus. Black hunter thrips, brown lacewings, dustywings, green lacewings, spider mite destroyer (a tiny, black lady beetle), and sixspotted thrips are also important mite predators. Predaceous mite midges (Feltiella species) and a rove beetle (Oligota oviformis) also prey on persea mite in coastal areas.

Releases of Galendromus species predaceous mites before persea mites become abundant has provided control in commercial avocado orchards. To check the viability of purchased predaceous mites, gently pour some mites and any shipping substrate into a clear jar and look for an abundance of fast-moving mites, which would indicate that the predators have arrived in good condition.

To improve the effectiveness of naturally occurring mite predators and any that are released, control ants, minimize dustiness (e.g., periodically hose off small plants), and avoid the application of miticides and broad-spectrum, persistent insecticides for all pests. See Protecting Natural Enemies and Pollinators and Natural Enemy Releases for Biological Control of Crop Pests for more suggestions.

Spraying the underside of leaves with a forceful stream of water can reduce mite populations on a few small trees where this is feasible. If mites become too abundant, horticultural (narrow-range) oil can be sprayed on foliage. Be sure to thoroughly cover the underside of leaves where persea mites occur. For more information, see Pest Notes: Spider Mites.

Discolored spots from feeding by persea mites.

Discolored spots and silken patches of persea mite.

Persea mites exposed with silk covering removed.

Adult (left) and immature persea mites exposed.