How to Manage Pests
UC Pest Management Guidelines
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.
Persea mite (family Tetranychidae) is a key pest that occurs in most avocado-growing areas of California except the Central Valley. It is most damaging to Hass, Gwen, and a few other varieties. Esther, Pinkerton, and Reed are of intermediate susceptibility. The Bacon, Fuerte, Lamb Hass, and Zutano varieties are much less affected. Many ornamentals and weeds also host persea mite. When persea mites were first introduced into California in the early 1990s, individual mites from heavy populations on avocado trees were seen drifting onto leaves of adjacent stone fruit trees, although they did not feed. Since that time, however, populations have been reduced and persea mites have not been observed on stone fruit trees or fruit, and Prunus species are not known to be a host of this mite.
Persea mite develops from an egg through a six-legged larval stage and two eight-legged nymphal stages before becoming an eight-legged adult. Adult females have an oval-shaped body that is slightly flattened and elongated. Females and immatures are yellowish or greenish with two or more small dark blotches on their abdomen. Old females that have ceased oviposition turn darker green and become somewhat smaller and inactive. Males are smaller than reproductive females. Males are somewhat pear-shaped, slightly flattened, and yellowish with or without small dark spots. Persea mites feed and reproduce mostly beneath webbed patches or silk-covered "nests."
Each female lays about 2 to 4 dozen eggs during her life. Eggs are round, pale yellow, and develop red eye spots as they mature. Egg to adult female development time is about 2 to 3 weeks when temperatures average 77° to 63°F. Generation time can be accurately estimated by monitoring degree-days.
Cool winter temperatures slow persea mite population growth. Mite densities are lowest about March and gradually increase through spring feeding on new leaf flush. Populations generally peak in July and August. Persea mite populations are suppressed, and populations may decline rapidly, when the daily high temperature is 100°F or more on several consecutive days and humidity is low.
High persea mite populations cause premature leaf drop and defoliation. Defoliation leads to sunburned bark and fruit, aborted or dropped fruit, and severely stressed trees, which later reduces yields.
Persea mite feeding on the underside of leaves causes discrete circular chlorotic to brown spots on the lower leaf surface. These spots become visible on the upper leaf surface. Persea mite colonies are small and can become very numerous. Each colony can produce dense webbing, which resembles a silvery spot on the underside of the leaf. High persea mite populations can often be recognized by numerous brown-spotted, green leaves hanging from trees and on the ground beneath infested trees. Heavily infested canopies can appear lighter colored overall when viewed from a distance.
Persea mite damage early in the season can be confused with sixspotted mite damage. Sixspotted mite webbing is less dense and usually does not occur in small circular patches. Sixspotted mite feeding causes brown to purplish irregularly shaped blotches, in comparison with the roundish, mostly scattered spots created by persea mite. Damage from sixspotted mites is generally confined to areas immediately adjacent to veins, while persea mite often feeds throughout the lower leaf surface. Persea mite also sometimes feeds on the upper leaf surface, but mite feeding on the upper leaf surface is usually caused by avocado brown mite. Avocado brown mite feeding causes the upper leaf surface to appear bronzed or scorched and damage does not occur in discrete circular spots.
Minimize tree stress to reduce the effect of persea mite feeding on trees. Appropriate irrigation frequency and amounts, good management of avocado root rot and other key pathogens, and harvesting fruit early will reduce the adverse impact of mite feeding. If treating, whenever possible choose pesticides that have low residual toxicity or are non-toxic to natural enemies.
In the early stages of a significant infestation, highly refined or narrow-range petroleum oils or certain other materials can be applied. Treat only where necessary and leave unsprayed areas to conserve beneficials and provide refuges from which natural enemies and pesticide-susceptible pests can recolonize treated trees. Maximize the interval between treatments and alternate applications among pesticides with a different mode of action to reduce the rate at which pesticide resistance develops.
Commercially available predators include predatory mites (Amblyseius californicus, Galendromus annectens, and G. helveolus, family Phytoseiidae) and green lacewing larvae (Chrysoperla spp.). Often relatively few predaceous mites are present through the winter because populations of their persea mite prey have been suppressed by hot summer weather. Introducing Galendromus helveolus helps to control persea mite if sufficient numbers of predators are introduced and releases are well-timed. If predator releases are planned, monitor persea mites regularly in late February through summer and release predaceous mites when about 50% of leaves have one or more active-stage pest mites. 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 indicates predators arrived in good condition.
Monitoring and Treatment Decisions
Consider the effect of weather on treatment decision-making. Heavy winter rains and high winds can substantially reduce subsequent mite populations and damage. Persea mite populations are suppressed or may crash when the daily high temperature is 100°F or more on several consecutive days and humidity is low.
There are no research-based thresholds for when persea mite treatment is warranted. Develop treatment guidelines satisfactory for your situation by keeping good records and adapting your monitoring and management methods as appropriate. Regularly monitor and record mite densities and compare these numbers from year-to-year with records of your control actions and their effectiveness. See MONITORING PERSEA AND SIXSPOTTED MITES for additional information.
UC IPM Pest Management Guidelines: Avocado