Mite Pests of Avocado—General Information
Spider mites (family Tetranychidae) and predatory mites (Phytoseiidae) are tiny eight-legged arthropods (larval stages have only 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.
Description of the Pest
Persea mite (family Tetranychidae) is a key pest that occurs in most avocado-growing areas of California except the Central Valley. Many ornamentals and weeds also host persea mite. When persea mites were first intro-duced into California in the early 1990s, individual mites from large populations on avocado trees were seen drifting onto leaves of adjacent stone fruit trees, although they did not feed. Since that time, however, numbers 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 laying eggs 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 growth of persea mite numbers. Mite densities are lowest around March and gradually increase through spring feeding on new leaf flush. Numbers generally peak in July and August. Persea mite populations are suppressed, and their numbers may decline rapidly, when the daily high temperature is 100°F or more on several consecutive days and humidity is low.
Persea mite 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.
High persea mite numbers causepremature 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 numbers 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.
|Damage||Persea mite||Sixspotted mite|
Mite feeding on the upper leaf surface is usually caused by avocado brown mite and results in the upper leaf surface to appear bronzed or scorched. 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.
Numerous predators feed on persea mite. Predaceous mites include Amblyseius (=Neoseiulus) californicus, Euseius hibisci, Galendromus annectens, and G. helveolus. Black hunter thrips (Leptothrips mali), sixspotted thrips (Scolothrips sexmaculatus), brown lacewings (Hemerobius spp.) and green lacewings (Chrysopa and Chrysoperla spp.), dustywings (family Coniopterygidae), a predatory midge (Feltiella sp., Cecidomyiidae), a rove beetle (Oligota oviformis, Staphylinidae), and the spider mite destroyer lady beetle (Stethorus picipes) are other common predators. Most predators are not highly effective because of persea mites' protective webbed nests. However, conserve natural enemies because they can reduce persea mite numbers, and predators often provide good biological control of avocado brown mite and sixspotted mite.
Commercially available predators include predatory mites from family Phytoseiidae (Amblyseius californicus, Galendromus annectens, and G. helveolus) and green lacewing larvae (Chrysoperla spp.). Often relatively few predaceous mites are present through the winter because presence 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.
For more information about obtaining and effectively releasing natural enemies, see Natural Enemy Releases for Biological Control of Crop Pests.
Eliminate or reduce persea mite alternate host plants growing near avocado, including mite-susceptible ornamentals, non-commercial fruit trees, and 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. However, be careful not to overfertilize. Excess fertilization, especially with quick-release formulations, may increase persea mite numbers and damage during late spring and summer due to increased foliar nitrogen. Spraying the underside of leaves with a forceful stream of water can reduce mite presence on a few small trees where this is feasible. Whitewash trunks and major limbs to protect bark and wood from sunburn after premature leaf drop.
Organically Acceptable Methods
Use biological and cultural controls and sprays of certain oils in an organically certified crop.
Monitoring and Treatment Decisions
- Inspect leaves for mites, mite damage, and natural enemies about every 7 to 10 days from mid-March through at least August, and perhaps through October.
- Coordinate monitoring and treatment decision-making for persea mite and avocado thrips, which are usually the key invertebrate pests feeding on leaves. Mite monitoring frequency, and the need for treatment and choice of material, can be affected by thrips management decisions.
- Consider the effect of weather on treatment decision-making. Heavy winter rains and high winds can substantially reduce subsequent mite presence and damage. Persea mite numbers are suppressed or may crash when the daily high temperature is 100°F or more on several consecutive days and humidity is low.
- Certain materials applied (usually earlier in the season) to control avocado thrips can also control or suppress mite numbers (which are usually treated later in the season if needed).
- Some materials can adversely impact natural enemies, so applying a less selective material early for thrips may increase the need to later treat mites.
- Only one application per season may be permitted or recommended for certain materials (e.g., abamectin) to reduce the development of pesticide resistance.
- Rotate among chemical classes when making multiple applications to reduce the development of pesticide resistance.
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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|. . . or . . .|
|COMMENTS: Make a single release of 2,000 mites per tree, or two releases each of 1,000 mites/tree, when regular monitoring of leaves for mite presence-absence shows that about 50% of leaves have one or more active-stage pest mites, typically in spring or early summer. The most effective release strategy is to dispense predator mites and carrier (e.g., corn grits) in small paper cups attached to branches. Attach four cups per tree evenly distributed around the canopy on avocado branches that are shaded from the sun. Add about 250 to 500 predators per cup depending on the release rate. The predators will disperse from the cups.|
|(Agri-Mek SC)*||Label rates||12||14|
|MODE-OF-ACTION GROUP NUMBER1: 6|
|. . . PLUS . . .|
|NARROW RANGE OIL|
|(415)||1%||See label||See label|
|MODE-OF-ACTION GROUP NUMBER1: Improves translaminar movement and persistence of insecticide.
|COMMENTS: Use with 1 to 2% narrow range (415) oil in a minimum of 50 gal water/acre for aerial applications (higher water volumes are more effective in controlling high numbers of mites) and 100 gal water/acre for ground applications. On large trees aerial applications may require larger volumes of water to achieve desired efficacy. Control may last 3 or more weeks. Only use in an alkaline or slightly acidic solution. Do not tank mix with nutrients. To avoid promoting pesticide resistance, and because of past heavy use, do not make more than one application of any abamectin product every 2 to 3 years in each grove. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|(Envidor 2 SC)||18–20 fl oz||12||2|
|MODE-OF-ACTION GROUP NUMBER1: 23|
|COMMENTS: Only one application is allowable per crop season. Apply in a minimum of 50 gal/acre by air; higher water volumes are more effective in controlling high numbers of mites. Contact material (no translaminar or systemic activity).|
|MODE-OF-ACTION GROUP NUMBER1: 10B|
|COMMENTS: Maximum of one application per year. Material shows some translaminar activity and addition of a surfactant or oil (more research is needed) may improve efficacy (stickers appear to limit translaminar activity and should be avoided). Apply in a minimum of 20 gallons/acre by air (higher water volumes are more effective) or 50 gpa by ground. This material works mainly as an ovicide (i.e. against mite eggs).|
|(Danitol 2.4 EC) *||16–21 1/3 fl oz||24||1|
|MODE-OF-ACTION GROUP NUMBER1: 3A|
|COMMENTS: Apply a minimum of 100 gallons by ground and 50 gallons by air. If oil is added to a fenpropathrin application, no more than 1% NR–415 oil may be used. Workers who enter for the purpose of harvesting within 7 days of application must wear specific harvest PPE, even though the PHI is 1 day (see label). To avoid promoting pesticide resistance, do not make more than one application of fenpropathrin every 3 years in each grove. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|MODE-OF-ACTION GROUP NUMBER1: 21A|
|COMMENTS: Contact material (not translaminar or systemic). Apply in a minimum of 95 gal/acre by ground and 50 gal/acre by air.|
|G.||NARROW RANGE OIL#||Label rates||See label||See label|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|COMMENTS: Requires good coverage to be effective. Check with certifier to determine which products are organically acceptable.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|#||Acceptable for use on organically grown produce.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their website at http://irac-online.org/.|