Agriculture: Almond Pest Management Guidelines

Webspinning Spider Mites

  • Pacific spider mite: Tetranychus pacificus
  • Strawberry spider mite: Tetranychus turkestani
  • Twospotted spider mite: Tetranychus urticae
  • Description of the Pest

    These three spider mites are difficult to distinguish as adults, have similar life histories, and are controlled in the same manner. However, Pacific mite is often the most difficult to control with miticides.

    The overwintering female mites are red- or orange-colored and are found under rough almond bark, in ground litter, and on winter weeds. During the season the color ranges from yellow to green to black depending on age and host food. All have dark spots. Adult males do not overwinter and are smaller than females. Eggs are laid on the foliage. Immature mites molt three times. Early in the season, mites are found in lower to central areas of the tree. The mites reproduce rapidly during warm weather between June and September. During favorable conditions, mites develop within 7 days, with 8 to 10 generations per season.

    Damage

    Mites damage foliage by sucking cell contents from leaves. The damage begins with leaf stippling. Leaves can turn yellow and drop off. High numbers of mites cover tree terminals with webbing. Crop reduction and reduced vegetative tree growth shows up the year after damage occurs.

    Management

    Spider mites are often a problem in water-stressed orchards. Mites can also become a problem when their natural enemies are disrupted by the application of broad-spectrum insecticides including pyrethroids, which kill many types of predatory insects and predator mites. Almond trees can tolerate moderate mite numbers without suffering economic damage. Predators are important in managing mites, so consider their presence and relative abundance before treatments are applied. Orchards with high predator-to-pest mite ratios will not require treatment.

    Monitor orchards for both predators and spider mites at least once every 2 weeks from March to early May; monitor at least once a week after that. When treatments are required, choose selective miticides that cause the least harm to predators.

    Biological Control

    Several species play an important role in mite control, including western predatory mite (Galendromus [= Metaseiulus] occidentalis), sixspotted thrips, the spider mite destroyer (Stethorus sp.) and minute pirate bugs (Orius sp.). In recent years the sixspotted thrips has become the most common insect predator of spider mites. Sixspotted thrips is an extremely good mite predator. It is highly mobile and both nymphs and adults feed on spider mites. However, movement of sixspotted thrips into orchards can be unpredictable and sometimes does not occur until mites have already exceeded treatment thresholds. If sixspotted thrips is present in an orchard, avoid using pyrethroid or spynosyn insecticides or miticides containing abamectin to preserve their presence and maximize biological control.

    The most widespread predatory mite is the western predatory mite. It is about the same size as spider mites, but the western predatory mite lacks spots and its color ranges from cream to amber red. It often can be observed moving quickly over the underside of leaf surfaces in search of spider mites. This predator maintains good control, unless the proportion of leaves with spider mites is higher than the proportion with predatory mites.

    Predators will typically control webspinning mites if presence-absence sampling indicates equivalent numbers of leaves with predators and with webspinning mites. When predator mites are present, but are not controlling the spider mites, a lower-than-label rate of a selective miticide may be applied to create a more balanced ratio (i.e., a 1:1 ratio of one leaf with a predatory mite for every leaf with a webspinning mite). If predatory mites are present in an orchard, choose a miticide with reduced toxicity to predatory mites. See RELATIVE TOXICITIES OF PESTICIDES USED IN ALMONDS TO NATURAL ENEMIES AND POLLINATORS for a list of pesticides used on almonds and their toxicity to the western predatory mite.

    Predatory mites can be purchased from insectaries and released to augment natural populations of predatory mites. Before releases, be sure that insecticide residues have sufficiently broken down to allow the survival of predatory mites. For example, research has shown that some pyrethroid insecticides can be highly toxic to predatory mites for several months to more than a year after application. In some cases, predatory mites from insectaries have been reared from stocks with tolerance to one or more pesticides, such as organophosphates and carbaryl, enabling survival in orchards if residues are present. Keep purchased mites cool when they are received from the insectary and release them as soon as possible. Release predatory mites when spider mites are present (to allow predatory mites to feed) but well before treatment thresholds are reached. Predatory mite releases are not advised in areas where sixspotted thrips are present, since the predatory thrips are known to eat both spider mites and predatory mites. For more detailed information on predatory mite releases, see Integrated Pest Management for Almonds, UC ANR Publication 3308 and Natural Enemy Releases for Biological Control of Crop Pests.

    Cultural Control

    Reduce dusty conditions in orchards by oiling or watering roadways and maintaining a ground cover. Prevent water stress, as this condition results in higher mite numbers and makes trees more susceptible to infestation and damage.

    During the season, avoid using broad-spectrum pesticides such as pyrethroids, carbamates, and organophosphates (unless organophosphate-resistant predator mites are present in the orchard); the use of these insecticides will often result in spider mite outbreaks.

    Organically Acceptable Methods

    Biological controls, including predator releases, cultural controls, and various types of oil sprays are organically acceptable ways of managing spider mites.

    Monitoring and Treatment Decisions

    • Monitor for mites at least weekly from May through August. Treatment is not necessary after August because mites begin to migrate off trees to prepare for overwintering.
    • If the orchard has problem areas, such as trees along roads or water-stressed trees, monitor every few days.
    • Before July 1, focus monitoring on hot spots—that is, the areas that develop mites first; these are often dusty or water-stressed areas of the orchard. Once the treatment threshold has been reached in these areas, sample the remainder of the orchard to determine if a spot treatment is sufficient or the entire orchard requires treatment.
    • After July 1, monitor the whole orchard, dividing it into sampling areas that could be treated separately.

    Within each sampling area, sample a minimum of five trees. Select 15 leaves from each tree, randomly picking leaves from both the inside and outside of the canopy as you walk around it. Examine both sides of each leaf under a hand lens looking for spider mites and eggs, western predatory mites or eggs, sixspotted thrips, and other predators.

    To sample trees that have not yet been treated for mites during the current season, use the presence-absence sampling form (PDF)to note the number of leaves on each tree with pest mites or their eggs, and the number of leaves with predators. There is no need to count total numbers of mites. Once you have sampled 5 trees, compare your total to the numbers in the "Don't Treat" and "Treat " columns on the form. Be sure to take into account the presence or absence of predators as noted on the form.

    If treatment is required determine which natural enemies are present and choose a product that is compatible according to the RELATIVE TOXICITIES OF PESTICIDES USED IN ALMONDS TO NATURAL ENEMIES AND POLLINATORS table.

    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.
    PREDATOR RELEASES
    A. GALENDROMUS OCCIDENTALIS#
    COMMENTS: Predatory mites can be released early in the season to establish or to augment resident populations. If an acaricide is needed and predators are present, be sure to use a selective miticide. Monitor to ensure that pest numbers remain in balance with predator numbers.
    MITICIDES
    A. BIFENAZATE
    (Acramite 50WS) 0.75–1 lb 12 7
    (Vigilant 4SC) 14–24 fl oz 12 7
    MODE-OF-ACTION GROUP NUMBER1: un
    COMMENTS: Contact toxin that targets all stages. Relatively safe for beneficial and predaceous mites. Apply with ground equipment; requires complete coverage of both leaf surfaces for effective control. A good choice for post-hullsplit control.
    B. HEXYTHIAZOX
    (Onager) Label rates 12 7
    MODE-OF-ACTION GROUP NUMBER1: 10A
    COMMENTS: Apply after sampling indicates pest mites are increasing, but before significant damage or webbing is present. A mite growth regulator: a contact toxin to eggs and young larval stages, so it is best suited for an early-season application if needed; causes adult females lay sterile eggs. Believed to have same effect on predator mite females as well. Do not make more than one application per year.
    C. SPIRODICLOFEN
    (Envidor 2SC) 16–34 fl oz 12 7
    MODE-OF-ACTION GROUP NUMBER1: 23
    COMMENTS: Contact toxin that targets all stages. Most effective when applied with oil at 0.5 to 1% concentration. More effective against twospotted spider mites than against Pacific mites. Low-to-moderate harm to natural enemies. A good choice for post-hullsplit control of twospotted spider mites.
    D. CYFLUMETOFEN
    (Nealta) 13.7 fl oz 12 7
    MODE-OF-ACTION GROUP NUMBER1: 23
    COMMENTS: Contact miticide that targets all stages. Most effective when applied with oil at 0.5 to 1% concentration. More effective against twospotted spider mites than against Pacific mites. Low-to-moderate harm to natural enemies. A good choice for posthullsplit control of twospotted spider mites.
    E. ACEQUINOCYL
    (Kanemite 15SC) 21–31 fl oz 12 7
    MODE-OF-ACTION GROUP NUMBER1: 20B
    COMMENTS: Contact toxin that targets all stages. Most effective at high rate. Do not use lower label rates for moderate to high numbers of spider mites. Safest of miticides to beneficials. A good choice for post-hullsplit timing.
    F. ABAMECTIN
    (Agri-Mek SC*, others) Label rates See label See label
    MODE-OF-ACTION GROUP NUMBER1: 6
    COMMENTS: Contact or ingestion toxin that paralyzes juveniles and adults. Applications are more effective with an oil (at least 1% v/v) or a penetrating adjuvant. Applications are most effective before hullsplit or until leaves harden off. Can be used effectively for treatment in May based on mite monitoring guidelines, but it is not recommended for preventive use at this time. Do not make more than two applications per growing season and allow at least 21 days between treatments. Do not exceed 20 fl oz/acre per application. Resistance has been found in the San Joaquin Valley where abamectin products have been applied for many years. Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations. Regulations affect use for the San Joaquin Valley from May 1 to October 31, 2018 and 2019. Review the Department of Pesticide Regulation's updated fact sheet.
    G. ETOXAZOLE
    (Zeal) 2–3 oz 12 28
    MODE-OF-ACTION GROUP NUMBER1: 10B
    COMMENTS: Inhibits molting of juveniles and causes adult females of both pest and predator mites to produce sterile eggs. Do not apply more than once a season. Long residual activity that can be used early- to mid-season in the absence of concern for predator mites.
    H. NARROW RANGE OIL#
    (Omni Supreme) Label rates See label See label
    MODE OF ACTION: Contact including smothering and barrier effects.
    COMMENTS: Use as an in-season treatment; oils do not control webspinning mites during winter dormancy. Be sure that trees are well watered to avoid phytotoxicity. Works by contact activity only, so good coverage is essential. Will affect natural enemies that are contacted with the spray, but there is little residual effect on remaining beneficials. Repeat applications may be necessary to control rapidly increasing numbers. Check with certifier to determine which products are organically acceptable.
    I. FENPYROXIMATE
    (Fujimite 5EC) 2–4 pt 12 14
    MODE-OF-ACTION GROUP NUMBER1: 21A
    COMMENTS: Contact toxin to juveniles and adults with long residual activity. Residues are toxic to both pest and predator mites for several weeks. A good choice under extreme mite pressure in the absence of natural enemies.
    J. FENBUTATIN-OXIDE
    (Vendex 50WP)* 1–2.5 lb 48 14
    MODE-OF-ACTION GROUP NUMBER1: 12B
    COMMENTS: Contact toxin that targets juveniles and adults. Do not apply more than twice a season. Good coverage is essential. Toxic to predator mites at full label rates but becomes less toxic at 10 to 25% maximum label rate when western predatory mites are present and if the development of resistance to this material by webspinning mites is not a concern (see ANR Publication 3308 for additional information). Below-label rates are intended to balance predator and spider mite populations. Most effective with 1% oil combination.
    K. CLOFENTEZINE
    (Apollo SC) 4–8 oz 12 14
    MODE-OF-ACTION GROUP NUMBER1: 10A
    COMMENTS: A growth regulator that targets eggs and some immature stages. Research is lacking in California as to its effectiveness and impact on predator mites.
    L. PROPARGITE
    (Omite 6E)* 32–64 fl oz 528 (22 days) 28
    (Omite 30WS)* 5–10 lb 528 (22 days) 28
    MODE-OF-ACTION GROUP NUMBER1: 12C
    COMMENTS: Contact toxin that targets juveniles and adults. Do not apply more than twice a season. Do not apply less than 40 days after, or 30 days before, an oil application. Toxic to predator mites at full label rates but becomes less toxic at rates that are 1/2 to 1/10 maximum label rate when western predatory mites are present. Below label rates are intended to balance predator and spider mite populations.
    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 these two intervals is the minimum time that must elapse before harvest may occur.
    * Permit required from county agricultural commissioner for purchase or use.
    # Acceptable for use on organically grown produce.
    1 Rotate pesticides 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; pesticides with a 1B group number should be alternated with pesticides that have a group number other than 1B. Mode-of-action group numbers (un=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).

    Important Links

    Text Updated: 06/19
    Treatment Table Updated: 08/17