How to Manage Pests

UC Pest Management Guidelines

Almond

Webspinning Spider Mites

Scientific names:
Pacific spider mite: Tetranychus pacificus
Twospotted spider mite: Tetranychus urticae
Strawberry spider mite: Tetranychus turkestani

(Reviewed 3/09, updated 11/12)

In this Guideline:


DESCRIPTION OF THE PESTS

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 populations 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. Orchards that are properly irrigated usually do not require treatment for mites, with the possible exception of dry-down prior to harvest, which can stress trees. 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 insects and predator mites. Almond trees can tolerate moderate mite populations 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 and once a week or more after that. When treatments are required, choose selective miticides that have the least negative impact on predators.

Biological Control

Several species play an important role in mite control, including western predatory mite (Galendromus [= Metaseiulus] occidentalis), sixspotted thrips, and spider mite destroyer (Stethorus sp.). The western predatory mite is the most widespread mite predator in almonds, but others may also be present and effective depending on location. About the same size as spider mites, the western predatory mite lacks spots and ranges in color 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.

A good rule of thumb is that predator mites will 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). For example, if predatory mites are sufficiently abundant, it has been shown that rates as low as one-eighth to one-tenth of the propargite (Omite) label rates can be used to balance predator-to-prey ratios, although actual control of the spider mites may only reach 50 to 60%. Similar research has not been conducted for newer miticides, but the concept might hold for other products.

Monitoring spider mite and western predatory mite populations and using miticides sparingly will help to delay the development of resistant spider mites. The other mite predators are often not abundant until pest mite numbers are high, and therefore their populations are difficult to manipulate. While the best course of action is to conserve and promote existing western predatory mite populations in orchards, commercially available western predatory mites can be released to establish or augment resident populations. (See Integrated Pest Management for Almonds, for detailed information on releasing and managing predator mites. See Relative Toxicities of Pesticides Used in Almonds to Natural Enemies and Honey Bees for a list of pesticides used on almonds and their toxicity to the western predatory mite.)

Organically Acceptable Methods

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

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 populations and makes trees more susceptible to 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 materials will often result in spider mite outbreaks.

Monitoring and Treatment Decisions

From May through August, monitor for mites at least weekly. 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 5 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 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, avoid pyrethroid, organophosphate and carbamate insecticides. All can cause upsets of biological control. Soft materials such as oils, acequinocyl (Kanemite) abamectin (Agri-Mek) and bifenazate (Acramite) are good choices for protecting natural enemies.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name) (conc.) (dilute)
(hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. Consequently, they are not necessarily listed in order of greatest efficacy against target pest. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. 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 material. Monitor to ensure that pest populations remain in balance with predator populations.
 
Note: This list includes most registered miticides listed in an order that takes into account both efficacy and effects on predatory mites and other beneficials.
 
MITICIDES
A. BIFENAZATE
  (Acramite 50WS) 0.75–1 lb 0.1875–0.25 lb 12 7
  MODE OF ACTION GROUP NUMBER1: 25
  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. ABAMECTIN*
  (Agri-Mek 0.15 EC, etc.) Label rates See label See label
  MODE OF ACTION GROUP NUMBER1: 6
  COMMENTS: Contact or ingestion toxin that paralyzes juveniles and adults. Direct contact toxicity to predators but not long lasting. 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 2 applications per growing season and allow at least 21 days between treatments. Do not exceed 20 fl oz/acre/application.
 
C. ACEQUINOCYL
  (Kanemite 15 SC) 31 fl oz 7.75 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 populations. Safest of miticides to beneficials. A good choice for post-hullsplit timing.
 
D. SPIRODICLOFEN
  (Envidor 2SC) 16–18 fl oz 4–4.5 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 impact on beneficials. A good choice for post-hullsplit control of twospotted spider mites.
 
E. NARROW RANGE OIL#
  (Omni Supreme and others) Label rates See label See label
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Be sure that trees are well-watered to avoid phytotoxicity. Works by contact activity only, so good coverage is essential. Will effect beneficials that are contacted with the spray, but there is little residual effect on remaining beneficials. Repeat applications may be necessary to control rapidly increasing populations. Check with certifier to determine which products are organically acceptable.
 
F. PROPARGITE*
  (Omite 6EC) 1.5–2 qt 0.375–0.5 qt 22 days 28
  (Omite 30WS) 7.5–10 lb 1.9–2.5 lb 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.
 
G. ETOXAZOLE
  (Zeal) 2–3 oz 0.5–0.75 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. HEXYTHIAZOX
  (Onager EC) 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.
 
I. FENPYROXIMATE
  (Fujimite 5EC) 1–4 pt See label 12 14
  MODE OF ACTION GROUP NUMBER1: 21
  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 beneficials.
 
J. PYRIDABEN/SULFUR
  (Desperado) 1–1.1 gal 0.25 gal 24 7
  MODE OF ACTION GROUP NUMBER1: 21/unknown
  COMMENTS: Contact toxin to juveniles and adults. Toxic to both pest and predator mites.
 
K. FENBUTATIN-OXIDE*
  (Vendex 50 WP) 1–2.5 lb 0.25–0.5 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 1/4–1/10 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.
 
L. CLOFENTEZINE
  (Apollo) 4 oz 1 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.
 
** For dilute applications, rate is per 100 gal water to be applied in 300–500 gal water/acre, depending on the label; for concentrate applications, use 80–100 gal water/acre, or lower if the label allows.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 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 Web site at http://www.irac-online.org/.

IMPORTANT LINKS

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431

Insects and Mites

F. G. Zalom, Entomology, UC Davis
C. Pickel, UC IPM Program, UC Cooperative Extension, Sutter/Yuba counties
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
D. R. Haviland, UC IPM Program, UC Cooperative Extension, Kern County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
R. E. Rice, Kearney Agricultural Center, Parlier
L. C. Hendricks, UC Cooperative Extension, Merced County
R. L. Coviello, UC Cooperative Extension, Fresno County
M. W. Freeman, UC Cooperative Extension, Fresno County

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