How to Manage Pests

UC Pest Management Guidelines

Walnut

Webspinning Spider Mites

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

(Reviewed 7/17, updated 7/17)

In this Guideline:


Description of the Pests

Pacific and twospotted mites overwinter as reddish orange, mature females in protected places on the tree, in soil, and in trash on the ground. During warm weather in spring, overwintered females begin feeding on walnut leaves and ground cover in the orchard. During periods of active feeding twospotted mites have a dark spot on each side of the body. Pacific mites have a second pair of dark spots near the posterior end. Often, however, the spots are barely visible or may coalesce to large dark areas, making it difficult to distinguish the two species.

Colonies develop on the underside of leaves and also on the upper sides when heavy numbers build up. Eggs are spherical and translucent when first laid, becoming opaque soon before hatching. Immature mites molt three times before becoming adults. The first stage mites have six legs; later stages and adults have eight legs. These mites reproduce rapidly in hot weather and may become numerous in June or July. They produce many generations per year. If temperature and food supply are favorable, a generation can be completed in 7 days.

Damage

Mite feeding causes stippling and browning of leaves. Clusters of brown leaves are often the first sign of an increasing mite numbers. High numbers produce copious webbing, and their feeding causes leaves to desiccate and drop. Defoliation early in the season can reduce nut yield and quality; defoliation late in the season will interfere with harvest.

Management

Natural enemies usually keep spider mites below damaging levels unless broad-spectrum pesticides are used. Use selective pesticides when treating other pests and monitor carefully for mites, predators, and mite damage.

Biological Control

The most dependable mite predator of spider mites is the western predatory mite, Galendromus (=Metaseiulus) occidentalis. Under optimal conditions, this predatory mite can produce a generation in 7 days. Because walnut orchards often lack an alternate food source such as European red mites early in the season, the western predatory mite sometimes may be late in building to numbers sufficient to control webspinning mites, which become abundant later in the season.

Sixspotted thrips can be very effective in reducing high numbers of webspinning mites. Thrips, however, usually do not move into orchards until mite populations are high. Both the adults and the small, yellowish larvae prey on mites.

The spider mite destroyer, Stethorus picipes, is a small lady beetle that feeds on mites. The larvae are small, dull black and have a velvety appearance. The spider mite destroyer, like the sixspotted thrips, generally does not become numerous until spider mite numbers are high. But it is an active feeder and can reduce mite numbers quickly when it is abundant. Minute pirate bugs also feed on spider mites.

Cultural Control

Orchard management practices can reduce mite problems. Minimize dust by oiling orchard roads and maintaining a ground cover. Well-irrigated, vigorous trees are less troubled by mite infestations. Choose selective pesticides when controlling other pests and try to avoid pyrethroids, organophosphates, and carbamates until later in the season.

Organically Acceptable Methods

Biological and cultural controls and sprays of narrow range oil are acceptable for use in an organically certified crop. Caution should be taken when applying oils in walnuts to avoid injuring trees (see narrow range oil comments in the table below).

Monitoring and Treatment Decisions

Begin looking for spider mites in late spring. Map out areas of concern for summer monitoring.

In June or early July start sampling for spider mites once a week through August:

  • Select randomly 10 trees in the orchard and pick 5 leaflets from low branches and 5 leaflets from high branches in each tree.
  • Look for the presence of webspinning spider mites, predator mites, and sixspotted thrips and record your observations (PDF) separately for low and high branch leaves. If predaceous mites are present on at least half of the leaflets that have mites, then there is a good probability that natural enemies will suppress the population unless broad-spectrum sprays are used for codling moth management. Monitor again in one week.
  • If predaceous mites are not present on at least half the leaflets that have mites, monitor again in 3 to 4 days (3 days if weather is hot) to determine if the webspinning mite population is increasing or declining. If mites do not build up in walnuts by mid-August, then a pesticide application may not be warranted.
Treatment Thresholds

Take into consideration populations of predators and whether or not you plan to apply an organophosphate or pyrethroid treatment against other pests later in the season, before you make a treatment decision.

For orchards where NO organophosphate or pyrethroid applications are used:

  • Predators present on fewer than 10% of mite-infested leaves: spray if 30 to 40% of leaves are mite infested.
  • Predators present on 20 to 50% infested leaves: spray if 40 to 50% of leaflets are mite infested.
  • Predators present on more than 50% mite-infested leaves: don't spray.

For orchards where organophosphate or pyrethroid applications are used: (If these materials are used, a tank-mix with a miticide may be warranted in warmer, dry areas on light soils.)

  • Predators present on fewer than 10% of mite-infested leaves: spray when 10% of leaves have spider mites.
  • Predators present on more than 10%of mite-infested leaves: spray when 20% of leaves have spider mites.
Common name Amount to use** REI‡ PHI‡
(Example trade name) (conc.) (dilute) (hours) (days)
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
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. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. BIFENAZATE
  (Acramite 50WS) 0.75–1 lb 12 14
  (Vigilant 4SC) 16–24 fl oz 12 14
  MODE-OF-ACTION GROUP NUMBER1: 20D
  COMMENTS: Apply in a minimum of 50 gal/acre. Do not apply more than once per season.
 
B CYFLUMETOFEN
  (Nealta) 13.7 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 25
 
C. SPIRODICLOFEN
  (Envidor 2SC) 16–18 fl oz 4–4.5 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 23
  COMMENTS: Kills all mite stages, but most effective on juveniles.
 
D. ETOXAZOLE
  (Zeal Miticide) 2–3 oz 0.5–0.75 oz 12 28
  MODE-OF-ACTION GROUP NUMBER1: 10B
  COMMENTS: Acts as a contact toxin to eggs, inhibits molting of juveniles, and causes adult females to produce sterile eggs. Do not apply more than once per season. Use for bearing trees allowed under a Supplemental Label.
 
E FENPYROXIMATE
  (FujiMite 5EC) 2–4 pt 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 natural enemies.
 
F. FENBUTATIN-OXIDE*
  (Vendex 50WP) 2 lb 0.5 lb 48 14
  MODE-OF-ACTION GROUP NUMBER1: 12B
  COMMENTS: Do not apply more than twice per season.
 
G. ABAMECTIN*
  (Agri-Mek SC) 2.25–4.25 fl oz 0.5–1 fl oz 12 21
  MODE-OF-ACTION GROUP NUMBER1: 6
  COMMENTS: Use in combination with a horticultural spray oil at a minimum of 1 gal oil/acre. Is effective against mites that are resistant to propargite. Apply after sampling indicates pest mites are increasing but before significant damage or webbing is present. A locally systemic material that is most effective is applied before July when foliage is still young and tender enough to absorb it. To delay development of resistance, use only once per season. 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, 2017. Review the Department of Pesticide Regulation's updated fact sheet (PDF).
 
H. HEXYTHIAZOX
  (Onager) 16–24 oz 4–6 oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 10B
  COMMENTS: A growth regulator that is a contact toxin to eggs and juveniles; adult females lay sterile eggs. For resistance management, do not apply more than once per year.
 
J. PROPARGITE*
  (Omite 30WS) 4–6 lb 1.5 lb 720 (30 days) 21
  MODE-OF-ACTION GROUP NUMBER1: 12C
  COMMENTS: Do not use within 14 days before or after an oil treatment or damage may occur. Do not apply more than twice per season. Do not graze animals on vegetation under treated trees. The twospotted spider mite is resistant to this material in the San Joaquin Valley. These rates are lower than the manufacturer's label rate.
 
J. NARROW RANGE OIL# See label See label
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Provides short-term control. Use label rate in 200 to 400 gal water/acre. Apply oil to walnuts only when soil moisture is adequate and trees have not been water stressed at anytime during the growing season. Should also provide control of frosted scale but is destructive of the walnut aphid parasite, Trioxys pallidus. Do not apply when temperatures exceed 90°F; make applications in evening or at night. Check with certifier to determine which products are organically acceptable.
 
K. ACEQUINOCYL
  (Kanemite 15SC) 21–31 fl oz 5.25–7.75 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 20B
  COMMENTS:  Do not apply more than twice per year and more than 62 fl oz (0.6 lbs a.i.)/acre per season.
 
L. CLOFENTEZINE
  (Apollo SC) 2–4 oz 0.5–1 oz 12 30
  MODE-OF-ACTION GROUP NUMBER1: 10A
  COMMENTS: Is effective against mites that are resistant to propargite. Apply after sampling indicates pest mites are increasing but before significant damage or webbing is present. Kills eggs and young larval stages. Good coverage is a must; use a minimum of 50 gal water/acre for concentrate sprays and a maximum of 400 gal water/acre for dilute. To delay development of resistance, use only once per season.
 
M. ROSEMARY OIL/PEPPERMINT OIL#
  (Ecotrol EC) 2–4 pt 1 pt 0 0
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Kills all stages, but good coverage is essential.
 
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 (un = unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).
** For concentrate application, use the amount given in 80 to 100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300 to 500 gal water/acre, according to label.
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 crops.

IMPORTANT LINKS

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Insects and Mites

J. A. Grant, UC Cooperative Extension, San Joaquin County
J. K. Hasey, UC Cooperative Extension, Sutter, Yuba, and Colusa counties
W. W. Coates, UC Cooperative Extension, San Benito County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
E. J. Symmes, UC IPM Program, Butte County
S. J. Seybold, Entomology, UC Davis (walnut twig beetle)
R. M. Bostock, Plant Pathology, UC Davis (walnut twig beetle)

Acknowledgment for contributions to Insects and Mites:
W. J. Bentley, UC IPM Program, Kearney Agricultural Research and Extension Center, Parlier (Emeritus)
L. C. Hendricks, UC Cooperative Extension, Merced County (Emeritus)
W. H. Olson, UC Cooperative Extension, Butte County (Emeritus)
C. Pickel, UC IPM Program, Sutter and Yuba counties (Emeritus)
G. S. Sibbett, UC Cooperative Extension, Tulare County (Emeritus)
D. Light, USDA, Albany, CA (Emeritus)(Codling Moth)

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