Agriculture: Plum Pest Management Guidelines

Webspinning Spider Mites

Description of the Pest

Twospotted and Pacific spider mites are not easily distinguished: both have two black spots on their yellow-green bodies. In fall they turn orange red before overwintering. The twospotted mite is most common in the Sacramento Valley and the Pacific mite in the San Joaquin Valley. They overwinter as adult females under bark and on weeds. When weeds dry in spring, the mites move to trees and feed on lower leaves towards the middle of the tree first. They eventually become distributed over the entire tree. There are many overlapping generations each summer, with eggs being laid in a fine webbing on the undersurface of leaves.


Webspinning spider mites are the most damaging mite species to plums. Spider mites feed by sucking the contents out of leaf cells. Such leaf damage reduces tree vitality and can adversely affect fruit size. Defoliation as a result of spider mite damage often allows the tree and fruit to become sunburned. Leaf injury caused by spider mites begins as a mottling and browning of leaves. Defoliation usually follows if control measures are not initiated. Both mites produce abundant webbing on both sides of the leaves.


In many cases biological control keeps spider mites under control. Miticides may be necessary in some orchards in summer, but only when mite populations reach damaging levels, which often occurs after pesticides have been used that disrupt natural enemies. Use timed searches from June 1 to July 15 to assess need for treatment.

Biological Control

Predaceous mites, Typhlodromus caudiglans and Galendromus spp., and the sixspotted thrips, Scolothrips sexmaculatus, feed heavily on webspinning mites and may give complete control in the orchard. Control is most successful when the presence of predators and mites per leaf is close to a ratio of 1:1. The presence of these predators can be encouraged by avoiding the use of disruptive insecticides. These beneficials are also commercially available for release in the orchard. A narrow range oil (e.g., Superior) spray will suppress low levels of mites without harming these predators. When predatory mites are present, low rates of the selective miticide, fenbutatin oxide (Carzol), may be used to reduce spider mite populations and hopefully better balance predator/prey ratios.

Cultural Control

Vigorously growing trees are much more tolerant to mite attack than trees under stress. Maintain trees with optimum irrigation and fertilization. Reduce dusty conditions in orchards by oiling or watering roads and by maintaining a ground cover. Do not allow the ground cover to dry in mid-summer or mites will move up into the trees.

Organically Acceptable Methods

Biological and cultural control and oil sprays are organically acceptable management tools.

Monitoring and Treatment Decisions

Soon after bloom, start looking for spider mites and predatory mites on first emerging leaves on scaffolds. Use this information to map out areas of concern for future monitoring in the fruit development season.

Take weekly samples using the timed search method below, from June 1 to July 15 until the treatment threshold has been reached and a treatment has been applied. Record observations on a monitoring form (PDF). If a treatment is applied on or after July 15, monitoring is no longer necessary. Continue to monitor untreated orchards until harvest.

How to Monitor

  1. In each orchard up to 40 acres, conduct a 5-minute search in two separate areas of the orchard, for a total sampling time of 10 minutes.
  2. For each 5-minute search, examine at least 2 to 3 leaves on 10 trees. Note presence or absence of spider mites or predators. Sample leaves from both inside and outside the tree.
  3. If mite population is spotty, continue to do two 5-minute searches throughout the summer. If you determine the mite population is consistent throughout orchard, one 5-minute search is adequate.
  4. Keep records of sample results on the monitoring form.
  5. Use the guidelines below to determine need for treatment.

Mite Ratings (percent of leaves with one or more mites)

  • low (1-20%) = an occasional mite on occasional leaf; hard to find.
  • low/moderate (21-39%) = mites easier to find but no colonies or webbing and few eggs.
  • moderate (40-60%) = some leaves without mites, other leaves with small colonies; eggs easy to find but very little webbing.
  • moderate/high (61-79%) = mites on mostleaves, colonies with eggs, and webbing on some leaves.
  • high (80-100%) = lots of mites on most leaves; eggs and webbing abundant.

Predator Ratings

  • low = hard to find; less than one predatorper six leaves (only a few leaves will have predators).
  • moderate = easier to find; one predator per three leaves (about half the leaves will have predators).
  • high = one or more predators per leaf (most leaves will have predators).

Treatment Decisions (Treat if the rating from at least one 5-minute search indicates)

  • low/moderate mite rating with low/moderate predator rating, or
  • moderate/high mite rating with moderate/high predator rating
Common name Amount to use** REI‡ PHI‡
(Example trade name) (conc.) (dilute) (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.
  (Acramite) 50WS 0.75–1 lb 0.1875–0.25 lb 12 3
  COMMENTS: Relatively safe for beneficial predaceous mites. Apply with ground equipment. Requires complete coverage of both leaf surfaces for effective control.
  (Vendex) 50WP 2 lb 0.5 lb 48 14
  COMMENTS: This selective material appears to be most effective when applied earlier in the season rather than later. Do not apply more than twice a season in not more than 400 gal water/acre. Use reduced rates to balance predator and pest mite populations if sufficient numbers of predators are present but pest mite populations are approaching the threshold level.
C. NARROW RANGE OILS# 4–6 gal 1–1.5 gal 4 0
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Oil used alone will only provide partial control for about a 2 week period. Always apply oil to well-watered trees and never when trees are stressed by hot (above 90°F), windy, dry (relative humidity lower than 20%) conditions or when such conditions are likely to occur within a few days after application. Additional applications may be needed at 2 week intervals, which may increase the potential for phytotoxicity. If fruit is present on the tree, treatment with oil may dull the fruit finish. Do not apply oil within 2 weeks of captan. Not all oil products are organically acceptable; be sure to check individual products.
  (Onager) 12–24 oz 3–6 oz 12 28
  (Savey) 50 DF 3–6 oz 0.75–1.5 oz 12 28
  COMMENTS: Apply after bloom but before adult mite buildup. Controls eggs and immatures that are sprayed or move onto treated surfaces; does not kill adult mites but will kill eggs laid on treated surfaces. Do not make more than 1 application/year.
  (Nexter) 8.8–10.67 oz 2.2–2.67 oz 12 7
  COMMENTS: Apply to each row for maximum protection when populations are building and most of the mites are in the immature stages. Do not make more than 2 applications/season or apply by air or through any type of irrigation system.
* Permit required from county agricultural commissioner for purchase or use.
** For dilute applications, rate is per 100 gal water to be applied in 300-500 gal water/acre, according to label; for concentrate applications, use 80-100 gal water/acre, or lower if the label allows.
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.
1 Rotate insecticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers for insecticides and miticides (un=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).

Important Links

Text Updated: 04/09
Treatment Table Updated: 04/09