How to Manage Pests
UC Pest Management Guidelines
Webspinning Spider Mites
Scientific names: Twospotted spider mite: Tetranychus urticae
Pacific spider mite: Tetranychus pacificus
In this Guideline:
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.
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
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
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
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
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.
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.
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.
Keep records of sample results on the monitoring form.
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
(80-100%) = lots of mites on most leaves;
eggs and webbing abundant.
- 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
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
||Amount to Use**
|The following materials
are listed in order of usefulness in an IPM program, taking into account
on natural enemies and honey bees,
of the timing on beneficials. When
choosing a pesticide, also consider information relating to environmental
impact. Not all registered pesticides are listed. Always read label of product being used.
||MODE OF ACTION GROUP NUMBER1: 25
||COMMENTS: Relatively safe for beneficial predaceous mites.
Apply with ground equipment. Requires complete coverage of both leaf surfaces for effective control.
||MODE OF ACTION GROUP NUMBER1: 12B
||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.
||NARROW RANGE OILS#
||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.
||(Savey) 50 DF
||MODE OF ACTION GROUP NUMBER1: 10B
||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.
OF ACTION GROUP NUMBER1: 21
||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.
UC IPM Pest Management Guidelines: Plum
UC ANR Publication 3462
Insects and Mites
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
K. R. Day, UC Cooperative Extension, Tulare County
Acknowledgment for contributions to Insects and Mites:
R. E. Rice, Kearney Agricultural Center, Parlier
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