How to Manage Pests

UC Pest Management Guidelines

Plum

San Jose Scale

Scientific name: Diaspidiotus (Quadraspidiotus) perniciosus

(Reviewed 5/06, updated 4/09)

In this Guideline:


DESCRIPTION OF THE PEST

San Jose scale, a major pest of fruit trees, is inconspicuous and usually not noticed until it builds up to large numbers on limbs. Limbs supporting large populations often ooze gum and exhibit rough bark and dieback. Dead leaves adhering to fruit spurs during dormant season indicate the presence of scale. Partially mature nymphs overwinter on limbs and trunks. In spring the nymphs develop into winged male and sessile female scale insects. Female scales have gray circular scale coverings. If the covering is removed, the lemon yellow body beneath can be seen. In May females lay eggs that hatch immediately and the young emerge from under the edge of the adult scale cover. The young crawlers settle on shoots where they feed and become adults or overwinter as partially grown scales. In California there are five overlapping generations each year. Crawlers first appear in late April and May, followed by continuous overlapping emergence from late June through December.

DAMAGE

San Jose scale can infest branches, shoots, leaves, and fruit. Adults and nymphs suck plant juices and cause considerable damage. They have been known to seriously weaken branches and main scaffold limbs, thus causing permanent injury to mature trees. Crawlers settling on fruit may cause fruit spotting.

MANAGEMENT

San Jose scale has many natural enemies that can frequently keep the pest under control if not disrupted by application of broad-spectrum insecticides. Many orchards that have not used broad-spectrum sprays for 2 or 3 years do not have San Jose scale problems. The best time to spray is during the dormant season when low-to-moderate populations can be managed with oil sprays, which don't destroy the scale parasites. The scale is monitored as part of the shoot sample during the dormant season and with pheromone traps in spring.

Biological Control

Natural enemies that feed on San Jose scale include two predaceous beetles: the twicestabbed lady beetle, Chilocorus orbus, and another small beetle, Cybocephalus californicus.

Parasites include a number of small chalcid and aphelinid wasps, including Aphytis and Encarsia (Prospaltella) sp. These predators and parasites are helpful in reducing scale populations, but broad-spectrum insecticides used during the growing season for other pests disrupt this natural control, and scale populations can build as a result. Low winter mortality due to mild temperatures will also permit a buildup of scale populations.

Organically Acceptable Methods

Oil sprays and biological control by native scale parasites are acceptable in organically managed orchards.

Monitoring and Treatment Decisions
Dormant season

Monitor San Jose scale during the dormant season by collecting 100 spurs and examining them for live scale as well as for tiny emergence holes, which indicate parasite activity. For details on sampling, see DORMANT SPUR SAMPLING and the monitoring form (PDF).

There is a correlation between infested spurs and infested plums, however, plums harvested in mid-June don't generally need a treatment. Expect more damage in late-harvested plums and treat them with oil at greenbud or popcorn if 3 spurs out of 100 are infested. If over 10% are infested, add an insect growth regulator (pyriproxyfen) to the oil sprays. Follow the guidelines below:

Dormant Treatment Decision Table (% infested spurs)

Treatment threshold Treatment
Harvested before
June 15
Harvested after
June 15
 
Below 20% Below 5% No treatment
20-60% 5-10% Oil at 6 gal/acre
Over 60% Over 10% Oil at 6 gal/acre plus insect growth regulator

Oil alone can be effective in controlling low-to-moderate populations (apply before January 21). If populations are high, include an insect growth regulator (pyriproxyfen-Esteem, Seize) with the oil. Organophosphates are available, but are associated with environmental problems and should be avoided. When the dormant organophosphate and oil spray is first omitted, San Jose scale populations may increase the first year, but by the second and third year, parasite populations have increased to levels where they reduce San Jose scale populations and maintain them at low levels. If you notice parasitized scale in your dormant sample, be sure to only use an insect growth regulator during the growing season.

Growing season

Monitoring with pheromone traps during the growing season will help you keep track of the appearance and development of scale populations as well as the level of parasitism (Aphytis and Encarsia) but does not tell you if treatment is needed. It mainly tells you how to time treatment for best control in spring using degree-days and pheromone traps to predict the crawler stage or sticky traps to trap the crawlers. Need for treatment is better assessed during the dormant season. Delayed dormant sprays are the preferred timing for treatment.

If inadequate control is achieved with the dormant spray, or the dormant spray is not applied, treatment is also effective when applied soon after the emergence of crawlers in May. Monitor scales by putting up pheromone traps around February 25 (see PHEROMONE TRAPS) and placing sticky tape in the trees in April. Record results on a monitoring form Place pheromone traps well within the canopy to keep them out of the wind. San Jose scale traps will attract both male San Jose scales and scale parasites (Aphytis melinus and Enarsia perniciosi). Adult male scales can be distinguished from the parasites by the presence of a dark line across their thorax where the wings attach.

When the traps begin to catch males consistently, start accumulating degree-days using a 51°F lower threshold and a 90°F upper threshold. If it is needed, apply a treatment for crawlers 600 to 700 DD after you catch the first males. Be aware that the traps may fail to catch any adults if weather is cold, rainy, or windy. Total generation time for San Jose scale is 1050 DD.

If May sprays are required, use a high-volume (dilute) application at 400 gallons or more per acre for best coverage; do not use a low volume application.

Take a fruit damage sample at harvest to assess the effectiveness of the current year's IPM program and to determine needs of next year's program. See FRUIT EVALUATION AT HARVEST. Record results on a monitoring form (PDF).

Common name Amount per 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, impact on natural enemies and honey bees, and impact 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.
 
DELAYED DORMANCY
 
A. DORMANT OIL such as:
  DORMANT FLOWABLE EMULSION 6 gal 1–1.5 gal 4 0
  NARROW RANGE OIL# 4 gal 1.5 gal 4 0
  MODE OF ACTION: Contact, including smothering and barrier effects.
  COMMENTS: Cover all parts of the tree. Will control low-to-moderate infestations. See Dormant Treatment Decision Table for rate to use based on % infested spurs. Oil applications at this time may cause some young shoots to burn or dieback, especially in years when trees are water-stressed, or have recently been subjected to freezing temperatures or to dry winds. Dormant flowable emulsion is less likely to cause burn. Some varieties, especially those that are weak growers or low in vigor because of soil or other location-related issues, can be especially sensitive to oil. Not all oil products are organically acceptable; be sure to check individual products.
 
B. DORMANT OIL such as:
  DORMANT FLOWABLE EMULSION 6 gal 1–1.5 gal 4 0
  NARROW RANGE OIL 4 gal 1.5 gal 4 0
  MODE OF ACTION: Contact, including smothering and barrier effects.
  COMMENTS: Cover all parts of the tree. Use with an insecticide for high infestations. Oil applications at this time may cause some young shoots to burn or dieback, especially in years when trees are water-stressed, or have recently been subjected to freezing temperatures or to dry winds. Do not use oil sprays on water-stressed trees. Dormant flowable emulsion is less likely to cause burn. Some varieties, especially those that are weak growers or low in vigor because of soil or other location-related issues, can be especially sensitive to oil.
  . . . PLUS. . .
  PYRIPROXYFEN
  (Esteem) 0.86 EC 13–16 fl oz/acre 12 14
  (Seize) 35WP 4–5 oz/acre 12 14
  MODE OF ACTION GROUP NUMBER1: 7C
  COMMENTS: An insect growth regulator. Do not apply more than once per growing season. Good coverage is essential for good control. Preferred treatment of high populations of scale in an environmentally sound program. Use allowed under a supplemental label.
  . . . or . . .
  DIAZINON* 50WP 4 lb 1 lb 24 21
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Resistance may be found in areas of the southern and south central San Joaquin Valley. Avoid drift and runoff into surface waters or choose alternative materials. Diazinon has been found in surface waters at levels that violate federal and state water quality standards.
  . . . or . . .
  CHLORPYRIFOS*
  (Lorsban) 4E 2 qt 1 pt 4 14
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Apply concentrate spray with a minimum of 100 gal/acre. Do not apply in the following counties: Butte, Colusa, Glenn, Solano, Sutter, Tehama, Yolo, and Yuba. Do not use more than 4 gal narrow range oil/acre with this material, apply more than once during dormant season, or allow meat or dairy animals to graze in treated orchards. Do not apply until winter rains or irrigation has replenished soil moisture. Do not allow livestock to graze in treated orchards. Avoid drift and runoff into surface waters or choose alternative materials. Chlorpyrifos has been found in surface waters at levels that violate federal and state water quality standards.
 
SPRING
 
A. PYRIPROXYFEN
  (Esteem) 0.86 EC 13–16 fl oz/acre 12 14
  (Seize) 35WP 4–5 oz/acre 12 14
  MODE OF ACTION GROUP NUMBER1: 7C
  COMMENTS: Apply when scale crawlers first emerge. Do not apply more than once per growing season. Good coverage is essential for good control. Use allowed under a supplemental label.
 
B. NARROW RANGE OIL# 6–8 gal 1.5–2 gal 4 0
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Use this option if bloom sprays are used for control of caterpillars. Plum trees tolerate oil treatments better in spring than during dormancy. Do not apply oil within 2 weeks of captan or within 30 days of a sulfur treatment. Not all oil products are organically acceptable; be sure to check individual products.
 
C. DIAZINON* 50 WP 3 lb 1 lb 24 21
                        4 EC 3 pt 1 pt 24 21
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Resistance may be found in areas of the southern and south central San Joaquin Valley. Avoid drift and runoff into surface waters or choose alternative materials. Diazinon has been found in surface waters at levels that violate federal and state water quality standards. Where plums are grown near waterways, do not use diazinon.
 
** 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 two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
+ 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 two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce.
Not recommended or not on label.
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: 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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