How to Manage Pests

UC Pest Management Guidelines

Citrus

Citricola Scale

Scientific Name: Coccus pseudomagnoliarum

(Reviewed 9/08, updated 6/13)

In this Guideline:


Description of the Pest

Citricola scale is a soft scale. Crawlers of the citricola scale appear from June through August. They settle primarily on the underside of leaves, but in severe infestations they also settle on the upper leaf surface and on twigs, rarely on fruit. Young scales are flat and almost translucent; they grow slowly over the course of the summer and fall, molting only once during that period. by November, immature scales turn a mottled dark brown color and begin migrating to twigs; this migration peaks in February and March. Once on twigs, they develop faster than they did on leaves and they turn a gray color. by late April, citricola scales molt and mature into the adult female stage. Females lay 1,000 to 1,500 eggs during the time from early May to early August. Eggs hatch after 2 to 3 days and crawlers move to leaves. There is only one generation a year, and there are no males.

Brown soft scale, another soft scale that is similar to citricola scale, may be found in the same areas as citricola scale but it has multiple generations and its colonies are composed of mixed instars and adults.

Damage

Citricola scale can be a serious pest of citrus in the San Joaquin Valley. A severe infestation may reduce tree vigor, kill twigs, and reduce flowering and fruit set. As they feed, citricola scale excrete honeydew, which accumulates on leaves and fruit. Sooty mold grows on honeydew and interferes with photosynthesis in leaves and causes fruit to be downgraded in quality during packing.

Management

Citricola scale is completely controlled by Metaphycus and Coccophagus parasites in southern California and is almost never seen. Even though these parasites are established in the San Joaquin Valley, biological control is not effective there, and treatments may be necessary in groves where broad-spectrum pesticides are not regularly used to control other pests. In groves practicing biologically based pest management (e.g., releasing Aphytis melinus for California red scale control), growers may consider withholding broad-spectrum citricola sprays until Aphytis activity is over in the fall (e.g., late October or November).

Biological Control

Introduced and indigenous parasitic wasps, Metaphycus luteolus, M. stanleyi, M. nietneri, M. helvolus, and Coccophagus spp., control citricola scale in southern California. Several of these parasites occur in the San Joaquin Valley but are unable to control citricola scale except in groves near urban areas or in those with high populations of brown soft scale, which serves as an alternate host for the parasites when citricola scales are not in the stage that the parasite attacks.

Organically Acceptable Methods

Biological control and organically approved petroleum oil sprays, such as 440 oil PureSpray Green, are acceptable for use in organically managed orchards.

Selectivity

Oil is the most selective pesticide available for control of citricola scale. However, oil simply reduces the scales' overall numbers and in many cases must be applied 1-2 times every year. The organophosphates chlorpyrifos (Lorsban) and methidathion (Supracide) and the carbamate carbaryl (Sevin) are broad-spectrum and toxic to most natural enemies. Chlorpyrifos can be selective if used at a low rate (2 pt/acre) and can more effectively reduce citricola scale than oil. A full rate of Lorsban (6–12 pt/acre) is not selective, but it can suppress densities so effectively that another spray is not needed for 2 to 3 years.

The neonicotinoids imidacloprid (Admire, Nuprid) and acetamiprid (Assail) are fairly broad-spectrum (i.e., toxic to most natural enemies) and will suppress citricola scale for one year. The insect growth regulator buprofezin (Applaud) is fairly selective, but toxic to vedalia beetles, and will suppress citricola scale for one year. The neonicotinoids should not be used in cottony cushion scale-infested orchards because they have no effect on cottony cushion scale, but they are highly toxic to the predatory vedalia beetle and can disrupt biological control of cottony cushion scale populations.

Resistance

In the San Joaquin Valley, a number of populations of citricola scale have been found to be resistant to the organophosphate chlorpyrifos (Lorsban). Thus, low rates of this insecticide would be ineffective and high rates only suppress citricola scale for a single year. Growers experiencing chlorpyrifos-resistant scale should use the other chemicals.

Monitoring and Treatment Decisions

Citricola scale is very sensitive to organophosphates and carbamates (if not resistant) and generally does not become a problem until growers stop using these insecticides for control of other pests. Check for citricola scale at all times of the year when monitoring for other scales, but look especially closely at the twigs in April and at the underside of leaves in mid- to late August.

Be sure to distinguish the citricola scale from brown soft scale. Brown soft scale has multiple generations and all stages will be present on leaves and twigs year round, whereas citricola scale has only one generation and is found on leaves only in the summer and fall, and the nymphs will be uniform in size.

April-May Sampling

Examine one 24-inch twig on the northeast side of 10 trees in each of four rows (for a total of 40 twigs) distributed throughout the orchard. On each twig count the number of scales and determine the average number of scales by dividing the total number in the sample by 40. If there is more than an average of one scale per twig and heavy production of sooty mold is occurring, the orchard may require an immediate treatment. If the population on twigs or leaves is observable but sooty mold is not a problem, then it is best to postpone treatments until fall when scales are small, positioned on leaves on the outside of the tree, and generally easier to control.

August-September Sampling

To sample for citricola scale in late summer, walk down four evenly spaced rows of the block. In each row, pick one leaf from the northeast corner of 25 trees. Examine the scale on the underside of the leaf to determine if they are alive or dead. Count the number of leaves in the 25-leaf sample that are infested with live scale (presence-absence sampling). Record results (example form(PDF)). A treatment is warranted if one or more of the four rows has 13 or more leaves infested with live citricola scale in a 25-leaf sample. If 5 to 12 leaves are infested, the population is building and should be watched very closely. If four or fewer leaves are infested, then a treatment is not needed this season.

Alternatively, count the number of nymphs on those 100 leaves (4 rows x 25 leaves) and if there are more than 0.5 nymphs per leaf then treatment is needed.

Treatment Timing and Relative Efficacy

If resistance is not a problem, chlorpyrifos (Lorsban) is the most effective insecticide for citricola scale control, followed by acetamiprid (Assail), buprofezin (Applaud), imidacloprid (Admire), and lastly oil.

Spring Treatments

Assail is the only insecticide effective against adult scales in spring before petal fall. However, it is fairly broad-spectrum and at this time of year only suppresses the females. Thus, it should not be used unless there is a severe problem that cannot wait until a summer treatment.

Summer-Fall Treatments

Most foliar treatments are applied during August-September because at that time of year the female scales have died, the nymphs are small and located on the outside leaves of the tree, and temperatures are warm, which makes the insecticides more effective. Because citricola scale are very sensitive to organophosphates and carbamates, rates lower than those recommended for armored scale can be effective.

Lower rates (2-3 pt chlorpyrifos [Lorsban] per acre) and delays in treatment until late October are suggested when parasites of California red scale need to be preserved. However, lower rates may result in a shorter period of control (1 year). Higher rates (6-12 pt chlorpyrifos [Lorsban] per acre) or application earlier in the season (August through September) are recommended if the population of citricola scale is very high or if California red scale must be controlled simultaneously.

Recently, some populations of citricola scale have been determined to have resistance to the organophosphate chlorpyrifos (cross-resistance to methidathion and carbaryl is likely a problem as well). These populations are not controlled by low rates of chlorpyrifos and high rates only suppress the population for one year. In these situations, avoid using organophosphate and carbamate insecticides. The insect growth regulator buprofezin (Applaud) is fairly selective (toxic only to vedalia beetles) and will suppress citricola scale during the season that it is applied. The neonicotinoids imidacloprid (Admire, Nuprid) and acetamiprid (Assail) are fairly broad-spectrum and will suppress citricola scale for one year. Admire and Nuprid are applied systemically in spring and the best use of Assail is in fall (September-October) after natural enemies have completed their activities. Citricola scale populations grow more rapidly and survive the summer better when the San Joaquin Valley experiences a cool, wet spring. When these conditions occur, the higher rates and earlier applications (August) are more effective.

Common name Amount to use R.E.I.‡ P.H.I.‡
(example trade name) (type of coverage)** (hours) (days)

  Calculate impact of pesticide on air quality
 
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.
 
LIGHT-TO-MODERATE INFESTATION ONLY
 
A. NARROW RANGE OIL (UR 92%)
  (415, 440) 1.2–1.4% (IC or TC) 4 When dry
  . . . or . . .
  NARROW RANGE OIL (UR 99%)
  (415, 435, 440, 455) 1.2–1.4% (IC or TC) 4 When dry
  RANGE OF ACTIVITY: Pests: broad (unprotected stages of insects and mites); Natural enemies: most
  PERSISTENCE: Pests: short; Natural enemies: short
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: For use on grapefruit, lemons, navels, Valencias. Use higher rate for July or August applications only. To avoid phytotoxicity, use same treatment timings as given for oil sprays for California red scale in central California. Narrow range 440 (or higher) spray oil is preferable in the Central Valley during warmer months because of greater persistence, but risk of phytotoxicity increases unless using products with 99% unsulfonated residues (UR). Do not apply oil until hatch is complete (late July to early August). Caution: Serious hazards are associated with oil treatments to green lemons because of phytotoxicity after sweating; check label for preharvest interval.
 
B. BUPROFEZIN

(Applaud) 35–46 oz/acre (IC or TC) 12 3
  RANGE OF ACTIVITY: Pests: narrow (scales, whiteflies); Natural enemies: predatory beetles
  PERSISTENCE: Pests: intermediate; Natural enemies: intermediate
  MODE OF ACTION GROUP NUMBER1: 16
  COMMENTS: For use on all varieties. Toxic to vedalia beetles. Most effective if applied after peak emergence of the first generation of crawlers. Apply after the crawlers have settled down and formed white caps. Slow-acting; This product does not kill the scale until they molt, so decline of the population is usually not observed until the next generation.
 
MODERATE-TO-HEAVY INFESTATIONS
 
A. CHLORPYRIFOS*

(Lorsban Advanced) Prebloom: 1 pt/100 gal (IC or TC) 5 days See comments
  Prebloom: 5–7 pt/acre (LV)  
  Late summer: 2–6 pt/acre (LV)  
  RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
  PERSISTENCE: Pests: short (low rates), intermediate (high rates); Natural enemies: short (low rates), intermediate (high rates)
  RESISTANCE: Some San Joaquin populations
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: For use on all varieties. In the prebloom period, use 1 pt/100 gal in 500 to 1,000 gal/acre; when using the low-volume treatment at this time, thorough coverage is essential. During late summer use 1.5 pt in 100 gal/acre (LV). Preharvest interval is 21 days for up to 7 pt/acre and 35 days above 7 pt/acre. Do not apply during Dec., Jan., or Feb. See label for additional restrictions.
 
B. IMIDACLOPRID
  (Admire Pro) 7–14 fl oz/acre 12 0
  (Nuprid 1.6F) 10–20 fl oz/acre 12 0
  RANGE OF ACTIVITY: Pests: narrow (aphids, glassy-winged sharpshooters); Natural enemies: predatory beetles and parasites
  PERSISTENCE: Pests: long; Natural enemies: intermediate
  RESISTANCE: None
  MODE OF ACTION GROUP NUMBER1: 4A
  COMMENTS: Apply to soil; remains effective 4 to 5 months. Moderately effective against nymphs and adults. Pre-wet soil before treatment is applied. Very toxic to bees; do not apply during bloom because bees may be drawn to irrigation water. For optimum uptake, apply to newly planted trees or trees irrigated by drip, microsprinkler, and low-pressure irrigation systems. Emitters must provide even, uniform distribution of water. Lightly pre-wet soil for several hours before application to break soil surface tension. Once the irrigation system reaches operating pressure, inject the treatment into the system over a calculated time interval (generally 2 hours) to allow uniform distribution throughout the system. The use of a dye marker in the treatment solution is recommended to determine when lines are clear of the treatment. Once the solution has cleared all irrigation lines and emitters, continue irrigation to move the insecticide into the active root zone but do not overirrigate or cause runoff. Wait 24 hours before subsequent irrigations. Apply in citrus orchards May–July. Toxic to vedalia beetle and should not be used in cottony cushion scale-infested orchards.
 
C. ACETAMIPRID
  (Assail 70WP) 3.4–5.7 oz/acre (IC or TC) 12 7
  RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
  PERSISTENCE: Pests: intermediate; Natural enemies: long
  RESISTANCE: None
  MODE OF ACTION GROUP NUMBER1: 4A
  COMMENTS: Residues last for 4 to 6 weeks. Apply in 300 to 1000 gal water/acre; use higher volume if insects are inside the canopy on the wood. Toxic to bees exposed to direct treatment; apply only during late evening, night, or early morning. Toxic to vedalia beetle and should not be used in cottony cushion scale-infested orchards.
 
D. METHIDATHION*
  (Supracide 25W) 1 lb/100 gal (IC or TC) 30 days 14
  RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
  PERSISTENCE: Pests: long; Natural enemies: long
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: For use on all varieties. Apply after bloom or at least 14 days before harvest.
   
E. MALATHION 8 Spray
  (Malathion 8 Flowable) 7.5 pt/acre (IC or TC) 72 7
  RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
  PERSISTENCE: Pests: intermediate; Natural enemies: intermediate
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: For use on all varieties. Do not apply during bloom.
 
F. CARBARYL*
  (Sevin XLR Plus) 2 qt/acre (IC or TC) 12 5
  RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
  PERSISTENCE: Pests: long; Natural enemies: long
  MODE OF ACTION GROUP NUMBER1: 1A
  . . . PLUS . . .
  NARROW RANGE OIL
  (415) 0.5–1.4% 4 When dry
  RANGE OF ACTIVITY: Pests: broad (unprotected stages of insects and mites); Natural enemies: most
  PERSISTENCE: Pests: short; Natural enemies: short
  MODE OF ACTION: Contact including smothering and barrier effects; also improves translaminar movement and insecticide persistence.

** LV - Low-volume uses 20–100 gal water/acre.
  IC - Intermediate coverage uses 250 to 600 gal water per acre.
  TC - Thorough coverage uses 750-2,000 gal water or more/acre, depending on tree size.
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.
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 additional information, see their Web site at http://www.irac-online.org/.

IMPORTANT LINKS

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441

Insects, Mites, and Snails

  • E. E. Grafton-Cardwell, Entomology, UC Riverside and Kearney Agricultural Center, Parlier
  • J. G. Morse, Entomology, UC Riverside
  • N. V. O'Connell, UC Cooperative Extension, Tulare County
  • P. A. Phillips (emeritus), UC IPM Program, UC Cooperative Extension, Ventura County
  • C. E. Kallsen, UC Cooperative Extension, Kern County
  • D. R. Haviland, UC Cooperative Extension, Kern County
Acknowledgments for contributions to Insect, Mites, and Snails:
  • J. Barcinas, E.S.I., Corona, CA
  • R. Dunn, Badger Farming Co., Exeter, CA
  • J. Gorden, Pest Management Associates, Exeter, CA
  • H. Griffiths, E.S.I., Corona, CA
  • D. Machlitt, Consulting Entomology Services, Moorpark, CA
  • C. Musgrove, retired entomologist, Riverside, CA
  • K. Olsen, S & J Ranch, Pinedale, CA
  • T. Roberts, E.S.I., Corona, CA
  • T. Shea, UC Cooperative Extension, Riverside County
  • J. Stewart, Pest Management Associates, Exeter, CA
  • P. Washburn, Washburn & Sons Citrus Pest Control, Riverside, CA

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