Agriculture: Citrus Pest Management Guidelines

Citricola Scale

  • Coccus pseudomagnoliarum
  • 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, second-instar 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 scales 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 insecticide applications 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 numbers 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

    Use biological control and organically approved petroleum oil sprays, such as 440 oil PureSpray Green in organically managed orchards. Oils alone only suppress citricola scale so multiple applications may be required.

    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 or 2 times every year. The organophosphate malathion and the carbamate carbaryl (Sevin) are broad-spectrum and toxic to most natural enemies. The neonicotinoids thiamethoxam (Actara) and acetamiprid (Assail) are fairly broad-spectrum (i.e., toxic to most natural enemies). The insect growth regulator buprofezin (Centaur) is a fairly selective pesticide, but it does affect vedalia beetles.

    Resistance

    IIn the San Joaquin Valley, 40% of populations of citricola scale have been found to be resistant to organophosphate insecticides. There is likely cross-resistance to carbamates. Thus, low rates of these insecticides would be ineffective, and high rates only suppress citricola scale for a single year. Growers experiencing resistant scale should use other insecticides (buprofezin, thiamethoxam, or acetamiprid).

    Monitoring and Treatment Decisions

    Citricola scale is only a problem in the San Joaquin Valley. It 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 late July.

    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 pesticide application. If the population on twigs or leaves is observable but sooty mold is not a problem, then it is best to postpone pesticide application until fall when scales are small, positioned on leaves on the outside of the tree, and generally easier to control.

    July-September Sampling

    To sample for citricola scale in summer:

    1. Walk down four evenly spaced rows of the block.
    2. In each row, pick one leaf from the northeast side of 25 trees.
    3. Examine the scale on the underside of the leaf to determine if they are alive or dead.
    4. Count the number of leaves in the 25-leaf sample that are infested with live scale (presence-absence sampling). Record results (example formPDF)

    A spray is warranted if one or more of the four rows has five or more leaves infested with live citricola scale in a 25-leaf sample. If four or fewer leaves are infested, then a pesticide application could wait until the following 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

    Acetamiprid (Assail), thiamethoxam (Actara) and flupyradifurone (Sivanto) are the most effective insecticides for citricola scale control.

    Spring Treatments

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

    Summer through Fall Treatments

    Most foliar insecticides are applied during late July through 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. Monitor eggs under the adult females during June and July and apply the insecticide after hatch is completed.

    About 40% of populations of citricola scale in the San Joaquin Valley have been determined to have resistance to organophosphate insecticides. Avoid malathion, dimethoate, or cabaryl insecticides if resistance is known to be a problem. The insect growth regulator buprofezin (Centaur) is fairly selective (toxic only to vedalia beetles) and will suppress citricola scale during the season that it is applied. The foliar neonicotinoids thiamethoxam (Acatara) and acetamiprid (Assail) and flupyradifurone (Sivanto) are fairly broad-spectrum and will control citricola scale for 1 to 2 years. Foliar neonicotinoids are more effective against citricola scale than systemic neonicotinoids. 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, careful spray coverage of the tree, and properly timed applications (late July to early August, when eggs have completely hatched and the nymphs are small) are most effective.

    Common name Amount to use REI‡ PHI‡
    (Example trade name) (type of coverage)** (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.
    SELECTIVE
    A. BUPROFEZIN
    (Centaur WDG) 34.5–46 oz/acre (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: Slow-acting; this product does not kill the scale until they molt, so decline in scale numbers is usually not observed for several months.
    . . . PLUS . . .
    NARROW RANGE OIL
    (415) 0.5–1% See label See label
    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; also improves insecticide spread and persistence.
    BROAD-SPECTRUM
    A. 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: intermediate
    RESISTANCE: None
    MODE-OF-ACTION GROUP NUMBER1: 4A
    COMMENTS: Effective against both adults and nymphs. 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 vedalia beetle and should not be used in cottony cushion scale-infested orchards.
    B. THIAMETHOXAM
    (Actara) 5.5 oz/acre 12 0
    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: Effective against both adults and nymphs. 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.
    C. FLUPYRADIFURONE
    (Sivanto 200SL) 12–14 fl oz/acre (IC) 12 1
    RANGE OF ACTIVITY: Pests: sucking insects such as psyllids, soft scales and aphids; Natural enemies: parasitic wasps
    PERSISTENCE: Pests: short; Natural enemies: short
    MODE-OF-ACTION GROUP NUMBER1: 4D
    COMMENTS: Safe for bees, and can be used during bloom. Do not exceed 28 fl oz Sivanto (0.365 lb a.i. flupyradifurone)/acre per year.
    D. MALATHION
    (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.
    RESISTANCE: 40% of San Joaquin Valley populations are resistant to organophosphate insecticides.
    E. CARBARYL*
    (Sevin XLR Plus) 3–5 qt/acre (IC or TC) See label 5
    RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
    PERSISTENCE: Pests: long; Natural enemies: long
    RESISTANCE: 40% of San Joaquin Valley populations are resistant to organophosphate insecticides.
    MODE-OF-ACTION GROUP NUMBER1: 1A
    . . . PLUS . . .
    NARROW RANGE OIL
    (415) 0.5–1.4% See label See label
    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 effects; also improves insecticide spread and persistence.
    ORGANIC
    A. NARROW RANGE OIL
    (415, 435, 440) 1.2–1.4% (IC or TC) See label See label
    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 effects.
    COMMENTS: For use on grapefruit, lemons, navels, Valencias. Use higher rate for July or August applications only. To avoid phytotoxicity, use same spray 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 sprays to green lemons because of phytotoxicity after sweating; check label for preharvest interval. Oils alone only suppress citricola scale, so they may need to be applied multiple times.
    ** IC - Intermediate coverage uses 250 to 600 gal water per acre.
    TC - Thorough coverage uses 750 to 2,000 gal water (or more)/acre, depending on tree size.
    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.
    * 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 are assigned by IRAC (Insecticide Resistance Action Committee).

    Important Links

    Text Updated: 02/17
    Treatment Table Updated: 02/17