Agriculture: Citrus Pest Management Guidelines

Purple Scale

  • Lepidosaphes beckii
  • Description of the Pest

    Purple scale is one of the armored scales. The cover of the adult female purple scale resembles a mussel shell in shape. The immature male cover is shorter and much narrower than the female cover. Mature males are winged insects that search out the immobile females. After mating, females lay 40 to 80 eggs under the cover. After egg hatch, crawlers emerge from under the cover and settle on branches, twigs, leaves, or fruit and begin to form their covers. They are covered with a mass of waxy threads until about half grown; at that time a brown cover with a purplish tinge forms. Purple scales prefer the cooler, shady parts of trees; temperatures above 80°F (27°C) greatly reduce scale numbers. Two generations occur between May and October and a third may be partially completed before cold weather starts.

    Damage

    Purple scale is an occasional pest in certain coastal areas where the mild climate and humid conditions favor its buildup. It attacks all parts of the tree. Its feeding causes yellowish halos to develop on leaves; on young fruit, the feeding sites remain green. When there are high numbers of scale, defoliation and twig dieback can occur; this usually takes place in limited patches on the lower north side of trees.

    Management

    Parasites usually provide good control of purple scale. Biological control may require supplementary insecticide applications at times, especially on dusty trees next to dirt roads. Controlling Argentine ants will also assist with purple scale biological control.

    Biological Control

    The most effective purple scale parasite is Aphytis lepidosaphes, a parasitic wasp that is generally distributed in areas where purple scale occurs. This parasite develops externally on the body of immature scales under the scale cover. Because this parasite is not commercially available, conserve naturally occurring populations of this beneficial in the grove. If insecticide applications are necessary, during August and September either spot treat (i.e., spray only those trees with high numbers of purple scale) or spray every fourth to sixth row at 4- to 6-week intervals if the entire grove is infested. This will assist in preserving natural enemies.

    Several predators including the twicestabbed lady beetle, Chilocorus spp., and the Australian lady beetle, Rhyzobius (Lindorus) lophanthae, are important.

    Organically Acceptable Methods

    Use biological control and organically acceptable oil sprays on organically certified citrus.

    Treatment Decisions

    If a spray is needed, it may be sufficient to spot treat (i.e., spray only those trees with high numbers of purple scale) with an oil spray or wash dusty trees with water. Oil sprays for the California red scale also control the purple scale.

    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.
    A. NARROW RANGE OIL (UR 92%)
    (415, 440) 1.2–1.4% (TC) See label When dry
    . . . or . . .
    NARROW RANGE OIL (UR 99%)
    (415, 435, 440, 455) 1.2–1.4% (TC) See label 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: Apply higher rate of narrow range oil in July or Aug. only. 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). Caution: Serious hazards are associated with oil sprays to green lemons because of phytotoxicity after sweating; check label for preharvest interval.
    B. CARBARYL*
    (Sevin XLR Plus) 3–5 qt/acre (TC) See label 5
    RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most
    PERSISTENCE: Pests: long; Natural enemies: long
    MODE-OF-ACTION GROUP NUMBER1: 1A
    COMMENTS: For use on all varieties. Do not apply during bloom. May increase citrus red mite numbers.
    C. CARBARYL*
    (Sevin XLR Plus) 3–5 qt/acre (TC) See label 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% See label When dry
    MODE OF ACTION: Contact including smothering and barrier effects; also improves translaminar movement and insecticide persistence.
    RANGE OF ACTIVITY: Pests: broad (unprotected stages of insects and mites); Natural enemies: most
    PERSISTENCE: Pests: short; Natural enemies: short
    COMMENTS: For use on all varieties. Reducing the rate of carbaryl increases survival of natural enemies. Do not apply during bloom. May increase citrus red mite numbers. Resistance exists in some California red scale and Euseius tularensis (predaceous mite) populations. Caution: Serious hazards are associated with oil sprays to green lemons because of phytotoxicity after sweating; check label for preharvest interval. Reducing the rate of the oil reduces the risk of phytotoxicity, especially in warmer growing areas of the state.
    ** LV - Low-volume uses 20 to 100 gal water/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 (un = unknown or uncertain mode of action)are assigned by IRAC (Insecticide Resistance Action Committee).
    Text Updated: 02/17
    Treatment Table Updated: 02/17