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Azalea Bark Scale Diagnostic Guide

Eriococcus azaleae

Discovered in 1881, the azalea bark scale, Eriococcus azaleas, is a prominent pest of rhododendron, azalea, and huckleberry (Vaccinium spp.). It has been reported in the Eastern US, as well as Belgium and Russia. This is a pest that generally does not kill hosts, rather it causes aesthetic damage. Honeydew can be an inconvenience as well.

Shrubs at Risk

Rhododendron, azalea, and huckleberry are all known to be infested by azalea bark scale.

Huckleberry

Rhododendron

Azalea

Signs of Damage

  • Leaf yellowing mid to late summer.
  • Sooty mold growing on honeydew can be found under the tree or on limbs.
  • Dieback from the tips inward is common.
Azalea Bark Scale

Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org

Physical Appearance

  • White waxy threads of the female egg sac near twig crotches are visible prior to June/July.
  • Small winged adults (males) are on leaves around August/September.
  • Crawlers are reddish, tiny, and mobile and found around June/July.
  • Adult females resemble mealy bugs.
Azalea Bark Scale

United States National Collection of Scale Insects Photographs Archive, USDA Agricultural Research Service Bugwood.org

Biology

  • Eggs are deposited into the white waxy egg sac in late April.
  • Crawlers hatch out of eggs in June-July or 957 growing degree days.
  • Crawlers prefer branch crotches and twigs where they insert their mouthparts.
  • Overwinter as nymphs.
  • There is one generation per year (possibly two in southern states).

Treatment Strategy

Nymphs must be targeted when they are unprotected by the waxy covering. This means dormant horticultural oil sprays may be used after the growing season is complete. A more effective option is to apply a systemic insecticide either during the prior fall or spring to control feeding crawlers from the inside of the plant.

  • Soil Application using Transtect

    Dosage: 1 packet/10-17 ft shrub height
    Timing: Apply early spring so AI is in place for feeding.
    Re-Treatment: Annually
    Transtect
    Transtect
    $326.00
  • Soil Application using Xytect 2F

    Dosage: 0.2 oz/foot of shrub height
    Timing: Apply during fall for next season control. Apply in early spring after ground thaws for current season control.
    Re-Treatment: Annually
    Xytect 2F
    Xytect 2F
    $69.60
  • Foliar Spray using RTSA Horticultural Oil

    Notes: Growing season
    Dosage: 0.25-2 oz of RTSA Hort Oil™/100 gal water
    Timing: In spring to target crawlers
    Re-Treatment: Apply as needed
    RTSA Horticultural Oil
    RTSA Horticultural Oil
    $39.00
  • Foliar Spray using RTSA Horticultural Oil

    Notes: Dormant season
    Dosage: 2-3 oz of RTSA Hort Oil™/100 gal water
    Timing: After growing season to control nymphs
    Re-Treatment: Monitor results and apply as needed
    RTSA Horticultural Oil
    RTSA Horticultural Oil
    $39.00
  • Foliar Spray using RTSA Horticultural Oil

    Notes: Combined with Distance
    Dosage: 0.5-1.0%/100 gallons water
    Timing: Apply late fall or early spring and monitor
    Re-Treatment: Retreat seasonally as needed
    RTSA Horticultural Oil
    RTSA Horticultural Oil
    $39.00
  • Foliar Spray using Distance

    Notes: Combined with RTSA Horticultural Oil
    Dosage: 8-12 oz./100 gallons water
    Timing: Apply late fall or early spring and monitor
    Re-Treatment: Retreat seasonally as needed
    Distance
    Distance
    $269.00
  • Soil Application using Xytect 75WSP 22 packets

    Dosage: 1 packet/24-48 feet of shrub height
    Timing: Apply spring or fall
    Re-Treatment: Annually
    Xytect 75WSP 22 packets
    Xytect 75WSP 22 packets
    $148.00

Treatment Expectations

Management of this pest is easiest when the nymph stage is targeted. Soil applied systemics have made application timing and methods much easier.

A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org

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