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Armored scale

Also known as Armoured scale, Diaspididae, Oleander scale (Aspidiotus nerii), Boisduval scale, Oystershell scale (Lepidosaphes ulmi), San Jose scale, California red scale

PestHarmful
Oleander scale (Aspidiotus nerii) on a leaf Photo: Grey Smith Depicted place: McCrae's Bush, Ashhurst, New Zealand (40°17′00″S 175°45′55″E / 40.2833940466°S 175.7653344923°E / -40.2833940466; 175.7653344923) · CC BY 4.0

In short

Armored scales are tiny insects that live under a hard, flat wax cover. They make no honeydew, so infestations often go unnoticed until leaves yellow and drop or shoots die back, and several systemic products that work on soft scales do not control them.

How it looks

Usually under 3 mm (1/8 in) across. Each insect sits under a flat, plate-like cover, round, oval or oyster-shaped depending on species, which can be lifted off to reveal a separate soft body underneath. Covers cluster on stems, leaves and fruit, sometimes forming a crust. The mobile crawlers are tiny yellow to orange specks; after settling they lose their legs and never move again. On orchids, the Boisduval scale female has a circular cover.

Signs

Yellowing or premature leaf drop, distorted plant parts, and dieback of shoots or branches; crusty encrustations of covers. There is no honeydew or sooty mould, and ants are usually absent.

Life cycle

Most species have several generations a year; in cold climates they overwinter as first-stage nymphs and adult females. In warm conditions the Boisduval scale completes a generation in about 50 days, and a female can lay up to about 200 eggs.

What favours it

Warmth allows generation after generation. Because adults do not move, infestations usually arrive on new plants.

Prevention

  • Inspect stems and leaves of new plants carefully, lifting a suspect cover with a fingernail, and quarantine new arrivals.
  • Keep plants well watered and cared for so they tolerate feeding.
  • Replace plants that are repeatedly infested.

Treatment

Start with the first step and move down only if the problem persists.

  1. Scrape covers off by hand, or prune out heavily infested shoots on small plants.
  2. Protect natural enemies: parasitoid wasps (such as Aphytis and Encarsia) and small lady beetles such as Chilocorus and Rhyzobius lophanthae.
  3. Before spraying, squash a few scales; dead ones stay attached but produce no fluid.
  4. Apply horticultural oil or insecticidal soap when crawlers and young nymphs are present; follow the product label.
  5. Many systemic neonicotinoid products do not control armored scales; if a systemic is needed, choose one whose label lists armored scales and follow the product label.

Pros and cons

Cons

  • No honeydew, so infestations are easy to overlook until damage is advanced.
  • Hard covers protect them from many sprays.

Look-alikes

Soft scales
Domed body that cannot be separated from a cover, with honeydew and sooty mould.
Mealybugs
Mobile, soft and covered in white powdery wax.

Affected plants

Orchids, citrus, fruit and ornamental trees and shrubs, and indoor plants.

Sources

  1. Scales — UC IPM Pest Notes
  2. Orchid Insect and Mite Pests in South Florida (ENY2114) — UF/IFAS
  3. Indoor plants: sap-feeding insects — RHS

Written in our own words from these sources. Check a product label before using any treatment.