Orchid and other diaspid scale insects

Small sap sucking diaspid scale insects can encrust the leaves and stems of glasshouse orchids and some other glasshouse plants.

Diaspid scales ( Diaspis boisduvalii )

Diaspid scales (Diaspis boisduvalii)

Quick facts

Common name: Orchid and other diaspid scales
Scientific name: Diaspis boisduvalii and other Diaspis species.
Plants affected: Glasshouse orchids and some other glasshouse plants
Main symptoms: Small whitish brown scales on leaves and stems
Most active: Year round

What are orchid or diaspid scale insects?

Diaspid scale insects are small sap sucking bugs that can infest glasshouse orchids and some other glasshouse plants. There are many types of scale insects that may be encountered by gardeners.

Symptoms

There are several species of diaspid scale in Britain that have a similar appearance and lifecycle. Adult female diaspid scales are covered by flat rounded whitish brown shell, about 2mm in diameter, and occur on the leaves and stems of host plants. The narrow elongate adult males are usually covered with a fluffy white wax; they generally cluster together at the base of the leaves. No honeydew is produced. Reproduction occurs throughout the year.

Heavy infestations can weaken plants and the scales and white wax produced by the males can be unsightly.

Control

Diaspid scale insects can be difficult to control, replacing severely infested plants should be considered.

Non-pesticide control

Physically squashing or removing scales may control light infestations.

A small black ladybird Chilocorus nigritus, which predates on all stages of diaspid scales is sometimes available from some suppliers of biological control (Adobe Acrobat pdf document).

Pesticide control

  • The waxy covering produced by scales gives them some protection from insecticides and the best results are achieved by spraying against newly hatched crawlers
  • Note that dead scales can remain firmly attached to the plants. The success of any treatment can be gauged by the extent to which new growth remains free of infestation
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep scale insect nymphs in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
  • More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
    The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener
  • Do not spray near plants in flower due to the danger to pollinating insects

Download

Pesticides for gardeners  (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

Diaspid scales feed by sucking sap from the leaves and stems of orchids and other glasshouse plants. Scales are named for the waxy shell-like casing which covers most of their body. When mature, the females lay their eggs under the protection of this shell. The eggs hatch into small active nymphs, known as crawlers, which wander over the plant surface until they find a suitable place to feed. They then become immobile and begin to produce their characteristic scale covering.

Infestations of scale insects are spread by the crawler stage, which may travel quite long distances before stopping to feed, and which can be spread by wind currents. Diaspid scales reproduce throughout the year on glasshouse plants.


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