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?
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 populations can weaken plants and the scales and white wax produced by the males may be considered unsightly.
Check susceptible plants frequently so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
Light infestations are of little consequence and can be tolerated. 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 scale insects.
- Where possible tolerate populations of scale insects. Well-tended healthy plants are able to tolerate light populations of these insects and so they do not necessarily require control
- It can be worth considering replacing heavily infested plants
- Adult scales can be removed when seen but this may not reduce large populations
- A 2mm long black ladybird, Chilocorus nigritus is availabel from some Biological control suppliers and can help control these scale in glasshouses during the growing season.
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.Spraying works best against the more vulnerable newly hatched scale nymphs, this scale breeds all year and so there is no ideal time to spray.
- Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of scale insect nymphs. These pesticides have a very short persistence and so may require reapplication to keep scale numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
- Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic
- More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
- A systemic containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles
- The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Pesticides for gardeners (pdf document)
Biological control suppliers (pdf document)
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.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.