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.
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?
Scale insects are sap sucking true bugs belonging to several families in the Hemiptera. Typically the adults are immobile having a flattened or raised appearance, with no visible legs. They often look like a ‘scale’ on a leaf or stem, many species produce a white wax often covering egg masses. There are more than 100 species found in Britain, 26 of which have been introduced. More than 25 species can be found in gardens or on houseplants.
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 using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.
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 recommends that you don’t use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.
Where you cannot tolerate scale, manage them using the information above as your first course of action.
Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.
Shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife.
The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.
Always follow the instructions on the products. For edible plants, make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.
Homemade products are not recommended as they are unregulated and usually untested.
Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.
Plants in flower must not be sprayed due to the danger to bees and other pollinating insects.
- Organic sprays, such as natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear 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 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 and SB Plant Invigorator). These are not considered organic
- Further information about the use of pesticides available for management of scale is available on the pesticides for gardeners leaflet
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.
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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