Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

sap

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.

Hemispherical scale rarely causes direct damage to plants. Apart from the presence of the insects the main symptom is the presence of honeydew on which sooty mould can grow.

It can appear similar to other diaspid scales.

Hemispherical scale insects can be difficult to control; replacing severely infested plants should be considered.
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.
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.

Non-pesticide

• Where possible tolerate populations of scale insects. Not all scale insects affect the growth of plants and so they do not necessarily require control
• Adult scales can be removed when seen but this may not reduce large populations
• Encourage predators in the garden, some parasitoid wasps that affect these scale can be found indoors
• The nematode Steinernema feltiae can be purchased to as a  biological control of hemispherical scale and is available from some Biological control suppliers. Nematode controls have the potential to infect non-target animals. They should therefore be used with care and only when there is a specific problem to treat

Pesticides

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 hemispherical 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.
 
The shorter persistence products (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 should 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

Downloads

Pesticides for gardeners  (pdf document)

Biological control suppliers (pdf document)

Hemispherical scale insects feed by sucking sap from the leaves and stems of various 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.

All hemispherical scales are female and reproduction is continuous throughout the year and entirely parthenogenetic. 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.