Soft scale

Soft scale is a very widespread and common scale insect attacking a wide variety of garden and glasshouse plants. It is a flat, oval pale yellow/brown insect usually found near the midribs of leaves and on stems and produces large amounts of honeydew.

Soft scale on the underside of a bay leaf

Soft scale on the underside of a bay leaf

Quick facts

Common name Soft scale
Scientific name Coccus hesperidum group
Plants affected A wide variety, common on citrus and bay
Main symptoms Sticky leaves and sooty mould
Caused by Sap sucking scale insects

What is soft scale?

There are many types of scale insects encountered by gardeners, they are sap sucking true bugs. Soft scale is a flat, oval, pale yellow/brown insect. The pale covering of the insect is waxy and repels liquids. The adults reach up to 4mm long, the immature (nymph) stages are much smaller. It is found near the midribs of leaves and on stems.

It produces large amounts of honeydew and breeds throughout the year. It attacks garden and glasshouse plants, including bay, Ficus and Citrus species, ivy, ferns and many others.

Symptoms

Heavy infestations of soft scale are characterised by heavy deposits of sticky honeydew upon which a black non-parasitic fungus sooty mould often grows. The 4mm long pale yellow brown insects will be evident upon examination of the undersides of leaves and stems.

Control

Non-pesticide control

Soft scale is difficult to eradicate. Whilst scraping the insects off the leaves can help reduce infestations this is unlikely to give good control. It can be worth considering replacing heavily infested plants.

Biological control

  • Parasitoid wasps (Metaphycus helvolus, Encyrtus spp. and Encarsia citrina) are sometimes available for control of soft scale in glasshouses. Some of these parasitoids occur naturally, blackened scales is a sign of parasitoid activity
  • The insect pathogenic nematode Steinernema feltiae is also available for soft scale control
  • These controls are available from some biological control suppliers

Pesticide control

Soft scale can be difficult to control with sprays as the waxy covering gives them some protection and the best results are achieved by spraying against the newly hatched crawlers. Heavily infested plants may need several applications. Scales often remain attached to the plant long after they have died, but new growth will remain free of scales and honeydew if the treatment has been successful.

  • 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 scale insect nymphs. These pesticides have a very short persistence and so may require reapplication to keep scale insects in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. These organic sprays can be used on most edibles. 
  • 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. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval
  • Do not spray plants in flower due to the danger to pollinating insects
  • 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

Note: Only lambda-cyhalothrin and deltamethrin can be used on bay leaves used for culinary purposes and there are no synthetic pesticides available for use on citrus although organic types can be used. One application/year of deltamethrin is permitted on culinary herbs; two for lambda-cyhalothrin and there is a 7-day interval between treatment and using the foliage.

Downloads

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

Biological control suppliers (Adobe  Acrobat pdf)

Biology

Female soft scales can lay up to 200 eggs underneath their body. These hatch into small pink crawler nymphs which move over the plant surface before settling down to feed. They are small enough to be easily blown around in wind currents and this is the main way that plants are colonised. The nymphs suck sap from the leaves and stems and, once feeding has commenced, they usually remain immobile for the remainder of their lives. 

In warm conditions breeding is continuous throughout the year.

A waxy layer is secreted over their bodies to form the scale, and this layer gives them some protection from pesticides.


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