Scale Insects



These small sap sucking insects are generally most troublesome on greenhouse and house plants, but certain species attack outdoor ornamental shrubs and fruit trees.

Infestations are often first noticed when the leaves become sticky and sooty, or when the plant leaves begin to yellow and distort. this is caused by the sticky honeydew they produce as they feed.

Close examination of stems and the underside of the leaves may reveal the waxy brown, yellow or white scales that frequently form thick encrusting colonies.

These scales are the female insects, which remain in the same position for most of their lives.

Males are rarely seen but are often winged and midge-like in appearance.

There are two types of scale insects, there is the soft scale coccidae, and the hard scale diaspididae.
Soft scale coccidae
Hard scale diaspididae.

Soft scale insects breed all year round on protected crops and produce large amounts of sugary honeydew when they feed on the plant's sap.

Hard scale insects lay eggs under their scales.

Once the nymphs hatch the female dies in early summer.

The newly hatched young scales then spread infestations over adjacent plants.

This variety does not produce large amounts of honeydew.

Female nymphs become immobile once they find a suitable feeding site until such times as they become adults.


Prevention is often the best option.

Check newly purchased plants thoroughly for signs of infestation.

Check stock plants regularly for signs of infestation and deal with them as soon as they appear.

Look for leaf drop, honeydew, sooty mould or ants feeding on the honeydew.

If an infestation is in its early stages, the scales can be cleaned off using a brush and water, or just a thumbnail.

Remove and destroy any infested leaves,as these may harbour mobile juvenile scales.

Similarly, keep the area around the base of the plant/s free of plant debris.

Chemical control:

Spray the affected plants with a natural fatty acid insecticide when the crawlers are hatching.

In heated greenhouses this can be at any time of year, but in the open and in un-heated greenhouses infestations usually occur during the summer.

Thorough spraying with a tar oil winter wash in December or January gives good control of scales on deciduous trees and shrubs, including fruit.

Biological control:

Use a natural predator such as chilocorus nigritus for hard scale insects, and metaphycus helvolus for soft scale insects.

Top of the Page