Scale insects are tiny, sap-sucking creatures which attack many varieties of plants. Each one is protected by a scaly, wax shield which can be either brown, white or yellow.
Plants at Risk
Scale insects will attack many of the same plants as mealy bugs, although there are several species of the insect with different preferred hosts. Hardy European plants are as vulnerable as tropical and sub-tropical species. Check plants carefully for both these pests before buying, and be most vigilant when plants are settled in their new homes. If an attack is discovered, treat the plant at once as scale insects multiply quickly.
Plants most at risk are:
All types of fig, particularly Weeping Fig and Fiddleleaf Fig. All varieties of palms, ivies, cacti and succulents. Cast-iron plants, Dragon Tree, Striped Dracaena, Croton, Seville Orange and Kangaroo Thorn.
Scale insects are easy to recognise, even by a novice gardener. They are small, about 5mm (1/4in) long, oval or round in shape, and have a brown, white or yellow protective covering or shield which is hard and scaly. Several species are found in this country, and they will attack both indoor and outdoor plants.
Recognizing the Symptoms of Scale Insect Damage
Scale insects live on the undersides of leaves, at leaf axils and on young, tender stems, clustered together in small colonies. Most species excrete a substance called honeydew which spreads over leaves and stems and feels sticky to the touch. Scale insects appear in many different forms. A prickly pear cactus suffers an attack, visible as white, crusty accretions. Scale insects weaken plant growth by sucking out and feeding on the sap, as well as looking unattractive.
How Scale Insects Multiply
Mature scale insects usually remain in the same position on a plant, but may occasionally move to a new feeding site when the flow of sap dries up. Each mature female produces several hundred eggs at a time, laying them under a smooth or woolly wax coating for protection. The eggs hatch after several weeks and the young insects or nymphs, commonly known as ‘crawlers’, move round the plant looking for a suitable place to settle, feed and form their own protective shield. The females live for about 3 months, during which time they may have laid as many as 1,000 eggs.
Protective Shield
The hard shield gives protection from predators and most insecticides, and stays in place after the insect has died. They are firmly attached to the plant and should be removed carefully to avoid damage to sensitive foliage. This hardened outer shell remains fastened to a plant even after the mature insect has died.
Combating an Attack
If there are just a few adult scale insects on a plant you can remove them by hand. Pick them off with tweezers or the tip of a sharp knife, taking care not to damage the foliage. Use a cotton swab or small brush dipped in soapy water or dilute methylated spirit to wipe small numbers of insects off plants with strong, robust leaves. Follow up by wiping the leaves down with a soft cloth moistened with water.
For severe attacks, you will have to use an insecticide. This is most effective when used to kill the crawlers before they begin to form their protective shield. Use a contact insecticide like malathion or imagediazinon when the crawlers have hatched out and are moving round the plant. Repeat after two weeks. Systemic insecticides such as formothion can be used to eliminate both crawlers and mature insects, but may damage some plants. Always check with your garden centre or with a specialist before using these insecticides.
As scale insects mature their protective coat darkens and hardens, reducing the effect of insecticides. This fern leaf is host to a mature specimen (top) and a number of younger, wandering insects.
Checklist
Do
- Isolate affected plants, keeping them well away from other plants especially after the eggs have hatched.
- Treat scale insect as soon as the first ones are seen. Individual insects are easy to remove with tweezers or a cotton swab dipped in dilute methylated spirit, but can soon multiply if neglected.
- Catch the young ‘crawlers’ before they develop a hard, protective shell.
- Examine new plants very closely before introducing them into your home.
Don’t
- Use a systemic insecticide like formothion or dimethoate without checking with your gardening shop or nursery that it is suitable for the plant you need to treat. This type of insecticide can cause damage to some plants, particularly ferns, fuchsias and begonias.
- Forget to apply insecticide at regular intervals to ensure that none of the insects have been missed.
