Biological controls can be attempted during the summer in greenhouses with a parasitic wasps, Metaphycus helvolus, Encyrtus spp. and Encarsia citrina. These attack two species of scale insect, soft scale (Coccus hesperidum) and hemisperical scale (Saisettia coffeae).
The parasitic wasp is available from some mail order suppliers of biological controls.
The shell or scale protects the older insects from insecticides, so spraying is more effective against the newly hatched nymphs. With scales on outdoor plants there is usually one generation a year and in most species the eggs hatch in late June to July.
Scales in greenhouses or on houseplants breed throughout the year so all stages in the life cycle may be present at the same time. Scale insects can remain attached to the plant long after they are dead but new growth should be free of scales once they have been brought under control.
- Deciduous fruit trees and roses can be treated with plant oil winter tree wash (e.g. Growing Success Winter Wash or Vitax Winter Tree Wash) on a mild dry day during December to control overwintering scale nymphs
- Deltamethrin (e.g. Bayer Sprayday Greenfly Killer) and lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) are contact sprays that can be used on ornamental plants and a wide range of edible plants listed on the product packaging
- Ornamental plants in leaf can be sprayed with the systemic insecticides thiacloprid (e.g. Provado Ultimate Bug Killer), which is available as a spray concentrate and a ready to use spray or acetamiprid (e.g. Bug Clear Ultra). Some formulations of these sprays can be used on some fruits, including apple, plum, gooseberry, currants, raspberry and other cane fruits, check the manufacturer’s instructions
- Sprays based on natural substances include fatty acids (e.g. Bayer Organic Pest Control or Doff Greenfly and Blackfly Killer ) or plant oils and extracts (e.g. Growing Success Fruit & Veg Bug Killer, Bug Clear for Fruit & Veg and Vitax Organic 2 in 1 Pest & Disease Control). These pesticides have very little persistence and so may need several applications during the scale's egg-hatching period, but they can be used on fruit trees and bushes
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Biological control suppliers (Adobe Acrobat pdf document)