Scale insects are brown wart-like creatures that tend to congregate on the underside of leaves and suck sap. They are most visible when on a light-coloured area - as shown with this photo of a fruit. Scale insects are not easy to remove or to kill.The scale forms a kind of shell over the insect and protects it from predators and insecticides. The scales are not only unsightly - over a period of time they debilitate the plant and can even kill it.
There are two divisions of scale insects - soft scale and hard scale. This web site offers a control for soft scale, which is a mix of two different parasites: Metaphycus helvolus and Encyrtus infelix. These are all very small flying insects which target scale insects, sucking the juices out of some and laying their eggs in others. They each live about a month and lay approximately 6 eggs a day, the young of which emerge through a small hole after 3 or 4 weeks.
They work best between 20-30°C, so are best introduced from May when temperatures and light levels are at their highest. None of them will be easily noticeable flying around the conservatory as they are only a mm or two across. As with other biological pest controls, they are not interested in humans or pets!
Use a single introduction of these beneficial insects on the most heavily infected plant. They will be breeding their own within a month Scale can also be cleaned off leaves by hand, using your finger nails or cotton buds using Soft Soap. Remember that larger Scale insects can often shelter their young underneath their bodies.
Cottony Cushion Scale
Cotton Cushion Scale is an unusual form of scale which often attacks citrus plants. Its latin name is Icerya purchasi. You will recognize them from the huge ribbed egg sac with can be as much as twice as big as the rest of their body. If you have a microscope, and turn this beastie over on its back you will find lots of little crawlers wriggling away under the shelter of their parent. Cotton Cushion Scale is not attacked by our soft scale mix, but you can clean off the adults by hand and then treat the crawlers (see below).
The problem with scale (and mealybug) is that they spread through the tiny, tiny, crawling 'babies' (properly called 'larvae') who are only active for a short while and are very difficult to spot. For that reason it is good to wipe down your plants with horticultural soft soap or SB Invigorator if you bring in plants in the Autumn - a house or greenhouse is a great refuge for these troublesome pests.