Mealybugs

Signs - small fluffy white lumps about 5mm (1/4") appear on plants, often in the axil - where the leaf meets the stem. Leaves turn yellow and may wilt and die. There are a large number of different types which are frequently specific to the host. Usually affect house and greenhouse plants, but also Pyracantha and fruit trees. Stick honeydew and black sooty moulds may accompany. Despite being insects, they don't look like insects, just a shapeless piece of cotton wool.

Damage - plants are rarely killed unless very heavily infested over a long period. Commonly weakened, a heavy infestation is very unsightly from the pests themselves and from the sticky honeydew that they secrete and possibly even black moulds that grow on the sticky honeydew. Root mealy bugs damage the roots.

Treatment - difficult to attack from the outside, any of those organic soapy sprays just fall off from the hydrophobic (water repellent) hairs that cover the insects. Powder form insecticides sit on the hairs and again don't get the insect inside. In small numbers they can be picked off manually before they build up a larger population, otherwise it's biological control (the predators are hungry critters so you need a heavy infestation) or a systemic insecticide that is taken up by the plant and passed along in the sap right to the mealy bug.

Biological control glasshouses or conservatories only

There are different species which can

be distinguished by the length of their tail filaments but they are all oval shaped. The most common species are the citrus mealy bug, glasshouse mealy bug and long tailed mealy bug. Nymphs and female adults cause damage. Most feed on higher parts of plants but some extract food from roots.

Biological control of mealy bug is with the predatory beetle, Cryptolaemus.
The adults and larvae of these beetles eat the mealy bug completely.
The larvae are similar in appearance to the mealy bug so be careful not to remove!

The beetle should be introduced once the mealy bug are observed and a second treatment applied later to ensure the number of predators is high enough to compete with the mealy bug population.

Buy mealy bug killer

Insect deterrents

Plants

Citron scents are particularly effective at deterring 'midges'; hence the recent rise in popularity of citronella - fragranced garden candles. Try growing plants with citrus-scented foliage, such as Geranium macrorrhizum and lemon balm. Another option is lemon verbena, which has wonderfully aromatic foliage - not fully hardy though, so best to grow it in a pot so that it can be brought into the greenhouse or conservatory to over winter.

Other aromatic herbs have insect-repellent properties - lavender (or here) has been used for this purpose for centuries, either in dried or fresh form. Mints can also help to repel aphids - spearmint or peppermint can be grown near roses to deter these pests.

Chemical insecticides

If your plants get a heavy infestation, then I think it's acceptable to use a chemical insecticide as a "smart missile" just on that particular plant. I resort to this when the aphids build up under cover on my favourite plum tree, or other plant/s.

Don't overdo it and spray too often and don't spray the whole garden "just-in-case".

Mealy Bugs