Mealy bugs are common, sap sucking pests that thrive especially well in warm conditions. Although they will infest a whole list of house plants - as well as greenhouse plants - they have developed a particular taste for begonias, coleus, African violets, cacti and succulents. They are active and will continually breed all year round, and tend to live in clusters, usually in protected parts - like leaf sheaths and axils, between twining stems or closely layered leaves, or underneath loose pieces of bark - of plants.
Colonies feed on the plant's sap and destroy its tissue in the process. The honeydew secreted by mealy bugs attracts sooty mould, which will form unsightly black patches on the plant, as well as ants. Some species also feed on roots, and can often be found at the bottom of pots or as woolly white masses around a plant's roots.
The adult female has an oval, soft body of up to around 4 mm (1/4 in) in length. While she may be pink, the waxy powder her body is covered in will give her a white appearance, which is enhanced by the waxy filaments projecting from her body's edges.
She will lay her eggs in a cotton-like pouch and protects them with the waxy substance she secrets. The nymphs hatching from these eggs about a week later look like tiny adults and will quickly crawl to a protected spot (earning them the commonly used name of crawlers) allowing them to feed in safety until they are mature, which will be a month or two later.
Male mealy bugs are extremely small, have two wings, do not feed, and basically live just long enough to reproduce. Females can not fly and do not crawl far, either, so as a rule, infestations are introduced through new plants that are infested. Keeping such plants under quarantine for at least 4 to 6 weeks, regularly checking for infestations and, if necessary, treating the plant before placing it among existing plants should help to prevent problems, as will promptly removing dead leaves, bits of pruned-off stems and other waste material.
Signs of Infestation
Specks or patches of fluffy white wax are typically the first sign of a mealy bug infestation. Both the females and their orange-pink eggs can be found below this substance. Accumulations of honeydew may also be visible, in particular if the infestation is heavy. In this case, blackened patches of sooty mould, which is attracted by the honeydew, may also be present. Root-feeding mealy bugs can be detected by easing the plant out of its container and looking for the tell-tale white mass among the roots. Mealy bug activity may also cause loss of vigour in the plant, as well as stunted growth and prematurely dropping leaves.
Dealing with Infestations
Infestations on plants kept in greenhouses can often be resolved by introducing Leptomastix spp., a parasitic wasp, or Cryptolaemus montrouzieri, a ladybird with larvae that have an appearance similar to adult mealy bugs. It should be noted, however, that both of these natural predators require comparatively high temperatures, making them effective only from May to September. They are also susceptible to pesticides, and should therefore not be used in conjunction with chemical solutions.
Naturally, they are also not suitable for use on plants kept in the home. Here, minor infestations can often be dealt with by using a solution of insecticidal soap, a little cooking oil and water. A cotton swab soaked in rubbing alcohol will also work, as will many home-made sprays. Root-feeders can often be effectively removed by washing the soil - complete with the unwelcome visitors - off the roots, removing affected parts and replanting the plant in fresh soil. It is vital to use either a new pot or wash the old one thoroughly before replanting to ensure no eggs likely to re-colonise the soil are hidden in it.
If the use of chemical solutions becomes necessary, it is worth remembering that organic pesticides with plant oils/ extracts and/ or fatty acids, as well as synthetic contact pesticides such as, for instance, Deltamethrin, will require repeated, regular applications to be effective, as they work only for short periods of time. Diazinon will also work, but it is essential to thoroughly wet the invaders with it.For particularly heavy, stubborn infestation, systemic insecticides tend to be far more effective. Containing thiamethoxam, thiacloprid or acetamiprid, these pesticides need to be administered with careful attention to instructions/ warnings. Gardening/ house plant experts will be happy to offer advice on the most suitable solution.