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Mealybugs are a type of unarmoured scale insect, where most species in the family are regarded pests. This is because they are sap-feeders, typically being concentrated around the roots so they can impact the vigour of plants. Species are associated with citrus, sugarcane, grapes, pineapple, coffee, cassava, ferns, and cacti (and the list goes on!). There are around 30 species of mealybug native to Britain, which around a third live on grasses. Other exotic species are thought to have intercepted quarantine authorities in the past couple of decades.
Many species of mealybug are garden and agricultural pests, doing exceptionally well in glasshouse conditions.
Many species are invasive.
Female mealybugs produce a white 'fluffy' wax, and this almost covers the whole body. They're usually detected from the powdery dusting left over the leaves of plants. They're oval-shaped insects with a textured body covered in white wax. Short, white, protrusions extend from around the body. Male mealybugs look a lot different compared with females. They look more like an aphid, but with the wings folded flat and outwards. Little males are produced by females, so they can be rare to see. Nymphs look very similar to females but smaller.
Symptoms can be variable and are dependent on the species. Balls of a cotton-like wax may be evident near nooks and crannies of plants. Plant surfaces may be covered in a clear sticky substrate. This is honeydew, its a byproduct of mealybug feeding. Honeydew can facilitate the growth of black sooty mould. Severe infestations may stunt growth or cause premature leaf fall. Infested plants may lack vigour.
Females make a large part of mealybug populations, and they can't fly, unlike males. This means the main route of transmission is by bringing newly bought, infested plants into the greenhouse or garden. Plants should always be inspected carefully before any money spent and purchased from a trusted source. To be extra safe, you can quarantine plants for a month before joining it with others. Fallen leaves should be collected and disposed of accordingly in case there are eggs or bugs on them. Sometimes it is just easier to dispose of the plant if the infestation is heavy. Always regularly monitor plants infested with mealybugs. The ladybird Cryptolaemus montrouzieri offers a solid form of biocontrol for glasshouses. Their larvae look a lot like mealybugs, so don't be alarmed if there seems to be more at first! Chemicals should not be used in conjunction with chemical sprays.
Chemical alternatives include organic sprays (containing natural pyrethrum), winter washes (containing natural plant oils) which are particularly good for vines, and the more persistent chemicals containing synthetic pyrethroids. The waxy coating on mealybugs is extremely protective of these chemicals, so if chosen plants should be coated thoroughly. Likewise, before any application be sure to scrape away any wax or dead insects, as this too serves as an added layer of protection. Please consider if chemical control is really necessary. If a chemical option is sought, check with your local garden centre and please take care to follow the manufacturers' instructions. Check with your local regulating body for guidance on active ingredients and their authorisation for use. Plants that are in flower should never be sprayed due to the danger they pose to pollinators.