Plants affected
Mealybugs are found mainly on glasshouse and houseplants, especially most cacti and succulents, Bougainvillea, Saintpaulia, orchids (especially Phalaenopsis), Fuchsia, tomato, Hoya, Nerium oleander, Passiflora, peach, citrus plants and grape vines.
Some other mealybug species can attack outdoor plants, such as Ceanothus, redcurrant, laburnum and Phormium.
Symptoms
Mealybug
infestations are usually first noticed as a fluffy white wax
produced in leaf axils or other sheltered places on the plant.
The insects or their orange-pink eggs can be found underneath
this substance.
Heavy infestations may result in an accumulation of sugary excrement known as honeydew. This makes the plant sticky and encourages the growth of sooty moulds.
Cause
Several species of mealybug occur in greenhouses or on houseplants. These include Pseudococcus affinis, P. calceolariae, P. longispinus and Planococcus citri.
The adult females have flattened oval-shaped soft bodies up to 4mm in length; they are sometimes pink in colour but appear whitish due to the white waxy powder that covers their bodies. Waxy filaments project from the edges of their bodies. Some species are all female; others have small winged males, but the latter are infrequently seen.
Mealybugs tend to live together in clusters in protected parts of the plant, such as leaf axils, leaf sheaths, between twining stems and under loose bark. They suck sap from plants, reducing their vigour and stunting growth. Heavy infestations may cause premature leaf fall.
Control
Female mealybugs do not fly or crawl far, so infestations are usually brought in on an infested plant. Inspect new plants carefully before putting them in a glasshouse/conservatory and, where possible, keep them in quarantine for a month or so before adding new acquisitions to an existing collection.
Dead leaves and prunings should be removed from the glasshouse as these may have mealybugs or eggs on them.
Imidacloprid (Provado Ultimate Bug Killer concentrate) is a systemic insecticide that is absorbed into plants and kills the pest as it feeds. A ready-to-use spray containing another systemic insecticide, thiacloprid, is available as Provado Ultimate Bug Killer Ready To Use. The former can be used on ornamental plants and glasshouse tomatoes, peppers and aubergines, but not other edible plants. Thiacloprid can also be used on these plants, plus glasshouse cucumbers. Other systemic insecticides that can be used against mealybugs are acetamiprid (Scotts Bug Clear Ultra concentrate or Bug Clear Ultra Gun) and thiamethoxam (Westland Bug Attack). These two pesticides are for use on ornamental plants only; thiamethoxam is restricted for use on container-grown plants.
Growing Success Winter Wash can be used on peach and grape vines in December while they are fully dormant. Scrape loose bark off the grape vines before treatment in order to expose hidden mealybugs. Spread newspaper under the vine to collect the bark scrapings for disposal.
Organic treatments include fatty acids (Bayer Organic Pest Control, Greenfingers Organic Pest Spray or Doff Greenfly and Blackfly Killer) or plant oils or extracts (Growing Success Fruit & Veg Bug Killer, Vitax Organic 2 in 1 Pest and Disease Control, Scotts Bug Clear for Fruit & Veg). These organic pesticides have a contact action and short persistence and so may require frequent use. They can be used on edible plants.
Biological control
A ladybird, Cryptolaemus montrouzieri, can be released into a glasshouse to control mealybugs. Note that the ladybird's larvae look like giant mealybugs. Both the adult ladybirds and their larvae are able to find and eat mealybugs and their eggs in confined spaces on the plants. Parasitic wasps are also available for use against this pest.
The ladybird and parasitic wasps need relatively high temperatures and so are only likely to be successful during late April to September. They are susceptible to most insecticides and should therefore be used as an alternative, rather than in addition to chemical control. They are available by mail order from suppliers of biological controls.