Mealybugs are common sap-feeding pests that infest a wide range of houseplants and greenhouse plants. Mealybugs weaken plants and excrete a sticky substance (honeydew) on foliage, which allows the growth of sooty moulds.
What is mealybug?
Mealybugs are common insect pests that tend to live together in clusters in protected parts of plants, such as leaf axils, leaf sheaths, between twining stems and under loose bark. They suck sap from plants and then excrete the excess sugars as a substance called honeydew. This lands on the leaves and stems were it is often colonised by sooty moulds, giving the surfaces a blackened appearance.
Mealybugs are found mainly on greenhouse plants and houseplants, especially cacti and succulents, African violets, bougainvillea, citrus plants, fuchsia, grape vines, hoya, orchids (especially Phalaenopsis), oleander, passion flower, peach and tomato. Some other mealybug species can attack outdoor plants, such as ceanothus, laburnum, New Zealand flax and redcurrant.
Most mealybugs thrive in warm conditions, which is why they are not usually a problem on outdoor plants. Mealybugs are active all year round on houseplants and in greenhouses.
You may see the following symptoms:
- Infestations are usually first noticed as a fluffy white wax produced in the 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 honeydew. This makes plants sticky and encourages the growth of sooty moulds, giving the leaf and stem surfaces a blackened appearance
- Severe infestations will reduce plant vigour and stunt growth. Heavy infestations may cause premature leaf fall
Infestations are usually first noticed as a fluffy white wax produced in the leaf axils or other sheltered places on the plant, such as on this Schlumbergera (Easter cactus).
The biological control, Cryptolaemus montrouzieri, larva with mealybug prey
Phormium mealybug (Balanococcus diminutus) on New Zealand flax (Phormium tenax)
Non chemical 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 greenhouse or 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 greenhouse as these may have mealybugs or eggs on them.
A ladybird, Cryptolaemus montrouzieri, can be released into greenhouses to control mealybugs. Note that the ladybird's larvae look like large 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 (Leptomastix spp.) 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 May 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.
- A systemic insecticide, thiacloprid, is available as Provado Ultimate Bug Killer Ready-to-Use or Provado Ultimate Bug Killer Concentrate 2. This can be used on ornamental plants and greenhouse-grown tomatoes, peppers, cucumbers and aubergines, but not other edible plants
- Other systemic insecticides that can be used against mealybugs are acetamiprid (Scotts Bug Clear Ultra concentrate or Bug Clear Ultra Gun) and thiamethoxam (Westland Plant Rescue Bug Killer Ormanental Plants). These two pesticides are for use on ornamental plants only
- Deltamethrin (Bayer Sprayday Greenfly Killer) is a contact insecticides that can be used on ornamental plants and most greenhouse fruits and vegetables, but not peach/nectarine
- Growing Success Winter Wash or Vitax Winter Tree Wash (contain plant oils) can be used on peach and grape vines in December while they are fully dormant. Scrape loose bark off grape vines before treatment in order to expose hidden mealybugs. Spread newspaper under the vine to collect the bark scrapings for disposal
- Other organic treatments for use during the growing season 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 more frequent use. They can be used on edible plants
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Several species of mealybug occur in greenhouses or on houseplants. These include Pseudococcus 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.
Female mealybugs lay eggs under a white, waxy coating. Mealybug nymphs resemble the adult insects and can complete their development in about a month in mid-summer. Breeding continues throughout the year in greehouses, but takes place at a slower rate in winter.
Common name Mealybug
Scientific name Planococcus citri, Pseudococcus longispinus, P. calceolariae and others
Plants affected Many houseplants and greenhouse plants
Main symptoms Fluffy white wax, honeydew and sooty moulds
Most active Year round