Q What are mealybugs?

A Mealybugs are small, oval, sap-sucking insects up to 4mm long. They look like tiny, pinkish or grey woodlice close up, but are usually covered with a fluffy layer of protective wax. Colonies resemble blobs of sticky cotton wool and may be accompanied by sooty mould.

Caption: Mealybugs protect themselves with a layer of white, fluffy wax

Q Which plants do mealybugs affect?

A There are three main groups: Glasshouse mealybugs (pseudococcus, planococcus and nippaecoccus) mostly come from the tropics and only survive here in protected environments (though one mealybug from Northern Europe can be found outdoors on shrubs in favourable districts). They attack a wide range of houseplants and greenhouse ornamentals, plus vines. Cacti and succulents, African violets, ferns and orchids are particularly vulnerable.

Root mealybugs (rhizoecus) build up into dense colonies on the roots of greenhouse ornamentals. They are 1-2.5mm long, greenish-yellow and surrounded by white wax. They tend to be most troublesome when the potting compost is kept on the dry side, so vulnerable plants include cacti and succulents as well as abutilon, African violet, begonia, ficus, hoya, hippeastrum, dracaena, ferns, fuchsia, palms and pelargonium.

Phormium mealybugs (Trionymus diminutus) attack phormiums and cordylines. They are 4-5mm long, grey or dark red, with a white wax covering. Unlike other mealybugs, they can survive low winter temperatures.

Q Tell me more about mealybugs.

A In warm greenhouses, or in the house, mealybugs can feed and reproduce all year round. Females lay batches of 100-150 eggs, protected by wax. These hatch out in a few weeks and the nymphs crawl around on the plant for a few hours before they settle to feed. Once they become adults they can still move but rarely do. Colonies of mealybugs tend to congregate in leaf axils, at the base of cactus spines or underneath leaves.

Q What damage do mealybugs do?

A Glasshouse mealybugs suck the sap of their host plants which weakens and can kill them. The colonies of mealybug with their associated wax, honeydew and sooty mould can be very disfiguring to specimen plants.

Root mealybugs prevent the roots from operating properly and the plants may appear stunted, or wilt, especially in pots. Damage is usually worse if the plants are in dry compost or soil. With phormiums, heavy mealybug infestations will weaken the plant and eventually kill it.

Q When do mealybugs attack?

A Mealybug infestations can appear at any time, though they are likely to be worse in late summer and autumn.

Q Could I mistake mealybugs for anything else?

A An infestation of mealybug is often mistaken for a fungus, but if you squash a colony, it will be wet and sticky. Woolly aphids, which also hide under fluffy white protective wax, look similar but attack a different range of plants. They are hardy and most commonly found outdoors on apples, pyracantha or cotoneaster.

Q How do I prevent mealybug infestations?

A Check new plants thoroughly before bringing them home. If you have a precious collection of, say, cacti, African violets or orchids, it is worth quarantining new plants for a month or so before mixing them with the others. Stripping old bark off indoor vines in winter helps to dislodge incipient mealybug colonies. Keeping the compost moist will deter root mealybug, though unfortunately this will not suit some of the most vulnerable plants.

Q How do I treat mealybugs?

A Eradicating mealybug requires persistence and it may be better to discard badly infested plants. Quarantine any pot-grown plants you are going to treat and put them outdoors if it is mild enough. In fact, leaving plants outdoors over winter is a good way to destroy colonies on plants that are hardy enough, such as camellias. Scrape, hose or brush off as many adults as you can, or cut out badly affected shoots.

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.

Organic treatments for use during the growing season include fatty acids or plant oils or extracts These organic pesticides have a contact action and short persistence and so may require more frequent use. They can be used on edible plants if the plants are washed thoroughly before consumption.

Caption: Mealybugs often hide in nooks and crannies on plants

Q What about biological control for mealybugs?

A If mealybug gets established in a heated greenhouse or conservatory, biological control using Cryptolaemus montrouzieri beetles or Leptomastix dactylopii, a parasitic wasp, should do the trick. Cryptolaemus is a black and orange ladybird from Australia. Its larva looks rather similar to a mealybug but is twice as big. An adult will consume about three mealybugs a day, and a larva nine. They only search a small area so you need to release at least one on each plant.

Leptomastix dactylopii is a wasp about 3mm long that kills mealybugs by laying their eggs in them. They are much better at finding small colonies than cryptolaemus but only attack citrus mealybugs.

Screen opening doors, windows and vents with fine netting so the adults won’t escape. Both controls thrive only in the summer as they need good light and temperatures above 21ºC.