lonies 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.