Sap-sucking insects
The problem Plants are weakened, foliage is distorted and bleached, and covered in a sticky honeydew on which thick, black sooty mould thrives. Virus diseases are spread by the mouth parts of pests, which often cluster around the growing point of the plants. The culprits Aphids (greenfly and blackfly), whitefly (those small white creatures that look like miniature Concorde aircraft on the undersides of the leaves) and red spider mites (pinprick-sized mites of yellow, green or red) are all to blame, as are mealy bugs, which surround themselves with white wool on greenhouse plants, and scale insects, the pests that look like miniature limpets or legless tortoises. The solutions Red spider mites can be discouraged by keeping a moist atmosphere in the greenhouse (they prefer warm, dry conditions) and all greenhouse pests can be kept in check by buying predatory insects, which vary depending on the pest (a chalcid wasp for whitefly, a predatory mite for red spider). Use sticky plastic traps to catch whitefly. Dab scale insects and mealy bugs with a tiny paintbrush dipped in methylated spirits or whisky. For greenfly and blackfly, use your fingers to rub them off, or spray them with very mild soapy water, or – on sturdy plants such as roses – squirt them with a powerful jet from a hose. Birds, wasps and ladybirds will do their bit, too.