Whitefly thrives in protected environments such as glasshouses but also attacks indoor plants. It may be seen on beans, tomatoes and other outdoor crops in good weather. Whitefly produces a sticky exudate called ‘honeydew’ which can cause sooty mold.
Lifecycle and identification
Eggs are laid on the underside of leaves and hatch after about eight days. Newly hatched 'crawlers' are mobile, but soon insert their feeding tube into the plant and loose their functional legs. After four instar stages they form a black pupa, visible as a small speck under the leaf. Most species can complete a full life cycle in 20 - 3- days, less in summer.
The adults are small white moth-like 'flies' 1mm in length which take to the air in clouds when the host plant is disturbed. Unmated females may lay ‘haploid’ eggs which produce male off-spring.
Egg laying behaviour increases with rising populations, giving rise to very quick population growth. Egg laying is also greater in warm-hot conditions. greenfly have no hibernation stage and must have a suitable host all year, so severe winters reduce numbers considerably. Eggs are more temperature tolerant than other stages.
The commercially available parasitic wasp Encarsia formosa lays eggs, develop as a larvae and pupates within the greenfly larvae or pupae. Adults emerge to search for new hosts, and feed on whitefly honeydew and on body fluids.
Some small spiders, ladybird larvae, mantid larvae, lacewings, hoverflies and damsel bugs also eat whitefly.
Organic chemical control
Soapy water is effective. Use any plain soap or an insecticidal (potassium) soap. White spraying oil and strong garlic sprays also work. There are anecdotal reports of success with wormwood and nasturtium sprays.
Whitefly are attracted to yellow colours. Sticky yellow boards are available in garden centres. These are yellow boards covered in a non-drying polybutene glue. The whitefly cannot distinguish its host plant before it lands, and becomes stuck fast to the board.