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A widespread pest for gardeners but, according to agricultural scientists, of little significance in commercial brassica crops. As a consequence, little research is currently being done on controlling this 'garden' pest.
The cabbage whitefly, Aleyrodes brassier is common on many brassica such as Brussels sprouts, broccoli, kale and cabbage.
The glasshouse whitefly, Trialeurodes vaporarioum differs from its close relative in being corniced to the indoors or Typical symptoms
Whitefly can be found at most times of the year. Small white winged insects live on the underside of leaves of brassica plants, and fly up in clouds when disturbed. Adult whiteflies are structurally similar in appearance to aphids, but they are covered in mealy grey hairs and have white wings. The young whitefly, known as 'scales', remain on the leaves. The flies themselves do not often cause severe damage, but the sticky honeydew or sugary excretions they produce can disfigure the plants when an attack is severe. This is not so much to do with the honeydew itself but the sooty or black moulds which grow on the honeydew. The sooty moulds will spoil flower buds, e.g. on Brussels sprouts, and will prevent leaves from photo synthesizing, or in other words feeding the plant. Kale leaves can look particularly unappesiting covered in mould!
The entire life cycle of the whitefly can take place on one single brassica plant. This in itself is a due to its control and why they are such a successful garden pest. Having over wintered on brassica, the adults move to further brassica in the spring and lay eggs in semi-circular groups on the under surface of leaves from mid May onwards. The tiny nymphs which emerge are mobile for a short while, but then settle down to feed. They insert their needle-like mouthparts into the plant tissues and suck the juices. The nymphs become immobile and develop a scale-like protective covering, which darkens with age. The nymph inside this 'scale' goes through four growth stages or instars. In the fourth and final instar, the nymph will pupate (during this stage it is making a physical change from a crawling insect into an adult capable of flight), and the scale becomes much thicker and darker. From this final instar will emerge an adult whitefly. The life-cycle from egg to adult lasts about four weeks in the summer, and will continue through to the autumn. when both eggs and adults will overwinter on the brassica plants.
Prevention and control
A healthy soil: the first line of attack is to create a healthy soil, which will product strong vigorous plant growth. Plants are then able to withstand much greater pest attack.
Sensible feeding :feeding excessively high levels of nitrogen to the soil, be it organic or otherwise, will favour the pest.
Encourage a natural balance: help nature to help you by planting simple attractants flowers. Flowering umbellifers such as fennel, carrot cow parsley and lovage will attract parasitic wasps such as Apheinus. These insects lay eggs inside the whitefly and consume it from the inside out. In this way the lifecycle of the whitefly is broken. Phacelia tanacetifolia is also an attractant for ApheLnus. Lacewings, predator which feed on whitefly, are attracted by members of the Compositae family e.g dandelions, yarrow and shasta daisy.
Do not panic: assuming your plant is in a healthy soil, this particular pest can be tolerated to quite high populations anyway.
Break the cycle: dig up winter brassicas as soon as cropping is finished and bury the plants in a trench or in the compost heap. Do this before planting out new brassicas.
Hand picking: whitefly eggs are laid on lower leaves. Remove infected leaves before the immobile young whitefly 'scales' turn into adults. A battery powered -hoover can be used to suck up the adults.
Disturbance: walking through your crop regularly will disturb the adults ant disrupt their feeding patterns.
Hosing town: a good jet of water can wash off honeydew and sooty mould.
Spray - a last resort: sprays, even those approved for use in an organic garden, can kill both the pest you are trying to control and beneficial natural predators, so think twice before using them. Insecticidal soap: the spray must hit the pests directly to be effective so it is best applies when temperatures are low (e.g. early morning when the adults are not so active. Spray under the leaves, using a good quality sprayer. Spraying once a week for 3-4 weeks may be necessary to see a significant effect. Soft soap is a useful wetting agent which also has some whitefly control side effects. 220g (8 oz.) of soft soap in 2251 (5gal) of rain (soft) water. Note: Be careful if you decide to use soapy dishwater. Washing up liquids are not designed for pest control; some can cause severe damage to plants.
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