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Whitefly

Ravageurs et maladies
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Whitefly.
Photo: Jardin botanique de Montréal (APS Press)

Onglets

Description

Summary

Whiteflies are major greenhouse and houseplant pests. In North America, three tropical and sub-tropical species cause particular problems in greenhouses: greenhouse whitefly (Trialeurodes vaporariorum), sweetpotato whitefly (Bemisia tabaci) and silverleaf whitefly (Bemisia argentifolii=Bemisia tabaci biotype B). These tiny insects feed by sucking the sap from their hosts and produce a sweet, sticky substance (honeydew) on which a black fungus known as sooty mould develops. As they pierce and suck the sap from plants, the leaves become discoloured and limp, and growth is stunted. Whiteflies can also transmit viruses.

Signs and symptoms

  • Whiteflies form colonies on the underside of leaves. They fly away quickly when disturbed.
  • Small yellow dots or pale spots form on upper leaf surfaces. In serious cases, leaves may become completely discoloured, wilt and drop prematurely.
  • The honeydew produced by these insects may cover the leaves, stems and fruit. It encourages the formation of sooty mould, which interferes with photosynthesis and plant respiration.
  • Infested plants are weakened and stunted.
  • These insects may transmit viruses to plants.

Latin name (genus)

Trialeurodes vaporariorum, Bemisia tabaci, Bemisia argentifolii=Bemisia tabaci biotype B

Host plants

Whiteflies attack different greenhouse ornamental and food plants, as well as various indoor plants. They may also damage outdoor plants in summer, but do not survive over the winter.

Ornamental plants: angel's trumpet, aster, begonia, bougainvillea, chrysanthemum, coleus, flossflower, flowering maple, flowering tobacco, geranium, gerbera, heliotrope, hibiscus, hortensia, Jerusalem cherry, lady's eardrops, miniature rose, ornamental pepper, passion flower, plectranthus, poinsettia, pot marigold, primrose, rhododendron, shrub verbena, etc.

Fruit and vegetables: bean, broccoli, cabbage, cucumber, eggplant, grape, lettuce, melon, pepper, potato, pumpkin, squash, strawberry, tobacco, tomato, etc.

Development cycle

Description and life cycle

Whiteflies are not true flies. They belong to the order Hemiptera and the suborder Homoptera, along with aphids and scales. They undergo incomplete metamorphosis (hemimetabolic insects). After the eggs hatch, the larvae go through four stages before becoming adults. At the end of the fourth larval stage they form a false pupa.

Eggs: The tiny eggs (about 0.25 mm long) are oval or conical. They are light-coloured at first, but generally turn dark before hatching.

Larvae: The oval, flattened larvae resemble scales. They are usually yellowish or greenish and almost transparent. In greenhouse whiteflies, the false pupa is covered in waxy filaments. This characteristic is used in distinguishing different species.

Adults: These tiny insects (about 1 mm long) have yellowish bodies and two pairs of membranous wings with a whitish powdery wax coating. They have biting-sucking mouthparts.

Whiteflies reproduce both sexually and by parthenogenesis (without fertilization by a male). In the course of her life (about 1 to 2 months), a female may lay up to 400 eggs, which she attaches to the undersides of leaves. Greenhouse whiteflies often lay their eggs in well-defined circles or half-circles. Incubation lasts 5 to 10 days, depending on the species. The 1st instar larvae crawl for a short time before becoming immobile and starting to feed. The subsequent larval stages are immobile. At the end of the 4th stage, the larvae stop feeding and form a false pupa. Adults emerge about 1 week later, through a T-shaped opening.

The length of whiteflies' life cycle depends on the temperature – the hotter it is, the shorter their life cycle. Under normal conditions, these insects complete their life cycle in about 1 month. There are several generations per year.

Prevention and control

Favourable conditions

Whiteflies multiply more quickly and are more active when it is warm.

Identification

  • Inspect plant leaves regularly, especially the underside of leaves. Use a magnifying glass to detect egg clumps and larvae.
  • Install yellow sticky traps near vulnerable plants to attract adults. Such traps are available from garden centres, but you can make your own by brushing strips of yellow cardboard with a sticky substance like petroleum jelly. Remember to clean off or replace the traps periodically.
  • The presence of honeydew and sooty mould may also indicate a whitefly infestation.

Prevention

Because the main source of infestation is newly introduced plants, it is important to inspect all new plants and isolate infested ones.

Regularly disinfect tools with a 70% isopropyl (rubbing) alcohol solution.

Inspect all susceptible plants regularly in order to act promptly.

Physical control

  • Cut off and dispose of heavily infested parts.
  • Use yellow sticky traps to catch as many adults as possible.
  • Pick off flying adults with a small vacuum, early in the morning, when temperatures are cooler and the insects are less active. Afterward, seal the vacuum cleaner bag in a plastic bag and place it in the freezer for 24 hours, to kill the whiteflies inside.

Biological control

In greenhouses, introducing natural predators and parasites (Encarsia formosa, Eretmocerus eremicus, Delphastus pusillus, Dicyphus hesperus) gives good results. This control method is difficult to apply at home, however.

Chemical control

  • Whiteflies quickly develop resistance to pesticides.
  • As a last resort, use a low-impact pesticide with insecticidal soap as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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