Glasshouse whitefly Trialeurodes vaporariorum

This small insect, looking like a tiny moth, was originally introduced from the tropics, but now causes serious problems on a range of glasshouse food and flower crops. It should not be confused with the very similar, but slightly larger cabbage whitefly on brassicas.

Damage. All stages after the egg have sucking stylets, which extract a sugary liquid from the phloem, often causing large amounts of honeydew and sooty moulds on the leaf surface. Plants that are seriously attacked include fuchsias, cucumbers, chrysanthemums and pelargoniums. Chickweed, a common greenhouse weed, may harbour the pest over winter in all stages of the pest's life cycle.

Life cycle. The adult glasshouse whitefly (Figure 14.10) is about 1 mm long and is able to fly from plant to plant. The fertilized female lays about 200 minute, white, elongated oval eggs in a circular pattern on the lower leaf surface over a period of several weeks. After turning black, the eggs hatch to produce nymphs (crawlers), which soon become flat immobile scales. The last scale instar is thick-walled and called a ' pupa' from which the male or female adult emerges. Three days later, the female starts to lay eggs again. The whole life cycle takes about 32 days in spring, and about 23 days in the summer.

Spread is mainly by introduced plants, or more rarely by chance arrivals of adult through doors or vents.

Control of glasshouse whitefly is achieved in several ways.

Amateur and professional horticulturalists should remove weeds (such as chickweed or sowthistle) harbouring the pest from crop to crop. Careful inspection of the lower leaves of introduced plants achieves a similar aim.

There is a reliable form of biological control. This involves a minute wasp (Encarsia formosa) which lays an egg inside the last scale stage of the white-fly. The developing whitefly is eaten away by the wasp grub and the scale turns black and soon releases the next generation of wasps (see Chapter 16 for more details). Control is usually most effective when whitefly numbers are low.

Amateur gardeners can use spray products containing specially formulated fatty acids to control young and adult pest stages. Professional horticulturists can choose between an insecticide that physically blocks the insect's breathing holes (see p204) such as alginate/polysaccharide, or a contact insecticide such as deltamethrin, or the above-mentioned fatty acids.

It is suggested that serious infestations of this pest receive a regular weekly chemical spray to catch the more sensitive scale and adult stages.

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