How to recognise it
Adults are white and look like a tiny moth. Adults, eggs and scales can be found on the underside of leaves. The adult is about 1mm long and winged, enabling it to fly from plant to plant. It is smaller than the cabbage whitefly.
Foliage can appear pale and curled, sometimes with yellow spots or other discolouration.
The best way to identify whether there is an infestation is to hang yellow fly catcher cards in the glasshouse above the plants to monitor insect levels.
The glasshouse white fly undergoes an ‘incomplete’ lifecycle; putting it in the ‘Endopterygeta’ group of insects. Each female lays about 200 white, elongated oval eggs in a circular pattern on the lower leaf surface over a period of several weeks. The eggs turn black then hatch about 10 days later to produce nymphs (crawlers) which soon settle to feed. Their legs and antennae degenerate and they become flat, immobile scales. The nymphs feed for about 2 weeks then stop feeding and enter into a pupal stage from which the adults emerge. Within three days the females are ready to start laying eggs again. The whole lifecycle takes about 32 days in spring and only 23 in summer.
This asexual breeding continues throughout the year in glasshouses. Outdoors the glasshouse whiteflies are killed off by severe winter weather, although some may survive by hibernating through mild winters.
Why it’s a problem
At all stages of its lifecycle (other than as eggs) the glasshouse whitefly has a stylet which it uses to extract sugars from the phloem, often depositing large amounts of honeydew on the leaf surface which can attract sooty moulds.
Persistent infestations can reduce plant vigour.
Where you are likely to find it
In glasshouses, on both food and flower crops. It particularly likes fuchsias, cucumbers, tomatoes, chrysanthemums and pelargoniums. Chickweed (a weed often found in glasshouses) can harbour the pest over winter.
It is mainly introduced on other plants, although occasionally adults can simply fly in through open doors and vents.
It can be found on outdoor crops in a particularly warm season.
How to deter it
Remove weeds (especially chickweed and sowthistle) which can harbour the pest.
Carefully inspect the lower leaves of new arrivals to ensure they are not carrying nymphs, scales or adults.
How to get rid of it
Biological control with Encarsia formosa or Eretmocerus eremicus (parasitic wasps) can be effective, usually used between March and October. The wasp lays an egg in the last scale stage of the glasshouse whitefly. Once hatched, the wasp grub then eats its way out of the developing whitefly – the scale turns black and releases this next generation of the wasp. Usually most effective for less severe infestations. These are sometimes found to be already established in glasshouses where pesticides have not been used.
Additional biological controls are also being developed including the small black ladybird Delphastus sp. and the small green predatory bug Macrolophus sp.
Insecticides containing fatty acids can be useful in controlling young and adult whiteflies. Additional insecticides such as alginate/polysaccharide or deltamethrin are also available to professional horticulturalists. Weekly spraying is recommended to catch all the pests in their more susceptible scale and adult stages.
If found outside spraying with pyrethrum (or a synthetic alternative such as bifenthrin) should remove the problem. Several other chemical options are also available including plant oils (which should not be used on fuchsias or begonias), plant extracts, acetamiprid, imidacloprid, thiaclopird or thiamethoxam. However, populations are becoming resistant to many insecticides, particularly in southern England.
The insects can be manually removed from glasshouses using battery operated vacuum cleaners which suck up the adult whiteflies.
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