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The warm temperatures of the
summer bring on a rush of new foliage growth, attracting a wide
variety of pests. Whitefly, one of the most difficult pests to
control, pose a special challenge to gardeners. Whitefly numbers
grow dramatically in the heat, most strains are resistant to
pesticides, and the pests infect a huge range of hosts including
bedding plants, strawberries, tomatoes, and poinsettias.
Just as the name implies, whitefly
are small, fly-like insects with white coloured wings. They hide
on the underside of leaves where they multiply rapidly. Whitefly
feed on plant juices and, in large numbers, can consume a considerable
amount of nutrients, causing plants to pale in colour. Like aphids,
they also excrete honeydew, attracting black sooty mold fungus.
Recently, these pests have been found to spread viruses.
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Identification
Although all whitefly tend to
look alike, there are two major types - greenhouse whitefly and
silverleaf whitefly (formerly sweetpotato whitefly). The greenhouse
whitefly has been recorded on over 250 plants while the silverleaf
whitefly attacks over 500 hosts. Adult greenhouse whitefly are
slightly less than 1/8 inch long. They have a white, waxy coating
and hold their wings parallel to the leaf surface. Adult silverleaf
whitefly are slightly smaller than the greenhouse whitefly and
have a yellowish hue. They hold their narrow wings at a 45-degree
angle to the leaf surface. |
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Although not as prevalent, other
whitefly types include the bandedwing whitefly; the spiraling
whitefly which attacks star apple and ornamental banana; the
cloudywinged whitefly which can infest citrus, gardenia and Ficus;
and the citrus whitefly which occurs on citrus and gardenia.
Life Cycle
Whitefly eggs are spindle-shaped
and stand vertically on the leaf surface. For many species, eggs
are white when first laid and turn dark grey (greenhouse whitefly)
or amber-brown (silverleaf whitefly) with time. Eggs are deposited
on the underside of leaves, sometimes in a circle or crescent-shaped
patterns. After hatching, the "crawler" stage moves
about on the plant looking for a feeding site.
At temperatures between 65 and
75F, it takes less than 40 days to develop from egg to adult.
Mating is not necessary for egg production and a female may lay
up to 200 eggs.
Control Techniques
Thankfully, there are a number
cultural techniques and organic sprays to help keep whitefly
from over-running your plants. A combination of these control
techniques is the best method to keeping whitefly numbers down.
Cultural Practices
When purchasing new plants, inspect
both the upper & lower surfaces. Preventing whitefly infection
this way saves headaches in the long term. Floating row cover,
placed over plants, will also prevent them from becoming infected.
If you do find whitefly, don't
panic! A cylinder vacuum can bring numbers down considerably.
Since the adult moths can fly easily from one plant to another,
do not place infested plant material next to clean plants.
Sticky traps
Whitefly are attracted to the
yellow colour of sticky traps. For best results, hang or place
the cards close to the top of the plant. Doors, vents and other
openings where whiteflies can enter greenhouses are other good
sites to hang yellow sticky cards.
Sprays
Whitefly began showing resistance
to synthetic insecticides in the 1980s. Sprays containing pyrethrum
or neem give the best control for organic growers. Both the surface
& the underside of the leaf need to be covered and during
the summer, a spray regime every 5 days is required.
Biological Control Agents
Encarsia formosa, a small wasp,
parasitizes to up to 100 immature greenhouse whitefly each. Encarsia
populations prefer temperatures above 70 degrees F for development.
The best time to use Encarsia is between mid-March and mid-September.
Delphastus pusillus, a small
black ladybird beetle, will control both greenhouse and silverleaf
whitefly. The larvae will consume up to 1000 whitefly eggs in
its lifetime but will also feed on nymphs. Like Encarsia, the
beetles perform best at temperatures between above 65F.
For more information on whitefly,
look to the following website: http://www.tvorganics.com
Terra Viva Organics - supply of yellow sticky traps |
