The Stuttgart giant beans are growing wonderfully up the trellis on the fence. But the white flies have come out in force, sucking on the underleaf, resulting in damage to the leaf such as yellow mottling, and some will wilt and drop off. They lay eggs on the underside of leaves which hatch in 8 days. The whitefly population suck the sap, and also excrete 'honeydew' which can cause problems with black sooty mould. They can also carry other diseases from plant to plant. Eventually they form a black pupa that looks like a small speck under the leaves. The white fly rapidly multiply - most species can complete a full life cycle in 20-30
days, and even faster in summer. Each adult female may lay 200 eggs, which only increases in warm weather.So, it's not a pest that can co-exist happily in garden taking only its fair share. It's a pest that needs to be eliminated early in the season.
There are some organic solutions:
The parasitic wasp Encarsia formosa is a biological control. Probably not so effective outside but can be obtained commercially in SA through Biological Services, PO Box 501, Loxton, SA 5333 Ph 08 8584 6977. It kills whitefly nymphs in one of two ways: they either lay an egg inside the nymph, providing food for their young, or they kill the nymph right away and feed on it. Once the whitefly nymphs are parasitised they turn black and no longer feed.
Other predators often present in the garden are spiders, ladybird larvae, lacewings, hoverflies and damsel bugs. Some of these are also available from commercial distributors.
There's also the good old sticky yellow strips. They can simply be hung near the beans (or other plants vulnerable to white flies), or I've found it effective to go out in the morning when the white flies are less active and manually place the sticky yellow strip on the underside of each leaf so the white flies get stuck on the yellow strip. As the white flies travel in 'packs', it's easy to get a whole lot in one morning swoop. Kind of satisfying in its own way! Whiteflies are strongly attracted to the colour
yellow, so don't wear yellow clothing around whiteflies or you may carry them from plant to
plant.I've also read that vacuuming in the early morning when whiteflies are cold and slow-moving with a small hand-held battery operated vacuum is useful for the adults before a great deal of egg-laying is done. After vacuuming enclose the section of the machine containing the bag with plastic and put in the freezer for 24 hours. I might investigate a dust buster or similar to have on hand in the garden!
Good old companion plants can also be effective. Nasturtium in addition to lovely flowers and leaves that can be used in salads in summer can also help to repel the pest.
Fertilizers can also be an issue. Fertilizers rich in nitrogen including manures, will cause lovely green growth which will attract more whiteflies. Check the phosphorus and magnesium levels in your soil, as deficiencies in these are believed to contribute to whitefly infestations.
You can also try a high pressure hosing under the leaves in the early morning, 3 days in a row, but in my experience they simply fly away and happily land elsewhere until its safe to return.
Some suggest that whitefly is a modern pest, created by the over-use of pesticides that have killed off its natural enemies. One study has even demonstrated an increased whitefly reproductive capacity when sprayed with certain insecticides! So avoid synthetic insecticides.
Here's some ideas for more organic approaches:
- Insecticidal soap sprays such as Natrasoap are a good choice of control for the home gardener; spray every 2-3 days for 2 weeks.
- Spray Eco-Oil or try making your own oil spray by mixing 1 tablespoon dishwashing liquid detergent with 1 cup of cooking oil; add 1 to 2.5 teaspoons of this solution to 1 cup of water, spray onto plants every 10 days.
- Botanical insecticides such as pyrethrum plus garlic are useful. Research undertaken in NZ on neem’s effectiveness for whitefly found that it had a major impact by preventing the 'nymph' stage from developing into an adult; the nymphs tend to disappear from the treated plants.
