It seems like Knysna’s gardens have been invaded by Whitefly, the suburbs most effected are those that were burnt last year. Could the reason be that all the natural predators were destroyed by the fire or are the predators killed of by the use of insecticides, herbicides and fungicides? Host plants include more than 250 ornamental and vegetable plants. Citrus, squash, pointsettia, potato, cucumber, grape, tomato and hibiscus are commonly infested.
Also common on indoor plants, tomatoes and in greenhouses, the whitefly is a sap-sucking insect that is often found in thick crowds on the undersides of leaves. When infested plants are disturbed, great clouds of the winged adults fly into the air.
Description: Very small wingspan 2 – 4mm, body and wings covered in fine wax. Very few veins, a few species with thin red-banded wings. Mature nymphs resemble scale insects and cannot move. There are 16 species known from the region.
Biology: unusual among bugs in having a resting stage when they lose their legs, but remain feeding. Produce large amounts of honeydew. Generally live in groups on the underside of leaves, often in very large numbers.
The Citrus Blackfly (Aleurocanthus woglumi) is an alien species lacking white wax deposits. Tobacco Whitefly (Bemisia tabaci) and Greenhouse Whitefly (Trialeurodes vaporatium) are agricultural pests, and attack a wide range of plants, causing them to dehydrate. They transmit viruses that cause yellowed leaves and stunted growth.
Exert from article by Bill Kerr, vegetable specialist and a breeder of a range of vegetables.
Like aphids, whitefly nymphs excrete a sticky, sweet honeydew. Ants collect this, which protects the nymphs from natural enemies, keeping their food sources secure.
Honeydew is also a good medium for sooty mould to develop. In this case, as the name implies, black soot-like mould covers the leaves, preventing them from using sunlight.
The honeydew drops from the feeding nymphs affect the leaves the flies feed on. It is easy to imagine how the large amounts of sap consumed can hamper plant growth.
Worse still, more harm is often done to the crop by the diseases whiteflies transmit than by the physical damage they inflict. For example, the relatively newly introduced curly stunt virus, found in tomatoes in our warmer areas, is transmitted by whiteflies. The disease can cause close to 100% crop loss.
The wide range of host plants means whiteflies can potentially be everywhere, so conserving their enemies is all the more important.
I receive many emails from desperate growers wanting to know what to spray against this insect. While some new pesticides have proven effective, they are not always registered for the crop being grown. Then there are the organic growers who are not permitted to spray chemical pesticides.
If it were not for the natural enemies of this pest, whiteflies would probably be our main vegetable pest. An outbreak of whitefly is usually the result of using crop chemicals harmful to its natural predators.
Chemical Control: Initially, insecticides were used successfully for control, but the whitefly’s short life cycle means that resistance builds up quickly and new chemicals are constantly needed to control the pest.
Natural enemies include ladybirds, lacewings, damsel bugs, also other pests including aphids, mealybugs, leafhopper larvae, spider mites, moth , scales and thrips.
Organic Neem Oil can be sprayed on vegetables, fruit trees and flowers to kill eggs, larvae and adults. Mix 28gr/3,7 lt of water and spray all leaf surfaces (including the undersides of leaves) until completely wet
Whitefly Insecticide Recipe
You can mix up a simple recipe for an insecticide spray for whiteflies right in your kitchen. First, make a base mixture with 1 tablespoon of regular dish soap (not one with added bleach or an antibacterial soap) and 1 cup of vegetable oil. Pour it into a bottle that can be sealed, and shake well to mix.
Note: This recipe must be diluted before using it on your plants.
Diluting the Recipe
Now that you have your base, you can mix 1 1/2 teaspoons of it into a cup of water. Mix this in a spray bottle, and shake it well. Oil and water do not mix, so it needs to be shaken often as you are using it.
Using the Spray
Once your spray is properly diluted and mixed, you can spray your affected plants. Be sure to spray the undersides of leaves, since this is where the whiteflies tend to be. This mixture kills adult whiteflies and smothers the larvae and eggs as long as they are covered in the spray, so be sure to apply it liberally. Reapply as needed after rain or watering your plants for the best results. Be sure to spray plants early in the morning or late in the day when temperatures are cooler, as the heat may cause an adverse reaction in your plant.
Another recipe, from The Old Farmer’s Almanac, recommends a mixture of five parts water, two parts rubbing alcohol and 1 tablespoon of dish soap. Mix well, and spray it on the leaves of affected plants. You might want to test this on one leaf first, in case the rubbing alcohol is damaging to the plant