Farmer's Weekly (South Africa)
Controlling whitefly
Parasitoids and other biological control agents are the most effective and sustainable way to manage whitefly, says Bill Kerr.
There are two major species of whitefly in South Africa: the sweet potato whitefly ( Bemisia tabaci) and the greenhouse whitefly ( Trialeurodes vaporariorum).
Whitefly damages plants by feeding on their sap and excreting honeydew, as aphids do. This covers the leaves, encouraging the growth of sooty mould, a black fungus that inhibits photosynthesis. Honeydew also attracts ants, which feed on it and reward the whitefly by keeping predators away.
PLANTS AND PARASITOIDS
Both species of whitefly infect a wide range of crops, weeds and flowers;
B. tabaci is known to attack more than 1 000 species of plants and
T. vaporariarum more than 700 species.
The good news is that because they target so many plants, they also have a wide range of natural enemies. This means that when these pests enter a crop, the predators and parasitoids soon follow. Were it not for these natural enemies, whitefly would wipe out huge swathes of vegetation.
A few years ago I visited a couple who had a shrub in front of their veranda. The wife asked me to identify the “white things” flying around the plant and suggest what to spray. I turned a leaf over and exposed whitefly pupae, many of which were already darkened. I told her not to spray, as the pest had already been brought under control by a parasitoid. On my next visit, there was no sign of whitefly at all.
BLACK VS GREEN LARVAE
The main parasitoid used for whitefly control is Encarsia formosa, a minute black wasp with a yellow abdomen. It punctures whitefly pupae to feed on the juices and lays eggs in other pupae, which subsequently turn black from the developing larvae. Unaffected pupae remain green in colour, making it easy to determine if the parasitoid is at work in the crop. (The parasitoids themselves are difficult to see with the naked eye.)
Between feeding on the whitefly pupae and laying eggs, E. formosa can drastically reduce the population of this pest.
CURLY STUNT VIRUS VECTOR
In certain areas, whitefly is a major problem as it is the vector of curly stunt virus. This is a devastating disease and no whitefly should be allowed on the crop at all. If you farm in an area free of the virus, you can safely allow some whitefly on the plants while inspecting for signs of E. formosa activity.
If necessary, you can spray a soft pesticide until E. formosa starts to manifest. I usually find a few whitefly on my crops but never have to spray for it.
I don’t have E. formosa in my tunnels at present, as the predators and parasitoids that control the tomato leaf miner ( Tuta absoluta) are controlling the whitefly as well.
PLAY IT SAFE!
In areas overrun by T. absoluta, many farmers spray a variety of chemicals in an endeavour to get the pest under control. By doing so, they kill off the natural enemies of whitefly; at the same time, exposure to these chemicals gives the whitefly the opportunity to build up resistance to these products! It’s a vicious circle that’s expensive and ultimately ineffective.
The only really sustainable solution is to work with nature; use safe insecticides and harness the power of the pest’s natural enemies. You can even introduce them artificially;
E. formosa is commercially available from biological companies.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables. Email him at farmersweekly@caxton.co.za. Subject line: Vegetable production.
WHITEFLY HAS MANY NATURAL ENEMIES; IF LEFT ALONE, THESE PREDATORS WILL CONTROL THE PEST