It seems that whitefly really loves tomato plants. Ever since I planted my first tomato seeds they seem to come at regular intervals. Initially I treated them with a garlic and pyrethrin based pesticide; that generally kept them away for some weeks.
More recently I had a very heavy infestation on mature tomato plants, and the garlic based pesticide appeared to be ineffectual. I’m not sure whether the whitefly got used to the taste of garlic, the pesticide had expired or if I just wasn’t covering the rather unruly plants well enough, but it didn’t seem to help.
An attempt at regaining control
So, I looked for some alternatives. I got sick of smelling garlic all the time, and tried to use a soap spray. One teaspoon of Sunlight liquid dish-washing soap per litre of water was recommended to me. I applied it once, and it seemed to kill some whitefly, but it didn’t really seem to get rid of them.
Apparently the soap has no residual action after the spray dries, so that might have been part of the problem. The other part of the problem was the pathetic hand-sprayer I was using. Apart from only holding a paltry 500 ml of liquid, it doesn’t really work when upside down; makes covering the bottom of leaves quite hard.
The final solution
I did some more research, and decided to A) buy a decent pressurised pesticide sprayer B) make sticky whitefly traps. Apparently they are attracted by the colour yellow. Some site I read claimed that they look for the “healthiest” leaves, and somehow yellow indicates that to them; makes no sense to me. In any case, the traps definitely work.
After bringing the whitefly infestation down to manageable proportions by consecutive daily soap sprayings (the pressurised sprayer made that a joy), I mounted my whitefly traps. Within minutes of mounting a couple around my tomato plants (and giving them a good shake to bother the whitefly) they were covered by a large number of soon-to-be whitefly corpses.
A week later there are virtually no whitefly left on my tomatoes! As an aside, the soap solution seems to work even better on red spider mite than on the whitefly. It seems to finally have rid my black eyed Susans of spider mite.
Building the whitefly traps
Combining ideas from the two links above, I decided to cover yellow poster board with plastic, and coating that with a thin layer of oil. Most people seem to use motor oil for some reason, but cooking oil seems to work just as well and is non-toxic to boot! I somehow got confused when shopping, and bought cling-wrap rather than plastic bags. I might have been reading about Roy Orbison that day.
To make the traps, I cut the yellow poster board into roughly 20x15cm pieces and then wrapped them in cling-wrap. I also punched some holes for strings to hang them from using a paper punch. I tied some rope through the holes, and sealed them using prestik. Just to keep it water tight. Finally I applied a light coating of cooking oil using a very advanced brush system. Ok, so I used toilet paper. Below you can seem some pictures of the traps in action.
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