I had a really bad infestation of whiteflies a couple of years ago and used neem oil. It was fairly effective if you sprayed every 3 to 4 days but after a while became less effective. I ended up rotating different sprays and had more success. I think the one thing that every successful whitefly control has in common is frequent spraying. I'm sure there are chemicals strong enough to kill them outright; but I would be very hesitant to use them on my tomatoes.
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