Let's start with some basics: These small, sucking insects have developed a resistance to many synthetic pesticides. That's why I recommend alternating at least two spray products, synthetic or organic. It's advice I've given out for over 15 years, and I've never had anyone tell me it didn't work. The key to success is breaking the egg cycle with multiple applications of the controls. If you follow the protocol, you will get a handle on your whitefly problem.
In fact if you don't get control at this time of year, you could lose an entire tomato crop in a matter of days. And you could be fighting black sooty mold on ornamentals for the rest of the summer.
So, how do you know if you have whiteflies? If you rustle the plant in question, the tiny white flies flutter up and around, then settle back down in a matter of moments. When heavily infested plants are rustled, a great cloud of winged adults fly into the air. Both the nymphs and the adults are damaging the plants by sucking juices from new growth, resulting in stunted growth, yellowing leaves, reduced yields and, ultimately, black sooty mold. That's because whiteflies, like aphids, secrete honeydew which leads to the mold. And on some plants, especially vegetables, whiteflies are known to transmit several viruses.
So, let's control 'em, eh?!
If you are not opposed to using chemical controls, one regimen I've recommended for over 15 years is alternating malathion and any other synthetic pyrethroid, such as bifenthrin, cypermethrin, resmethrin, or permethrin. Use two pump-up sprayers - one with malathion and one with the synthetic pyrethroid. Spray one of the controls, wait a few days, and spray the other. Then, do it again. And again. Try to get at least four sprayings over about two weeks, alternating between the two controls. That will break the egg cycle and kill any adults. Be careful, though ... malathion is not approved for some plants like hibiscus. Read the label to learn those on which it should not be used.
Now, what if you want to remain completely organic? I have good news and bad news. I've noted that whiteflies can develop a resistance to chemicals, and there are a few organics they'll thumb their noses at as well. However, you can use a great many liquid organics in the same alternating procedure. But it's been learned that it can take twice as long to control the egg cycle with organic controls. So, whether you use soapy water sprays, insecticidal soaps, spinosads or neem oils, you'll need to keep using them for as long as 3 or 4 weeks. And my advice when using organic liquids is to alternate between three of them, not just two.
Another great tool for whitefly control, whether you do it organically or synthetically, is adding "sticky traps." You can purchase them at garden centers and feed stores, but why not have some science class-type fun and make your own with some yellow plastic drink cups? (Flies and aphids are attracted yellow.) Pick up some sticky substances like Tangled Foot or Tangle Trap organic to spread on the outside of the cups. You can also use plain old Vaseline.
There are also some predatory insects you can purchase that work on whiteflies if they're detected early. Aggressive, non-stinging parasitic wasps are the most well-known. But once the populations are full-blown, such beneficials are a waste of money. Plus, while they may sound like the ultimate organic control, they are hard to come by. And in many cases, they're ridiculously expensive.