Many gardeners are fighting a losing battle with whiteflies, which not only suck sap from plants but also transmit viruses. My peppers and tomatoes are under constant attack. The unseasonably warm weather has encouraged these pests, which are resistant to most eradication efforts. Soap & oil spray only kills the flies it directly hits. Insecticides kill the natural predators of whitefly. The best thing to do is to check your plants frequently and remove infested leaves. It’s much easier to deal with an incipient infestation than a full-blown one.