I hear a lot about white flies, aphids and spider mites. What attracts them? What are the signs to look for? And how do I solve the problem?
You have named three of the most prolific and perennial pests of greenhouses and production areas. All of the ones you have named feed on the sap of leaves, so they are all considered “sucking” insects as opposed to “chewing” insects, which have movable jaws. If it looks like your leaves are being chewed on, the pests you are asking about are not the culprits. If you notice speckled, mottled, curled or puckered leaves and fine webbing, then you likely have one of these pest infestations.
Whiteflies are most often, as their name implies, white in appearance and about an eighth of an inch long. They are found, like aphids and mites, on the undersides of leaves. With heavy infestations, you can gently shake the plant and be greeted with a cloud of whiteflies. Whiteflies love Solanaceous plants such as tomatoes, eggplants and peppers, and are a common pest of tropical plants as well. The key to controlling whiteflies is in the frequency of your chosen remedy, not so much your choice of application. There is no pesticide that I am aware of that will kill all of the life stages of whiteflies, meaning repeated applications will be necessary since you will not kill their eggs even if you kill all of the adults.
Aphids are the largest of the three pests you mentioned and are the easiest to spot. They feed on almost any kind of plant. Their damage is usually found on new growth, and their feeding distorts leaves, causing them to pucker or curl. Aphids are fairly easy to control, but thorough applications must be applied. These little suckers are born pregnant, so if you miss a few, a few days later you will have hundreds or thousands show up again. Aphids can be controlled by a strong stream of water, insecticidal soaps, any insecticide containing pyrethrins, or by releasing predatory lady bugs.
Spider mites, which are not actually insects, cannot be controlled with insecticides. They are arachnids, and their biology is completely different than aphids or whiteflies. Spider mites tend to favor outdoor evergreens, but if you find them indoors, they will feed on almost anything. They are best controlled with suffocants, such as horticultural oils. If you catch them early enough, hosing your plants down may suffice, but mites multiply quickly and can cause a lot of damage in short order. If you see a small spider where you suspect spider mites, leave it alone. It is probably eating some of the mites that are too small to be seen without the aid of magnification. To see if you have mites, take a white sheet of paper and tap the suspicious-looking leaves or needles over top of it. The sight of tiny, moving specks will validate your suspicions.