Red mites on tomato plants may not be noticeable at first, because they are so tiny. If a you look closely, you will see minuscule red dots running around on the leaves, and sometimes on the soil. A magnifying glass or pair of reading glasses can help if you suspect red mite activity but do not see the mites right away.
Red mites are one of the many species of spider mites that may infest tomato plants. Red mites look like tiny little red specks with eight legs. They build tiny, thin, silky webs, much like spiders do. You can find them on the undersides of leaves, not the top. While they are incredibly tiny, red mites are advantageous to gardeners over green mites for a single reason--they are much easier to see.
Unlike spiders, spider mites do not eat insects. Instead, they suck the chlorophyll (green juicy matter) out of the individual cells of plant leaves. Each feeding leaves a tiny yellow/bronze spot on a leaf, where the juices have been sucked out. As more feeding occurs, these spots grow larger. Eventually, if an infestation is severe enough, leaves fall off and die. Red mites generally start from the bottom leaves and work their way up a tomato plant, unless they are stopped.
Spider mite life cycles are short. A single life cycle can be completed in a week. Because their individual damage is so small, gardeners may not notice any problems until they become large. By that point, red spider mites in all stages of life may be present all over a single tomato plant, or multiple plants if they are planted close together.
Red mites, unfortunately, withstand many pesticides. Applications of insecticidal soaps and oils can kill them by coating their bodies in soap or oil, asphyxiating them. This solution only works if the soap or oil is applied consistently, and directly to spider mites when you see them. Both types of insecticide lose their effectiveness when they dry. The organic integrated pest management approach to insect control often uses predatory mites. These mites do what their name implies, and hunt down other insects, including red spider mites. They do not cause harm to tomato plants.
If you see minor red spider mite damage, you may think the problem is nothing to be concerned about. If the population stays small, you will be right. But because their life cycles are so short, a red spider mite population may increase exponentially in a short span of time, causing considerable tomato plant damage and/or death. Take steps to remedy a red spider mite problem as soon as you notice it.