Controlling two-spotted spider mites by introducing predatory mites

Question: We always have spider mites on our flowering garden phlox and a few other perennials in the summer. The leaves look speckled with tiny pin-pricks of white, and we can see their webs on the undersides of the leaves. We’ve read that spraying can lead to other problems (and we certainly don’t want to spray around the bees and butterflies!). Is there a beneficial insect we can release to help control the spider mites?

Answer: Two-spotted spider mites are common pests of the garden, though typically their numbers don’t grow large enough to cause significant damage to plants. However, if you’re seeing their fine webbing on leaf undersides and noticing the discoloration of leaves, you’re correct to look into resolving the problem.

Spider mites have many natural predators, which is typically how they’re kept under control. But when conditions are right, or when there aren’t enough natural predators around, damage of the nature you describe is possible, especially with susceptible plants. Introducing predatory mites is an excellent line of defense against damage from spider mites.

Though multiple families of mites contain predatory species, the most important family of predatory mites is Phytoseiidae (its members are called phytoseiid mites).

Mites are not insects but rather members of the arachnid class. They do not have the antennae, wings and segmented bodies of insects. Adult mites have eight legs and are very small; generally between 0.004-0.031 inches (0.1-0.8 millimeters) in length. Mites are often thought of as being plant pests and indeed many of them are, including the seemingly ever-present two-spotted spider mite you have in your garden. However, there are scores of predatory mite species that feed on pest mites and many different insects.

Get the right kind

If you’d like to introduce predatory mites to your garden, it’s important that you get the right kind. Phytoseiid mites are often categorized based on their preferred prey.

Type I mites are specialist predators that feed on only one type of pest mite (the two-spotted spider mite, for example). They are very efficient predators and can significantly reduce pest mite numbers quickly. However, once the prey population declines, they often do too.

Type II mite species are not as specialized as Type I and will feed on other mite prey if their preferred species isn’t available. Their populations are far better at self-sustaining as long as alternative prey species are available.

Type III mites are generalist predators that eat various mite species as well as other pest insects like thrips, whitefly nymphs, scale crawlers, leafhoppers, fungus gnats and psyllids, among others. Type III predatory mites are also able to survive on pollen and plant fluids when prey is absent. These species are a constant in most gardens as they can survive on such a varied diet, though they are less effective at gaining rapid control over pest numbers.

Predatory mites are quite good at maintaining populations of pest mites and other pest insects at manageable levels. They produce multiple generations per year due to their rapid reproduction rate and short lifespan. Depending on the temperature and humidity levels, predatory mite species can complete their lifecycle in as few as three weeks. During its development, each predatory mite can consume up to twenty pest mites per day.

Avoid chemicals

Predatory mites are extremely prone to the effects of miticides and chemical insecticides. The use of such chemicals often causes a rapid rebound in pest mite numbers due to the elimination of predatory species. Avoid using these products.

Phytoseiid mites can be purchased online from various insectaries and introduced into your garden in the spring or summer (I recommend buglogical.com, groworganic.com, or arbico-organics.com). The species Amblyseius andersoni is among the most widely available.