Mites are a common pest in indoor gardens. The two-spotted spider mite (Tetranychus urticae), named for the two distinct patches of dark brown or black on its otherwise yellow or brown body, is one of the most common varieties seen in the garden.
The “spider” part of the spider mite name comes from the webbing they produce, which is similar to a common house spider. Two-spotted spider mites use the silk that they produce in several ways. They cover the leaf with it and live in the web to protect themselves from predators.
They also lay their eggs in the webbing to protect the eggs from predators. Lastly, spider mites use their webbing to get blown from place to place. The mites will produce a strand of silk and then use it like a sail or parachute to have the wind blow them around, hoping they will land on a new plant.
Two-spotted spider mites are also extremely small, measuring just 0.4 mm or 1/100 of an inch—less than the size of the period on the end of this sentence. They damage the plant by piercing individual cells on the surface of the leaf and sucking out the cell juices.
These deflated, dead cells produce an ashy look on the surface of the leaf. If you find the characteristic ashy look on the leaf and can see webbing on your plants, you most likely have a spider mite problem. Luckily, there are several ways to prevent this from happening and deal with them if you do find them.
Preventing Spider Mites in the Garden
The first step in any pest control plan is prevention. If you are growing outside, this is a little more difficult, as pests will be blowing and flying in naturally. However, if you are growing in a contained facility, such as a greenhouse or growth chamber, make sure you have not been around other plants that have pest issues before entering.
Two-spotted spider mites are good at grabbing onto your clothing and then jumping off when you get near other plants. Many large growing operations require staff to wear lab coats or disposable jump suits and hair nets to prevent pests from hitchhiking in on workers’ clothing and jumping off. Also make sure any air intakes are screened with fine mesh, so there is less chance that your fans will suck in pests from outside.
Biological Control of Spider Mites
Biological control is a great way to use naturally occurring predator-prey interactions to protect your crop. There are several spider mite predators on the market with special adaptations that allow them to walk in the webbing the spider mites produce and eat both the eggs and the mites.
Amblyseius californicus (now named Neoseiulus californicus) and Phytoseiulus persimilis are both widely available predatory mites. They are similar in size to two-spotted spider mites, but have sleek-looking, tear-drop-shaped bodies that are red in hue. These mites come both as a loose material that you sprinkle on the plant and as sachets—little bags that hold a microhabitat for the predatory mites.
The predators live in the bag and then slowly emerge over a matter of weeks. This is a great way to ensure predators are in your crop before the mites arrive. The predators are then able to eat any two-spotted spider mites that happen to arrive before they can establish and damage the crop.
Another good predator is actually a type of maggot. As an adult, Feltiella acarisuga is a harmless little fly, but the larvae or maggots are voracious predators that love to eat two-spotted spider mites. The maggots are blind, but mother flies lay their eggs next to existing patches of mites.
The maggots then cruise around the leaf and on the webbing until they find a mite or egg to eat. They then latch on using a combination of their mouthparts and a slime they produce and suck the juices out of the mite.
Pesticides for Spider Mites
Pesticides are also a viable way to control spider mites and there are many different products available. I will not discuss these here, as that topic warrants its own separate article, but I will say always remember to read the label and follow the directions when using pesticides, as you can damage the crop and hurt yourself if they are applied improperly.
Two-spotted spider mites are a common crop pest and there are many resources online to help identify them and give you more information on control options. The University of California’s IPM website is a great resource for any gardener or professional. Remember that prevention is the best way to control pests; otherwise, you will always be playing catch-up.