Spider Mites

How to Identify Spider Mites

A two-spotted spider mite feeding on a leaf.

The spider mite is a very common pest in greenhouses. It is tiny, 1/16 of an inch or smaller. Most spider mites are oblong in shape, have eight legs, and are dull or dusty yellow to red colors. The two-spotted spider mite usually has two distinct spots visible on the creature. A magnifying lens is useful in identifying them, but they are usually much more noticeable due to their habits.

Adult spider mites can be seen in the webbing surrounding the growing shoot of this gardenia.
They can also be seen stationary on the shiny leaf surface.

Spider mites colonize on leaf surfaces near the midvein, usually near the upper new growth on plants. They often stay close together. Wherever spider mites are, their tell-tale webbing will also be on the plant. They are very slow creatures and will crawl very slowly when poked or if air is blown on them.


How to Identify Damage Caused by Spider Mites

Spider mite damage on these roses has caused a yellow stippling effect on the leaves.
Photo courtesy of UC Statewide IPM Project

These pests have rasping-sucking mouthparts that they use to eat the contents out of plant cells. A yellow stippling appears on the leaves as the plant cells die. The cosmetic damage can cause many greenhouse plants to be unsalable. Severe damage reduces the plant's ability to photosynthesize, reducing size and yield. Plant death can occur.

Greenhouse Conditions

The greenhouse is not just a place where growers produce crops. It is an environment that attracts, supports, and produces plant-eating pests. Keeping this fact in mind, we can realize and deal with the fact that our management of the greenhouse can control pests.

Some greenhouse conditions are known to encourage spider mite populations. Hot and dry conditions have this effect. When plants are water-stressed the mites feed on more cells to sustain themselves and end up spreading. The warmth of the greenhouse encourages rapid reproduction cycles. The closeness of greenhouse plants allows mites to crawl from plant to plant if the leaves are touching.

Keeping plants adequately watered will reduce damage from mites. As much as it is possible, temperatures should be kept below 85 degrees Fahrenheit. Simply spacing plants so that canopies do not overlap can stop the potential of spider mite populations to spread rapidly across the greenhouse.

While an application of insecticides is sometimes necessary, it will usually end up hurting beneficial arthropods in the area. Consider that an initial reduction in pests after insecticide application is often followed by a greater resurgance soon after. This is because all the predators of the pest have been killed and the few pests that survived the insecticide are now free to infest the greenhouse. Reducing chemical sprays as much as possible allows a healthy system of beneficials to remain in the greenhouse and control pests continually. Cutting back on insecticide applications will save time and money.


What Are the Natural Enemies of Spider Mites?

Interestingly enough, other mites are commonly used to prey on spider mites. In particular, the species Phytoseiulus persimilis and Amblyseius californicus are effective.

P. persimilis has caught this spider mite and is feeding on it.
Photo courtesy of UC Statewide IPM Project.

P. persimilis hunts down and kills spider mites. It prefers to feed on younger stages of the spider mite, but will attack older. P. persimilis can only feed on spider mites so you can be assured that they will hunt down and eat every spider mite they can as long as they are alive in the greenhouse.

A. californicus has sought out these spider mite eggs.
Photo courtesy of UC Statewide IPM Project.

A. californicus operates in much the same way as P. persimilis but has the added benefit of being able to survive without spider mite prey for weeks at a time. This makes it an option for preventative control. It is also tolerates a range of chemical pesticides, making it a valuable addition to conventional pest control.

The F. acarisuga larva is going after and feeding on two-spotted spider mites and their eggs.
Image courtesy of BioBest Biological Systems

Another predator of spider mites, Feltiella acarisuga , can be used to control them in the greenhouse. This insect is a gall-midge that lays its eggs into spider mite colonies. The larvae feed voraciously on eggs, nymphs, and adults of the pest. Not only does the gall-midge thrive in the humidity of a greenhous, but it is able to fly from plant to plant laying its eggs. It works well in conjunction with P. persimilus and A. californicus and has the ability to survive in the absence of the spider mites for some time.

How Can I Get These Natural Enemies?

All of these natural enemies are easily purchased online from several reputable companies. They are like any other pest control method in that they will come in easy-to-use packaging and have instructions on how to use them.