Mighty problems: Spider mites
Every so often, you may notice that a plant does not look as healthy as usual. At first glance the plant in question may seem slightly dried out and in need of a watering. If given a closer inspection, many small brown spots of damage can be seen on the leaves, you have a problem. And the most tell-tale sign of the plant being under attack is the presence of fine webbing all over, especially between the leaves and twigs. It is this webbing that announces that the plant is being damaged by spider mites.
Spider mites are very tiny creatures in the scientific class Arachnida, which also includes spiders and ticks. Like spiders, they have eight legs and two body parts. They are similar to aphids in the type of damage they do to plants. Spider mites have piercing, sucking mouth parts which they use to injest the liquids inside leaves and other soft tissues of plants. Left untreated, these mites will cause leaves to wilt and brown, and new growth to dieback. They are a fairly common pest of indoor house plants and can infest outdoor plants such as roses, raspberries and spruce trees. Most mites are extremely tiny and therefore difficult to notice on plants. They can be checked for by firmly shaking the affected plant or individual leaves over a blank white sheet of paper. Mites will fall onto the paper and appear as very small dark specks moving around.
Spider mites can be very difficult to control as they are quite prolific and under ideal conditions they can produce a new generation in less than a week. They prefer hot and dry growing conditions and are more common in summer months, both outdoors and indoors. Spider mite problems on house plants are often best fixed by isolating the affected plant and treating it by regularly rinsing it under running water and applying insecticidal soap. Badly damaged areas of the plant should be pinched off and discarded. They can also be killed off by increasing the humidity levels around the plant by placing it in a sealed plastic bag for a period of time.
Spider mites are always present outdoors but their numbers are usually kept down by various predators, including predatory mites. Sometimes though, weather conditions are ideal for spider mites and they begin to rapidly increase in number. However, their preference for hot, dry conditions means they can be controlled to a certain extent by regularly hosing down the affected plant. Like aphids, they are soft bodied and can be knocked off plants with a strong blast from the garden hose. This will also help to remove the unsightly webbing they create. Any remaining spider mites can be sprayed with a variety of available garden insecticides such as insecticidal soap. Although it is important to note that sometimes outbreaks of spider mite populations result from overuse of insecticides that kill off their naturally occurring predators. As with most plants, it is a balance of overall beneficial practices that ensure optimal plant health.
Sharon Moffat has a Plant Science degree from the University of Manitoba and has worked for the City of Winnipeg's Insect Control Branch for the last 24 years.
This plant is suffering from a bad case of spider mites.
A close-up of spider mites.