Field Guide     Pest Management     Spider Mites

Spider Mites

CROP IMPACTED: apples, beans, berries, cherry, melons, ornamental plants, peaches, pears, squash, sugar peas, watermelons

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Family: Tetranychidae

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About Spider Mites

There are many varieties of mites that are closely related species in the Tetranychus genus which have become a common problem in many gardens and landscape areas. Distinguishing between these many varieties of spider mites is unnecessary since their damage, management and biology are virtually the same. Typically on infested plants, webbing will cover the fruit, twigs and leaves. Spider mites are also called web spinning mites and are not considered insects but members of the arachnid class along with ticks and spiders.

The damage they inflict on plants is not a concern unless the population is very high. If the population does get out of control their will be visual damage to leaves. This damage is caused by spider mites sucking the content out of the cells from the leaves. The first sign of damage shows up as light dots on the leaves that may take on a bronze colour. As the feeding increases, leaves will turn reddish or yellowish and fall off. Damage is generally increased when the plants are compounded by the stress of minimal water.

The loss of leaves will not reduce your yield for that year in fruit trees unless the damage occurs in the spring to very early summer, but it has the potential to have an impact on next year’s crops. For annual vegetable crops, loss of leaves because of spider mites can greatly impact the yield of your crops leading to infested plants becoming sun burnt and dying.

Spider Mite Reproduction and Life cycle

The mating and feeding of spider mites begins with the warmer weather of spring. The life cycle of spider mites is composed of the egg, the larva, two nymphal stages and the adults. The full life cycle of a spider mite from eggs to adult is dependent on the temperatures. In optimal conditions with temperatures at approximately 26˚C, spider mites can development in 5 to 20 days. The life span of an adult female is about 2 to 4 weeks long and in that time period they can lay several hundred eggs that attaches to fine silk webs and will hatch in approximately 3 days after they are produced. When their reproducing starts to increase in hotter weather, spider mites commonly become a problem from June to September. You will notice a drastic decline of spider mites infestation in late summer when predators start to eliminate them, the condition of host plants becomes unfavorable and the weather begins to cool down.

Spider Mites Identification and Habitat

Identification

To the naked eye, spider mites look like tiny, moving dots. The best way to identify them is by their living conditions. To distinguish spider mites from other mites, look for the silk web they produce on infested plants, where massive colonies of spider mites live on the underside of leaves.

Adult spider mites have 8 legs with an oval body and 2 red eyespots near the head. They can come in a range of colours from red, brown to yellow and green. Female spider mites have large, dark blotches on both sides of their body and many bristles covering their entire body and legs. They produce round eggs that are translucent and change to a cream colour right before they hatch.

Varieties

  • - Twospotted spider mite: Tetranychus urticae
  • - Carmine spider mite: Tetranychus cinnabarinus
  • - Strawberry spider mite: Tetranychus turkestani
  • - Pacific spider mite: Tetranychus pacificus
  • - Spruce mites: Oligonychus ununguis
  • - Southern red mite: Oligonychus ilicis
  • - European red mites: Panonychus ulmi

Habitat

Spider mites favor hot, dusty conditions and can first be found on trees or plants that live on the edges of gardens or along dusty roads. They also have been known to cause the most damage to plants that are under stress from lack of water. Spider mites feed on the leaves of apples, beans, berries, cherries, melons, ornamental plants, peaches, pears, squash, sugar peas, and watermelons. Through the winter months, spider mites often turn more red or orange hibernating in protected areas like bud scales, bark cracks or under debris around your gardens.

Spider Mites Monitoring and Control Methods

Monitoring

Trying to detect spider mites is quite difficult due to the fact that they are very small. Typically you will notice the damage to the leaves before the bugs. Look underneath leaves for mites, their eggs and webbing. However, the only way to tell if it is in fact spider mites is to use a hand held lens. Once they are disturbed they will rapidly move around. Make sure spider mites are actually present before you treat them. Sometimes the infestation of mites is gone before you even realize they were there. Plants that have been infested by spider mites can often recover once the mites have left.

Cultural Control

With the size of spider mites come many natural enemies, often limiting the population and growth of them. Keeping the dust down by applying adequate amounts of irrigation and rain fall can be an easy way to reduce the infestation of spider mites. It is important that you have good coverage of water, especially on the underside of the leaves where spider mites spend most of their time.

Chemical Control

It is important to continuously monitor mite levels right up until you treat them. Spider mites may have decided to leave your plants before you get a chance to eliminate them. Avoid broad-spectrum insecticides, this form of treatment used to prevent other pests will eliminate spider mites natural enemies and has been shown to stimulate reproduction. Chemical control of spider mites can be done by spraying either insecticidal oils or soaps. Plant-based oils like canola, cottonseed or neem oil and petroleum-based horticultural oils are acceptable. There are a few plant extracts that have been shown to be effective in killing spider mites. Some include cinnamon oil, clove oil, garlic extract, mint oils, rosemary oil and others. Note: do not use soaps or oils on plants that are water-stressed or if temperatures are higher than 32˚C. These particular materials can injure some plants; make sure to check the labels. Soaps and oils have to come in contact with spider mites to kill them. That being said, excellent coverage, especially on the bottom parts of the leaves, is vital and repeated application may be necessary.

Sources

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7405.html

http://entnemdept.ufl.edu/creatures/orn/twospotted_mite.htm

http://www.ext.colostate.edu/pubs/insect/05507.html