Red Spider Mite can be a Problem for Tomatoes

7 Mar

An infestation of spider mite is generally brought about by hot, dry temperatures and low humidity.  Sometimes, if using pyrethroid-based or other pesticides that are broad-spectrum and which kill all insects – both beneficial and pest – an outbreak can arise, because all the natural predators of these mites, have been killed.

spider mite

Spider Mite Damage can look a little like a nutrient deficiency

To the uninitiated, the first signs of this pest may look like a nutrient shortage, as the leaves of the crop yellow and die off.  These mites generally choose the upper leaves first, where conditions are drier and warmer. The pest itself is not noticed because it lives on the undersides of the leaves and is very small.  However, if you turn the affected leaves over and look on the underside, in cases of severe infestation, small red mobile dots can be seen and quite frequently, also webs (because mites are closely related to spiders – hence the term “spider mite”).

Spider mites feed on plant cells and produce characteristic small, yellowish, speckled feeding marks.  If you examine the leaves closely, this speckling is quite obvious on the upper leaf surface, although the damage occurs below. Many insecticides have little or no effect in controlling spider mites. Their small size, ability to exist close to the veins of the plants, the webs and because they are a different species of pest than other insects, make control by spraying extremely difficult.

Spider mites are sensitive to day length and are capable of hibernating within cracks and crevices of the plant structure. If you had spider mites last year, it is highly likely that they will continue to be a problem this year.

Spider mite populations can explode under conditions of high temperature (above 30°C or 85° F) and low relative humidity (below 60% RH). Temperature affects the rate of the life cycle (see Table 1), hence the higher the temperature, the more rapid the rate of spider mite population growth.

Table 1.  The effect of temperature on rate of life cycle of spider mites

Spider Mite Life Cycle
Temperature (C & F) 16°C  (60°F) 18°C (64°C) 21°C (70°F) 32°C (90°F)
No days (egg to adult) 30 21 14.5 3.5

In the US, and some other countries, predatory mites can be purchased, which are extremely effective at cleaning up spider mites. These predatory mites multiply nearly twice as fast as red spider mites and only feed on other mites, and not on plants.

Where it is not possible to purchase predatory mites, it is possible to slow the rate of development of spider mites by increasing the humidity around plants by watering their leaves.

Products such as fish emulsions sprayed on the foliage of plants are also known to be effective.

When the crop is finished, it is important to clean up and destroy all plant residues to minimise the risk of carry over of these pests.

Discover more about controlling Spider Mites and other Pests in Chapter 11  of  How to Grow Juicy Tasty Tomatoes.