Spider Mites

General

In a nutshell: Major pest of many greenhouse and outdoor crops; warm, dry conditions allow this species to thrive; extremely rapid population growth; sap suckers—cause yellow spotting on leaves and cover the plant in white webbing; adult females become dormant to overwinter.

What are spider mites?

Worldwide there are more than 1 200 species of spider mite, many of which are major crop pests. Of these, the two-spotted spider mite (Tetranychus urticae) is, by far, the species with the greatest impact on crops grown both outdoors and in greenhouses. Additionally, most experts consider the red spider mite (also known as Tetranychus cinnabarinus) to be a different colour form of T. urticae. Despite their small size, they can cause severe damage in very little time because they reproduce very quickly.

What kinds of plants do they attack?

The two-spotted spider mite can affect a wide variety of plants, including vegetables, ornamentals, fruit trees, berries, grapes and field crops. Cucumbers, peppers, tomatoes and parsnips are particularly vulnerable.

What do they look like?

Adult spider mites, like all mites, are related to spiders and have eight legs. The female is oval shaped and about 0.5 mm long. The adult male’s body is smaller (~0.3 mm long), slimmer and more pointed at the rear end. The mites range in colour from orange, light yellow or light green to dark green, red, brown or almost black. The colour of the adult usually depends on the crop in which the mites occur (e.g. on cucumbers they are usually yellow-brown, on tomatoes they’re typically red-brown). Sometimes you can see the large dark spots that give the mite its name on either side of the body.

The female lays over 10 eggs per day on the undersides of leaves. The eggs are round and 0.14 mm in diameter, on average. They’re translucent when freshly laid, eventually become opaque and, when ready to hatch, straw-coloured.

After emerging from the egg, the two-spotted spider mite goes through three intermediate stages (larva, protonymph and deutonymph) before developing into an adult. Each larval and nymph stage has an active period and a dormant period that are roughly the same length. The larvae are three-legged and almost colourless with dark red eyes when first hatched. Once the newborn mite begins to feed, it changes to green or brownish-yellow.

How do I know if my plants are under attack?

Spider mite larvae, nymphs and adults damage the host plant by feeding on its sap. They appear mostly on the underside of leaves, where they pierce the plant’s cells and suck out the contents. The first signs of spider mite attack are small yellow dots on the upper surface of the leaves.

The cellular damage reduces photosynthesis, leads to moisture loss and stunts plant growth. As pest pressure increases, whole leaves turn yellow and the entire plant can whither and die. In tomatoes and cucumbers, as little as 30% leaf damage can lead to total crop loss. The nymphs and adults also produce a silk webbing that can completely engulf the infested plant. The appearance of ornamental plants, in particular, is negatively affected by this webbing and by spotting on the leaves.

Warm, dry conditions can cause spider mite populations to increase rapidly. In favourable environments, they can complete their life cycle in 1–2 weeks, and at 25°C the mite population can double in less than three days. Additionally, several overlapping generations can occur together.