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Two-Spotted Spider Mite

Ravageurs et maladies
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Two-Spotted Spider Mite.
Photo: Jardin botanique de Montréal (APS Press)

Onglets

Description

Summary

Two-spotted spider mites are not true insects, but tiny arthropods, just like other mites and ticks. With their four pairs of legs, they look like miniature spiders. They are so small that they are difficult to see, but they do weave tell-tale whitish webbing. These tiny parasites can cause considerable damage to a wide variety of ornamental and fruit plants. In a severe, uncontrolled infestation, leaves and fruit may drop and plants may die back quickly, particularly in hot, dry weather.

Signs and symptoms

  • Two-spotted spider mites are barely visible to the naked eye, but they do leave a number of telltale signs.
  • The first signs of damage appear on young, tender shoots, which are full of sap; the mites use their mouthparts to pierce and suck cell contents from the leaves.
  • In a light infestation, small yellowish dots followed by pale patches appear on upper leaf surfaces.
  • In a heavy infestation, the leaves may turn completely yellow or take on a bronze or silver tinge; you will see fine whitish webbing beneath the leaves and between the branches on new shoots; severely affected leaves dry out and drop prematurely.
  • Affected plants show delayed growth; fruit is smaller and drops before it ripens; a heavy infestation can interfere with fruit set in the current season and the formation of fruit buds for the following year.

Latin name (genus)

Tetranychus urticae

Host plants

Various ornamental and fruit shrubs and trees, including apple, ash, caragana, cherry, cinquefoil, crabapple, currant, elder, elm, grape, hawthorn, honey-locust, hydrangea, juniper, linden, maple, plum, poplar, rose and willow.

Various ornamental herbaceous plants, including alumroot, bleeding heart, carnation, castor bean, celosia, chrysanthemum, clematis, cornflower, cosmos, dahlia, foxglove, hibiscus, hollyhock, larkspur, lavender, monkshood, nasturtium, pansy, phlox, verbena, violet and yarrow.

Development cycle

Description and life cycle

Two-spotted spider mites belong to the class Arachnida, like spiders, and the order Acari, along with other mites and ticks. Before reaching adulthood, they pass through the egg, larva and nymph stage.

Eggs: Globular, about 0.13 mm in diameter. They are translucent when first laid, turning straw yellow before hatching.

Nymphs and larvae: Resemble adults, but smaller; nymphs have eight legs and larvae six.

Adults: Globular body, fairly translucent, greenish or yellowish with two darker spots on back. They have 4 pairs of legs but no wings or antennae. Females are about 0.3 mm long and males slightly smaller. Overwintering females turn orange with metallic green.

Adult females overwinter on trees, sheltered between the scales on buds and in crevices in the bark or in plant litter on the ground.

In spring, they wake up. During their short lifespan (about two months), they may lay one hundred to two hundred eggs, attaching them to the underside of leaves. Incubation takes five to twenty days, depending on the temperature. After the eggs hatch, the young larvae move to the new leaves to feed. They reach adulthood in ten to twenty days. Higher temperatures shorten the life cycle.

Depending on the climate, there may be as many as five to nine generations a year. Populations normally reach their maximum in late July and early August.

In fall, the fertilized females turn orange and form colonies on plants or the ground, where they overwinter.

Prevention and control

Favourable conditions

Two-spotted spider mites multiply faster during hot, dry weather, in poorly aerated sites. They tend to proliferate after mild falls and winters and after drastic pruning, excessive feeding with high-nitrogen fertilizer and anything else that encourages the growth of tender shoots.

Identification

Use a magnifying glass to carefully examine the underside of leaves, particularly on young, tender shoots. Spraying the leaves and stems will make it easier to see the fine webbing.

Prevention

  • Keep plants vigorous by fertilizing them adequately, pruning them properly and watering them during dry spells.
  • Avoid excessive feeding with high-nitrogen fertilizers and drastic pruning, both of which promote the growth of tender tissues that are more susceptible to infestation.
  • Keep your garden free of plant litter.

Physical control

  • During the growing season, rake up leaves and fruit as they drop, to break the insects' life cycle.
  • Remove and destroy heavily infested shoots.
  • Spray the foliage, trunk and branches with a strong, steady stream of water to dislodge the mites and increase humidity levels.
  • During dry spells, spray the leaves of susceptible plants regularly to maintain humidity levels.
  • Water early in the morning, to allow the foliage to dry out during the day; avoid overwatering, however, so as not to encourage fungal infections.
  • In fall, remove all plant litter, to reduce the overwintering population, which is the first source of infestation in the spring; do not compost infested plant litter.

Biological control

Encourage natural predators (plant bugs, ladybird beetles, syrphid fly larvae, predacious mites) by increasing the variety of plants in your garden.

Chemical control

As a last resort, use a low-impact pesticide with insecticidal soap or mineral oil as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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