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Spruce Spider Mite

Ravageurs et maladies
English
Spruce Spider Mite on needle.
Photo: Ward Strong, BC Ministry of Forests, Bugwood.org
Oligonychus ununguis

Onglets

Description

Summary

Spruce spider mites attack several coniferous species. These tiny parasites are not true insects, but arthropods, 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 telltale webbing between the needles on a tree. In a heavy, uncontrolled infestation, needles may turn yellow and drop and trees may die back gradually, particularly in hot, dry weather.

Signs and symptoms

  • Spruce spider mites are barely visible to the naked eye, but they do leave a number of telltale signs.
  • Just like two-spotted spider mites, they excrete abundant silk and weave numerous webs between the needles, using them as shelters and nests.
  • The first signs of damage appear on older foliage on lower branches, and the damage gradually spreads upward and into new shoots.
  • As the mites suck out the contents of plant cells, the leaves gradually become discoloured.
  • Early in an infestation, small yellowish dots appear on the needles; they then turn grey or bronze.
  • In a heavy infestation, trees may turn completely brown and drop their needles.

Latin name (genus)

Oligonychus ununguis

Host plants

Several coniferous species, including arborvitae, fir, hemlock, juniper, larch, pine and spruce.

Name of host plants

Development cycle

Description and life cycle

Spruce 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 stages.

Eggs: Spherical, reddish and about 0.13 mm in diameter.

Larvae and nymphs: Resemble adults but smaller and paler green. Nymphs have eight legs and larvae six.

Adults: Oval, very dark green or brown body. They have four pairs of legs but no wings or antennae. Females grow to about 0.5 mm and males are slightly smaller.

The adults, nymphs and larvae do not survive in winter. Only the eggs overwinter, hidden between the scales on buds or at the base of needles.

In late April, the overwintered eggs start to hatch. Newly hatched larvae start by attacking the older needles while waiting for the new shoots to develop. The larvae develop into nymphs and the nymphs into adults. After mating, the first-generation females lay their eggs at the base of needles. The rate at which subsequent generations appear depends on the climate, with warmer temperatures shortening the life cycle.

Populations normally reach their maximum in late July and early August. There may be as many as three to eight generations per year.

Starting in late summer (September until the first frost), the females of the latest generations deposit their eggs between the scales on buds or at the base of needles.

Prevention and control

Favourable conditions

Spruce spider mites multiply faster during hot, dry weather, in poorly aerated sites. Eggs are more likely to survive mild falls and winters.

Identification

Use a magnifying glass to carefully examine the underside of foliage, particularly young, tender shoots. If you shake a branch over a sheet of plain paper, you will be able to see the dark pests scatter quickly.

Prevention

  • Keep your trees 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 excess growth of tender new shoots that are more susceptible to infestation.

Physical control

  • Spray the foliage, trunk and branches with a strong spray of water to destroy the nests and dislodge the mites.
  • During dry spells, spray the leaves of affected trees regularly to maintain humidity levels and prevent mites from proliferating.
  • 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 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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