Spruce spider mite
The adult conifer spider mites grow up to 0.6 mm long, have eight legs and an oval body shape. Due to their gut contents, the originally pink mites often appear dark green. They move quickly with characteristic shaky movement. Under the microscope, their legs show the claws and bristles on the terminal limbs that are very characteristic of spider mites. To feed, they pierce the needle tissue of their host plants with their stiletto-shaped mouthparts and suck out the cells of the uppermost tissue layers.
The eggs of conifer spider mites are onion-shaped, 0.15 mm in size, and red-orange to brown in color. As a distinctive feature, it has a short stalk on the outer pole.
On firs and spruces the eggs are laid at the base of the needle, on thujas they are preferably found in grooves and in protected places of the scale leaves. The mites overwinter as an egg, which is very resistant to cold. After hatching, the empty eggskins often remain. Larvae have three pairs of legs, later they molt to so-called nymphs, which have four pairs of legs.
Like other spider mite species, the coniferous spider mite can produce very fine webs that surround the sucking sites of heavily infested leaves. In the wild, however, such webs are seldom found - and they are rarely very pronounced. They probably serve mainly to protect the mites from their predators, such as lacewings, special ladybug species (e.g. Stethorus and Scymnus species), flower bugs(Anthocorus and Orius species) and predatory mites(Typhlodromus sp.). Only when these natural enemies can no longer do their job does mass reproduction set in.
The mite can multiply well, especially in dry years. Under such conditions, damage that has already occurred becomes more noticeable. Depending on the ambient temperature, the development cycle is completed in a short time (in only 15 days at correspondingly high temperatures), so that several generations can develop each year. Since each female mite lays about 50 eggs, the mites can reproduce very rapidly under favorable weather conditions, as early as mid-March, throughout the year.
The mites bite superficially located plant cells - these turn yellow and die. Infested scale leaves of thuja show a mottled appearance up close, from a distance they look dull green. Heavily infested thuja turn brown and may wither. In the course of mass development, the mites slowly migrate from heavily infested areas to areas of the tree that are still healthy.
In conifers, the sucking activity, usually beginning in the interior of the crown, causes the affected needles to dry out and, in the case of heavy infestation, to fall off. This damage is often confused with drought damage due to lack of water. A magnifying glass, or better still a stereomicroscope, is absolutely necessary for accurate diagnosis. This is because usually only remnants of empty egg cases or molting remains can be seen at the abandoned infestation sites.
An infestation lasting several years can considerably weaken young trees and cause them to die.
The coniferous spider mite is very common in Austria's gardens, which is probably related to the conditions that are unfavorable for conifers. In particular, dry-warm locations with low light availability and overfertilization form favorable conditions for the development of pest nests on plants.
Prevention and control
- Sufficient irrigation in dry locations.
- Avoidance of dense stand and overfertilization
- For the chemical control of spider mites (=acaricides) a number of well effective agents are available (see list of plant protection products approved in Austria)
- Many agents show little efficacy against some mite stages (e.g. eggs). However, since there is always a mixture of mites of all developmental stages, two treatments should be applied at intervals of seven to ten days
- Winter eggs: a sprouting spray with preparations containing oil is recommended.
Last updated: 01.12.2021