HYG  Pest newsletter

Issue Index

Past Issues

Cool-Season Mite Complex

April 25, 2001

About now, a complex of mites attacks conifers and broadleaf plants. These mites prefer the cooler weather of spring and late fall. Though feeding occurs in spring, symptoms may be expressed in summer when plants are stressed. Mites normally remove chlorophyll (green pigment) from leaves or needles, resulting in flecking/stippling or browning. Webbing may be present. The most destructive conifer-feeding mite is the spruce spider mite, Oligonychus ununguis, which feeds on conifers such as spruce, arborvitae, hemlock, juniper, Douglas fir, and some pines. Their piercing–sucking mouthparts remove chlorophyll from leaves or needles. Injured foliage generally looks bronzed to brownish.

Adults are oval, about 1/60-inch long, and black, red, or tan; nymphs, light gray–green. Round, brown eggs are laid under bud scales or in axils of needles. Overwintering eggs are laid on plants September to November. Eggs hatch into nymphs. Spruce spider mites are present in high numbers from April to mid-May; they may be present in June in northern Illinois. It takes about 3 to 6 days to go from egg to nymph. All active stages primarily feed on needles, preferring older ones. There can be three generations per year.

Presence of spruce spider mite can be verified by knocking them off branches onto white paper, where they are easily visible. They produce a green streak when smeared. Red streaks indicate predatory mites.

Managing spruce spider mite involves proper cultural practices, such as watering and fertilizing to minimize stress, and using pest-control materials. These include bifenthrin (Talstar), dicofol (Kelthane), dimethoate (Cygon), summer oil, or insecticidal soap. Each works only by contact, so thorough coverage is important. Improper use can lead to mite outbreaks because it may kill the mites’ natural enemies. If feasible, use a hard stream of water to remove mites from plants as this approach is less harmful to natural enemies. Be careful when using summer oils because they may discolor blue-needled conifers.

Another group of mites, eriophyids, damages plants early although they are not restricted to the cooler times of year. Eriophyid mites, also known as blister, bud, gall, and rust mites, are extremely tiny (less than 0.3 mm long), microscopic worm- or spindle-shaped mites, resembling cigars, with head and legs on one end. Eriophyids have two pairs of legs, a unique characteristic among mites (all other adults have four pairs). They cannot be detected with the unaided eye.

Eriophyid mites are a specialized group of plant feeders. Many feed on a few closely related species or genera. An attachment on their hind end allows them to hold on to the plant. They feed deep within the plant tissues, sucking out juices with their styletlike mouthparts and transferring a substance that deforms plant growth. Feeding generally results in densely packed or distorted growth that looks rough. However, it can result in galling, clustering or witches’-broom, swollen or thickened growth, leaf blistering, and russeting or bronzing of leaves.

Eriophyid mites tend to live together in large num-bers and reproduce within the folds of plant tissues. Spherical eggs are generally laid in groups but can be laid individually. They hatch in under 2 weeks into young mites that take about 2 weeks to a month to mature into adults. Reproductive potential of eriophyid mites is very similar to twospotted spider mite. A female lays up to 100 eggs.

Eriophyid mites may be classified into either foliar-feeders or gallmakers. Hemlock rust mite is a foliar-feeding mite, Nalepella tsugifoliae. It feeds on pine and hemlock needles. High mite populations can cause leaves to turn blue, then yellow-brown before they drop. It is primarily a pest in the spring, less so in summer. An example of a gall-making eriophyid mite is ash flower gall. Feeding by ash flower gall, Eriophyes fraxiniflora, causes a proliferation of flower buds, disfiguring male flowers with galls that turn brown in late summer. Mites overwinter as adults in the bud scales. Ash flower gall is normally a springtime pest.

Managing eriophyid mites is hard, as they usually go undetected until too late to prevent damage. Once noticed, they are established within the plant. As a result, preventive miticide sprays are needed. However, the number of effective miticides for controlling eriophyids is limited. Pest control materials that may be effective include endosulfan (Endosulfan), dicofol (Kelthane), and abamectin (Avid).

An important management strategy, even if applying pest-control materials, is to dispose of plants or prune portions showing symptoms.

Author: Raymond A. Cloyd


College Links