Spruce Spider Mite

Tap a branch over a sheet of white paper and watch for dark, slow specks that crawl, then smear green if you crush them, because those are spruce spider mites, not grit. Hose the inner, older needles hard in spring and again in fall, because this mite feeds in cool weather and sits still once days hit the 80s. Labeled insecticidal soap or horticultural oil can finish live mites if the spray actually wets the inside of the plant. Skip oil on a Colorado blue spruce, because it takes the blue off the needles. Do not run a hot-July houseplant mite calendar on a spruce.

People still say spruce mite, then spruce spider mite. The animal is Oligonychus ununguis, a cool-season spider mite, not an insect. Young mites are pale green, and adults go dark green to almost black, with salmon-pink legs, about the size of a pepper grain. They overwinter as tiny eggs at the needle bases. In the yard they show first on spruce — dwarf Alberta, Colorado blue, Norway, white — and they also feed on arborvitae, juniper, hemlock, pine, and fir. Fine silk between needles shows up when numbers are high. Older inner needles go off-green, then bronze, and they can drop. A healthy large spruce often lives. A small dwarf Alberta that is going bare cannot wait.

Hose the inner needles in spring and fall

This mite works when the weather is cool, so the session is April into May and again in September, not a July afternoon when two-spotted mites would be running on a rose. Once daytime heat sits in the 80s, spruce spider mites lay resting eggs and stop feeding, so a midsummer spray on empty needles misses them. Hose the inside first, because a hard stream knocks mites and eggs off the inner and lower needles, where rain does not reach, and on a small spruce or an arborvitae hedge you can repeat that wash several days in a row, or weekly through the cool weeks, because they do not all drop on the first pass.

Labeled insecticidal soap or horticultural oil can finish mites that are actually moving, because those sprays work only where they wet the body. Soak the inside of the plant, not just the pretty outer face. Skip kitchen dish liquid, because it can burn needles and has no plant label. Skip oil on a blue spruce, because the oil dissolves the wax bloom and the old needles stay green until new growth hides them. Soap does not strip that blue. Do not soap a drought-stressed plant, tender new growth, or needles in full sun above about 90°F. Skip a Sevin or permethrin bottle as the first move, because those insecticides kill the predatory mites that smear red on the paper and the outbreak often gets worse.

1. Confirm. Tap three or four places over white paper. Dark, slow specks that smear green are this mite. Ten or more live mites on a tap, with few fast red predators, is the treat threshold. Fast reddish mites that smear red are helpers, so wait.
2. Hose the inner and lower needles hard. On a dwarf Alberta or a small arborvitae, repeat several days in a row, then weekly while the weather stays cool. Keep the plant watered and mulched, because drought makes the feeding worse, and skip a heavy nitrogen feed that only fattens the mites.
3. Soap or oil only while mites are still crawling, and only where the spray wets the inside. Repeat if live mites remain, since dried soap does not keep working. Skip oil on Colorado blue spruce. Skip soap in heat, drought, or on brand-new growth.
4. A tree over about ten feet is a hire if you cannot wet the crown. A driveway fog wets the skirt and misses the mites. Dormant oil in midwinter can smother overwintering eggs after a bad fall, but skip that oil on blue spruce too.

Bronzed needles in July can be last spring's feeding

Conifers are slow to show this injury, so a spruce that looks dirty bronze in June or July may have been fed on in April, or even last fall, after the mites already laid resting eggs and stopped. Tap first. If the paper is blank, a miticide now does not rewind the color, because those needles will not green up again. The tree holds several years of needles, so old injury can sit there looking current. Treat only if mites are crawling now, or if you find a heavy band of overwintering eggs at the needle bases and you plan to catch them at hatch in spring.

A healthy, well-established spruce or hemlock usually lives through a year of this feeding. Lower inner needles drop, the skirt looks thin, and the plant looks rough until new growth covers it. Seedlings, a newly planted screen, a drought-stressed hedge, and dwarf Alberta spruce are a different judgment. That tight little cone is a favorite, and a heavy year can kill a small plant or leave dead patches that never fill. Arborvitae in dry, dusty soil bronzes the same way. Water those plants, hose them while mites are moving, and do not wait for the whole globe to go rusty. If the tap test is blank in July, mark the plant and come back in early September, because that is when this mite starts again.

Tap a branch over white paper; crush a dark speck

You will not see these mites well on the needle. Hold white paper under a branch that looks off-green and strike the wood hard, then wait a few seconds, because they sit still before they crawl. Adults are about a fiftieth of an inch, oval, and dark. Young ones are paler and start with six legs. Eggs at the needle bases are tiny, round, slightly flattened, with a hair sticking up from the middle. Fine silk between needles is a late sign, not the first one. Check the older inner growth on the lower third of the plant, then one or two more spots around the canopy, because a single clean tap does not clear a spruce.

Two-spotted mites run a summer calendar; wool on a twig is adelgid

Two-spotted spider mites are a different animal. They are pale, with two dark spots, and they love hot, dry weather on houseplants, vegetables, and deciduous shrubs. See spider mites for that summer job. Do not copy their July hose-and-isolate calendar onto a spruce in August, because spruce spider mites are usually resting then. Southern red mites are another cool-season kind on rhododendron and camellia, not this conifer silk.

Needlecast turns older needles purple-brown and drops them from the lower inside first, same neighborhood as mite bronze, but a hand lens shows black fruiting bodies in rows on the needle, not crawling specks on paper. A miticide does not finish a fungus. Uniform brown on the windward side, or a sharp line above last winter's snow, is winter burn or drought, and the tap test is blank. Cotton-swab tufts stuck to the twig at the needle bases are adelgids, not mite silk. See adelgids. Do not run their wool smear as the finish on this mite, and do not treat mite stipple as hemlock wool.

Related reading

• Spider mites — two-spotted mites on a hot, dry leaf or houseplant, not this cool-season conifer mite.
• Adelgids — cotton tufts on the twig, not silk between spruce needles.
• Scale insects — bumps on the needle or wood; dieback on a spruce is often this, not mites.
• How to grow spruce — site and water for a spruce that takes this mite on the inner needles.
• How to grow arborvitae — a hedge in dry dust bronzes with the same mite.