SPIDER MITES

Spider mites are not insects. They are eight-legged arachnids — nearer to a tick than to anything else in this guide — and that is the first practical fact about them: an insecticide you trust for every other pest on the list may not touch them at all.

They belong to the family Tetranychidae; the one you almost certainly have is the twospotted spider mite, Tetranychus urticae. Adults are normally green or yellow, turning red as day length shortens in autumn. The body is oval with two red eye spots near the head end; females carry a large dark blotch on each side and are covered in bristles over the legs and body. At the size they actually are, they read as bristly dark specks. The eggs are visible too — minute, white, and laid in tight clusters.

The name comes from their habit of spinning fine silk over infested leaves and new growth. That webbing is diagnostic, but it is also late: by the time you can see it without a loupe, the colony has been reproducing for a while.

They pierce the underside of the leaf and drain the contents of individual cells. Each emptied cell is a dead white point, and the points accumulate.

The first sign is fine stippling, clustered along the veins and heaviest around the midrib — as though the leaf had been pricked repeatedly with a needle. As numbers rise the speckles merge and the leaf takes on a whitish or greyish cast before it dies. On a dark, glossy aroid leaf the earliest stage often does not read as spots at all: it reads as a loss of sheen, a patch that has gone flat. Turn the leaf over and look with a loupe before you conclude it is a feeding problem.

Once reproduction is underway a distinctive webbing forms — usually under the leaf first, then at the geniculum or the petiole insertion, then at the growing tip. Mites leaving a plant whose food quality is falling let out threads of silk and drift, or are blown, onto whatever is next to it. That is why an infestation on one plant is a question about the whole shelf.

What makes this pest genuinely difficult is not toxicity. It is arithmetic.

A single female lays up to about a dozen eggs a day, and mating is not required for her to do it — an unmated female produces sons, which then mate with her. At around 70°F eggs hatch in roughly three days and the survivors reach adulthood in about two weeks; at the top of the mite’s range the whole egg-to-egg cycle compresses to somewhere near five to seven days. Adult females can also go dormant when the photoperiod shortens and re-emerge to lay weeks after it lengthens again, which is one reason spider mites keep reappearing on indoor plants that looked clean all winter.

The consequence is a population that can rebound between two applications spaced a fortnight apart, and one that meets a new chemistry with enough individuals, and enough generations, to find resistance quickly. A failed attempt to kill mites is not a neutral outcome. It is a selection event.

These will hold a light, early infestation. Once mites are established they will not, and persisting with them is how a manageable problem becomes a webbed one.

1. 01Quarantine every incoming plant for three to four weeks, away from the collection, and inspect the undersides twice in that time. Nothing else on this page comes close to it for value returned. Almost every indoor mite outbreak arrived on something.
2. 02Horticultural or dormant oil. A mechanical control — it suffocates, so there is no mode of action to develop resistance to. It only reaches the mobile stages, not the eggs, so it has to be repeated every few days to catch each hatch. Direct contact only, no residual. It can also be tank-mixed with many other products to improve their coverage. Oils burn leaves above roughly 80–85°F, and some foliage is sensitive regardless — test on a couple of leaves and wait a day before you spray a whole plant.
3. 03Soap and peroxide, with a cloth. Real soap — Dr. Bronner’s or Safer insecticidal soap — plus 3–4 tablespoons of 3% hydrogen peroxide per cup of water. Spray it on, then physically wipe the mites off with a soft damp cloth and rinse with plain water. The wiping is doing most of the work. Contact only, no residual.
4. 04Rinse the undersides. A firm shower removes a large fraction of a young population and costs nothing. It does not finish the job, but it lowers the number your next step has to deal with.
5. 05Know when to stop treating and start discarding. A heavily webbed plant sitting in a collection is a reservoir that reseeds everything you have just cleaned. That is a judgement about the whole shelf, not about the one plant.

Of everything in this guide, spider mites are the pest biological control handles best. They are also the pest most likely to defeat you chemically — which makes the trade worth taking seriously rather than treating as a curiosity.

Predatory mites arrive in a shaker bottle or a slow-release sachet and are sprinkled or hung on the plant. They do not bite people, they do not eat plants, and when the prey runs out the specialists simply die off. Nothing below is a rescue for a plant that is already grey and webbed end to end — knock the population down first, then release.

Read the matrix before the paragraphs. It answers the two questions that decide a purchase: which life stages does this reach, and what else does it kill.

Miticides. Four are worth a home grower’s attention.

1. 01Confirm it is mites. A 10× or 20× loupe, leaf undersides, along the midrib. Stippling without mites is a different problem and a miticide will not fix it.
2. 02Isolate the plant before you carry it anywhere. Bag it if it has to cross a room.
3. 03Knock the number down mechanically first — rinse, then soap and a cloth. Every mite removed by hand is one the chemistry does not have to reach.
4. 04Choose by life-stage coverage, using the matrix. A product with no egg activity leaves the next generation intact; an ovicide alone leaves the adults laying. Either pair them or pick one that covers all stages.
5. 05Repeat on the mite’s clock, not yours. At room temperature that means roughly every five to seven days, at least three rounds, timed to catch each hatch before it can lay.
6. 06Rotate the mode of action between rounds — see the note in §06. Rotating brand names within one group is how the resistant population gets built.
7. 07Re-inspect ten to fourteen days after the last round. One surviving fertile female restarts the whole thing, and she does not need a mate to do it.
8. 08Decide which outcome you are after. Control — zero or near zero — and suppress call for different amounts of chemistry, and on a plant that lives indoors permanently, control is usually the honest target. The definitions are in the guide’s key terms.

1. 01Spider Mites — UC Statewide IPM Program, Pest Notes 7405
2. 02Spider mites and their control — Ohio State University Extension
3. 03Spider mites — University of Minnesota Extension
4. 04Humidity and the survival of Tetranychus urticae — Nature PDF
5. 05Two-spotted spider mite — Koppert pest profile

1. 01Biological control and natural enemies of invertebrates — UC IPM, Pest Notes 74140
2. 02Phytoseiulus predatory mites — UC IPM natural enemies gallery
3. 03Western predatory mite — UC IPM natural enemies gallery
4. 04Spider mite destroyer, Stethorus — UC IPM natural enemies gallery
5. 05Mite midges, Feltiella — UC IPM natural enemies gallery
6. 06Phytoseiulus persimilis — Cornell Biological Control
7. 07Neoseiulus predatory mites — Cornell Biological Control
8. 08Stethorus punctillum — Cornell Biological Control
9. 09Neoseiulus californicus — Koppert
10. 10Amblyseius andersoni — Koppert
11. 11Feltiella acarisuga — Koppert
12. 12Spider mite control, the essential guide — Koppert
13. 13Temperature, humidity and the ability of Phytoseiulus persimilis to regulate Tetranychus urticae

1. 01Side effects of pesticides on beneficials — Koppert, searchable database
2. 02Side-effect manual — Biobest
3. 03Pollinator and beneficial insect toxicity — Intermountain Tree Fruit Production
4. 04Comparative toxicities of novel and conventional acaricides
5. 05Mode of action classification — Insecticide Resistance Action Committee (IRAC)
6. 06Gaining control over spider mites — Green Coast Hydroponics PDF · archived
7. 07Tank mixing revisited — GPN magazine PDF · archived
8. 08Managing houseplant pests — Colorado State University Extension
9. 09Insect control, soaps and detergents — Colorado State University Extension