Key takeaways
- Spider mites are arachnids, not insects — adults have four pairs of legs. They’re members of order Acari.
- Two trigger seasons: winter (greenhouse heating dries the air) and summer (oven-like temperatures). Hot and dry equals mite explosion.
- Damage signature: stippling, yellowing foliage, and (in heavy cases) silken webbing strung from midribs — especially from two-spotted spider mite (Tetranychus urticae).
- Reproduction is fast and largely female-skewed — populations can double every few days under ideal conditions.
- Preventive go-to controls: Phytoseiulus persimilis (the “PP” predator), Neoseiulus fallacis, Amblyseius andersoni, and good humidity management.
Spider mites aren’t insects — they’re arachnids (order Acari) with four pairs of legs. Tiny, fast-breeding and almost invisible without a hand lens, they explode in hot, dry conditions and can collapse a crop in days.
Background
Spider mites are not insects — they’re members of the order Acari. The key distinguishing feature is leg count: insects have three pairs of legs; adult spider mites have four pairs. They’re common pests of indoor plants worldwide, with perhaps thousands of mite species across several families known to be phytophagous (plant-eating).
Many growers use “spiders,” “mites,” or “spider mites” interchangeably. The species you’re most likely to encounter are the two-spotted spider mite (Tetranychus urticae), the spruce spider mite, members of the Acaridae, Eriophyidae and Tarsonemidae families — the latter three being so small they require a hand lens or microscope to spot. Cyclamen mites and broad mites belong to Tarsonemidae.
How They Become a Problem
Spider mite trouble can start anytime, but two windows are especially risky: winter, when humidity drops from greenhouse heating, and summer, when temperatures rise to oven-like extremes. In interiorscapes, winter is usually the trigger — lower light, dry air from building HVAC, and warm rooms add up to perfect mite conditions.
Spider mites are notorious hitchhikers. They ride in on people’s clothing, on incoming plants, on tools, and on air currents. Once inside, they explode in dry, warm pockets — usually somewhere along an edge or perimeter, then spread inward. Outdoor crops see seasonal pressure tied to weather patterns, with hot dry spells driving rapid population growth.
A Serious Pest
Most spider mite pests reproduce at alarming rates — populations can double every 3-5 days under ideal conditions. Their feeding collapses plant cells by drawing out sap, causing tissues to yellow and develop a stippled appearance. Symptoms include yellowing foliage, webbing, and ultimately defoliation.
Fortunately, most spider mites are visible to a careful scout with a hand lens. Some species — the Acaridae, Eriophyidae and Tarsonemidae families — are so small that you may only see them under high magnification. By the time their damage is obvious, populations are already advanced. Reproductive biology is largely parthenogenetic in heavy populations, with unmated females producing more females — another factor driving exponential growth.
Some Advice on Spider Mites
Two-spotted mites and a handful of others spin silk. In early infestations, this silken webbing runs from the midrib of one leaf half to the other — forming a tent-like haven where females lay tiny pearl-like eggs and raise their young. The best way to spot these meager beginnings is to eyeball along the midrib against a good light source.
If a spider mite infestation goes unnoticed, the webbing eventually engulfs whole leaves, flowers, and even entire plant tops. Once webbing is obvious, you’re late. Predator releases now will still work, but expect crop losses you wouldn’t have had with earlier detection.
Spider mites multiply geometrically — you don’t have weeks to react. Routine hand-lens scouting on leaf undersides is the single most valuable habit a mite scout can develop.
Signs of Spider Mites
If spider mites aren’t detected and controlled early, before damaged tissue can be outgrown by new plant development, they can cause considerable economic damage. A spider mite feeds by inserting its piercing mouthpart into the phloem and drawing sap. The collapsed cells appear as clusters of yellowish, sunken specks — the classic “stippling.” Mites can also transmit plant viruses, adding another dimension to the damage.
Hand-lens scouting on leaf undersides — especially along midribs and at the base of new growth — is the highest-yield monitoring tactic. Yellow sticky traps don’t catch mites well because mites don’t fly. Tap-test: hold a sheet of white paper under suspect foliage and tap the leaf sharply — mites and predatory mites both fall onto the paper for a quick count.
Solutions to Treat Spider Mites
There are many biological control agents available specifically for spider mites. Amblyseius andersoni and Neoseiulus fallacis can be used preventively — highly recommended given the speed at which mite populations explode. Phytoseiulus persimilis is the gold-standard curative predator, very good at collapsing established two-spotted mite colonies.
Sulfur sprays can help and are reasonably safe for many beneficials — check compatibility before mixing programs. Horticultural oils and insecticidal soaps work as contact knockdowns but offer no residual; they’ll also kill predatory mites already on the crop, so time applications carefully.
Most importantly: manage humidity. Spider mites hate humid air. Misting, evaporative cooling, and dense planting all raise humidity and slow mite reproduction. Combined with preventive predator releases, humidity management can keep spider mites at thresholds where they never become a crop problem.
Predators plus humidity is the formula — Persimilis for outbreaks, Andersoni or Fallacis for prevention, and a humidifier for everything in between.
For over 30 years, Green Methods has been a leader in the production and application of beneficial organisms used in biological pest control and integrated pest management programs.