Predatory mites
Also known as Phytoseiid mites, Phytoseiulus persimilis, Neoseiulus californicus, Neoseiulus cucumeris
In short
Predatory mites are tiny, fast-moving mites that hunt spider mites, other plant-feeding mites and thrips. They are one of the most widely used biological controls in greenhouses and for houseplants. Because they look like the pests they eat, they are easy to mistake for an infestation.
How it looks
Adults are under 0.5 mm, pear-shaped, shiny and eight-legged, and move quickly compared with the slow pest mites. Phytoseiulus persimilis is bright reddish-orange, about 0.5 mm, with pale salmon nymphs; its oval eggs are about twice the size of spider mite eggs. Neoseiulus californicus is pale orange, peach or pink and only visible well under a hand lens. Unlike two-spotted spider mites they have no pair of dark blotches on the body. They roam over leaf undersides and among webbing rather than sitting still.
Signs
Fast-running orange or pale mites among spider mite colonies, and spider mite numbers and fresh webbing declining.
Life cycle
Egg, six-legged larva, two eight-legged nymph stages and adult. Phytoseiulus eggs hatch in 2–3 days and development takes about 25 days at 15 °C but only about 5 days at 30 °C, so it can outpace spider mites; adults each eat 5–20 prey a day.
What favours it
Phytoseiulus needs humidity: development nearly stops at 25–30 % relative humidity and immatures struggle below about 70 %. Neoseiulus californicus tolerates drier, hotter conditions and can survive on pollen when prey is scarce. Leaves of neighbouring plants touching helps them spread.
Prevention
- Keep humidity reasonable around mite-prone plants; this discourages spider mites and helps the predators.
- Avoid broad-spectrum insecticides and miticides, and control ants.
- Check new plants for spider mites and release predators early, while pest numbers are low.
Treatment
Start with the first step and move down only if the problem persists.
- Release commercially reared predatory mites (for example Phytoseiulus persimilis for heavy two-spotted spider mite webbing, or Neoseiulus californicus as a hardier general option; N. cucumeris and N. fallacis are also sold) following the supplier's instructions.
- Rinse heavily infested plants with water before a release to knock pest numbers down.
- Do not use miticides after a release; if a spray is unavoidable choose a short-residual product and follow the product label.
Pros and cons
Pros
- Work well indoors and in greenhouses, including on houseplants.
- Some species, such as Neoseiulus californicus, also eat thrips and tarsonemid mites (the family that includes broad and cyclamen mites).
- Harmless to people, pets and plants.
Cons
- Phytoseiulus eats only spider mites and dies out once they are gone.
- Sensitive to dry air, some pesticides and residues.
- Hard to see, so success can only be judged by pest decline.
Look-alikes
- Two-spotted spider mites
- Slow-moving, often greenish-yellow with two dark blotches, and live under fine webbing; predatory mites are shiny, pear-shaped and run fast.
- Soil predatory mites
- Relatives that live in the potting mix and eat fungus gnat larvae and thrips pupae rather than hunting on leaves.
- False spider mites
- Flat, red, very slow and without webbing.
Where you find it
Houseplants, greenhouse crops, strawberries and ornamentals affected by spider mites, broad or cyclamen mites and thrips.
Related
Sources
- Phytoseiulus predatory mites — UC IPM Natural Enemies Gallery
- Neoseiulus predatory mites — UC IPM Natural Enemies Gallery
- Phytoseiidae — Wikipedia
Written in our own words from these sources. Check a product label before using any treatment.