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Rove beetles

Also known as Staphylinidae, Dalotia (Atheta) coriaria, Spider mite destroyer rove beetle (Oligota)

Beneficial or harmlessBeneficial
Rove beetle with short wing covers and exposed abdomen Photo: gailhampshire from Cradley, Malvern, U.K · CC BY 2.0

In short

Rove beetles are slim beetles with short wing covers that leave most of the abdomen bare; nearly all are predators or scavengers. One small species, Dalotia (Atheta) coriaria, lives in potting media and is sold to control fungus gnats and shore flies. A few tropical Paederus species carry a toxin that blisters skin if the beetle is crushed.

How it looks

Most are 2–8 mm (range under 1 mm to 35 mm), long and narrow, with very short wing covers and a flexible exposed abdomen. Dalotia coriaria adults are glossy dark brown, finely hairy and 3–4 mm long, running quickly over the surface of compost. The spider-mite rove beetle Oligota is black, about 1 mm, with a pointed, upturned abdomen; its larvae are pale yellowish with fine spines. Paederus species are strikingly coloured, with an orange-red body and metallic blue or green wing covers.

Signs

Small, fast, dark beetles on the surface of moist potting mix or under pots; fewer fungus gnats.

Life cycle

Egg, larva, pupa and adult, with both larvae and adults hunting. Dalotia takes about 21–22 days from egg to adult at 25 °C and 11–12 days at 30 °C.

What favours it

Moist organic material, leaf litter and damp growing media. Paederus beetles are active by day and are attracted to lights at night, which is when they end up indoors.

Prevention

  • Leave small rove beetles in potting mix alone; they are hunting gnat larvae.
  • Avoid soil drenches of broad-spectrum insecticides.
  • In the tropics, keep insect screens on windows and reduce bright lights near open doors at night to keep Paederus out.

Treatment

Start with the first step and move down only if the problem persists.

  1. No control needed. Dalotia coriaria can be bought for greenhouse and propagation use against fungus gnats; follow the supplier's instructions.
  2. For Paederus indoors: never crush it on skin. Brush it off gently or pick it up with paper, and wash skin with soap and water if one was squashed.

Pros and cons

Pros

  • Dalotia prefers fungus gnat larvae and also eats shore flies and thrips pupae in the growing media; it is sold commercially as adults and larvae.
  • Oligota larvae eat about 20 spider mites a day.
  • Paederus and many others prey on crop pests.

Cons

  • Crushing a Paederus beetle on skin releases pederin, which raises a burning rash and blisters within 12–36 hours that can last two to three weeks (Paederus dermatitis).
  • Experiments using general predatory rove beetles for biological control have mostly not been successful.

Look-alikes

Earwigs
Earwigs have obvious pincers at the tail end; rove beetles do not.
Ground beetles
Ground beetle wing covers reach the end of the abdomen.
Fungus gnat larvae
Gnat larvae are legless, translucent white worms with a black head living inside the mix; rove beetles have legs and run on the surface.

Where you find it

Seedling trays, cuttings, potted plants and greenhouse crops troubled by fungus gnats or shore flies; outdoor beds and leaf litter.

Sources

  1. Rove beetle — Wikipedia
  2. Spider mite rove beetle — UC IPM Natural Enemies Gallery
  3. Ecology and Role of the Rove Beetle, Dalotia coriaria, and Insidious Flower Bug, Orius insidiosus, in Greenhouse Biological Control Programs — Scientific Research Publishing
  4. Paederus — Wikipedia
  5. Biological Control of Fungus Gnats — UConn IPM

Written in our own words from these sources. Check a product label before using any treatment.