Fungus gnats in carnivorous plant pots: risks and control

How much harm fungus gnats do

The sources disagree, and the disagreement comes down to what kind of plant is in the pot. For ordinary houseplants, University of Maryland Extension describes fungus gnats as annoying but harmless, saying they do not significantly damage indoor plants ^[3]. UC IPM takes a middle line: adults do no damage, but large numbers of larvae can damage roots and stunt growth, especially in seedlings and young plants, and have killed houseplants in moist, organically rich soil ^[2].

The ICPS treats them as a real hazard for carnivorous plants, and especially for small ones. Fungus gnat larvae eat seedlings of most species grown in peat or coir, damage Pinguicula whose leaves touch the soil, and may tunnel into stems. Larvae of fungus gnats and shore flies have also been shown to spread root-rotting fungi, and the wounds they make can help pathogens in ^[1]. Wisconsin Extension likewise notes that larval feeding wounds give root rot organisms an entry point ^[4].

So a few adults around a mature Sarracenia are a minor issue. The same gnats in a pot of seedlings, leaf cuttings or butterworts need action.

How to confirm fungus gnats

• Adults: dark, delicate flies resembling tiny mosquitoes, usually about 1.6 to 3 mm (1/16 to 1/8 in) long, with long antennae. They are weak fliers and tend to run across or rest on the soil and leaves ^[2].
• Larvae: slender, whitish or translucent maggots with a shiny black head, up to about 6 mm (1/4 in) long ^{[4, 2]}. In very wet pots they can leave slime trails on the soil surface ^[2].
• Not to confuse with shore flies: these are sturdier, strong fliers with short antennae and spotted wings, and they feed on algae ^{[4, 2]}.

To check a pot, put a yellow sticky trap in it for adults, or lay pieces of raw potato, cut side down, on the soil for larvae and look under them after three or four days ^{[2, 4]}. If you cannot tell which pot the gnats come from, Maryland Extension suggests covering a pot with screening or cheesecloth for a few days to see whether flies appear ^[3].

Populations build fast. At 24 °C (75 °F), eggs hatch in about 3 days, larvae feed for about 10 days, and adults emerge about 4 days later, so a new generation can appear in roughly 17 days ^[2]. A female can lay up to 200 eggs ^[4].

Why the usual advice does not fit

Standard extension advice is to let the soil surface dry between waterings and improve drainage ^[2], or simply to dry the pot out until the larvae die ^[3]. That works for most houseplants but not for bog plants: the ICPS checklist tells flytrap growers never to let the water dish dry out ^[5]. As the ICPS puts it, the wet, peaty soil that fungus gnat larvae prefer is also the preferred soil of many carnivores ^[1]. Instead of changing the water, carnivorous plant growers change the surface.

A sand layer on the soil

What to do. The ICPS recommends a 5 mm layer of medium to coarse horticultural sand on top of the soil, which discourages adults from laying eggs. Straight play sand or builders sand will not do because the fine grains defeat the purpose; sieved and washed builders sand can work, and washed 12 or 16 mesh blasting sand is best because the grains are even. Pinguicula and Cephalotus seem to do even better with 5 to 10 mm of sand, and most mature sundews do not mind it. The layer also helps keep down moss ^[1].

Other ICPS pages use the same trick: Mexican butterworts get up to 5 mm of coarse silica sand after planting ^[6], repotted temperate sundews get a shallow sand layer that also deters crane flies, whose larvae can devastate a pot of sundews ^[7], and the CPN article on indoor flytraps notes that its pure sand top layer is not attractive to fungus gnats ^[8]. The soil guide covers sand types.

Plastic bags for seedlings and cuttings

What to do. Seedlings and leaf cuttings are too small for a sand layer, so the ICPS keeps them in plastic bags until they are big enough ^[1]. Its subtropical sundew guide uses bags both for humidity and to keep fungus gnats out, and suggests leaving seedlings in them until they are large enough not to be killed by the larvae ^[9]. Nepenthes seed pots are bagged for the same reason ^[10]. See sowing seeds and sundew leaf cuttings.

Biological drenches

Bti. The ICPS names a relatively safe product made essentially of spores of Bacillus thuringiensis subsp. israelensis (Bti), used as a soil drench ^[1], and its Mexican Pinguicula guide points growers to an ICPS webinar on Bti products for protecting plants from fungus gnat larvae ^[6]. UC IPM explains that Bti is toxic only to fly larvae, gives temporary control, and does not persist indoors, so repeat applications are usually needed. The Bt sold for caterpillars is a different subspecies and does not work on fly larvae ^[2]. Use the dose and interval on the product label.

Nematodes. UC IPM also lists the nematode Steinernema feltiae, which works in moist conditions between about 16 and 32 °C (60 and 90 °F) and can establish itself after several applications. It considers predatory Hypoaspis mites a poor choice for individual houseplants ^[2]. None of the sources reviewed here reports trials of nematodes on carnivorous plants.

Chemical insecticides

The ICPS advises against chemical pesticides for fungus gnats unless you are a commercial grower with special training, and even then to follow the label and test a few plants before treating the rest ^[1]. Butterworts are a particular concern: the ICPS warns that certain pesticides will harm and possibly kill Pinguicula ^[6]. UC IPM adds that insecticides are rarely warranted for fungus gnats in and around homes ^[2].

Preventing fungus gnats

• Top pots with 5 mm or more of coarse, washed sand ^[1].
• Keep seedlings and cuttings in bags until they can take a sand layer ^[1].
• Do not overfertilize, which feeds the fungi and algae the larvae live on ^[4].
• Use clean, pasteurized ingredients rather than incompletely composted organic matter, and do not move infested pots near clean ones ^[2].
• Feed small plants by hand rather than relying on adult gnats as food ^[1].

If you see other insects on the leaves rather than flies around the pot, see aphids, mealybugs and scale, or run the troubleshooter. Butterwort growers will find more in the Pinguicula genus page.