Matt Bertone
I was recently asked by a few plant pathologists to clarify what pest codes should be used for various flies called “fungus gnats.” I was delighted and thought, “This is my time to shine!” Fungus gnats were part of my dissertation research regarding the phylogenetics of the lower Diptera (=“Nematocera”), those gangly, fragile, true flies including mosquitoes and midges.
“Fungus gnat” is a general term for various members of the superfamily Sciaroidea. This group is hugely diverse, containing >13,000 species with tens of thousands of species that remain undiscovered or undescribed. In fact, a recent survey of a site about the size of two football fields in Costa Rica was found to have over 800 species of one of these families, the gall midges (Cecidomyiidae)! The superfamily Sciaroidea contains several families of flies. Some of them are rarely seen and not well known even among entomologists, including Ditomyiidae, Diadocidiidae, and Bolitophilidae. Others are very common, and these are the ones in which diagnosticians may come in contact.
The first one to get out of the way is Cecidomyiidae, or gall midges. Although more primitive members of this family are associated with fungi and rotting understory material (sometimes called “wood midges”), most people know gall midges for attacking plants and causing damage directly or through gall formation. Well-known members include the hessian fly (Mayetiola destructor) and the swede midge (Contarinia nasturtii), but rarely are Cecidomyiidae called “fungus gnats.” That brings us to the three main families of common flies that are regularly called fungus gnats: Mycetophilidae, Keroplatidae, and Sciaridae.
Mycetophilidae are considered the fungus gnats that don’t have qualifiers in their name, i.e. representing common or strict fungus gnats. Many of the families in Sciaroidea were lumped in this group before being split out over the years. Adult mycetophilid fungus gnats (Figure 1) are common at lights and in many natural areas, especially forests. They generally have a compact body with long, very spiky legs (the tibial spurs are particularly distinct). They will have separated compound eyes and often have two to three ocelli, the simple eyes near the compound eyes. Larval mycetophilid fungus gnats are mostly associated with the large fruiting bodies of various fungi, though some live in rotting wood or animal nests. Larvae of Mycetophilidae are often pale white with a dark head, and they often have transverse rows of small spicules on their belly used to help movement. Furthermore, the underside of their head is open in the back, without a bridge connecting the two sides (the underside looks like a “V”).
Keroplatidae are called predatory fungus gnats. Although a smaller family, they are fairly common. They are usually more diverse in adult form than the Mycetophilidae, some having amazingly long antennae (e.g., Macrocera; Figure 2) or large flattened antennae (e.g., Keroplatus). Adults are difficult to generalize, but larvae are extremely worm-like and commonly found when flipping logs or looking under bracket fungi (Figure 3). These larvae spin slimy, silk webbing to collect fungal spores and trap small arthropods to consume, hence the name “predatory fungus gnats.” Keroplatidae also exhibit some really interesting biologies, including species with bioluminescent larvae (e.g., Australian/New Zealand glowworms and a species in the Southeast U.S.) and even a genus that is a parasite of land flatworms (Planarivora)! Although adult predatory fungus gnats are separated from other fungus gnats mainly by characters in their wing venation, the long larvae are fairly distinct, especially with the numerous annulations along the body. These larvae will rarely be submitted to clinics, except in fungal fruiting bodies or rotting wood. They are not associated with plants or plant damage.
Figure 2.
Figure 3.
Finally, the family that most of us regularly get in samples: Sciaridae, or dark-winged fungus gnats. These flies are also called “black fungus gnats” by many experts, especially those in Europe, but both names refer to a group of fungus gnats whose members are diverse but largely similar looking as adults and larvae. Adults have a dark body, often also with dark or smoky wings (Figure 4). The most diagnostic features of adult Sciaridae are the tuning-fork-shaped central veins in the wing, along with eyes that almost always meet on the top of the head as a thin bridge (creatively called the “eye bridge”). Note that a few genera, including the economically important potato scab fly (Pnyxia scabei), do not have an eye bridge. Larval dark-winged fungus gnats are white or cream-colored with a very dark to black head (Figure 5). They lack the spicules found on the underside of Mycetophilidae larvae, but they have a narrow bridge almost fully connecting the rear of the underside of the head. Dark-winged fungus gnat larvae are common in potting soil of house plants, under the bark of decaying trees and logs, and elsewhere. However, of these three groups, they are most closely associated with plants, sometimes as primary plant miners (mostly of natural hosts) or as pests of roots and inside small stems, typically in greenhouses. Dark-winged fungus gnat larvae are also sometimes submitted for ID when they form large migration masses during particular times of year, frightening and disgusting homeowners.
Figure 4.
Figure 5.
When it comes down to it, most pathologists dealing with greenhouse pests will be seeing dark-winged fungus gnats (Sciaridae), and these flies probably make up >95% of all identifications of “fungus gnats” made in NPDN labs. We will be working to clean up the pest codes and make the names a little clearer. In general, I would suggest using “Superfamily Sciaroidea (IOBFAAB)” for all identifications where a fungus-type gnat is suspected but the specific family is unknown (including perhaps some Cecidomyiidae). Furthermore, dark-winged fungus gnats (both adults and larvae) are generally difficult or impossible to identify to genus/species, and there aren’t many keys out there to help. Control measures are typically not specific to the type of dark-winged fungus gnat, so even I almost always just use the general code for “Family Sciaridae (IOBFAAA)” when diagnosing them from homes or greenhouses.
I hope that helps clear up some nomenclatural issues in this group. Please feel free to contact me if you have questions!
