An Entomologist by training, it generally takes a lot to gross me out. (I’m constantly suppressing shrieks of “It’s adorable!” when growers show me aphids). But now that it’s dark and wet in the greenhouse, there’s been a sudden appearance of a rather unlovable pest some growers have been referring to as “Pot Worm”.
Not an actual worm at all, this pest is a lover of over-watering and fungal production. Read on to find out what it REALLY is, and how to control it.
Appearing over the last few weeks in crops like Kolanchoe and Christmas Cactus, these worm-like pests had a few distinct characteristics making them unlike anything else in the greenhouse:
- Long, dark brown/black larvae inhabiting the top layer of media
- “Slime trails” on the surface of the media
- Balls of loose webbing, either near the pot edge, or affixed to the underside of lower leaves
A quick inspection under the microscope revealed they definitely weren’t related to earth worms. (True “pot worms” can appear in compost and other material with high organic matter, but tend to be white and highly segmented near the ends, and are unlikely to occur in sterilized potting mix). Instead, these were fly maggots; most likely a species of fungus gnat or midge.
The balls of webbing were actually loosely protecting larvae that were pupating. Here’s what the pupae look like:
The National Identification Service in Ottawa confirmed that samples were indeed a type of fungus gnat. The “normal” fungus gnats we regularly see in the greenhouse are Dark-Winged Fungus gnats, in the family Sciaridae. The ones we’re suddenly seeing are in a closely related family, Mycetophilidae, which literally translates to “fungus-loving” (finally, my high school Latin comes in handy!!!). The actual species identified was Sclophila mississippiensis.
I’m guessing that no one wants to say “Mycetophilidae” on the regular, and these flies don’t have an established common name, so I’ve decided to call these guys “Pot Gnats”. (Oddly, neither of the growers involved wanted me to name it after them…)
Below is a picture of the adults side by side. They are fairly very similar looking, so it’s probably hard to distinguish Dark Winged Fungus Gnats from Pot Gnats when stuck to sticky cards. Identification is therefore best determined by signs on the soil media.
The good news here is that this type of fungus gnat is actually less damaging that dark-winged fungus gnats. Mycetophilids are only known to eat the fruiting bodies and spores of fungi or decaying organic matter. They are NOT known to burrow into or eat plant roots. Ultimately, their damage is likely purely aesthetic.
However, it is unknown at this time if these flies can transmit plant pathogens. (Dark-Winged Fungus Gnats are known vectors of diseases like Fusarium and black root rot; see this article), so controlling their spread is still important. Plus, they are yucky.
The easiest form of treatment for these guys seems to just to watch your watering; at this time of year, with the cloud cover and lower temperatures, it can be easy to over water. This promotes the production of fungi, mosses and algae that attract pests like fungus gnats, shoreflies, and even high populations of collembola (springtails) or potato bugs. It’s also likely to encourage plant diseases like pythium.
Other than that, we’re guessing the control of Mycetophilid fungus gnats is pretty much the same as dark-winged fungus gnats. Options include:
- Insect growth regulators like Dimilin (diflubenzeron) or Citation (cyromazine) that are fairly safe to use in conjunction with biocontrol programs
- Predators like Hypoaspis mites or the rove beetle Atheta (Dalotia) coriaria
- The use of entomopathogenic nematodes like S. feltiae or S. carpocapsae. Both like to inhabit the top 0-3 inches of soil, where Pot Gnat likes to hang out, but S. feltiae may be better this time of year as it performs better in cooler temperatures.
Overall, this “novel” pest is likely not one to be terribly concerned about, but it’s presence does point to issues such as over-watering or overly humid conditions in the greenhouse.