Gnats and Flies are Your Friends

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Where do those freakin’ gnats come from? How can I get rid of them? —

We have Fungus gnats and phorrid flies because we keep our vivariums too wet. The best (and easiest) way to get rid of them is to let our substrates dry down, and stop over watering. Simple, right?

Not quite so simple. Keepers over water their vivariums for a number of reasons – all well intentioned, almost all misguided. So it is probably just as important to understand how and why to stop over wetting our vivariums. The first mistake is installing a drainage layer (See: Why We Dont Need or Want Drainage Layers). The second mistake is over using a misting system, (See: How to Stop Abusing Your Misting System). Adding too much water is probably the single biggest mistake we herpetoculturists make with our vivs. There is a whole bioactive industry out there that encourages this mismanagement. (See: Why We Should Avoid Bioactive Suppliers).

A lot of over watering comes from a misunderstanding of the humidity needs of our animals and how to provide it (See: The Humidity Mystery Unraveled). The current paradigm mistakenly states that the best way to raise humidity is to apply water from above. In fact the best way to provide humidity for most of our animals is from the substrate – from below. Even dart frogs and tree frogs do not need daily misting in a properly designed enclosure. In fact daily misting is detrimental to the captive environment you have created as well as the species you are keeping – and not just because it attracts flies!

There is a lot of bad information floating around social media about fungus gnats and phorid flies. Keepers often seem confused about where they come from and how to get rid of them. Lets address each of those areas …

Both fungus gnats (Orfelia and Bradysia species) and phorid flies (Megaselia scalaris) are attracted to moist, decomposing organic matter – like the vivarium you insist on over watering.

Phorid flies are attracted to decomposing animal matter as well as vegetable matter. They are sometimes called coffin flies because of their attraction to dead things – like dead people in coffins (1) or scuttle flies because they tend to run across surfaces rather than fly. (2)

Fungus gnats are attracted to damp, decomposing organic matter. They lay eggs in places like our kitchen drains and under the rims of our toilets. And once established in those places they are very difficult to get rid of. In our vivariums they lay eggs in wet substrate, on rotting fruit or vegetables or decomposing leaves, moss, etc.

Where Do They Come From?

First, where they DON’T come from is potting mix. We often hear that they are brought into the home in bags of potting mix. But any quality potting mix is heated to 140F through the composting process used to create the product – a temperature that will kill insects and their eggs. If bags of potting mix have flies its because they have tears or holes in them that the flies entered probably while sitting on the retailer’s shelves. Just look over bags to make sure they dont have holes when you buy them. (See: Why Potting Mix is the Perfect Substrate)

But the flies certainly CAN come from the green house. Eggs laid in the pots while at the green house can hatch after we get the plants home and planted in our cages, or just while they are sitting in the window looking pretty. People will often bring a plant home and repot it with a bag of potting mix, then blame the potting mix for the flies when in fact it was the plant that introduced the pests.

They can also come from top soil, or products containing top soil that have not been sanitized. (See: Top Soil Doesnt Belong in Vivariums). By the way, in the US top soil is not sanitized or composted and so can contain all the pathogens we would find in nature: viruses like snake fungal disease, parasites like round worms, or pathogenic bacteria. It also quickly compacts, which drives out air pockets killing off aerobic bacteria that we have to have for bioactivity (3). In short – do not use top soil, or any product with the word “soil” in the name, or with “soil” listed as an ingredient.

And they can just arrive from outside like regular ‘ol house flies from the outside.

The problem is that by the time we see one, there are probably dozens of eggs already laid in the substrate or on rotting food or feces. So the most likely answer to the question “Where are they coming from?” is that they are probably coming from our poorly managed vivariums! This is also why trying to eliminate them by killing the adults is often futile – as long as one escapes the problem will continue (4)

How Can I Get Rid of Fungus Gnats and Phorrid Flies?

First what wont work: mosquito dunks. And yes, I know everyone suggests them. The problem here is that in order to be effective at killing the larvae in the substrate you need to create a “drench”, which soaks the soil and which makes the over all situation much worse. Now we have a saturated soil drenched with a bacteria (Bacillus thuringiensis subspecies israelensis or Bti, aka Gnatrol) that killed the fly larvae but which will take weeks to dry out, attracting flies all the time before it does. This approach is counter productive! (5)

The situation is similar with nematodes (Steinernema feltiae), which can work under some circumstances but which require a moist top layer of substrate, which we do not want in our vivs. Simpler solutions are at hand. (5)

The other thing that wont work is sticky traps, for the reasons mentioned above. They are also very dangerous around our herps. An animal that simply brushes up against a sticky trap is often hopelessly tangled by the time we discover the problem. Sticky traps can help reduce the adult population of flies, but the adult flies are already short lived and will be quickly replaced by the hatching young.

What will work is simple: Dry down your vivarium. If you have already saturated your substrate by over watering or over misting, open your cage doors, mix up the potting mix substrate to bring the lower, wetter strata up to the top. Keep the heat up to encourage evaporation. And then STOP OVERWATERING! Once we dry the substrate down to where it should be the eggs and larvae will die off. Problem solved.

We do not have to keep the substrate dry however. Water evaporating off substrate is important to maintain the humidity levels we may want. So when the substrate is dried down well we water again, and water well. Misting the top layer is not what we are after here. We want to water enough to wet the lower levels of substrate – but just barely. Then let the whole thing dry down again – much the way soil will in nature between soaking rains.

Because potting mix has been carefully designed to encourage water movement through it, the moisture in the lower layers will wick up and be evaporated off the surface. This produces humidity for the captives (See: The Humidity Mystery Unraveled) and allows the surface of the substrate to dry down while the lower levels stay slightly damp. Our friends the flies can only use the top inch of substrate to lay their eggs, so with that drier surface they have no way to reproduce. And that IS simple.

Now all you have to do is make sure you dont leave food (animal or plant) to rot in the cage. Flip waste into the substrate, or rinse it in with a hand mister, or spot clean if flies appear to be using the animal waste to their advantage. With a well designed enclosure you will rarely have to even spot clean – spraying waste into the substrate or flipping it over into the substrate with a scoop spoon will be more than sufficient. You may find you dont even have to do that!

1.) https://nhm.org/stories/coffin-fly

2.) https://extension.psu.edu/phorid-flies

3.) https://ag.umass.edu/home-lawn-garden/fact-sheets/bagged-potting-mixes-garden-soils-for-home-gardeners

4.) https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ENTO-433/ENTO-433.pdf

5.) https://ipm.ucanr.edu/PMG/PESTNOTES/pn7448.html

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