If there is one worm bin hassle that frustrates vermicomposters - especially new vermicomposters - more than anything else, it’s probably fruit flies and/or fungus gnats. In this “critter series” installment we’ll look at what they are, why they are such a pain, how to help prevent them, and what to do if/when they do arrive. We’ll also spend some time looking at a few other types of flies that can make their presence known in composting systems.
Let’s get things started by looking at how flies fit into the overall critter family tree!
Classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Suborder: Nematocera (gnats, mosquitoes)
Suborder: Brachycera (more typical flies such as house flies, fruit flies)
All flies and gnats are part of the order Diptera - referred to as the “True Flies”. This is an incredibly diverse order of insects that can appear - as larvae, pupae, or flying adults - in most composting systems, especially when located outdoors.
What do they ALL have in common?
- Fast reproduction and life cycle (when conditions are warm).
- Waste-feeding larvae and usually non-feeding, flying adults.
- Opportunistic breeding/feeding habits, often targeting rich food deposits.
People often confuse fruit flies with fungus gnats, but they are actually in completely different sub-orders (Brachycera and Nematocera, respectively) and are quite different in appearance, especially upon closer inspection. They also tend to thrive in somewhat different environments.
Fruit flies look like a miniature version of a house fly (see image at beginning of article). They have prominent, colorful eyes, and their bodies tend to be a somewhat lighter, brownish colour.
Fungus gnats tend to have a more delicate body - almost like a miniature mosquito (they are both in the same suborder) - that is often darker in colour.
Unfortunately, capturing really good images of either one of these organisms - let alone together - can be quite challenging, but the good news is that there are countless great photos of both online!
Here is a helpful page showing a visual comparison of the two:
Fruit Flies vs. Fungus Gnats - What's the difference?
Fruit flies typically thrive in newer systems with a lot of rich food scraps in them. Excess fruit and veggie waste, especially when it has already been invaded by eggs/larvae (before being added to the system), can really provide a launch pad for a population explosion.
Most gnats, on the other hand, tend to thrive in more mature, often lower-nutrition systems. This can make them especially tricky to get rid of (e.g. simply adding a lot more bedding and removing food scraps likely won’t have the same impact on a serious infestation of gnats as it will on fruit flies).
Fruit Fly & Gnat Prevention Strategies
They say an ounce of prevention is worth a pound of cure. This definitely applies to flies and gnats! The more you can completely prevent them from becoming established in the first place, the better.
How can we do this?
1) Ecosystem Diversity - a diverse ecosystem means you have more organisms, and usually more balance of different trophic groups as well. Predators, such as rove beetles (pictured on right), and predatory mites help to keep the fast-breeding consumers in check. Other consumers, such as springtails, mites and isopods can serve as competitors, helping to reduce the amount of available food for fly/gnat larvae. If you look closely at a rich, outdoor ecosystem - such as what you find in old heaps of bedded manure - you can see this sort of amazing diversity at work, and actually adding these types of living materials to your composting systems can help to boost populations of beneficial organisms (living materials offer other great benefits as well) .
2) Lots of Bedding - Fruit flies and gnats tend to be drawn to wetter conditions and materials that offer a lot of food value (again, these are often different for fruit flies and gnats). Maintaining an abundance of newer, absorbent bedding materials - especially at the top of the system - can help to make the environment less appealing to these pests.
It can also serve as a physical barrier, making it more challenging for them to make their way down to their food source.
NOTE: Apart from maintaining a thick layer of dry-ish, loose bedding, adding some form of worm blanket directly over the worm habitat zone can give you an extra layer of protection against flying pests.
3) Optimization & Moderation - Similar to the previous strategy, the more we optimize food and the environment for the worms (and other beneficials), and the less we cater to the flies/gnats, the greater our chances of avoiding a population explosion. Feeding in moderation is an important part of this, since we only want an amount of food that the worms can process in a timely manner. Lots of extra food means lots of extra material for the flies to invade.
Chopping/grinding, freezing/thawing, and aging (leaving scraps to sit for a period of time) are ways to help make food more microbe- and worm-ready. Freezing has an added benefit of ensuring that you aren’t introducing any contaminated foods (e.g. fruit fly eggs and larvae can arrive in the skins of fruits and vegetables during warmer times of year).
Speaking of which…
4) Avoiding Contaminated Materials - A very common way for pests to get established in a system is by adding material from another system, especially one that has been sitting outdoors. Be very careful about adding material from an active backyard composter or any other outdoor system to an indoor worm bin, for example. As touched on, freezing scraps can be another way to help avoid the introduction of unwanted eggs and larvae.
5) Strategic Location Selection - Where you put your worm bin can influence the likelihood of you ending up with a flying pest invasion, especially at certain times of year. E.g. Keeping a worm bin in your kitchen puts it closer to where fruit flies often appear during warmer times of year. Similarly, if your system is near potted plants, you may increase the likelihood of ending up with gnats.
Systems in outdoor locations are extremely challenging to keep pest-free. The good news is that they shouldn’t be nearly as annoying in that type of environment, but this is a very important consideration if you plan to keep regular worm bins outdoors for warmer parts of the year, and then bring them indoors.
Dealing with Established Populations of Fruit Flies & Gnats
Reducing the chances of ending up with a flying pest invasion is a great way to go, but it’s important to prepare yourself for the (virtually inevitable) arrival of these organisms somewhere along the way on your vermicomposting journey.
Once this does happen, the key is to act fast and to employ a multi-pronged strategy for dealing with the situation.
1) Stop feeding and remove all unprocessed food materials you see in the system - especially bigger chunks of fruit and veggie scraps. We recommend tossing these materials in a bag and putting them in a freezer temporarily (it will kill fruit fly eggs and larvae and will preserve the scraps until you are ready to use them again).
2) Add lots of new bedding to the system - this will help to soak up excess moisture and make it more difficult for fruit flies to get in and out.
3) Use trap and "bait" systems - fruit flies go crazy for anything that is starting to break down and ferment. If you happen to have some apple cider vinegar, that can attract them really well, but something like apple juice with some baking yeast in it can be incredibly effective. If you put saran wrap over a small dish with the attractive liquids and punch a few holes this can serve as a basic but surprisingly effective trap.
If you’re just as happy to purchase your traps, there are various options available (such as the fruit fly trap pictured to the right)
Aside from actually trapping them, when you are able to lure them in, you can also...
4) Vacuum up lots of adults - this might sound really funny, but vacuuming clouds of "breeders" can actually be a surprisingly effective way (when combined with other strategies) to bring about a population crash. Having your bait traps set up, ideally, close to your system, will make it a lot easier to concentrate the flies, but you should also be able to suck up a lot of them simply by turning on the vacuum and opening up your system. For best results, do this multiple times a day.
5) Use parasitic nematodes - this one is particularly well-suited for dealing with fungus (and other) gnats, in situations where a population has gotten really out of hand. There are various nematode species used for this purpose, but Steinernema feltiae is likely the most common choice for fungus gnats, and the one we’ve found to work very well.
While we’re on the topic of biological controls, it’s worth noting that some people have reported success with using “Mosquito Bits” (like Mosquito Dunks, but in granular form) to control various small flying pests, while others recommend predatory mites. We haven’t found success with either of these, but they may still be worth testing out.
Other “True Flies” You Should be Aware of
It is important to note that there are a wide range of different dipteran flies that can end up invading a vermicomposting system, especially outdoors. So, we recommend that you never assume you know exactly what flies you are dealing with. Below you’ll find a few, apart from fruit flies and gnats, that are worthy of mention.
Scuttle Flies - A very common type of fly that can show up - especially in high-nutrition outdoor systems - is the “Scuttle Fly”, also known as a “Coffin Fly”. These are definitely flies you want to prevent versus combat in indoor systems, since they can be very challenging to deal with once established. In terms of appearance, they basically look like very large fruit flies - but likely the easiest way to ID them is based on the way they move. Unlike fruit flies and gnats, you’ll never see clouds of scuttle flies in the air. They can fly, of course, but they actually spend a lot of time running. As a result, they are even better adapted for tracking down, and getting into, rich food deposits than fruit flies or gnats.
Soldier Flies - The most widely-known variety of this family (Stratiomyidae) is the Black Soldier Fly (Hermetia illucens). While they are reportedly found in southern regions of Ontario and BC during summer months, they are much more abundant in warmer regions of the USA and other parts of the world. A more common Soldier Fly in Canada is the Yellow Soldier Fly - Ptecticus sp (shown here).
If you find large, active maggots in your outdoor systems (especially systems containing rich food wastes), there’s a decent chance they are some type of soldier fly larvae. Black Soldier Fly larvae in particular are known for their incredible waste-processing abilities, and tolerance of very hot and foul conditions.
House Flies / Stable Flies - These are two closely related members of the family Muscidae. We’ve included them together since the adults are almost identical in appearance. Both are attracted to rich wastes, but house flies tend to be drawn to particularly foul materials (e.g. rotting meat), while stable flies tend to be found in close proximity to livestock (and their larvae and pupae are frequently found in livestock manure). One even more significant difference between them is that adult stable flies bite - so you’ll likely know fairly quickly if you’ve found them! 😉
If you end up with typical house fly maggots in a composting system, it likely means you’ve added waste materials not ideally suited for typical composting, or you’ve allowed the system to become putrid in some other way. Removing foul materials and adding plenty of bulky bedding and living materials should help to remedy the situation fairly easily.
As annoying as many members of order Diptera can be for us composters (especially when attempting to compost indoors), it’s always important to remember that they play a very valuable role in the global decomposition of organic wastes. Thankfully, with proper management and fast action (when needed), we can usually limit the major outbreaks to outdoor systems/habitats.
Stay tuned for more installments in our ongoing critter series!
Other Articles You May Be Interested In
An Introduction to Worm Bin (Macro) Invertebrates
Springtails - White Plague or Worm Ally?
How to Feed Your Worms to Get The Best Results From Your Worm Bin
Smelly Worm Bins - Possible Causes & Effective Solutions
What Is “Living Material”?
Bedding - The Most Important Material in Your Worm Bin?
DIY Plastic Worm Composting Bins
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