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Nov 15, 2009

How To Keep Insects Out of a Composting Toilet

The biggest myth that you will hear about composting toilets is that they do not attract any insects. The truth is, the compost in most composting toilets is very attractive to a particular type of insect; fungus gnats.

How To Guide

Composting toilets really are not attractive to other insects due to the fact that the waste is decomposing so rapidly. The fungus gnat is really the only pest that one needs to guard against.

Fungus gnats are primarily attracted to decaying organic matter, mostly because of the amount of fungi that is present on decaying organic matter. This means that the waste in a composting toilet will always be attractive to these pests.

Fungus gnats, while unsightly, are not biting insects, nor are they harmful to humans. For most people who own a composting toilet, the problem with fungus gnats is the “ick factor” of having an insect fly around that you know has been crawling over your poop. For this reason, most composting toilet owners want to get rid of them.

Keeping them Out

In order to prevent introduction of a fungus gnat population, specific moisture guidelines should be followed. If the composting toilet is too dry (which is most frequently the case), fungus gnats will populate like mad. They love dry but damp conditions. If you up the moisture level to where it should be for an ideal compost, which is between 40 and 60 percent, you will be far less likely to have a fungus gnat population.

For most dry composting toilet owners this means adding a bit of extra water to the pile (on top of urine) once every few days. The pile should look dark brown, not light brown as it would be if it was dry. If you want to be extremely adventurous you can purchase and use a moisture meter; the ideal range is between 4 and 6 for the meters which range between 1 and 10, with 5 being the most preferable number. You will also notice that toilet paper is not breaking down quickly enough when your compost pile is too dry; if your toilet paper is disappearing quickly then your moisture level is correct.

If you have a composting toilet with a fan, it should be turned off when not in use. If you don’t want to smell urine when you unplug the fan, you can set it up on a timer to turn off after you leave, and go on for a short time per day while you are gone. If a fan is constantly drying out the compost, you will get gnats.

This measure will not work to kill an already thriving population; it will only control it. To kill them, you actually do need pesticides.

Why Not Natural?

While it would be excellent to use more natural remedies like tea tree oil or pyrethrins in composting toilets to kill off an insect bloom, organic and natural ingredients do not have the necessary persistence to kill off the eggs. If you start applying pyrethrins after seeing just one or two gnats, you may be able to get them, but the reality is that naturally sourced treatments just biodegrade too fast in a composting toilet. A lot of consumers will try natural remedies before moving on to the harsher chemical treatments; I would argue that it is best to hit it with the chemicals initially to both ensure that the population doesn’t get out of hand, and to not waste resources.

Degradation of Pesticides in Compost

I had the pleasure of meeting with a scientist at a Canadian university to talk about this very topic. There have been studies which show that Resmethrin and Chlorpyrifros, two popular pesticides, actually break down in compost. This study details how it happens.

Malathion is also recommended in places where Resmethrin is not available. Chlorpyrifros should be avoided as it is more harmful than the other two, although Malathion and Resmethrin are by no means at all friendly.

This does not mean that these chemicals are happy and cozy; they should only be used when necessary. Some may even be banned in your region, and if that is the case, you are left with organic methods only. The environmental benefits that a composting toilet offers far outweigh the need to very occasionally use these chemicals on them. In my experience, a small insect outbreak occurs in each dry composting toilet once every year, if moisture levels are not “kept on top of”. If moisture levels are monitored, you may never see a fungus gnat in your life.

How To Apply

You will want to up the moisture level of the compost to the 40 to 60% mark. If insects have been introduced, then you may need a few good soakings with water to get it where it needs to be. Keeping it there will help control the adult population. The added moisture will also help the pesticide circulate throughout the entire compost mass, so soaking the pile before pesticide application is the best way to go.

The pesticide should be applied throughout the entire unit, not just in the composting chamber. Remember that your unit is vented to the outside, so you will be breathing in a minimal amount of the pesticide. Even so, tie a towel around your nose and mouth or use a mask during application just to be on the safe side. Put on rubber gloves as well to protect your hands.

This must be applied once a day until the insects are gone. If you see one or two, you still need to apply the pesticide as the eggs are still present. I would even advise doing it a day or two after you notice no visible population just to make sure you are hitting the eggs.

Compost Too Wet

You may also notice not just fungus gnats but other kinds of insects if the compost is too wet, or anaerobic. The compost will also smell bad, which it won’t if it is too dry. In order to dry out your compost, leave the fan on and have the males urinate somewhere else. If it is a perennial condition, you may need to introduce wood shavings (anything but cedar - this will kill your compost) into your bulking material to keep too much moisture from being trapped in the peat moss. The same protocol for pesticide application applies here, excepting of course adding water before you begin.

Which Designs Are Less Likely To Get Insects?

Designs that can maintain a good amount of moisture in the composting chamber will give you the greatest measure of success. The more moisture you can keep in the chamber without overwhelming it and making it septic, the less likely it is that you will get insects.

Effectiveness / Result

Insects will never invade a composting toilet where the compost mass has the correct moisture levels.


Gets rid of insects in a composting toilet.

The Facts

Remember that these are suggestions only and you should contact your composting toilet manufacturer for the best way of killing pests in your specific model. Some of these items, such as adding water, may be detrimental or unneccessary in one-chamber designs.

Other Possible Solutions
Au Naturel

If you need to go all natural, the only option is really pyrethrins. This is an ingredient in many "natural" insect sprays that you can buy in local hardware stores. In my experience, application of pyrethrins will only control the insect population, and will never get rid of it entirely unless you catch it really early on in the infestation.

Less Peat Moss

Fungus gnats are naturally attracted to peat moss. If you mix up your bulk as half peat and half wood shavings, you will automatically reduce the incidence of insects. -supergreenme


  1. This is a great post.. Very informative... I can see that you put a lot of hard work on your every post that's why I think I'd come here more often. Keep it up! By the way, you can also drop by my blogs. They're about Vegetable Gardening and Composting. I'm sure you'd find my blogs helpful too.

  2. Thank you for the info! We are getting frustrated and starting to be freaked out with the weird bugs.