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Black soldier flies might be the most human-friendly insects out there. They can bring your compost setup to a new level, allowing you to re-use more of your waste. They also produce unlimited fertilizer to improve your garden, and act as feed for your pets and other animals. They’re incredible insects, but they’re not invincible. Like all bugs, black soldier fly larvae (BSFL) have a certain tolerance to heat and humidity.
In the summer, it can be hard to keep your BSFL compost bin humid and cool, especially if you keep it outside. In this article, we’ll talk about some of the nuances of long-term maintenance and care with BSFL, so your compost bin can stay active year-round.
Black Soldier Fly Biology
The black soldier fly (Hermetia illucens), is maybe mankind’s best (though also most under-appreciated) friend. This unassuming little black fly should be a must on any homestead, urban farm, in any garden, or near any chicken coop. Even if you don’t have any of the above, black soldier flies can produce feed for reptile pets and other livestock.
It works like this. The black soldier fly starts life as a larva, where it lives in a specially-made compost bin in your yard. You supply your BSFL with whatever kind of organic waste you might have around. They can compost all the normal stuff (vegetable scraps, etc.) as well as dairy, meat, citrus, and even animal waste.
Their digestive systems are so efficient that they can turn just about anything into fertilizer and feed. As they eat, they turn your waste products into frass, which is as good as gold for your garden. They also make liquid fertilizer that you can spray on your plants.
When the larvae get big enough, they undergo a change and prepare to turn into flies. They will try to escape the compost bin, crawling up a specially-made ramp into a collection bucket. At this point, you can use them as feed for just about anything. They have good nutritional value and are the only insects currently approved by the FDA as animal feed.
If you let some of them pupate, they will become flies and keep biting, disease-spreading houseflies out of your house. It’s a win-win-win-win situation.
Risks of High Heat
Or at least, it all sounds pretty straightforward and simple. But how you set your BSFL compost bin up is going to depend a lot on your lifestyle. Most folks will opt to keep their BSFL in the garage or outside near their garden. Compost bins aren’t exactly squeaky clean, after all, and it’s nice to keep your BSFL close to the garden so you can collect and distribute their fertilizer easily.
But when the summer heat starts to beat down, you might run into issues. Black soldier fly larvae are fairly sensitive to heat and humidity, and need certain conditions to thrive. Ideally, you want your BSFL between 70 and 85°F with 50 to 70% humidity.
As you may know, the composting process can produce heat on its own. It’s not uncommon for your BSFL to raise their own temperature 10 or more degrees just by eating and doing their thing. This is helpful in the winter, when you’re worried about keeping the temperature up. But adding the summer sun to that can push the internal temperature of your bin into the 90s and 100s.
And that’s risky. BSFL will slow down and compost more slowly at higher temps. Above 85, their metabolism will slow, and they’ll stop eating. Above 100°F, they will quickly start to die off. A hot summer day in the wrong conditions could easily kill your whole BSFL colony.
Passive Temperature Control
The first thing you should think about is how to control the temperature in your compost bin passively. This means keeping it somewhere cool and well-ventilated.
Ideally, BSFL composting works best indoors. Some setups involve using a few different shallow bins, which you can store in shelves, for different sized larvae. As your larvae grow, you can dump your bins over a sieve to easily collect their frass and remove uneaten food scraps. Best of all, you’ll have the benefits of indoor temperature control to keep things perfectly adjusted.
But if you have to keep them outdoors, your biggest priorities should be shade and ventilation. Keep your bin out of direct sun. Consider swapping out the normal lid for a mesh screen, or making air holes to promote ventilation. When making your compost bin, avoid black plastic.
One issue to keep an eye on is the balance between ventilation and humidity. More ventilation means it stays cooler, but it can also dry out your compost bin. A more direct approach to ventilation is to cut a hole in the side of the compost bin and add a fan that blows air directly inside. This will ventilate without allowing so much humidity to escape.
Beating the Heat
Adding water to your compost bin can also keep the temperature down, depending on how you do it. Adding moisture to your compost bin directly can increase the humidity and keep the temperature down at the same time. We recommend opening your compost bin up fairly regularly and spraying it down to keep things wet inside.
Ice does the same job, but even more efficiently. You can give your BSFL ice cubes or frozen bits of food, which will slowly thaw in the compost bin and keep things cooler. Another method is to freeze water bottles or ice packs, which you can bury in your BSFL’s bedding.
But it’s important not to let things get too moist. Ideally, you don’t want to go over 70% moisture in your BSFL bin. Like with reptile pets, it can be helpful to add a thermometer and hygrometer in your BSFL compost to keep an eye on the conditions when it’s hot and dry.