From what I've seen online, I'm fairly sure that there are a lot of people putting thier BSFL bin in the sun, I mean it feels like the obvious move, right? The adult flies mate best with natural sunlight, the larvae grow faster when it's warm, and a sunny corner of the yard seems like the logical place to place the bin, The bin has a lid, right? Well you know what else has a lid? A boiling pot!
You've probably already put this together, but just in case you haven't. Your grubs are already making their own heat. A bin in full sun isn't a warm bin, it's an oven, and the larvae are just in there cooking.
Your larvae are little furnaces
A pile of feeding BSFL generates a surprising amount of heat all on their own, just from eating and moving around. Researchers who measured this found that larval activity pushed the substrate temperature at least 10°C above the surrounding air, that's roughly 18°F over ambient, coming from the grubs alone (Li et al., 2023, Insect Science).
So picture a mild 80°F afternoon. Inside a busy bin, the substrate where the larvae actually live can already be sitting near 98°F before the sun touches it.
Now put that same bin in direct sun. The lid heats up, the plastic walls heat up, the air inside heats up, and none of that internal furnace heat has anywhere to go. The sun's heat and the colony's heat stack on top of each other. It's not unusual for a sun-baked bin to run into the high 90s and past 100°F internally on a day that felt perfectly reasonable when you walked outside.
Where the danger line actually is
BSFL are tough, but they're not invincible, and the window between "thriving" and "dying" is pretty narrow.
The comfortable range for a working colony is roughly 70 to 85°F. In that band they eat hard, grow fast, and generally take care of themselves. Studies on development tend to agree that the optimal temp is somewhere around the mid 80s to mid 90s°F depending on what they're being fed, but that's the temperature of the larvae, not the air, and remember they're already running hot on their own.
Push past the mid 90s and things start to slow down. The grubs eat less. Above roughly 100°F and you're on borrowed time, that's where they stop feeding entirely, and start to migrate away from the food to find somewhere cooler, then they'll start dying off. The research on lethal limits puts the upper thresholds for the different larval stages somewhere between about 99 and 111°F (37 to 44°C), and a 2025 study found that sublethal heat damage stacks up well before the temperature ever gets high enough to kill them outright (Chia et al., 2018, PLOS One; Schow-Madsen et al., 2025, Functional Ecology).
Sunny bins get hot fast. Considering all of the factors, you can cross that danger line on an afternoon that never felt hot to you at all. By the time you notice grubs crawling up the walls and away from the food, the damage is usually already done.
But wait, don't the flies like sun?
I know what you're thinking, but everything says you need sun for the adult flies to mate. And you do, but they're not mating in the bin, and they don't mate while baking in the direct sun either. They need the sunlight, but they can move in and out of it. They aren't going to lay eggs in a place that they find hostile to their babies. And the sun is the most hostile thing for a BSFL.
Adult flies mating in the open air is a totally different situation from a plastic bin full of larvae sitting in that same sun. The flies can move. They warm up, they mate, they fly off to a cooler spot when they want to. Your larvae are trapped in a container with no way to escape the heat you've built for them. Good airflow and some ambient light around your setup is great. Baking the bin itself is not.
Just keep it in the shade
So do me a favor and just put your bin in the shade, under a tree, on a covered porch, along the north side of the house, in a shed or garage that doesn't cook in the afternoon. Full shade, not "morning sun and afternoon shade." The larvae will make all the heat they need on their own, that part is genuinely handled for you.
If you're somewhere that gets truly hot in the summer, shade alone might not be enough on the worst days, make sure there is plenty of airflow. I also recommend getting a little hygrometer/thermometer so you can see your bin's actual internal temperature and humidity. But sun exposure is the first and biggest mistake, and it's completely free to avoid.
Sources: Li et al., "Black soldier fly (Diptera: Stratiomyidae) larval heat generation and management," Insect Science, 2023. Chia et al., "Threshold temperatures and thermal requirements of black soldier fly Hermetia illucens: Implications for mass production," PLOS One, 2018. Schow-Madsen et al., "Crossing thermal boundaries: Quantifying the impact of sublethal heat stress on growth in black soldier fly," Functional Ecology, 2025.
