A lot of people search for a black soldier fly trap, and I get why. You hear about these grubs, you find out they can chew through your food scraps and turn into free chicken feed, and the obvious next thought is "okay, how do I catch some."
So let me clear up the trap part first, because it's a little different from what you'd expect.
A "trap" for black soldier flies isn't really a trap
When most people say trap, they mean something that catches an insect and kills it, like a fly strip or a wasp trap. That's not what you want here. You're not trying to get rid of these flies, you're trying to invite them to move in and lay eggs.
So what you're actually building is two things working together. One, an attractant, something that smells like a good place to raise larvae. Two, an egg laying site, a dry spot with little cracks right next to that attractant where a female will tuck her eggs. People call the whole setup a trap, but it's closer to a guest room with a sign out front.
The reason this matters is that female black soldier flies, the mamas, don't lay eggs in the wet food. They lay in dry crevices near it. If you only put out a bucket of slop, you might attract them but give them nowhere good to lay, and they'll go find a better crevice somewhere else, maybe in a gap in your shed you'll never find.
How to actually attract them
Here's what tends to work, roughly in order of how much it matters.
Start with the bait. Black soldier flies are drawn to fermenting, decomposing organic matter. Overripe fruit, vegetable scraps, spent grains from brewing, even a bit of chicken feed wetted down all work well. Fruit and anything fermenting seems to pull them in especially hard. Keep it moist, not soup, just moist, and let it get a little funky. That smell is the whole invitation.
Put it somewhere warm and somewhat sheltered. These are warm weather insects. They get active when it's warm out and they basically shut down when it gets cold, somewhere around 54 degrees Fahrenheit and below they mostly stop. So a sunny, sheltered corner in late spring through summer is your window. A spot with a little shade in the hottest part of the day is even better so the bait doesn't cook.
Give them a place to lay. This is the part most people skip. Set a piece of corrugated cardboard, with the open flutes facing the food, in a dry spot just above or beside the bait. Those little cardboard channels are almost exactly what a female is hunting for. This is the same idea behind a purpose built egg collector like The Coop, it just gives the mamas the dry crevices they want and gives you a clean way to pull the eggs out instead of peeling apart soggy cardboard.
Then leave it alone. Check it, but don't tear it apart every day. You're waiting to see small clusters of pale yellowish eggs packed into the dry crevices. That's the win.
The honest part, this doesn't always work
I'll be straight with you, attracting wild black soldier flies depends a lot on where you live.
In a lot of the southern and coastal US they're everywhere in summer and you can pull them in without much trouble. Here in Colorado, and anywhere with a shorter warm season or colder nights, wild populations are thinner and less reliable. Some years you'll get them, some years you'll set out perfect bait and barely see one. I don't want to promise you a yard full of grubs when the weather might just not cooperate.
If you've tried the bait and the cardboard for a few warm weeks and nothing's showing up, that's not you doing it wrong. That might just be your climate.
When to skip the trap and just buy a starter colony
This is the reframe I'd offer. Ask yourself what you actually want.
If catching wild flies is the fun part for you, the project itself, then go for it, it's a genuinely cool thing to pull off. But if what you really want is a working colony producing grubs and frass on a schedule you can count on, waiting on the weather and the local fly population is the slow, uncertain way to get there.
The faster, more dependable route is to start with larvae or eggs you bought, get a colony established in a contained bin, and then let that colony produce its own next generation. Once you've got a healthy population going, you control the breeding instead of hoping the neighborhood flies show up. A setup like our BSFL Bin Kit is built around that idea, give the larvae a place to work and the prepupae a way to crawl out and be collected, so you're not starting from scratch every season.
Frequently asked questions
Do I need a black soldier fly trap to get started?
No. A trap, really an attractant plus an egg laying site, is one way to gather wild flies, but it's the slow and weather dependent route. Buying a starter colony of larvae or eggs is faster and more reliable.
What attracts black soldier flies the most?
Fermenting, moist organic matter. Overripe fruit, vegetable scraps, and spent grain are all strong draws. The smell of active decomposition is what brings the females in.
Where do black soldier flies lay their eggs?
In dry cracks and crevices right next to a food source, not in the wet food itself. Corrugated cardboard with the flutes exposed mimics this almost perfectly, which is why egg collectors are built around it.
What temperature do black soldier flies need?
They're warm season insects. They get active in warm weather and mostly stop developing once it drops to around 54 degrees Fahrenheit, so late spring through summer is the window in most places.
Will a trap work in a cold climate?
Maybe, but less reliably. In shorter season or colder regions, wild populations are thinner. If a few warm weeks of baiting turns up nothing, starting with a purchased colony is the more dependable path.
Is attracting black soldier flies a bad thing for my yard?
No. They don't bite, they don't sting, and the adults don't carry disease the way houseflies do. Them showing up usually just means your compost is healthy and active.
So if you came here looking for a black soldier fly trap, the real answer is to bait them in and give them a dry place to lay, or skip the guesswork and start a colony you actually control. Depends on whether you want the project or the grubs.
