To tell the difference between male and female black soldier flies, we can start by looking at the lower body part. The male have a more round lower tail whereas the female have a more pointy tail.

The tail

The tail of BSF female is split up like a fork or scissor shape. Whereas the tail of the male is more round.

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If we take a closer look at the male BSF, we can see a more oval tail shape:

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If we look more closely at the female fly, you can see the two-pronged fork structure at the tail:

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Female black soldier fly | Source

A more accurate way to call these parts is the genitals. Let's see the differences next:

Genitals

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The genital of the female looks like a two-pronged fork. The male reproductive organ, as you can see, is more of a claw-like structure. When they mate, the claw-like part of the male will grab & suck the pointy fork of the female in. Then he moves some juices from the chest down to make the fertilization happen.

The stomach

From one researcher's sharing, we can also tell the difference by looking at their stomach or abdomen. The upper abdomen of the female is slightly more reddish. While the top part of the male abdomen is slightly more bronze.

But in a crowded love cage, how can we know which is which? Let's see:

In the love cage

In the breeding area, when we see the flies hanging around the more brightly lit area, then chances are those are the male flies. They need the light source to warm up their bodies for mating & also to see the signal from the female flies that she's ready.

When the female flies are in the mood to mate, she'll stretch her tails out. More bright areas help the male see this signal clearly & get to action. So they buzz around these spots more often. As you can see the flies on the red piece where it's brighter, they are more likely male flies.

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Underneath the red piece is a shaded area where the pregnant female flies are laying eggs. Female flies in general prefer a more cool or shaded area.

You can also tell by the movement of the flies. A black soldier fly, when it lands on something, usually stays very still or does not move around at all. When it lands on a netting or a piece of wood but keeps moving around here & there, you'll know that it's a female fly surveying the area and looking for a good place to lay her eggs.

Now when accidentally catching the flies mating, we'll know more clearly which is which. It was a bit confusing for us at first, but here you can see. The male is on the left & the female is on the right. The male is usually the bigger one.

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Can we determine the sex even before their pupation?

It's not scientifically proven yet or anything. But there's some info that a grower shares from her own experience. It's sort of like the temperature-dependent sex determination in some reptiles, insects or fish.

The grower observes that if we leave the darkened pupae in shade for about 10 days before placing it in the breeding cage, it will tend to pupate out more female flies.

This is one major concern of many growers, that is, they don't have enough female in their population to lay eggs. Although this might be totally wrong, it's worth giving it one or two trials. Maybe you'll find something that works better.

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