The reason black soldier flies are not mating may be because of the density in the raising area or the love cages. With many flies in one place, they may get confused while bumping into each other. They may not find a partner in time. BSF sometimes mate in-flight so too much flying around may disturb the event.

Many growers initially think that by putting lots of BSF in one area they can get more eggs in return. The results are quite counter-productive. If this is the case, divide the population up so they get good space to fly around, get some rest, find a mate & lay eggs.

A good density can be about 10-20 kg of larvae for a 6 square meter raising cage. If each blackened larvae weighs around 0.125 grams, then this is about 80,000-160,000 of flies in a 6m square space. Or specifically about 13,333-26,666 flies per square meter. If you have a love cage that measures 70 x 70 x 140cm or 27 x 27 x 55 in. (width x length x height), you can place about 6000-10,000 flies in there.

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BSF mating | Source

BSF love early morning light for mating. From 9am to 1pm is a great time for mating. A good temperature range is about 25-32C (77-89F). During the winter or rainy season, the mating may be delayed because of the lack of sunlight. With sunlight, the body temperature of the flies will be warm enough to get into reproductive mode. If you're growing it indoors, try some breeding lights.

Also, check the age of the flies. Are the blackened larvae about the same age with each other when you put them in? Although the same larvae from one same egg batch may not pupate on the same day, a more uniform population helps for better chances of mating & fertilizing. So we don't have flies that may die off earlier before the event occurs. For the first 3-4 days, the newborn flies will not be sexually matured yet. So they may not mate at this point. After they've matured, mating will occur the next few days of their lives.

It's good to have a love cage with good sunlight around–not too much or too little at one side. Sometimes, when there's too much sunlight at one side, the flies will be naturally attracted to get there. Some growers have noticed that as the flies buzz into one corner like this to get light, they may stack upon each other. This increases the density & may cause fly death without them ever mating or laying eggs.

Although adult black soldier flies don't eat, they do like to drink. It's not necessary but you can mix in some sugars in the water for them. You can use a wet towel or some kind of sponge in a bucket of water. Replace the water every 5-7 days so they don't give off a smelly smell, which may attract the mature female flies to lay eggs there. This water keeps the flies hydrated & energized throughout their short 7-day life span.  

In the love cage, some people hook on artificial plants, vines or pieces of fabric. These are the 'relaxing' places for the flies when they grow tired flying around. They are not totally necessary but it keeps the flies from crowding the four-sided mesh screen surrounding where they usually chill.

You can use a mosquito netting or mesh screen with a smaller-sized aperture (or opening) so no flies can escape out when you open or close the cage. This can ensure better chances of mating.

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