One reason why 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 (this may cause 'air' traffic jam), 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 have proven to be 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 for a good balanced population.

black-soldier-fly-mating
BSF mating | Source

Another aspect we can look at for this issue is:

The Light Source & Temperature

BSF love early morning light for mating. From 9 am to 1 pm is a great time for mating. A good temperature range is about 25-32C (77-89F). During the winter or rainy season, the mating activity may be slowed down because of the lack of sunlight.

With sunlight, the body temperature of the flies will be warm enough to get into reproductive mode. The light also helps the male flies see when the females poke their tails out for action. If you're growing BSF indoors, try some breeding lights.

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.

Also, check the age of the flies:

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.

This is so that we don't have flies that may die off earlier before the mating 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.

But also another reason could be maybe your BSF flies are thirsty?

Hydration for the Adult Flies

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, cotton 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.  

Some growers also design:

Relaxing Place for the Flies

In the love cage, some people hook on artificial plants, vines or pieces of fabric. These are the 'relaxing' places for the flies to land 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 hang out around.

Keeping The Flies From Escaping

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.

Responses to Readers' Questions

In most cases, I have problems with ants eating my BSF eggs and the neonates....How can I manage this?

--> You could try sprinkling some chalk, diatomaceous earth, neem oil, black oil or kerosene oil around your raising area. If it is really bad, some folks use boric acid ant traps or chemical ant deterrents. Spray around at a distance so it won't affect your BSF eggs or neonates. A bowl of sugary water might distract the ants' attention to the eggs. When your BSF is crowded in number, the ants and houseflies may visit less often. I hope this helps!

Inspired by your question, we've made a post here: 'Ants attacking BSF eggs & hatchlings what to do' https://zenyrgarden.com/ants-attacking-black-soldier-fly-egg-hatchlings/ Please check it out - hope it's helpful.

Thanks for the info

--> Thanks for the pictures, wow congratulations!

I have about 30 solid BSF in the love cage and l have been putting them in the sun for the 3 days now but they are still not mating. Is it possible that they are all females or all males? What else can I do?

--> Hey first of all, I'm sorry to hear about that. Now that it has probably been two days more, how are they now? Have you noticed any mating yet in the cage? Did you put any egg collectors in the cage? And have you seen any white stuff (egg) between the wood gap. BSF are notoriously hard to mate according to some. Some say it may only be 30% mate rate in the population. Please first though, be patient and give them a bit more time to see. Please also check that there's light but it's not too hot. About 25-32C would be good enough for them. Remember to keep the flies hydrated. A moisture or humidity level of 70% is good to lengthen the life span of the flies, and thus, increasing chances for reproduction. You could also consider misting water during the hot hours of the day to help cool it down. Also make sure that your flies are about the same or similar age so they could more likely mature and mate at nearly the same time. I have a post here about 'Uniform BSF Population' if you'd like to take a look at later. Please also make sure that there is attractant inside the cage. The smell signals to the female flies that there could be good food here for her babies. It thus could possibly motivate her to connect with a male fly and lay eggs, theoretically speaking. I would also like to suggest you to have a look at where the flies are hanging out. Are there more flies sitting on the net where the light is, or are there more flies gathering in the more shaded corners of the love cage? Please see. The flies that like more light are probably the male and the flies that like more shade are probably the female. You could then have an overall look or estimation of the proportion of male/female in the 30 flies. I would also like to ask, what feed did you have them on? Stronger protein foods could give them strength for reproduction. Have a look overall. I hope the flies mate. Keep us posted. I hope this could give you some ideas to begin with. Keep us updated, and See you again next time.

Update note: Very important, something I just learned today. From farmers who have had similar issues with fly mating and reproducing: remember to place your darkened pre-pupae in sand or a similar dry material so it's dark and cool. Very important at this stage to note again. This is so the pre-pupae don't move around too much, which could make it lose energy and become smaller. Doing so could help the pre-pupae not turn too small and for it to form a chrysallis (hard shell, becoming pupae) for good reproduction! And from my limited personal experience, I remember that when the pre-pupae or pupae is small, the emerged fly will also turn out to be small. So I guess one aspect is it could start from the beginning of the process that we may need to get a hang of. You might take note of this for future reproduction next time!

If you would like to send us some pictures or follow up with some more details, please just shoot me an email. Would be more than happy to have a look into this and share the experience. Our contact is at https://zenyrgarden.com/contact/

All the best.

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