Black soldier fly's slow pupation may due to the temperature being too hot or too cold, the foods, and sometimes human interference.

Here are some possible reasons below:

#1. Temperature

When temperatures drop below 65F (18C) for quite a while, the metabolism system of the BSF larvae will slow down. This happens a lot during the winter months. As they eat less, they'll grow slower. Consequently, it takes longer for the larvae to pupate. Sometimes, it can take up to 5-6 months for the larvae to turn into flies as the cold months go by.

On the other extreme, too hot a pupation area may also cause the larvae to slow down pupation. When 15-days-old larvae turn a bit dark brown or charcoal, they will move themselves out of the feeding area to find a cool, dry place for metamorphism. They usually bury themselves under the ground to get some good shade.

Some people mix in sawdust, sand or coco peat to create a good dry environment. It helps them reach pupation just in time. Oat hulls & vermiculite are not very good medium because it's hard to separate them out from the pupae by flotation (separating the larvae and residue by dipping them in water).

You'll be able to tell this by seeing the darkened larvae stop wiggling. For example, if tomorrow morning they're ready to turn into the hardened cocoon or chrysalis, they'll automatically crawl up from the sand or sawdust late tonight. When we see them again the next morning, they'll all be laying there non-moving. This is a good sign of they are ready to turn into flies. About 7 more days, they'll transform into BSF flies. You can see part of the transformation here:

A black soldier fly cracking out of its shell

During the time when the pupae have crawled up from the sand, they still like it a bit cooler. Some good shade at this point can also help. In general, a good temperature range for BSF larvae (when they are still cream in color) is 25-32C (77-89F). They'll grow & pupate fast within this temp range.

The second aspect we may look at for slow pupation is:

#2. The Foods

Foods that take a longer time to consume may delay the BSF pupation process. For example, if we feed them with fish, it may take longer for them to finish the foods than veggies. When there's still food, the flies may stay there for longer to feed on some more. People find that cooked fish tends to get chewed up faster than raw fish. Although it's not entirely necessary, we may shred the foods to make it easier & faster for them to digest.

Another point a grower shares about this issue is:

#3. Touching

Too much touching may be part of the reason for slow pupation. Some growers disturb or touch the larvae a bit too much or too often. It's not really clear how this may affect the growth rate. According to some raisers, just letting them lay there & do their thing will most likely help the process.

For example, when the darkened wiggling pre-pupae are about to turn into motionless pupae, if we bring them up above the ground or shed some light over the box, it may disrupt some of the things that's going on inside them. They may need to start over or do some fixings here and there.

To Help the Pupation Process

In summary, to help with the pupation process check the temperature around your BSF raising box. If it's too hot, you may want to scoop some out and place them into different boxes. If it's too cold like in the winter months, then we may need to wait till the weather warms up.

Check also the foods. Make it easier for them to digest by shredding or cooking. And finally, don't touch or disturb them too much during their transformation process. It's okay to watch and observe as the flies emerge from their shells. Hopefully this has given you some ideas to fix the problem. Let us know if you have any comments. Cheers & have a good time raising BSF.

Responses to Readers' Questions


--> Thanks for your question. To attract black soldier flies, you could use the liquid leachate from the BSF frass (residue bsf poop + undigested foods + exoskeletons). Female black soldier flies may be attracted to come. Or use the smell of the bsf larvae themselves. I've met on one occasion a female black soldier landed from far away on one of my bsf box as she senses the smell of her own specie. This is possibly one reason why some people separate the feeding place & the breeding place, as the smell could attract flies to lay eggs randomly in the feeding area, which would be unwanted.

Other stuff you could try: fermented maize, some people use chicken poo, bad fermented foods, animal guts, pet kibble, coffee ground, etc.

For more information, check out this post here 'How to attract black soldier fly': . I hope this helps!

Hello , this is the first time we are trying to utilize BSF to digest down food waste and we were surprised about how quickly 1,000 larve ate our kitchen scraps. But we purchased the larve and would like to have them cycle . That said our winters are mild 50-60 degrees . But I have read that they need 75-85 in other to pupate into flies and restart the cycle vs having to pay 25 dollars per thousand larval. Wondering if they will pupate at that temperature or will require higher temps. Will they hibernated if temperature aren’t between 75-80 or not hatch/die? Currently I have them in a cardboard box in a saw dust medium for the larval that has climbed out of our bin

--> Hello, thanks for your question. For the larvae that have climbed out of the bin, what color are they could you please have a check? AFAIK, the larvae that are cream in color can survive temperatures from 50-60F (10-15C), under this range they may not survive well. And below 20F (-6C), they'll more likely die off. When temperature drops below 65F (18C), the larvae will tend to hibernate. I'm not sure what might happen to the darkened larvae (prepupae) in cold weather. However, for the cream larvae, the good news is they will not die. They just grow slower, become smaller, and eat less. If the rest of your larvae are cream, then you don't have to worry too much in this case. They will survive the cold, pupate and become flies when the weather warms up again, but may not do so during this cold time. And overall, the temperature between 75-86 will be good for their overall development through all the stages.

(Update): From a member in a group that I asked, the bsf will pupate (become flies) in cold weather but at a much slower rate.

I'll drop a relevant link here if you're interested for more: 'How to take care of BSF in the winter":

I hope this helps!

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