Many growers have experienced the early death of black soldier flies sometimes only hours after pupation. In the morning the flies buzz around & land on the netting around the cage. When it's near noon or in the afternoon, the flies slowly die off & fall down to the ground. These black soldier flies never mate or lay any eggs. Let's see what might be going on.

Immature Crawl-Offs

Usually, in a feeding box, the little cream larvae that we see spend most of their time digging into the food. They almost never crawl out of the box or to the sides if there's good space and good food for them. The only ones that move out naturally are the blackened larvae that have eaten enough.

That being said, when you notice some cream larvae moving out to the edges or out of the bin, we know something might be going on. Otherwise, they'll be gathering inside enjoying their food.

You can see an example in the pic below. The cream larvae are trying to move out & crawl off a cliff. It's a bit blurry, sorry folks, as these guys wiggle non-stop. The reason why they are self-evacuating may be because of the heat issue or over-population.

Cream larvae trying to get out of the feeding area

If it gets too hot in the box, the cream larvae will naturally move out to the sides to find a cooler place. This may cause the larvae to pupate early, or in this case, people call them immature crawl-offs. Even though they may still turn into blackened pupae & eventually flies, these guys are very weak. Which may explain for their early death after pupation.  

Over-population may cause the temperature to rise, especially in an environment with thousands of heat-generating wiggling larvae. In one feeding box where a grower feeds them with oatmeal, it could get up to 43C (110F) in the center of the box.

The growing medium may also be a contributor to the heat. Rice bran, for example, tends to retain quite a bit of heat. The larvae poop, if left unfiltered, may also heat up the space. If this happens, mix in some beer grains, spray some water to cool it down or filter the poop out. We can also divide the larvae out to other boxes to keep the heat down.

The other aspect that could lead to early crawl-offs is the lack of food for the larvae. Again, this may be tied to the over-crowded growing space where there's not enough food for everyone. The larvae near the bottom may not get enough food or oxygen as the larvae on the top. When they crawl out early & pupate, their health inevitably will be weaker than the fully mature ones. You could turn the food sometimes with a windshield wiper or trowel so the food is evenly distributed. A non-sharp tool can help not to cut or damage the larvae.

Also, regarding this early death after pupation, look out for the:


Check around for any chemicals that may be harmful to the adult flies. Although young cream larvae are quite resilient, the adult flies are more chemically sensitive. Maybe it's the ant spray or some other insecticide stuff that may get carried around by the wind.

It may be the pesticides in the foods they eat. As a small note, if you feed the larvae apples, you could possibly soak the apples in clean water for about 20-30 minutes, it will help remove some of the pesticide residues. I'm not sure yet if this could work for other types of veggies or if soaking them in EM or salt could help reduce the harmful stuff or undesirable bacteria.

Or more specifically it could be the insecticide treated directly on the net where the BSF usually land. Because for some net (like mosquito netting), they treat it with chemicals to kill the insects and prevent malaria. The BSF may die by just having a sniff of this stuff around.

Natural Environment

Sometimes, it's not because of the density of the growing larvae but it may just be the natural weather in the local area. For example, in some places it may get too hot during some time of the year (e.g. heat waves in the summer). When this happens, we may see a mix of cream & blackened larvae crawling out to the harvesting box. This also leads to early crawl-offs, which may then die young because of their weaker physical health.

As this can be seasonal, you can prepare before the heat waves come. Some folks use the ice packs in the fridge to cool the feeding box down. Or when it's that time of the year that you know it will be hot, space the larvae out & provide full shade for them. You could also spray some water to hydrate the flies or make it cool down. Doing these little things can help the larvae grow up strong after pupation. It maximizes the chances of successful breeding & egg laying.

Responses to Readers' Questions

What is the remedy for premature death of bsf.

--> Thanks for your question. The remedy for premature bsf death can be a not-too-hot growing area, sufficient food supply and pesticide-free surrounding. The very max temperature BSF can withstand is up to 43C (110F). They like it at around 24-30C (75.2-86F). To make sure the growing box is not too hot, you could space out the population density. Create air flow under it for them to stay cool and breathe. Some substrate like wheat bran or rice bran has a tendency to heat up the place. So you could add spacers like cashew nut shells, beer grains or cassava to dissipate the heat. When it's too hot, you may notice a slight ammonia smell around the growing place and the condensation around the edges. The skin of BSF turn dry and you may see speckles of the dried skin in the food. Some people also put ice packs in the box to cool it down, especially during heat waves.

As for food supply, BSF larvae can eat up to 5-10 times their weight. So you can feed them at a 1:1 or 1:10 ratio. Put enough for them to consume so the food won't turn moldy. As your feeding box is not too crowded, food will be shared more for each larvae. In a non-crowded box, the larvae will grow quite fast and fat. They will have enough strength to pupate and not die prematurely. Also, check around your surrounding for any insecticide or pesticide residue. Some fruits/veggies may contain pesticide residue. Ant killer could also be lingering around killing the bsf. Please check back often. I hope this helps!

Hi! My BSF larvae are dying after they turn into prepupae. What might be the reason?

--> Thanks for your question. To begin with, have your prepupae turned into a hard black/charcoal coating on the outside? And are they moving or not moving? Have a look at this blackened prepupae right here:


If the prepupae have formed a hard blackened shell outside and they are not moving, then things are in the natural flow. It is normal at this point for them to remain motionless. The good news is they are not dead. They are still alive! And about 2 weeks from here, the prepupae will emerge into adult black soldier flies. At this stage, they like to bury themselves under something dark and dry like sand or coco peat as a substrate. On the last night, before they jump into the next phase of becoming pupae and then flies, the pupae will emerge themselves back up to the surface automatically. You literally don't have to do anything or worry that they may suffocate themselves under the sand. It's the beauty and magic of these creatures.

In some other cases, if you observe that your BSF larvae are not moving, it might be they have gone into their sleeping mode. This happens when temperatures drop below 15C (60F) or in a low-light environment. Gradually introducing light back to them as the temperature warms up will wake them up. They'll be wiggling again. Also, check your source to see if the incoming BSF eggs are of good quality. If it is another case, let me know. I hope this helps!

>> Relevant info: Black soldier fly life cycle

My bsfs die minutes after pupetting why

--> Hi, we have received your question and pictures. Let's give us some time to think about this and possible aspects/questions. I will get back to you. Meantime, any more descriptions/details you have, please share with us.

Hi there again. May we run through some questions to see if we can find the answer:

  • What is the cage netting material you use? Please be sure to check the netting as some mosquitoes netting may be pre-treated with chemicals to help fight malaria. A swift smell of this might also kills insects, including the bsf new pupetted flies. Make sure the air and environment is also free of sprayed chemicals around.
  • Please also see how hot is it in your area around? Are the corners in the room brightly lit evenly or is it brighter on this side than that? Are you seeing little flies gathering more in one corner? Are you seeing any of them stacking on top of each other? This may create a heat issue causing the death of flies. Also, are you giving them some spray of water especially on hot times of day? From experience of some farmers share, moisture could help lengthen the lifespan of the flies.
  • Tracing back, what kinds of food are you giving them? Although it is waste food, please be sure it is from a good trusted source (even better if free of pesticide residue). A balanced diet profile of fruits, veggies and some starches give the larvae enough strength for growth and size to pupet (pupate) into flies.
  • How many pupae are you placing in the cage?
  • Is it the cardboard as the flooring, is it good for air flow.
  • I am curious but not sure if I should raise the question up: do you have the dead young flies bodies (or carcasses)? We might have a look to see what could be going on with them.
  • Is this sudden death a first-time occurrence or has it been happening again and again for a few times already?

Please have a check. Any more info, please share for accurate diagnosis. Thanks farmer, see you again next time!

Why reason my Bsf larvae is ded

--> Hi thanks for your question. Can you please provide more details. At what stage did it die, is it at the cream larvae, the pupae or the fly stage?

If it's the cream larvae stage, can you please check:

  • The food source: no toxic residue, balanced diet (starch + protein + vitamin)
  • Heat issue: crowded larvae, not good airflow, food heating up

If it's dead when it's in the black pre-pupae stage:

  • You see some sticky white string shoots out of the pre-pupae bottom: maybe there's too much protein in the food source --> the pre-pupae needs to excrete it out (I've seen this occur in some of my pre-pupae).
  • When pre-pupae turns into pupae, it turns motionless. The motionless pupae are still alive and not dead. They just stop moving waiting for fly emergence.

If the flies are dead, then check:

  • The cage netting if it has been sprayed with insecticide (mosquito killer)
  • The environment around if there are any toxic chemicals
  • Is the temp (25-32C) and humidity (40-70%) on point?

Have a check & see. See you again next time!

Mera BSF larva khane ko dhang se nahin Kha Raha hai aur FIR sust hokar mar ja raha hai

(translation from the Hindi language: my bsf larva is not eating food properly and then sluggishly dying)

--> Hi thanks for your question. I'm sorry to hear about that. What food are you giving them? Are they in cream color when they die? Please see.

हैलो, आपके प्रश्न के लिए धन्यवाद। मुझे इसके बारे में सुनकर खेद है। आप उन्हें क्या खाना दे रहे हैं? क्या वे मरने पर क्रीम रंग में होते हैं? कृपया देखें।

hailo, aapake prashn ke lie dhanyavaad. mujhe isake baare mein sunakar khed hai. aap unhen kya khaana de rahe hain? kya ve marane par kreem rang mein hote hain? krpaya dekhen.

Share or pin this post!


Cover image source