Many growers have experienced the early death of black soldier flies 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 is 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. 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. Or more specifically it's the insecticide directly on the net. BSF usually land on the net. And 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. 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 it 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 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 folks also put ice packs in the box to cool down, especially during heat waves.

As for food supply, BSF larvae can eat up to 5-10x 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. 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. 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. You can ask on our forums. I hope this helps!

>> Relevant info: Black soldier fly life cycle

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