If the BSF larvae have turned black or charcoal and are non-moving, then they may be in a state ready to turn into flies. They usually lie still on day 26-33 (or 1-2 weeks after they have turned dark).
This is totally normal & is part of its natural life cycle. So don't worry too much in this case. The pupae will stop wiggling at this point. After about 1-2 more weeks, they will turn into flies.
In other cases, there may be some issues. If it is so, let's see the ideas together.
#1. Density
It's likely that the non-moving black soldier fly larvae is dead or dormant. In some feeding areas where the density of the larvae is way too high, the little ones below may get too hot.
BSF larvae themselves generate a lot of heat from the wiggling movement. In a hot crowded area, they may not get enough of the oxygen they need. This can cause early larvae death at 7 or 8 days old, when they are still in cream-like color.
For small-scale farming, a population density about 3-5 grams of eggs per 1 meter square is good. For larger scale, people put 50 grams of eggs per 3-5 square meter. If you go heavy-duty, try 100 grams per 3 square meter & observe the population growth and re-actions.
#2. Temperature
A good temperature range for the larvae is 25-32C (77-89F). During the winter or in low-light intensity, the larvae may turn motionless. If it drops below 15C (60F), they'll be inactive & go into hibernation mode. If this is the case, it is completely normal and you don't have to worry anything about it.
These guys are pretty tough and resilient however, they will not die in this case. They are just sleeping. They'll still survive in this dry, dark, cool environment for up to 6-8 months.
In a little research, people submerge the larvae in isopropyl alcohol (IPA) for about 6 hours. When they take the larvae out, amazingly these little guys are still kicking and wiggling and very much alive like they've never been drenched in the alcohol.
For dormant motionless larvae, a good source of light introduced gradually should wake them up & make them active again. If it gets below 20F (-6C) and the larvae are left unprotected, then they may die. So make sure to keep it cool but not terribly cold.
Also, have you checked the:
#3. Air Flow
Make sure there's good air circulation underneath & around your feeding areas. This is important because BSF larvae like an aerobic (oxygen-abundant) environment with good air flow & not an anaerobic (oxygen-lacking) space which can be suffocating for them. An oxygen-lacking (not good air flow/exchange) environment may induce the growth of other molds/bacteria.
To create good air flow, you can try stacking up the feeding boxes with spaces between each other. Alternatively, use something like coconut shells, rice hulls, sawdust or wood chips as the bottom bedding with a mesh screen on top.
Cedar wood may not be a good choice because it can be too aromatic. These spacers will create air channels inside your bin, making it easier for the larvae to live and breathe.
#4. Turning the Food
It's good to turn the food in your boxes once in a while so everyone gets a fair share. However, don't mess it up too much too frequently.
Too much touching or disturbing can slow down the pupation process (i.e. the larvae turning black) & sometimes damage the little bodies.
Another important aspect to keep your larvae happy is:
#5. Food for the Larvae
Check if there's enough food in the bin for them. If there's too many larvae but too little food, they may crawl out & pupate early.
The early pupae in this case are usually very weak. When they pupate out into flies, they may die off very fast without ever mating or reproducing anything.
See if the food is too wet because excess water may drown the tiny larvae or make it difficult for the blackened larvae to crawl out. They may lay there unmoving. A good moisture/humidity is around 60-70%. For 1 part food, we can add about 0.6 part (or half a part) water.
At the other extreme, check if the food is too dry. This may be caused by the heat issue in the feeding area. You may see pieces of their dry skin scattered around the surfaces. Sometimes, they'll move out to the four corners or the edges to cool down. We may get a slight smell of ammonia around (as protein is breaking down) or see some condensation around the box. The very max they can go is about 105-110F (40-43C). These are the crisis points. They may also die because of this overheating.
Also, have you checked that the food source for the larvae is free of chemical residue? From some growers' experience, it may be the chemicals or pesticide residue in the veggies/fruits that could harm the young larvae. If you grow your own veggies, that's better because you know the source. Or you could get food waste from a trusted, reliable source.
If you use tap water to mix in with the food, let it off gas for 1 day so the chlorine evaporates out. Alternatively, you can pre-treat the foods with microbial enzymes or effective microorganisms so the harmful stuff gets broken down before the larvae eat them.
To find out more about the benefits of EM or effective microorganism and how to make it yourself, check out >> How to make EM effective microorganism
It may sound intuitively unbelievable at first, but sometimes the smell may kill:
#6. The Smell of an Old BSF Box
In another case, a grower observes that when he leaves new young BSF larvae to grow in an old box with the dead skin or bodies of the previous generations, they tend to die young most of the time. He didn't have time to clean all nooksnand crannies of the boxes every time and so he dumped the new ones into the old boxes. Some other growers don't experience this issue though.
This is still hard to explain according to the growers. Maybe you can try checking around the feeding environment for any invaders. Ants, rats or houseflies may steal away some food. Maybe it's the smell that scares the young larvae. Try airing out the box & taking the dead skin or bodies out. If you want to recycle the skins of the flies, try using them as attractant or feed for the fish. It seems to work well for some people. They are also good fertilizer.
We found a good explanation for this young larvae death on the BSF forum. When previous generations die, all of their guts/intestines are purged (discharged out). This means all the bacteria/acid in their stomach are now released into the whole bin & the area around. These bacterial guys may hang around there for a while & they may be the reason that affects the growth of the younger generations. It's unrelated, but this reminds me of why in some countries, this could possibly be the reason they have people's guts taken out when the person passes away.
Anyway, for BSF in this case, wash the bin & dry it somewhere. Place some black soil in so the acid/bacteria can cling on to it & get washed out with it.
Just another small tip for preventing motionless larvae or early larvae death:
#7. Shade When Eating
Also, BSF larvae are photo-sensitive (not that they don't like taking pictures of themselves, kind of, but they are creatures sensitive to light). They like a bit of privacy (darkness) when eating.
The babies usually sneak around the underside of a carrot or a piece of meat. When you lift the pieces of food up, you'll see a lot of larvae hanging around there on the underside. So give them good shade, not too much direct sunlight when they're eating. A dark bin, a lid cover or a piece of burlap is good for them.
And finally, after all boxes have been checked, see if you're having:
#8. Chemically-treated Larvae
In some other cases, the BSF larvae (usually named as Calci-worms or phoenix worms) that people get from the pet stores may be sprayed with some chemical so they won't pupate or turn into flies.
No matter how much we feed or care for these guys, they just won't make it into adult BSF flies. Make sure to get good larvae from a reliable source for good chances of survival & reproduction.
And for Happy, Healthy BSF Larvae
So there we go, as you have just seen, if your BSF larvae is darkened and in a non-moving state, that is completely normal. You can be happy and excited at this point because that means the larvae (or more accurately the pupae) are getting ready to turn into BSF flies.
However, if the larvae stop moving when they are still cream in color, you may want to check the density in your growing environment. See if the food is good and pesticide-free. Check the temperature around to make sure they live in a happy and cool house. Create good air flow all around for them & clean the boxes to get rid of any old smell if need be. Finally, give them some privacy (shade) while they're eating and get good larvae from the start so you won't end up with chemically treated ones that may not turn into flies.
Hope this brief post has sparked some ideas as to why your BSF larvae may be in a motionless state. Good luck with your growing & have good returns or just fulfillment.
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