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.
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 we 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.
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. 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 space out the larvae 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.
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. May be it's the ant spray or some other insecticide stuff that may get carried around by the wind. The BSF may die by just having a sniff of this stuff around.
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. Doing these little things can help the larvae grow up strong after pupation. It maximizes the chances of successful breeding & egg laying.
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