The reasons black soldier fly larvae may be growing slow & small may be because of the foods, the space for each larvae or the temperature & humidity. Together, let's come see why and how to help the larvae grow bigger and faster.

#1. Temperature for larvae

BSF do well in temperatures from 24-30C (75.2-86F). If it gets too hot, they may need to spend extra energy to find a cooler space around. Which takes some more time & may cause them to grow slower and small.

The heat issue is usually caused by a dense growing population. This means less food for each larvae in one space. The larvae at the very bottom may not get as much foods and oxygen as the ones on top. This leads to larvae growing big & small in one growing area and the larvae may not be in even sizes.

At the other extreme, cold temperatures may cause the larvae growth to slow down. Below 65F (18C), the larvae may go into dormancy. Their metabolism works slower at this point. They eat less & thus grow much smaller and slower. The larvae won't die but they are just in sleeping mode. When it warms up, they will naturally wake up and be active again.

So to fix this:

If it gets too hot

Try spacing out your larvae. Larvae raised in a more spacious place tend to grow bigger & faster than those in a crowded area.

Also, check your growing medium. Does it get too hot when the larvae are munching on it? Rice bran or wheat bran, for example, has a tendency to get quite hot in the feeding area. You can try mixing in some spacers like the fruit of the cashew, beer grains or some cassava to dissipate the heat.

You can also tell that it's getting too hot by noticing a slight ammonia smell around your larvae feeding area. You may see some condensation around the edges of the box. Also, larvae may be trying to get up & out of the box to find a cooler place to live. These are signs that we may need to make some changes.

Secondly:

If it gets too cold

At night, when temperatures drop below the ideal point, cover your larvae with a piece of fabric to keep them warm. A little light bulb will do the job. In the winter months, you can make the population more dense. Each larvae is a little heater themselves. So their body temperature will hopefully keep the other larvae in the family warm.

Also, to help your BSF larvae grow bigger and stronger, check the:

#2. Foods for larvae

One reason why the larvae is taking a long time to consume the foods is because the foods may have a higher protein amount than the other kinds. For example, in a little experiment, they find that it takes longer for the larvae to eat on a fish-only diet than a veggie-only diet.

Diet Days to finish eating
Standard 17-19
Veggie 22-26
Fish 33-39

This may be the reason that many growers share with each other, that is, if there's still food the larvae may stay to eat for a bit longer. In this case, the standard mix they use is a mix of:

  • Alfalfa meal (30%)
  • Wheat bran (50%)
  • Cornmeal (20%)

They then mix that with pelleted peanut hulls at a 1:1 ratio. Finally, water is mixed in at 5 parts diet : 6 parts water (~60% moist) to get the final mix. The larvae finish this food the fastest out of the three.

On the other side of the story, growers share that the larvae growing small may be because there's little nutrient in the foods. For example, larvae fed on a plants-only diet tend to be smaller than those fed on meats or starch. Some foods that have given good results are:

  • Yam/potato
  • Soybean meal
  • Chicken feed
  • Fish/meats
  • Banana tree

The larvae don't seem to grow very big on cow dung. You can see the result of a small experiment here. The larvae are both ten days old. One box is fed using soybean meal & the other is fed on cow dung.

ten-days-old-larvae-cow-dung-soybean-meal-size-comparison.jpg

The results are interesting to investigate further.

The study we talked about can be found here:

The Intestinal Microbiota of Hermetia illucens Larvae Is Affected by Diet and Shows a Diverse Composition in the Different Midgut Regions
The larva of the black soldier fly (Hermetia illucens) has emerged as an efficient system for the bioconversion of organic waste. Although many research efforts are devoted to the optimization of rearing conditions to increase the yield of the bioconversion process, microbiological aspects related t…

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