If you're using the usual household refrigerator, it might be too cold for the BSF grubs. To save the larvae alive, people put them in a wine fridge. In the 50-60s F (10-15C), the larvae will be alive and well. They'll actually go into hibernation mode and may turn motionless at this point.
But don't worry, these grubs are very hardy and they won't die easily. The point that may freeze them to death is when temperature drops below 20F (-6C). If you keep the larvae in a cool dark dry place, these guys will hang in there for 8-12 weeks. Some folks have even seen them alive & wiggly for 6-8 months after waking them up from their dormancy.
Simple ideas for BSF larvae storage
Storing the larvae is very easy and straightforward.
Grab some larvae that you want to store and pop them in a Ziploc bag. Pour in some of that juice from the feeding box (to get the smell that can attract more pregnant BSF when we release the larvae out again). And swing the bag in the fridge.
Alternatively, you can try a soda can. Put some straw in for aeration stuff and drop the larvae in. Then stuck the can in the fridge.
And that's pretty much all there is to it!
Do you need to feed the grubs during dormancy?
Not necessarily. During this 'sleeping' period, their metabolism will tend to slow down. So you won't see much wiggling or munching on the foods. If you don't feed the larvae (like many busy growers don't sometimes), they won't die. They'll just grow much skinnier and smaller.
To wake the larvae up
When it's time you want to use the larvae, gradually introduce light back to them. They'll react to the warmth and begin to kick-start their bodily functions again.
Some folks store the black pupae and use them this way to feed their birds, fish or bearded dragons in the winter. If you decide to freeze these black pupae, they will die. But the nutritional goodness is still preserved.
Others save the cream newborn larvae for use later in winter production. For example, you can collect and zip the larvae in the fall. Then 2-3 months later when the cold hits, you can take them out and gradually put them in a feeding box in a greenhouse where it's warm enough during the day. The larvae will then wake up and do what they do best. With this, we can still get grubs growing and active even in the wintertime.
If you'd like to see how to over-winter your BSF larvae, check out this post:
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