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 Overwintering indoors in brewer's yeast slurry 
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Post Overwintering indoors in brewer's yeast slurry
In Seattle, the winters get cool enough that BSFL stop self harvesting mid-October, and I don't usually start noticing the wild population in my compost until late June. I am playing with methods to early inoculate my compost and grub harvester so that I can get a longer harvest season.

On October 3rd, I pulled out about 200 mature grubs from the grub bin & put them in a Mason jar with about 2 inches of yeast cake from a batch of beer I had recently brewed, with the hopes of keeping them alive indoors over the winter. Yeast is a common fruit fly substrate, but it is usually autoclaved & thus the substrate is sterile (& obviously the yeast are dead!). I used the trub/yeast cake from primary fermentation, which contains denatured barley proteins, hop sediment and live yeast. Yeast are very good at resisting mold and other fungi, so I reasoned that live yeast might also work even better. Previous similar attempts ended in smelly, moldy jars (though with some grub survival over winter). The yeast slurry successfully resisted other molds for this entire time, but also had its own funky smell (better than my other attempts which went really moldy).

Today (April 21st), I poured the jar of yeast and grubs into the outdoor compost to see if it will jumpstart it earlier this year. It is still hitting 4-8°C overnight these days, so I imagine they will remain dormant until it gets warm enough. My other jar with grubs in cornmeal paste had about 10 adults emerge indoors, so I thought it would be a good idea to get them out before they pupate, as any live adult flies would likely drown in the slurry without much room to move around. The relatively high moisture content of the yeast cake seems to have prevented them from pupating, whereas the cornmeal paste I had covered with ground corn cob pet bedding, so the grubs are able to find a dryer spot to emerge.

After 6.5 months in yeast slurry, the grubs were dormant, but would still move around if I swirled the jar, and could occasionally be observed climbing the sides of the jar. The jar was on a windowsill for the duration of this time, so light cycle was seasonal, and temps were between 12-18°C. I had a mesh lid for the top of the jar, so the yeast slurry had a tendency to dry out a bit, but still retain some moisture. I added fresh yeast slurry from subsequent beer batches 3 times during this 6.5 month period to increase the moisture content.

I don't know if the number I have inoculated into my compost will have a significant impact -- I can only hope that they will lay eggs in my compost earlier than the migrating/hibernating ones would, and possibly attract more wild BSF to my compost. The take home lesson from this experiment was that the grubs do quite well living and surviving for extended periods in a live yeast slurry, that may be preferable to other moldier options :lol: Next year, I think I would try to reserve an even greater number of grubs at the end of the season & really get a big jump start.


Sun Apr 21, 2013 5:13 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
rockjetty wrote:
... The take home lesson from this experiment was that the grubs do quite well living and surviving for extended periods in a live yeast slurry, that may be preferable to other moldier options
Thanks for experimenting and for updating us with the results. Could you estimate the percentage of larvae that survived?

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Next year, I think I would try to reserve an even greater number of grubs at the end of the season & really get a big jump start.
It would probably take quite a few (thousands) to get a decent jump start?

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Sun Apr 21, 2013 5:26 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
Thanks for sharing this rockjetty.

Have you considered enclosing the mature larvae in screen for captive reproduction? Theoretically, if 200 grubs equaled 100 male/female pairs and you contained them from emergence to egg laying you would get 100 females X 600-900 eggs each = 60,000-90,000 new larvae.

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Sun Apr 21, 2013 5:45 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
rockjetty wrote:
... After 6.5 months in yeast slurry, the grubs were dormant, but would still move around if I swirled the jar, and could occasionally be observed climbing the sides of the jar. The jar was on a windowsill for the duration of this time, so light cycle was seasonal, and temps were between 12-18°C. I had a mesh lid for the top of the jar, so the yeast slurry had a tendency to dry out a bit, but still retain some moisture. I added fresh yeast slurry from subsequent beer batches 3 times during this 6.5 month period to increase the moisture content.
Mmmm - overwintering in a beer bath :lol:

Did you notice any changes in the grubs (size, color, etc) that would indicate they were active and feeding?

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Sun Apr 21, 2013 6:12 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
BorealWormer wrote:
rockjetty wrote:
... After 6.5 months in yeast slurry, the grubs were dormant, but would still move around if I swirled the jar, and could occasionally be observed climbing the sides of the jar. The jar was on a windowsill for the duration of this time, so light cycle was seasonal, and temps were between 12-18°C. I had a mesh lid for the top of the jar, so the yeast slurry had a tendency to dry out a bit, but still retain some moisture. I added fresh yeast slurry from subsequent beer batches 3 times during this 6.5 month period to increase the moisture content.
Mmmm - overwintering in a beer bath :lol:

Did you notice any changes in the grubs (size, color, etc) that would indicate they were active and feeding?


When I read rj's original post I almost asked for a definition of terms because the dark (aka mature) larvae don't eat.

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Sun Apr 21, 2013 6:14 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
Yeast slurry from homebrewed beer ought to be a good feedstock. Alcohol levels are low and a variety of proteins and unfermented dextrins are still in the slurry.

In an ideal world, Jerry's captive breeding estimates might be correct, but just like with people, a number of BSF seem unable to "hook-up" with a willing partner when in the mood. (Hey! I remember what it was like back in my bachelor days! 8-) )

In my captive breeding experiments, I estimated it took 20 couples (40 flies) to generate 1 egg cluster, so 100 flies would be 50 couples which would be more likely to generate 2 or 3 eggs clutches. Naturally, individual breeding conditions vary from system to system, but I recall being quite disappointed there wasn't more "he-ing and she-ing" going on in my captive breeding area.


Sun Apr 21, 2013 6:57 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
Tarvus wrote:
In my captive breeding experiments, I estimated it took 20 couples (40 flies) to generate 1 egg cluster, so 100 flies would be 50 couples which would be more likely to generate 2 or 3 eggs clutches. Naturally, individual breeding conditions vary from system to system, but I recall being quite disappointed there wasn't more "he-ing and she-ing" going on in my captive breeding area.

I guess there is a lot of "hit and miss" in nature. :P

I haven't done captive breeding before, but I am this year.

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Sun Apr 21, 2013 7:10 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
Jerry wrote:
Tarvus wrote:
In my captive breeding experiments, I estimated it took 20 couples (40 flies) to generate 1 egg cluster, so 100 flies would be 50 couples which would be more likely to generate 2 or 3 eggs clutches. Naturally, individual breeding conditions vary from system to system, but I recall being quite disappointed there wasn't more "he-ing and she-ing" going on in my captive breeding area.

I guess there is a lot of "hit and miss" in nature. :P

I haven't done captive breeding before, but I am this year.


Your results will probably be better than mine - given that you plan to provide them chardonnay and pipe in Barry White music! :shock: ;)


Sun Apr 21, 2013 7:18 pm
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Post Re: Overwintering indoors in brewer's yeast slurry
Tarvus wrote:
Your results will probably be better than mine - given that you plan to provide them chardonnay and pipe in Barry White music! :shock: ;)

Your forgot the chocolate! ;)

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Sun Apr 21, 2013 7:20 pm
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