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 Lethal temperature for BSF larvae 
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Post Lethal temperature for BSF larvae
I've read reports that the lethal temperature limit for black soldier fly larvae (BSFL) is between 113º-115ºF/45º-46ºC. I've also read empirical data from trusted forum members that BSFL have been observed thriving in environments that were significantly warmer. I have a theory that allows for both to be true.

Today I did an experiment to determine the lethal limit for BSFL body temperature. I found the lethal temperature to be approximately 119ºF/48ºC.

The way I reconcile this result with observations of BSFL thriving in hotter environments is that they can tolerate higher temperatures for short periods, but must move to a cooler area before their internal body temperature reaches the lethal limit. A good analogy might be a human who walks across hot coals; they can do this with minimal damage, but if they stood for several minutes on the coals they would be severely burned.

For the test I added a variety of BSFL to water and slowly increased the temperature to the point of lethality. The temperature was increased by approximately 1ºF/.5ºC every 2 minutes. The larvae included both juvenile and prepupal stages, with juveniles ranging from .3 inches/8mm to .75 inches/19mm. The larvae died when the water reached 119ºF/48Cº. I assume that the body temperature of the larvae lagged the water temperature somewhat, but the smallest larvae died at about the same time as the larger individuals.

My tools and methods were not up to laboratory standards, but the test was good enough to confirm in my mind that the data reported by others is relatively accurate.

I found an article about thermoregulation by fly larvae that might help us understand better how temperature relates to BSF behavior: LINK

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*I'm not an entomologist, and much of what I write about BSF is an educated guess.


Sun Jul 06, 2014 6:19 pm
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Location: sand hollow idaho
Post Re: Lethal temperature for BSF larvae
I really hadn't thought about the high temperatures much. I will have to check on that some more. I have read 148f in the bin where they were very actively feeding. On the other hand I did have a massive die off a couple summers ago in the greenhouse when the air temp reached 125f.
It now has me wondering if they can survive higher heat if it's generated in their bin and bleeds off to the environment versus being the ambient temperature of their surrounding.


Mon Jul 07, 2014 1:27 am
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Post Re: Lethal temperature for BSF larvae
I think we need to consider two things; body temperature and that of the environment. What I directly measured was temperature of the water (environment), but since I raised the temp slowly and the larvae were completely immersed, we can assume that the body temp of the larvae was very close to the recorded temp of the water. I have no doubt that BSFL could survive in environments that exceed their lethal body temp, but they could not stay in that environment past the point where their body was raised to the lethal limit.

I assume that black soldier fly larvae everywhere will have about the same lethal body temperature, just like humans everywhere share similar limits. If none of my larvae could survive a body temperature of 119ºF or slightly less, then I don't believe any BSFL would survive it.

My larvae died quickly at 119º measured by my (marginally accurate) thermometer, but I don't know what would happen if they were help at a slightly lower temp for a longer period. It might be just as lethal. That might explain why some researchers report different, but similar limits.

I think this indicates that it's fine to maintain a BSFL environment with areas well above the lethal temperature limit, but the larvae must have cooler areas to retreat to before their body temp reaches the lethal limit.

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*I'm not an entomologist, and much of what I write about BSF is an educated guess.


Tue Jul 08, 2014 2:24 pm
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Post Re: Lethal temperature for BSF larvae
You may want to consider the availability of oxygen as cause of death. Oxygen levels in warm water drops sharply, possibly suffocating the larvae. Just a thought.


Wed Jul 09, 2014 10:56 am
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Post Re: Lethal temperature for BSF larvae
Hi Gil, welcome to the forum!

That's a good point, thank you. I think I took about 30-40 minutes to bring the larvae up to temperature, and I don't know how long it takes to suffocate BSFL. I might try bringing some water to 119ºF and then add some larvae and see if I get a similar result.

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*I'm not an entomologist, and much of what I write about BSF is an educated guess.


Wed Jul 09, 2014 5:59 pm
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Post Re: Lethal temperature for BSF larvae
I did another test of the lethal body temperature for black soldier fly larvae to eliminate the good question of oxygen starvation posed by Gil.

For this test I inserted the probe of a thermometer into an old crock pot and adjusted the temperature to a relatively consistent 118º-120ºF/48º-49ºC. To achieve that temperature the lid was raised above its normal closed position on the pot by setting two long pieces of wood across the opening, and resting the lid on them. This left a large gap that allowed for fresh air to enter the pot. About two dozen BSFL of varying sizes from a few mm to full sized pre-pupal mature larvae were added near the tip of the temperature probe. Their activity level increased almost immediately and continued to increase for about 10 minutes at which time they all died.

For me, this confirms that the lethal temp for BSFL is in the range of 115º-119ºF/46º-48ºC. I do believe they can survive for brief periods in higher temps, but that they must move to cooler areas before their body temp reaches the lethal limit. Even in a greenhouse with a recorded air temp of 120ºF/49ºC, there should be areas within a BSF composter that would be cooler. The more dense the waste is, the longer it would retain the lower temperature it reached during the night.

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*I'm not an entomologist, and much of what I write about BSF is an educated guess.


Sat Jul 19, 2014 7:14 pm
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Post Re: Lethal temperature for BSF larvae
Might BSF larvae do any of the maneuvers that at least one type of wasp's larvae do to survive high heat; but with BSF larvae's own temperature settings for when the tactic kicks in ? And might BSF larvae employ the second tactic even if they don't perform the first tactic the wasp larvae use ?

When temperature approaches 41*Celsius the larvae of Perga dorsalis first tactic is hiking up it's abdomen to get convection going that drops their body's temperature. But a second tactic occurs once higher temperature of 37*Celsius needs dealing with. The larvae put out from their anus "...a filtrate from the semiliquid midgut". This oozes over the larvae to add a surface property which makes their body more evaporative, leading to a greater loss of heat from the larvae. By coating itself a larvae keeps it's internal temperature up to 6*Celsius lower than it's environment temperature at that time. See (1974) "Convective and evaporative cooling in sawfly larvae"; link = http://www.sciencedirect.com/science/ar ... 1074900304

The Perga dorsalis apparently can safely lose even 17% of it's body weight water performing evaporation to shed heat. If BSF larvae do anything similar to evaporative cooling then the factor of how much body water that larvae can lose influences how long that larvae can weather the challenge of elevated temperatures. Similarly, vermicompost bins over loaded with food lets substrate microbes generate extremely high heat which forces so much a demand for evaporative cooling that compost worms can lose too much of their body weight; the worms that survive a climb to the surface are visibly dehydrated & appear to suffer some metabolic failure to have replenished a normal body coating.


Sun Jul 20, 2014 4:29 am
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Post Re: Lethal temperature for BSF larvae
The larvae were somewhat dry when I added them to the cooker; they were relatively clean when harvested, and they had been in a dry container for about 20 minutes prior to the test. During the test the only behavior I observed was the typical crawling we're all familiar with, and I saw no secretions of any type.

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*I'm not an entomologist, and much of what I write about BSF is an educated guess.


Sun Jul 20, 2014 12:24 pm
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