The Effect of Temperature on Emergence of Black Soldier Flies : The Life and Times of BSF (Black Soldier Flies)
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The Effect of Temperature on Emergence of Black Soldier Flies

by Terry Green on 04/02/13

Female Black Soldier flies (BSF, H. illucens) live just long enough to find a mate and lay eggs in propagating their offspring. Females live no longer than about four to five days from the time they emerge from their puparia while seeking a mate and laying eggs in clutches near decaying organic waste. New offspring hatch from the eggs (about the size of a small grain of sand) within three to four days and immediately seek out decaying organic waste to feed off of in building up  insect biomass. Larvae (also referred to as grubs) grow rapidly up to about ¾ inches (16 mm) in length. On approaching their sixth instar, the prepupa stage in their life cycle, they climb out of the decaying waste (self-harvest) in seeking dark, dry hiding places to pupate while undergoing metamorphosis. They emerge from their pupal shell (the puparium) as new adult BSF ready to mate and carry on.  This blog describes the effect of temperature on the  emergence rate of adult BSF in addition to showing how this can be easily measured simply at home and in scaled up processing facilities.

The average time elapsed between the prepupa stage and the time adults emerge from their puparium (the dwell time to adult emergence), can be measured by noting the time prepupae self-harvest and timing how long it subsequently takes for adults to emerge from the puparium. A simple way of determining the average emergence dwell time is to transfer self-harvesting prepupae into a plastic container as in Fig. 1. Record daily the number of adults clinging to the inside walls of the container and netting covering the container. After each daily count, open up the container and let the adults fly free and close it up again. Record progressive rates of emergence over the next days that follow until no further adults are seen emerging inside the container. You can also determine the viability of the self-harvesting prepupa population by counting the number of adults recovered in tracking emergence divided by the number of prepuae added to the container.

Fig. 1. Setup for monitoring changes in the dwell time between initial harvesting of Black Soldier fly (BSF) prepupae and their subsequent emergence as adults. Left, BSF adults emerging from puparia; Right, counting chamber fabricated from an empty plastic container secured with mosquito net covering the open top allowing for free exchange of air and environmental conditions under which the prepupae are stored, and through which a visual count of newly emerged adults can also be recorded. Copyright © 2013, Terry Green, All rights reserved.

Whether you are operating a BSF food scrap recycler at home, or interested in scaling up production, having this information available helps you to efficiently ramp up the propagation of BSF in recycling food scrap and farming BSF.  Without taking stock of and maintaining a healthy and abundant adult BSF population, your BSF operation is likely to fall short of achieving its full potential in meeting your goals. For example, culling off prepupae prematurely, without leaving sufficient time for mature adults to emerge can be counterproductive in achieving a high throughput in processing food scrap waste. Storing prepupae aside in the hopes of having more adults emerge and lay eggs beyond the optimal time adults can reasonably be expected to emerge under the conditions you are working in serves likewise serves no useful purpose and takes up valuable space in farming operations.

Fig. 2. Effect of temperature on the daily number of Black Soldier fly (BSF) adults emerging from their prepuparia and the time it takes between recovery of self-harvested prepupae and emergence of adult BSF. Peak emergence rates occur between the 13th and 16th day when prepupae are stored at an average day time temperature of 31 C (red bars), and between about the 24th and 29th day stored at 19 C (blue bars). Copyright © 2013, Terry Green, All rights reserved.

Fig. 2 shows that temperature has a remarkable effect on emergence of BSF adults. Measurements were made under two different sets of conditions. In the first, daily emergence rates of adults were recorded in following up on ~200 prepupae stored inside our BSF shed over a 30 day interval starting from the day the prepupae first exited our food scrap processing bins (Scaling Up Black Soldier Fly Food Scrap Processing | Phase III ). The average day time temperature was 31 C inside the processing shed. Evening temperatures bottomed out at ~20 C. Day time temperatures ranged from a high of 36 C to a low of 25 C. The first adult to emerge was seen on day 9 (see Fig. 2). By the 13th day, the emergence rate reached 44 adults. High emergence rates progressed on through the 19th day. By the 21st day and on emergence rates bumped around 1 to 2 adults per day up through the 30th day. Prepupa viability was 100% (all prepupa emerged as adults). Between the 13th and 14th day, the day time temperature dropped to 25 C inside our BSF shed, shifting the emergence rate to the following day as the temperature in the shed returned to 31 C. This suggests considerably sensitivity of adult emergence to temperature fluctuations at the end stage of metamorphosis as adult emergence is about to commence.

In the second temperature study, prepupae were stored at a constant temperature of 19 C without variation between day time and night time temperatures. This resulted in a marked delay in emergence rate of adult BSF. It took until the 24th day before the first adults emerged from their prepariums (see Fig. 2, blue bars). Emergence rates in this latter instance peaked between the 26th and 27th day and tailed off slowly thereafter. The last adult to emerge occurred on the 44th day of storage (monitoring was extended to cover a 60 day post harvest interval). The viability of prepupae stored at the lower temperature of 19 C was 68% indicating 30 C storage is far preferable in maximizing emergence rates of stored prepupae.

It is interesting that there is a near perfect doubling of the adult emergence rate in half the time with a 10 C rise in day-time temperature storage conditions above ambient room temperature (~20 C), and that optimal emergence rates can easily be achieved providing day time temperatures fall within the range of 30 C. Even under ambient room temperatures, significant numbers of adults however emerge albeit at lesser rates and in lesser yield.

These observations likely account for the tendency of BSF in the wild to thrive particularly well in tropical and subtropical regions of the world, and for their populations to fluctuate much more in temperate zones of the world. The results also account for the the fall off of BSF populations in the wild in much cooler northern regions of the world where conditions are not conducive for emergence of adult BSF from their puparia.

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