One of the reasons why black soldier fly is a very suitable insect for large-scale farming is its safety for humans and other animals. Everybody knows house flies, blow flies, horse flies, and black flies. Until recently, however, pretty much nobody even heard of black soldier flies. This is a clear indication that this species is not a pest for most people.
Adult black soldier flies are not attracted to people because they do not obtain any resources from us directly. On the contrary, they generally prefer open well-lit spaces; therefore, they do not like entering human-made structures such as houses and storage facilities. Moreover, unlike house flies and blow flies, black soldier flies lay their eggs next to decaying substrates, not right on top of them. Therefore, there is less of a chance for them to pick up pathogenic bacteria.
Black soldier fly larvae feed on already dead, decaying materials. While doing so, they suppress several species of pathogenic bacteria, including Salmonella, E. coli, and MRSA. They also outcompete and repel pest species of filth-dwelling flies, such as house flies and blow flies. In addition, black soldier flies have been shown not to accumulate many of the tested organic toxins present in their substrates.
Of course, the safety of any operation should never be taken for granted. A hazardous situation can always be created if enough effort is put into creating it. So, necessary precautions must always be taken when rearing black soldier flies, just as they must be taken when doing anything else.
First, while black soldier flies do suppress populations of many bacterial species, they should not be considered as some kind of a biological sterilizer. Bacteria of the affected species may be reduced in abundance, but not to the point of being unable to infect animals and cause disease. Furthermore, not all pathogens are affected, while the known suppressive effects may not occur on all substrates and at all stages of decomposition. So, microbial safety of harvested biomass should not be assumed as a given.
Second, black soldier fly larvae accumulate heavy metals from their food. Therefore, wastes that contain such metals should not be converted into components of animal feed using larvae. Similar situations may also exist with other pollutants. When contamination of larval substrate is suspected, the resulting products made from larval biomass need to be tested accordingly.
Third, there are several reports of occasional intestinal myiasis caused by black soldier fly larvae. Myiasis is the infestation of living vertebrate animals with fly larvae. These larvae survive on their host and feed either on its dead or living tissue or on the food ingested by the host and contained inside its gastrointestinal tract. Many different fly species can cause intestinal myiasis, which results from swallowing live eggs or larvae. Black soldier fly myasis is accompanied by abdominal pain, vomiting, and diarrhea, sometimes with blood.
Although myiasis sounds rather disturbing, it is very rare. Furthermore, there seems to be no uptick in the number of recorded cases despite the recent explosion in the number of facilities rearing black soldier flies. Basically, wearing gloves and washing hands while handling fly eggs and larvae prevents it from happening.
Overall, safety of producing black soldier flies is at least comparable, and may be exceeding, the safety of other farming operations. It should not be taken for granted, but there are no serious hazards that would put rearing large quantities of this species into question.