Trillions of insects are farmed around the world for different purposes, including as a source of food for humans and animals, as biofuel, and for waste management. For example, more than 200 billion black soldier flies (BSF) are raised each year for these purposes and more.  

The authors of this paper note that the insect farming industry is expected to grow. Thus, learning how to rear insects in high-welfare conditions is an important consideration. Although the debate about insect sentience is still unresolved, these authors adopt a precautionary approach and assume that farmed insects can suffer. In this article, they review some of the major farming conditions that lead to BSF death, illness, and other physical problems, which they assume would also impact their welfare.

BSF are typically killed as larvae before the pupation stage, and they may be sold alive for animal consumption, frozen, baked, roasted, microwaved, boiled or blanched, asphyxiated, or shredded for slaughter. However, there is limited research on the impacts of these methods. Beyond slaughter, the authors also explore issues with disease outbreaks, environmental conditions, nutrition, crowding and injury, handling, genetic modifications, and environmental contaminants.

Disease Outbreaks

The authors note that pathogens are a real concern, especially as the BSF farming industry continues to grow. For example, some fungi pathogens may cause up to 100% of larvae to die. When disease outbreaks occur, insects may show behavioral changes, lethargy, swelling, deformities, and feeding changes (among other signs). Beyond pathogens, parasites like mites and phorids may infect farmed BSF populations, and predators including ants, spiders, and cockroaches may pose a problem (albeit a less significant one). 

To mitigate disease outbreaks, the authors recommend that producers raise BSF in ways that mimic their natural environment. It’s also important to consider the environmental conditions of BSF rearing facilities (more on that below). For example, lowering humidity levels may prevent fungal diseases. To deter parasites and predators, producers should also use quarantines and prioritize good hygiene and cleaning practices.

Environmental Conditions

Factors such as temperature, humidity/moisture, oxygen, pH, pupal substrate, and light can affect welfare, although the ideal environmental conditions may vary depending on the BSF population.

Regarding temperature, adults and larvae have different optimal temperature ranges. In general, though, the authors argue that temperatures shouldn’t drop below 19 °C or above 30 °C to reduce mortality rates. What complicates things is that crowding and feeding can lead to metabolic heat production, which in turn raises heat levels and should be considered when farming BSF.  

Beyond temperature, humidity levels may affect BSF egg laying, mating practices, and flight. The authors argue that humidity levels should be set to a minimum of 70%. Moisture levels can be heavily affected by the substrate used in rearing bins. 

The authors point out that larvae are typically slaughtered before they use substrate for pupation, but it’s still important to think about substrate use for larvae reared into adulthood. They identify wood shavings as the ideal substrate at this stage, although additional research is needed to confirm this.

Oxygen and pH levels are additional considerations. According to the authors, producers should take steps to ensure oxygen levels don’t drop too low in rearing bins, which can impact larvae (especially when they’re feeding in substrate). From a biological perspective, pH levels may be affected by microorganisms living in the substrate, and sub-optimal pH levels can impact BSF growth and gut microbiomes. Although neutral-to-basic conditions tend to support growth, the authors note that growth is not a strong indicator of welfare. As a result, more research in this area is needed.

BSF larvae avoid light, and the authors argue that producers need to provide adequate substrate for larvae to avoid light sources. When larvae reach adulthood, they tend to live longer in no-light conditions, although light can help trigger mating activities. However, the authors also say that more research is needed to understand the best light sources and how variations in light (e.g., flickering and intensity) affect mating. 

Finally, regarding mating, spatial conditions can also impact adult BSF mating activity on farms. For example, BSF mating cages should be placed in areas with limited human activity, as they are sensitive to interruptions. Research suggests that cages should also be larger than 3m³, although the authors point out that many of these studies don’t specify how many individual flies should be placed in the mating cages (making the results confusing).

BSF Nutrition

Many adult BSF are not given food (due to a belief that they can simply live off of their energy reserves). However, the authors say that mated adults may live longer (up to 14 days compared to 8 days) when given access to sugar water. According to the authors, it seems that the current feeding practices on BSF farms often result in starvation, suffering, and premature mortality. 

When it comes to nutrition, it’s important to consider the insects’ life stage. For larvae, food sources can impact their development, weight, and body composition. Research suggests that larvae should be offered nutrient-dense diets with a mix of carbohydrates and proteins. However, it’s also important not to overfeed them, which may also harm their health (and potentially their welfare). 

Meanwhile, adult BSF (and especially mating/mated males and females) require different levels of energy. Studies have found that females benefit more than males from eating protein-dense milk and agar, while sugar-water may extend male lives more so than female lives.  

Injury And Crowding

Situations of larval crowding facilitate the spread of diseases and may increase the risk of injury. BSF larvae bodies are soft and very susceptible to injuries from sharp materials, inappropriate handling, and cannibalism. Even if injuries don’t kill larvae, they can highly increase the risk of infections.

The authors note that adults and larvae should be housed separately, as BSF larvae have been known to cannibalize adults. 

Handling-Associated Stress

Human handling and disturbance may be common on BSF farms, although these activities may also cause BSF to experience stress. There will always be some handling associated with rearing BSF, for example when changing their substrate, checking on their health, and “harvesting” live insects for exotic animal feed. However, limiting the frequency of disturbance, or handling larvae in dark rooms, may help mitigate the impacts of disturbance.

Selective Breeding And Genetic Modification

The authors explain that researchers are exploring ways to genetically modify BSF to support industry needs, including ways to extend their development time (to increase larvae weight), “improve” their nutrient concentration (to benefit their use as human protein sources), and remove their ability to fly (so they can be housed in smaller cages). On a similar note, some BSF are selectively bred for optimal traits, although this has been known to harm their populations. More research is needed to fully understand the welfare impacts of these practices. 

Environmental Contaminants

The authors note that products used to feed BSF may be contaminated with pollutants, such as chemical pesticides and herbicides that can impact their health. Furthermore, “garbage” can be found in BSF waste, including acids and chemicals, plastics, and sharp objects. These can harm both larvae and human workers. When garbage contaminates larvae food sources, it can be toxic and deadly to the insects.

Welfare During Slaughter

Research and regulations on BSF slaughter methods are currently non-existent. As a result, the authors consider which methods may cause death within one second, as they expect this would cause the least amount of suffering. Using time-to-death criteria, they believe that boiling/blanching, freezing in liquid nitrogen, and grinding would be the most humane methods for both larvae and adults. Although not a standard industry practice, they also recommend the use of anaesthetics. 

Less humane slaughter methods for larvae, also based on time-to-death, seem to be sand roasting, microwaving, sun baking, oven baking, freezing in air, and asphyxiation. Some producers starve and “wash” larvae before slaughtering them, which the authors believe may also harm their welfare.

Advocacy Takeaways

This paper suggests that there are many issues that can impact BSF welfare, and most of them are not yet fully understood by researchers and producers. As more animal advocates turn their attention to insect welfare, it’s important to think about the needs of individual insect species, including black soldier flies. This paper offers starting points for advocates to consider in future research, legal advocacy, and campaigns for BSF welfare.