Dietary preferences and impacts of feeding on behaviour, longevity, and reproduction in adult black soldier flies (Diptera: Stratiomyidae; Hermetia illucens)

Abstract The black soldier fly (BSF; Hermetia illucens ; Diptera: Stratiomyidae) is a novel mini-livestock species now reared in the trillions annually around the globe. As BSF farming continues to grow, research on how to protect these animals’ welfare in farmed settings is also essential. Currently, industry standard practice involves not feeding BSF adults and providing only water due to a misconception that BSF adults cannot eat. However, BSF adults have functional mouthparts and digestive systems, gravitate towards and consume food when it is provided, and live longer when provided with food. Therefore, current industry practices, where only water is provided, do not keep BSF adults free from hunger. In order to address this welfare concern, we studied the carbohydrate and protein preferences of BSF adults as well as their preferred protein to carbohydrate ratio and overall carbohydrate concentration. We assessed three carbohydrates (honey, white sugar, and molasses) and three proteins (yeast, agar, and casein). Using the maximally preferred diet (1:8 ratio of yeast to molasses, with molasses concentration set to 5%), we assessed BSF adult mating behaviour, egg mass, and sex-specific longevity on the diet vs in the water-only conditions that are currently standard in the industry. We found that BSF were observed to mate more frequently, laid more eggs on days 2-5 post-eclosion, and lived 4-6 days longer when given access to their preferred diet. Our data suggest that feeding adult BSF their preferred diet could improve animal welfare (by keeping the animals free from hunger and allowing for natural feeding-related behaviors) while also improving the number of eggs laid per day.