Behaviour of two fly species reared for livestock feed: Optimising production and insect welfare

The mass rearing of insects as animal feed is a new and rapidly growing component of circular agriculture, which offers the opportunity to develop it in such a way that it promotes insect health and welfare. Behaviour is an important indicator of animal performance and welfare. In this review, we synthesise the current behavioural knowledge on two saprophytic dipteran species that are increasingly being used as mini-livestock, the black soldier fly (Hermetia illucens) and the housefly (Musca domestica). We evaluate which behaviours need to be considered to optimise insect production and welfare under mass-rearing conditions. We distinguish between the different life stages (adults and larvae), and describe their feeding behaviour, social interactions (adult mating, larval aggregation), oviposition behaviour and possible cannibalism. For each species, we review what is known about these behaviours in natural environments, and how this is affected by abiotic factors or interactions with conspecifics and heterospecifics. We also address how the flies’ microbiome and pathogens can influence various aspects of behaviour. Notable differences in natural behaviours between the two species, such as their courtship and mating behaviour and the larval distribution within feed substrates are identified. These behavioural differences have important implications for how we should rear the two fly species in industrial settings, as a mismatch in mass-rearing conditions may induce environmental stress or compromise insect productivity and welfare. Escape behaviour, larval aggregation behaviour, possibly cannibalism in the larval stage and mating frequency and reproduction rate are identified as behaviours providing information on welfare of larval and adult flies. Finally, a number of aspects are identified for which behavioural knowledge is currently still sparse, while this may be important to safeguard insect welfare. We conclude with recommendations for future research to promote insect welfare.