Contrary to popular belief, adult black soldier flies can eat
Implications for the farming of these animals
This post is a summary of “Dietary preferences and impacts of feeding on behaviour, longevity, and reproduction in adult black soldier flies (Diptera: Stratiomyidae; Hermetia illucens)”, written by M. Barrett, N. Patel, B. McCarry, G.K. Shellenberger, E.R. Schwartz, K. Fiocca, and E.A. Waddell, recently published in Journal of Insects as Food and Feed. It was adapted from the announcement published by the lab behind the research. The original research received funding from Rethink Priorities.
Background
A common misconception is that adult black soldier flies (BSF) cannot or do not eat. This misconception may take one of three forms:
Adults lack functional mouthparts.
Adults receive no fitness benefits from consuming nutrients, as they gather all the resources they need during the juvenile life stage and are now solely focused on reproduction.
Adults only “drink” and liquid diets are not “feeding.”
Recent research has shown that all three of these claims are empirically false1.
First, adults have functional mouthparts of the ‘sponging’ or ‘lapping’ type that work to consume liquid and semi-liquid diets via capillary action. These mouthparts are characteristic of many other flies, like the fruit fly and the house fly, both well known for feeding. These mouthparts lead to a functional digestive system with all the enzymes necessary to break down both carbohydrates and proteins.
Second, feeding adults increases their longevity and, depending on the diet, increases the likelihood of laying multiple clutches of eggs or a larger number of eggs. This suggests there are fitness benefits of feeding at the adult life stage (even if juvenile nutrition is also important to adult fitness) and may explain why a recent preprint showed that 100% of wild adult BSF were found with nutrients in their stomach.
Third, adults bring in nutrients with their liquid diet and benefit from those nutrients and not just the water. This is liquid feeding behavior, much like adult butterflies, vampire bats, mosquitoes, or aphids that consume exclusively liquid diets like nectar, blood, and plant sap. Accordingly, a recent preprint demonstrated that BSF have the necessary neural architecture to detect the difference between water and sugar water, and actively prefer the sugar water when provided with a choice (see Figure 1 in the linked preprint).
If adult BSF are not fed on farms (they are reared as breeders), this may pose a welfare challenge, as the animals may not be free from hunger or allowed to engage in natural foraging and feeding behaviors.
The study
The authors set out to investigate BSF adult nutrition, asking:
What carbohydrates and proteins do BSF prefer?
In what ratios do they prefer these dietary components?
How does a preferred diet impact mating behavior, egg laying, and longevity?
Key findings
In a series of nutrition-preference tests, the study demonstrated a strong preference among adult BSF for yeast and molasses, provided at a 1:4 or 1:8 protein-to-carbohydrate ratio, with 5% molasses, 1.25% yeast, and 93.75% water (hereafter referred to as ‘adult feed’). These ingredients were preferred to casein and agar (for protein) and to honey and dextrose (for carbohydrates), although dextrose at 10% was not different from molasses.
Longevity increased on the preferred diet by ~25% for both males and females compared to water-only treatments
Additionally, mating behavior increased when provided with adult feed compared to only water
Egg laying increased on the preferred diet
Egg laying increased by 3.8-fold in the first 8 days in the adult feed condition compared to the unfed condition (echoing results of increased egg laying in protein-carb diet conditions in other BSF studies).
Egg laying also began sooner than in the water-only condition (in contrast to other BSF studies on feeding adults).
Implications for BSF farming
Keeping BSF adults free from hunger by providing life-stage-appropriate liquid feed with both protein and carbohydrates is good for their welfare. It also serves the industry’s production goals.
Further dietary optimization efforts could identify even more effective diets for BSF welfare and reproduction, thereby increasing egg yield and reducing variability in reproduction.
Cost-benefit analyses to producers, that factor in the time and labor costs of diet production and hygienic maintenance versus the benefit in reproduction efficiency, will be needed to better understand if and how feeding adult BSF could be implemented by producers.
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In addition to the resources linked in-text below, see: Bruno et al., 2019; Furman et al., 1959; Bertinetti et al., 2019; Chia et al., 2018; Klüber et al., 2023; Macavei et al., 2020; Nakamura et al., 2016; Romano et al., 2020.
Wow I don't know most of the things you think of as "popular belief!"