The hidden sweet tooth of the black soldier fly ( Hermetia illucens )

The black soldier fly, Hermetia illucens, is increasingly studied for its ability to convert organic waste into protein, offering solutions for waste valorisation and livestock feeding. Adult performance is critical for egg production, yet the behavioural and molecular bases of sugar feeding in adult black soldier flies remain poorly understood. To fill this gap, we combined behavioural assays, morphological analyses, electrophysiological recordings and gustatory receptor (GR) repertoires and expression. All the experimental tests conducted in this study converge to show that adult flies can detect and consume sucrose, with females responding more strongly than males. Genome analysis identified 28 GRs, a surprisingly small number for a generalist fly, including only three putative sugar-specific GRs homologous to those of the eight known sugar GRs in Drosophila. Moreover, one of these GRs showed a general high level of expression including in the head and in the antennae whereas the other two displayed tissue-specific patterns of expression. We also identified a high number of GR pseudogenes, including four putative sugar receptor pseudogenes, indicating multiple gene loss events of GRs compared with other dipterans sharing a similar ecological niche. Despite this reduced GR repertoire, adult black soldier flies retained strong behavioural and physiological sensitivity to sugars in their environment. The set of behavioural, morphological and electrophysiological tools developed here provides a foundation for deeper investigations into feeding behaviour in this species of growing agroecological importance.