The hidden sweet tooth of the black soldier fly ( <i>Hermetia illucens</i> )

The black soldier fly Hermetia illucens (BSF) 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 BSF 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 the adults 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 show a general high level of expression including in the head and in the antennae whereas the two others display tissue-specific patterns of expression. We also identify a high number of GR pseudogenes, including four putative sugar receptor pseudogenes, indicating multiple gene loss events of GRs compared to other dipterans sharing a similar ecological niche. Despite this reduced GR repertoire, adult BSF retain 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.