The Hidden Sweet Tooth of the Black Soldier Fly (Hermetia illucens)
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. While egg production depends on adult performance, the biology of adult BSF remains poorly understood. In particular, growing evidence suggests that adults can feed and benefit from sugar supplementation but the behavioural and molecular bases of sugar detection remain unknown. To fill this gap, we tested whether adult BSF can detect and prefer sugar by using behavioural assays, binary-choice feeding tests and proboscis extension responses. We also described numerous gustatory sensilla on the proboscis with scanning electron microscopy, and characterized the responses to sugar in some of them with electrophysiological recordings. These experiments were complemented by an analysis of the gustatory receptor (GR) gene repertoire and the expression levels of sugar-responsive receptors. 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 and substantial reduction of the GR repertoire compared to other dipteras sharing a similar ecological niche. We conclude that although BSF possesses a reduced set of GR genes, including only three putative sugar-responsive GRs, adults have retained a strong 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.