Standardized Protocols for Global and Local Thermal Nociception in Black Soldier Fly (Hermetia illucens; Diptera: Stratiomyidae) Larvae

Abstract

Nociception, the capacity to detect tissue-damaging stimuli such as noxious chemicals or high heat, is increasingly studied across insect orders and life stages, informing our understanding of its adaptive value and molecular mechanisms. The black soldier fly (Hermetia illucens; Diptera: Stratiomyidae) is widely recognized as the star of the growing insects as food and feed industry. Black soldier fly larvae (BSFL) are reared at high densities that, combined with their extraordinary metabolism, can generate lethal overheating on farms. Data on the thermal nocifensive capabilities of BSFL could inform our understanding of larval behaviors during overheating events (and potential welfare impacts of thermal slaughter), and provide a comparative datapoint to the well-studied vinegar fly (Drosophila melanogaster; Diptera: Drosophilidae). Accordingly, we adapted global and local thermal nociception methods from larval vinegar flies for use with BSFL. We find that global assays (akin to boiling) adapt easily to both first and late instar BSFL, that BSFL exhibit slightly different nocifensive behaviors than last instar vinegar fly larvae, and that BSFL have higher thresholds (thrashing begins at 39.70 C in late instar BSFL versus 26.6 C in D. melanogaster). In contrast, the classical local nociception assay did not adapt easily to BSFL, so we modified the assay to test a time-bounded, continuous exposure to the temperature probe instead of response-latency. This assay generated a unique, gradual pattern of increasing responsiveness to the probe in late instar BSFL (>95% responsiveness above 66 C) rather than the sharp cutoff in responsiveness at 52 C demonstrated in D. melanogaster during response-latency assays. Altogether, these protocols open the door for standardized research on BSFL thermal nociception for fundamental and applied purposes.