Black Soldier Flies (BSF), Hermetia illucens (Linnaeus, 1758) (Diptera, Stratiomyidae) are large flies with brownish wings and a pair of characteristic translucent pale patches (windows) on the abdomen . The larvae of this synanthropic species are very effective decomposers of an exceptionally wide range of decaying organic matter including manure, food waste, agricultural by-products, organic leachates and cadavers (including human) [2, 3, 4, 5]. Not surprisingly, the BSF have thus emerged as the dominant source of insect protein and are now utilized on large scale commercially worldwide due to its ability to convert organic matter into oil- and protein-rich feed (for a recent review see ). In nature, adult flies rarely feed but may occasionally imbibe water or honeydew. Therefore, the larvae need to accumulate enough protein and fat during the six larval stages in order to provision the females with sufficient resources for producing 500–1000 eggs  and males with sufficient energy for sperm production and mating on the wing. Commercial producers in China, Europe, South East Asia, Africa, South America and North America presently rear large numbers of larvae that are used as feed in aquaculture and poultry farms [8, 9]. Many cultures were started using ‘Phoenix Worms’ (https://phoenixworm.com/), which have been commercially available for 20 years , but the origin of many additional cultures is unknown. This is problematic because it is common for cultures to be shared without prior genotyping.
The native range of BSF is considered to be New World with the northern-most native populations being from the southwestern parts of the US (at least up to 40º N) and northern South America . However, the species is now cosmopolitan and occurs in all other tropical, many subtropical, and some temperate regions (between 46°N and 40°S) [11, 12, 13]. All the Old World populations are considered introduced. Indeed, numerous historical records from the late 19th and early 20th century document the presence of the species in the New World  while the earliest records for the other regions are considerably younger. For the Afrotropical region, it is a South African record from 1915  and several subsequent ones from Madagascar (1930s) ). The first record for the Palaearctic region is from Malta in 1926:  but by the 1960s the species was known from Spain, France and Italy [12, 16]. In Asia the first confirmed records are from the 1940s (e.g., Malaysia) no confirmed records from China until 1960 . With regard to the Australasian region, reports of H. illucens specimens from Australia collected in 1915 [16, 17] have now been revealed to be erroneous because they belong to a related species (H. pallidipes Hill, 1919) . The oldest known specimen for this conspicuous species is thus from 1948 (in Australian National Insect Collection). According to  many Pacific islands were colonized by H. illucens by the1940s. This includes Hawaii, the Solomon Islands, New Caledonia, Mariana Islands, Palau, and Guam. Overall, it is striking that many of the earliest known localities outside of the New World are on islands or close to the coast. This may indicate that worldwide trade played a major role in the spread of BSF . Introduction via shipping is likely responsible for the fast spread of the species throughout the Pacific region in the 1940s (e.g., Guam and Palau) given that the species’ arrival coincided with troop and supply movements during WWII.
Upon closer study, species with exceptionally large distributions are frequently revealed to consist of species complexes, or, to comprise several evolutionary significant units that deserve recognition in order to protect their genetic integrity [19, 20, 21]. Fragmentary evidence has been emerging that BSF may be such a species. For example, a regional study of mitochondrial cytochrome c oxidase I (COI) barcode variability in South Korea  revealed 10 highly diverged haplotypes. If confirmed for the populations and cultures worldwide, such high genetic diversity would have considerable implications for scientific research and commercial use of the species. For example, it took decades until genotyping revealed the presence of several species in widely used experimental cultures of leeches in the genus Helobdella. By the time of discovery, the lack of timely genotyping had done serious harm to the scientific literature because results had been filed under incorrect scientific names . One year later, similar problems were discovered for a “medical device”, the medicinal leech Hirudo medicinalis. Many leeches used for medical purposes were revealed to belong to species that were not approved for commercial use . Yet, BSF captive cultures used for scientific or commercial purposes still use flies that have not been genotyped and very little is known about the genetic diversity of the species worldwide. We here carry out a first systematic survey across the species’ current range. We compare the genetic diversity of wild-caught flies with samples obtained from cultures maintained for academic or commercial purposes. In addition to mitochondrial data, we assess the genetic diversity for two fast-evolving nuclear markers (ITS2 and a loop region of 28S rDNA; [25, 26]. Lastly, we test whether two cultures with high genetic divergence for COI are reproductively compatible.