Are BSF an invasive species?
Austin from Olympia asked this question, and it's one that has come up before on my blog. It's been a while since I addressed it, so here goes. Of course much of this is speculation on my part and I welcome differing perspectives.
I have read that black soldier flies originated in the southeast, and I assume that they have naturally expanded their range over time. I believe that because their current range seems to fit a pattern where they would have migrated into regions that support their environmental needs. Perhaps the BSF on the northern fringe of their range slowly adapted to progressively cooler climates, and also to drier climates on the western edge, allowing their range to expand.
To directly answer the question, no, in my opinion BSF can not be described as an invasive species. (Non-native species disrupting and replacing native species) BSF are native to the US, and I don't believe they displace native species. Instead, I believe that their population has grown with the human population, since BSF are well suited to consume our plentiful waste. If I'm correct about that, then BSF haven't displaced other species. If they do displace another species it would be houseflies and other types of flies which have also benefited from increased human activity.
It is thought that BSF expanded around the world by hitchhiking on ships, starting with Columbus, but, even on a continent where BSF are non-native, I don't think they fit the definition of an invasive species. That's for the same reasons I gave above, about how their expansion and success is linked to the growth of human generated waste. More about that below.
Black soldier fly larvae are different from many fly species in that they spend much more time in the larval stage, and do all of their eating in that stage. (BSF adults don't eat) Therefore, BSF require large amounts of rotting food or manure to thrive. In the absence of human activity (copious food waste, livestock manure) I imagine that an otherwise healthy population of BSF would probably die out.
I've read that house fly larvae can fully develop in as little as 8 hours under optimal conditions. Compare that with BSF larvae which can mature in two weeks at best. That represents about forty times longer spent eating. This may mean that in an environment with little manure or food waste available (absence of human activity), house flies could survive, but BSF would not. All of that is point out that BSF are not likely to be invasive, even as non-natives, because their population has grown as a result of the increasing availability of food provided by humans.
Maybe we need a classification that is related to "invasive species", but instead would describe non-native species that are beneficial, or at least neutral, to the environment.