of human efforts to control those pests. Work conducted at Portsmouth University in 1986–1987 indicates Simulium spp. create highly acidic conditions within their midguts. This acidic environment provides conditions ideally suited to bacteria that metabolise cellulose. Insects cannot metabolise cellulose independently, but the presence of these bacteria allows cellulose to be metabolised into basic sugars. This provides nutrition to the black fly larvae, as well as the bacteria. This symbiotic relationship indicates a specific adaptation, as fresh-flowing streams could not provide sufficient nutrition to the growing larva in any other way. Regional effects of black fly populations In the wetter parts of the northern latitudes of North America, including parts of Canada, New England, Upstate New York, Minnesota, and the Upper Peninsula of Michigan, black fly populations swell from late April to July, becoming a nuisance to humans engaging in common outdoor activities, such as gardening, boating, camping, and backpacking. They can also be a significant nuisance in mountainous areas. Black flies are a scourge to livestock in Canada, causing weight loss in cattle and sometimes death. Pennsylvania operates the largest single black fly control program in North America. The program is seen as beneficial to both the quality of life for residents and to the state's tourism industry. The Blandford fly (Simulium posticatum) in England was once a public health problem in the area around Blandford Forum, Dorset, due to its large numbers and the painful lesions caused by its bite. It was eventually controlled by carefully targeted applications of Bacillus thuringiensis israelensis. In 2010, a summer surge of insect bites blamed on the Blandford fly required many who had been bitten to be treated in a hospital. The New Zealand "sandflies" are actually black flies of the species Austrosimulium australense and A. ungulatum. In parts of Scotland, various species of black flies are a nuisance and bite humans, mainly between May and September. They are found mainly in mixed birch and juniper woodlands, and at lower levels in pine forests, moorlands, and pastures. Bites are most often found on the head, neck, and back. They also frequently land on legs and arms. In Peninsular Malaysia 35 species of preimaginal black flies were discovered in a study in 2016, including Simulium digrammicum, which had been considered locally extinct. Public health Only four genera in the family Simuliidae, Simulium, Prosimulium, Austrosimulium, and Cnephia, contain species that feed on people, though