Some flies like crowds

Global population growth and migration from rural areas to cities has resulted in human settlements being concentrated in urban areas. A consequence of these patterns is an increase in consumption of food and other natural resources, as well as an increase in the production of waste. In Africa it has been calculated that close to 50% of solid waste to landfill is organic, largely comprising food waste. This has led some researchers to devise ways in which to "close the loop" between waste and food production by recycling nutrients that would otherwise be lost.

Agricultural, industrial, and household organic waste can be transformed into value-added products using a process termed "bioconversion". One promising approach is to use flies that are naturally associated with the breakdown of decaying material to convert waste into body mass. The flies can then be harvested and used as a source of proteins and fats that can be fed to livestock and farmed fish. Wild birds and fish would normally eat insects and a feed enriched with insect components represents a "paleo diet". Alternatively, flies reared on waste can also be converted into biodiesel.

To be able to convert the large quantities of organic waste produced by agriculture and other human activities and produce these value-added products, enormous numbers of flies are required. For this reason, and to reduce the space and resources required to house them, flies need to be kept at very high density. But just like some people who prefer the cosmopolitan lifestyle of living in cities, some fly species are better able to handle the pressures of living in crowded conditions and thrive when given the opportunity. For this reason, Nina Parry and Chris Weldon from the Department of Zoology and Entomology at the University of Pretoria (UP) have worked to establish which of four blowfly species could cope best when living at high density. Blowflies are easy to work with and are naturally associated with decaying meat and manure, and therefore could be a good option for the bioconversion of these organic waste types.

The four fly species that have been tested are commonly found around your garbage bin: the African latrine blowfly, oriental latrine fly, copper-tailed blowfly, and green bottle fly. Flies of each species were kept at densities of 20, 50, 100, 250, 500, and 1 000 flies per 30 × 30 × 30 cm cage with an even number of females and males. Observations were recorded of the time it took for 50% of the flies to die, the number of eggs laid, and the number of eggs that hatched.

The species that had the lowest mortality while maintaining high egg production at a high density was the African latrine blowfly, which makes it the best-suited species that was assessed for the conditions experienced in bioconversion facilities. The green bottle fly had much higher egg production than any of the other species, but only at low density, which is not ideal for efficient use of space within a bioconversion facility. The copper-tailed blowfly also produced high numbers of eggs, but survival and egg production were low at higher densities. Fertility of the oriental latrine fly did not change in response to density, but this species had low egg production, making it less suitable for the production of large numbers of larvae for bioconversion of waste when space is limited.

Further research is required to test the suitability of blowfly species for the degradation of particular waste materials and conversion into usable body mass. However, results so far indicate that the African latrine blowfly is a promising candidate for use in bioconversion facilities due to its adaptability to crowded conditions. These flies could well be the new savvy city-goers of the fly world.

These results are published in full in: Parry, N.J., Pieterse, E. and Weldon, C.W. 2017. Longevity, fertility and fecundity of adult blow flies (Diptera: Calliphoridae) held at varying densities: Implications for use in bioconversion of waste. Journal of Economic Entomology. DOI: 10.1093/jee/tox251

Researchers

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  Professor Chris Weldon

  Professor Christopher Weldon is an Associate Professor in Applied Entomology at the University of Pretoria (UP). He completed his undergraduate studies at the University of Newcastle in Australia, graduating with a Bachelor of Environmental Science (honours) and the Dean’s Medal. He has been pursuing research at UP since he was appointed as a lecturer in Applied Entomology in 2012.
  
  Prof Weldon says his field of research would contribute to the betterment of the world because human health and livelihoods often intersect with the activities of insects. Using a foundation in insect behaviour, ecology and physiology, his research aims to address these interactions, whether negative or beneficial, with a focus on flies but also insect pests of horticulture. Results from these studies inform sustainable management practices that can reduce reliance on pesticides but may also facilitate the beneficial use of flies.
  
  He leads a research group of postgraduate students at honours, master’s and PhD level. “We pursue projects on the traits that make fruit flies destructive and invasive pests; the behaviour and movement of fruit flies that damage our fruit for consumption and exports; the biology and management of mosquitoes and other blood-feeding flies that affect human and animal health; and the use of flies for bioconversion of organic waste to promote a sustainable, circular economy,” Prof Weldon explains.
  New research that the professor and his research group have been working on over the past 18 months has been the practical interventions that can be used by hotels to reduce mosquito abundance, the nuisance of mosquito bites and the potential spread of diseases. Such practical interventions will improve the experience of hotel guests and encourage tourism.
  
  Prof Weldon and his group have also done research on how body condition and environment affect fruit fly attraction to lures and movement in the field; the diversity, abundance, and damage caused by thrips in avocado and macadamia orchards; and how to improve surveillance of invasive pests of citrus. All this work is being done in partnership with South African horticulture industries as well as international organisations to improve the productivity and competitiveness of South African horticulture.
  His advice to school learners or undergraduates who are interested in his field is to read widely about the diversity and importance of insects. There are plenty of interesting and reputable sites featuring that kind of content, Prof Weldon says, such as Entomology Today, as well as great books on insects in southern Africa. He also suggests they join a society like the Entomological Society of Southern Africa or the Lepidopterists’ Society of Africa to learn more about insects from experts and have opportunities to work with them. “Register for an undergraduate degree programme in Entomology like UP’s BSc Entomology, he adds, “and be sure to take the right subjects in school to be admitted.”
  

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Related Photo

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  To be used for bioconversion, flies need to tolerate crowded conditions