Sciomyzidae
Creators: name: Williams, C. D.
name: Knutson, L.
name: Gordh, G.
name: Mc Donnell, R. J.
name: Kula, R.
name: Gates, M.
name: Buffington, M.
name: Broad, G.
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Diptera
Sciomyzidae

description: Role of Sciomyzidae in biocontrol and evaluation of biocontrol implications of natural enemies of Sciomyzidae
Other than data on percentages of field collections of Sciomyzidae eggs and larvae/ puparia parasitised by some Hymenoptera reviewed above, there is little quantitative information on the impact of natural enemies on sciomyzid populations. Obviously, the impact of, at least, Trichogrammatidae on egg populations and of Braconidae and Ichneumonidae on emergence of adults from puparia might be significant. However, the impact of the latter two families on the standing crop of larvae feeding in/on snails may be low, because the larvae continue to feed and develop to pupariation even when parasitised by these wasps. It should be noted that physiological reactions of hosts and death of larvae as a result of parasitoid feeding may be non-negligible.
About 240 of the 548 valid, described species of Sciomyzidae, worldwide have been well documented (in most cases including complete life cycles) as obligate parasitoids/predators of Mollusca (except two predators of oligochaete and one parasitoid of millipedes). Thus, Sciomyzidae are well recognised as potential biocontrol agents of disease-carrying freshwater snails and terrestrial snails and slugs of agricultural importance. Their potential has been reviewed by WHO (1961), Berg (1964), Greathead (1981), Appleton et al. (1993), Coupland and Barker (2004), Knutson and Vala (2011), Murphy et al. (2012), and others. Knutson and Vala (2011) extensively reviewed the field trials and laboratory experiments to date against freshwater Biomphalaria snails that are obligate intermediate hosts of Schistosoma spp. worms affecting man [in Iran (Tirgari and Massoud 1981) and South Africa (Appleton et al. 1993)] and against Lymnaea freshwater snails that are obligate intermediate hosts of Fasciola spp. worms affecting livestock [in Hawaii (Chock et al. 1961) and Ireland (Gormally 1988a, 1988b; McDonnell 2004)]. Research results and reviews of Sciomyzidae that are natural enemies of terrestrial snails and slugs that are agricultural pests are presented by Coupland (1996), Coupland et al. (1994), and Coupland and Barker (2004). Baker (1985) and Bailey (1989) presented the life cycle and biology of Pelidnoptera nigripennis (Fabricius) as a potential biocontrol agent of millipede household pests in Australia. PhD thesis research on Tetanocera spp. as biocontrol agents of slugs in greenhouses in Ireland has been conducted at the National University of Ireland, Galway [see papers from The Applied Ecology Unit (Gormally’s lab) by Hynes et al. (2014a, 2014b), Bistline-East et al. (2018, 2020a, 2020b) and D’Ahmed et al. (2019)].
There are studies underway on the role of natural populations of Sciomyzidae in providing ecosystem services (ie their effects on natural populations of pest molluscs and as bioindicators). Some earlier population studies (eg Eckblad and Berg 1972; Arnold 1978) and more recent studies (eg Williams, Moran et al. 2009, Williams, Sheahan 2009, 2010; D’Ahmed 2021) give some indications of the value of data on Sciomyzidae in those regards.
In terms of modern ecological and biocontrol theory, the most comprehensive analysis of Sciomyzidae is that by Barker in Coupland and Barker (2004). In reviewing the requirements for successful biocontrol agents, Barker detailed that Sciomyzidae meet, or potentially meet, some of these requirements. More research is needed in some areas.
Ease of laboratory rearing
McLaughlin and Dame (1989) reared the endemic North American freshwater predator Dictya floridensis Steyskal on a large scale, continuously through the F8 generation, availability of snails and manpower being the limiting factors. Appleton et al. (1993) demonstrated that production of large numbers of endemic South African freshwater Sepedon spp. predators could be achieved with relatively modest investment and nonspecialised workers in laboratory rearings.
Successful long-distance transport
Sciomyzidae adults, eggs and pupae are very generally robust animals, with viable cultures being easily transported (eg the freshwater predator Sepedomerus macropus (Walker) from Nicaragua reared at Cornell University and shipped to the Department of Agriculture, Hawaii Chock et al. 1961). Knutson successfully carried many rearing containers of studies in progress on several species, 2–7 January 1964, from Harpenden, England, to Almeria, Spain, under less-than-ideal conditions.
Favourable recipient environment
Sciomyzidae exploit most freshwater and terrestrial snail habitats (eg Speight and Knutson 2012). Also, many species reproduce continuously at favourable temperatures, having several generations per year; many species have a broad north–south range which allows for selection of natural enemies from a range of climatic/environmental conditions.
Pest suppression
There have been no studies on Sciomyzidae (and few on gastropods) using the procedures and criteria proposed by Waage and Mills (1992), Kidd and Jervis (1997), or Jaenike (1998). Such studies – especially highlighting polyphagous parasitoids of Diptera – are essential.
Stability of pest suppression
Barker et al. (2004) commented extensively on the plethora of features of Sciomyzidae and their potential freshwater and terrestrial gastropod target prey. He noted that some research has been conducted on some aspects – this is a particularly important aspect of his analysis.
Minimal adverse impacts on biodiversity
While information on Sciomyzidae meets requirements 1–3, there is insufficient information concerning pest suppression (4) and pest suppression stability (5) and the impact of natural enemies (that attack sciomyzids?) on those aspects. Requirement 6 is discussed by Barker et al. (2004) and extensively by Knutson and Vala (2011). Barker et al. (2004) discussed the potential impact of Sciomyzidae on non-target gastropods in relation to four scenarios proposed by Hopper (1995). He concluded that no sciomyzid could be guaranteed to have no direct non-target effects (his scenario 1) and that many might have considerable impact on biodiversity (his scenario 4). However, with careful selection of sciomyzid species for introduction based on niche requirements and prey preferences, scenario 3 (some small but generally unpredictable mortality if the natural enemy also utilises one or more co-occurring species in addition to its normal hosts/prey) or even scenario 2 (no impact if the natural enemy utilises other hosts/prey but is temporally or spatially isolated from them) may be the expected outcome. The methods, protocols and facilities for testing the safety of introduced agents have improved greatly over the past few decades; if these are followed we can be assured of the safety of introduced agents (Knutson and Coulson 1997). Van Driesch and Hoddle (2017) further outline the possible direct and indirect impacts of introduced biological control agents. They list the following potential impacts: (1) direct attacks on native insects; (2) negative foodweb effects, such as competition for prey, apparent competition, or displacement of native species; (3) positive foodweb effects that benefit non-target species; (4) hybridisation of native species with introduced natural enemies; and (5) attacks on introduced weed biocontrol agents. All these need to be considered before Sciomyzidae are introduced as classical biological control agents of molluscs.
Barker emphasised the need for six kinds of information to estimate levels of gastropod mortality caused by the larvae, (1) the average level of predation or parasitism per host/ prey generation, (2) its variability from generation to generation and whether or not this source of host/prey mortality is a key factor, (3) the extent to which predation or parasitism tends to act as a density-dependent factor, (4) other prey mortalities that combine with that caused by the sciomyzid larvae to counter the host/prey’s potential rate of increase, (5) any important mortalities suffered by the sciomyzids that reduce their effectiveness, and (6) the density of searching sciomyzid adults.
Research needs for further understanding the impact of natural enemies in the use of Sciomyzidae as biocontrol agents
● Other than the publications on Trichogrammatidae (Juliano 1981, 1982), Diapriidae (Knutson and Berg 1963; O’Neill 1973) and parasitoids of Anticheta melanosoma (Knutson and Abercrombie 1977), there is essentially no quantitative data or information on populations of natural enemies of Sciomyzidae.
● The internal pathogens of adult and immature Sciomyzidae are essentially unknown; larvae, pupae and adults should be examined for pathogens, for example Wolbachia (Proteobacteria: Rickettsiales: Rickettsiaceae).
● An investigation of potential endosymbionts which are known to occur in other Diptera (eg Sciaridae) might also yield useful results.
● Review of museum collections of pinned adults such as Asilidae, Empididae, Scathophagidae and some predatory wasps holding adult prey will provide information on sciomyzid species that are preyed upon by such predators.
● The major advance in knowledge of parasitoid natural enemies of Sciomyzidae will come from further collecting immature stages in nature and holding them in the laboratory for the emergence of natural enemies. Methods for collecting larvae and puparia of Sciomyzidae are detailed by Knutson and Vala (2011). Larvae are held with food snails, until pupariation, in containers simulating the conditions where they were found. Puparia are held individually in vials with a slightly moist substrate until emergence of the parasitoids or adult flies. The hosts of many of these parasitoids could be identified, especially by study of the third-instar cephalopharyngeal skeleton and other larval vestiges, as well molecular methods such as DNA barcoding.
● Egg, larval and pupal parasitoids could be obtained by exposing laboratory-reared stages in small containers in the precise sciomyzid micro-habitats in nature, then retrieving and holding them for emergence of parasitoids. This technique was successfully used by O’Neill (1973), who obtained the new species Trichopria atrichomelinae (Diapriidae) from puparia of Atrichomelina pubera reared in the laboratory and exposed for seven days in A. pubera habitats.
● Laboratory tests of the host range of the more common, polyphagous parasitoids, including appropriate stages of potential sciomyzid biocontrol agents, should be carried out in areas where biological control projects are planned. Techniques as described in the ‘Review of the literature’ section above could be used to monitor natural enemy impact on sciomyzid biocontrol agents during such projects.
● Preliminary to introduction/augmentation of Sciomyzidae for biocontrol purposes, assessments of the impact of endemic parasitoids (especially polyphagous species) on Sciomyzidae should be made by rearing parasitoids from fly larvae/puparia found in dead snails and from eggs of insects in the target microhabitats, as described in our Section ”Behavioural groups and microhabitats” - see above. Rearing of parasitoids from other Diptera in the biocontrol target area will help to reveal the potential extent of impacts of polyphagous parasitoids.
● A review of the super-parasites that attack parasitoids of Sciomyzidae is needed.
description: Published as part of Williams, C. D., Knutson, L., Gordh, G., Mc Donnell, R. J., Kula, R., Gates, M., Buffington, M. & Broad, G., 2025, First tabulation and analysis of natural enemies of snail-killing flies (Diptera: Sciomyzidae), their position in the fly / mollusc ecosystem, and implications for use of sciomyzids in biological control, pp. 331-396 in Journal of Natural History 59 (5 - 8) on pages 384-387, DOI: 10.1080/00222933.2024.2443125, http://zenodo.org/record/14798887