The Muscidae and Calliphoridae are commonly referred to as “filth flies” because of their association with refuse, biological waste, and other decaying and microbe-rich substrates (Fig. 1). The imminent potential of house flies (Musca domestica L.) and blow flies (Diptera: Calliphoridae) to carry and disseminate “germs” is clearly understood by laypersons, who almost instinctively know to shoo and swat these annoying pests when they enter the domestic realm or land on our food. Blow flies also are familiar to the public for their role in forensic entomology, making guest appearances on popular television shows like “Forensic Files” and “CSI” (e.g., Forensic Files 1996, 2003; CSI 2000). In areas of the world where livestock are reared, filth flies are well-known pests that negatively impact animal production. Adult horn flies and stable flies feed on blood, causing decreased weight gain and delayed milk production. Houseflies and face flies feed on animal secretions and cause annoyance in the process. More importantly, blow fly larvae can cause myiasis, and adults disseminate pathogens from filth to animals as well as between animals, and even contaminate their human food products (milk, eggs). In addition to these costs, there are added expenses associated with fly control including insecticidal ear tags, pesticide sprays, traps for adult flies, and manure management to reduce larval breeding sites. Estimates of the economic impact that flies have on livestock producers have been reported elsewhere (e.g., Kunz et al. 1984, Hogsette and Geden 1994, Taylor et al. 2012).
Our knowledge of the biology of filth flies, in particular their associations with microbes, still is a blossoming area of study. The fairly recent availability of “omic” resources such as genomes, transcriptomes, and microbiomes for the most important of these fly species, and advent of new and improving techniques for studying fly–microbe interactions on a biochemical, cellular, and molecular level, collectively provide exciting opportunities to discover novel targets for fly control and mitigation as well as avenues to exploit filth fly biology for human benefit. The papers in this special collection have the common theme of using these next-generation resources and innovative approaches to study the biology of fly–microbe interactions.
Interactions Between Larval Filth Flies and Microbes
Larval filth flies have a trophic association with substrates that are teeming with bacteria. For the Muscidae, in agricultural settings (Fig. 1) this involves development directly in animal waste (e.g., fresh dung for horn flies and face flies) or materials contaminated with animal waste such as bedding and feed (e.g. stable flies and house flies, although dung, manure, and decaying plant materials can also be utilized). Muscid larvae ingest bacteria and other microbes in the substrate. The ability for muscid larvae to be artificially reared on agar monocultures has demonstrated that bacteria are indeed the source of larval nutrition (Lysyk et al. 1999, Zurek et al. 2000, Perotti and Lysyk 2003). The composition of the microbial community and its ability to be digested and used as food directly impacts larval survival to pupa, eclosion rates, and adult fitness. Thus, for these flies, the microbial ecology of the site directly impacts the reproductive potential of local populations.
Scully et al. investigated the microbiomes of larval stable flies (Stomoxys calcitrans L.) and their developmental substrates from different sources (calf bedding, artificial media) and found that the larval microbiomes were more similar to each other than they were to their matched substrates. This suggests that larval stable flies digest certain microbial species in the substrate, rendering them unlikely to be identified in the microbiome analysis and causing the disparity between those isolated from the substrate and those isolated from the larvae themselves. Similar findings have been reported with house flies and horn flies, as reviewed by Nayduch and Burrus in this collection. With house flies, the survival of some indigestible species of bacteria lends to their increased possibility of being carried trans-stadially by newly emerged adults, which is important in the epidemiology of house fly-disseminated pathogens.
Horn flies develop in fresh dung pats and, like other muscids, their larvae require live bacteria for successful development (Perotti et al. 2001, Temeyer 2009). While it has been demonstrated that horn flies can be reared on monocultures of bacteria (Perotti and Lysyk 2003), in this collection Olafson et al. investigated whether different strains and doses of bacteria, Salmonella enterica ser. Montevideo ("Salmonella"), would affect horn fly larval development. Similar to other studies in this issue (see: Nayduch and Burrus, Thomson et al., Pace et al.) and previous studies (Olafson et al. 2014), green fluorescent protein (GFP)-expressing recombinant strains of bacteria were used in order to facilitate selective culture and visualization. Horn flies were reared on manure inoculated with different strains and doses of GFP-Salmonella. The study concluded that a combination of dose and strain may contribute to decreased survival, evidenced by low adult emergence, due to proliferation and invasion of certain strains during late stages of pupal development. During this time, bacteria have the opportunity to escape entrapment in the (previous) larval gut during histolysis and histogenesis, subsequently invading the tissues of the developing imago.
For the Calliphoridae, larval developmental sites contain decaying materials such as carcasses or rotting garbage. In contrast to the muscid flies, blow flies subsist on the substrate itself, and likely compete with microbes for this resource (reviewed by Tomberlin et al. in this collection). Indeed, blow flies can be reared aseptically on blood agar for subsequent medicinal purposes, such as maggot debridement therapy (Andersen et al. 2010). As blow fly larvae develop and ingest the substrate they secrete products that eliminate other microbes and thoroughly destroy many of those that are ingested in the gut (Mumcuoglu et al. 2001). Although microbes are not a nutritional requirement for successful larval development in this family of flies, gravid female blow flies are attracted to decomposing carcasses, and this is likely mediated by volatiles from microbes (reviewed in Tomberlin et al.).
Brundage et al. describe how the presence of eggs from conspecific and heterospecific blow flies (Calliphoridae), and their associated microbes, may impact succession on an ephemeral resource. In regard to colonization of vertebrate carrion, Cochliomyia macellaria (F.) is a primary colonizer while Chrysomya rufifacies (Macquart) is a secondary colonizer in Texas. In addition, Ch. rufifacies larvae are facultatively predacious on C. macellaria larvae and can be cannibalistic. Because arrival time impacts larval success and fitness, the authors hypothesized that the presence of eggs and their associated microbes serve as cues to assist gravid female blow flies locating, and assessing, resources to be used as oviposition sites. Their results showed that C. macellaria and Ch. rufifacies responded differently to eggs of conspecifics and heterospecifics, and speculated that this was mediated by microbial volatiles. Further, because attraction and the microbial community both changed over time, ovipositing adults may have altered behavior in response to these changes in microbe volatiles. Interestingly, the study also revealed that taxa associated with Ch. rufifacies eggs were similar to those from C. macellaria, which may camouflage them from C. macellaria.
Weatherbee et al. examined how microbes and larvae interact to recycle the biomass of the decaying substrate (in this case, pig carcasses) by taking snapshots of the microbiome from the carcass surface and both internal and external to the maggot mass over time and space during decomposition. Using high-throughput 16S rDNA sequencing, they found that the abundance of microbial taxa changed over decomposition time as did the abundance of maggot species, and interestingly found multiple blow fly species occupying the same maggot mass including Phormia regina (Meigen), Lucilia coeruleiviridis (Macquart), and Cochliomyia macellaria (F.). Further, microbial taxa varied across sampling locations, which was attributed to significant interactions between the microbes, maggots, and the environment during the decomposition process.
Interactions Between Adult Filth Flies and Microbes
Although adult filth flies apparently do not have a nutritional requirement for consuming microbes, as evidenced by the capacity to rear them aseptically in the laboratory, they still frequent microbe-rich sites for reproductive purposes. While visiting these sites, flies become surface-contaminated with microbes, and additionally will opportunistically (e.g., via grooming) or intentionally ingest the substrate and associated microbes. In contrast to larvae, adult flies are highly motile and are zoophilic and synanthropic (Fig. 1). Adults of zoophilic species feed on blood (e.g., stable flies, horn flies) and secretions and excretions (e.g., blow flies, house flies) of animals. These animal products provide proteins necessary for commencing egg development in anautogenous females, and therefore are critically impact the reproductive capacity of local populations.
Thomson et al. addressed the question whether female house flies acquire and harbor a greater abundance of bacteria than male house flies by looking at sex-specific acquisition of E. coli and Salmonella ser. Typhimurium from manure. Female and male flies were exposed to manure inoculated with of GFP-expressing strains of bacteria in two assay conditions: one with inoculated manure being the only food source for the flies and the other where flies were exposed to inoculated manure and sterile sugar water was also provided. Flies were collected at three time points over 24 h and bacteria were enumerated. Compared to male flies, female flies harbored more bacteria of either species both on their surfaces and internally over 24 h in all assays. The authors proposed that female house flies acquired and harbored more bacteria than males due to their increased interest in manure for oviposition and nutritional purposes.
Because blow flies and house flies can acquire protein from animal and nonanimal sources, they have the potential to disperse to sites away from animals and their excrement. Adult blow flies and house flies will seek protein from domestic sources such as human food and pet excrement, and likewise utilize peridomestic sites such as decaying refuse, decaying flesh (especially blow flies), and even compost for oviposition and larval development. While all filth flies are of veterinary importance, the synanthropic behavior of blow flies and house flies makes them additionally important to food safety and human health. Pace et al. compared the potential of house flies (Musca domestica L.) and blow flies (Phormia regina Meigen) to acquire and transmit GFP-expressing strains pathogenic bacteria, E. coli O157:H7 (EC) and Salmonella enterica (SE), to lettuce (Lactuca sativa L.). After a 10-s exposure to manure inoculated with either microbe, house flies acquired more SE than EC and blow flies acquired more EC than SE. However, there was no difference in the amount of bacteria acquired when exposure time was increased to 30 s. Blow flies harbored more EC and SE than house flies, and consequently transferred more bacteria to lettuce during transmission bioassays. Further studies showed that blow flies deposited the greatest abundance of bacteria to lettuce during the first 30 s of exposure. The differential acquisition and transmission rates of pathogens by these two synanthropic filth fly species and the potential of blow flies to transmit pathogens in such a short time window have great implications for fly management at postharvest facilities.
Future Approaches to Understanding Filth Fly Microbe Interactions
Several studies featured in this collection used culture-independent microbiome analyses. This tool is cost-effective and allows for large-scale snapshots of microbiota that can be applied to other studies aimed at understanding the spatiotemporal dynamics of microbe–fly interactions. In addition, utilization of next-gen approaches like RNAseq, comparative transcriptomics, and proteomics can help in characterizing the biological responses of filth flies while they interact with this changing microbiome across life history, decomposition, and other conditions. Several studies in this collection used recombinant microbes (e.g., those with a GFP tag) in order to track the acquisition, interactions, and result of those interactions between pathogens and flies. Similar approaches could be used to study microbe–microbe interactions with flies by tagging microbes with different markers, e.g., GFP and RFP (red fluorescent protein).
Modeling the acquisition and dissemination of microbes by filth flies both in the agricultural setting and in regard to human health is a research area that is largely unexplored. Abiotic factors that impact microbial communities, such as temperature as it relates to climate change, should be incorporated in these models, especially those for muscid flies whose larvae are nutritionally dependent on microbes. Predictive biology of filth fly resource utilization, population structure, geographic distribution, and disease dissemination potential should take a multidisciplinary approach including the human and animal host biology, since many of these fly species are dependent on host organisms either as adults (blood feeding) or larvae (development in their waste or carcasses). Because microbes are important in blow fly attraction to carrion as well as working competitively or synergistically in recycling the biomass, they must continue to be considered in estimating time of colonization. Next-gen approaches used in studies featured here would be useful in investigating the interactions between blow flies, other necrophagous insects, and the carrion microbiome from colonization to decomposition completion.