Black Blow Fly Research Articles

Black blow fly (Diptera: Calliphoridae) bacterial symbionts inform oviposition site selection by stable flies (Diptera: Muscidae).

Larval habitats of blood-feeding stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), overlap with foraging sites of black blow flies, Phormia regina (Meigen) (Diptera: Calliphoridae). We tested the hypothesis that bacteria in blow fly excreta inform oviposition decisions by female stable flies. In laboratory 2-choice bioassays, we offered gravid female stable flies fabric-covered agar plates as oviposition sites that were kept sterile or inoculated with either a blend of 7 bacterial strains isolated from blow fly excreta (7-isolate-blend) or individual bacterial isolates from that blend. The 7-isolate-blend deterred oviposition by female stable flies, as did either of 2 strains of Morganella morganii subsp. sibonii. Conversely, Exiguobacterium sp. and Serratia marcescens each prompted oviposition by flies. The flies' oviposition decisions appear to be guided by bacteria-derived semiochemicals as the bacteria could not be physically accessed. Oviposition deterrence caused by semiochemicals of the 7-isolate-blend may help stable flies avoid competition with blow flies. The semiochemicals of bioactive bacterial strains could be developed as trap lures to attract and capture flies and deter their oviposition in select larval habitats.

Temporal population genetic structure of Phormia regina (Diptera: Calliphoridae).

The genetic structure of forensically important blow fly (Brauer & Bergenstamm) (Diptera: Calliphoridae) populations has remained elusive despite high relatedness within wild-caught samples. This research aimed to determine if the implementation of a high-resolution spatiotemporal sampling design would reveal latent genetic structure among blow fly populations and to elucidate any environmental impacts on observed patterns of genetic structure. Adult females of the black blow fly, Phormia regina (Meigen) (Diptera: Calliphoridae), were collected from 9 urban parks in Indiana, USA over 3 yr and genotyped at 6 polymorphic microsatellite loci. The data analysis involved 3 clustering methods: principal coordinate analysis (PCoA), discriminant analysis of principal components (DAPC), and STRUCTURE. While the PCoA did not uncover any discernible clustering patterns, the DAPC and STRUCTURE analyses yielded significant results, with 9 and 4 genetic clusters, respectively. Visualization of the STRUCTURE bar plot revealed N = 11 temporal demarcations indicating barriers to gene flow. An analysis of molecular variance of these STRUCTURE-inferred populations supported strong temporally driven genetic differentiation (FST = 0.048, F'ST = 0.664) relative to geographic differentiation (FST = 0.009, F'ST = 0.241). Integrated Nested Laplace Approximation and Boosted Regression Tree analyses revealed that collection timepoint and 4 main abiotic factors (temperature, humidity, precipitation, and wind speed) were associated with the genetic subdivisions observed for P. regina. A complex interplay between environmental conditions, the unique reproductive strategies of the blow fly, and the extensive dispersal abilities of these organisms likely drives the strong genetic structure of P. regina in the Midwestern US.

THE BISON AND THE BLOW FLY: USING PUPARIA OF THE BLACK BLOW FLY (PHORMIA REGINA: DIPTERA, CALLIPHORIDAE) TO CONSTRAIN THE SEASON OF DEATH AND TAPHONOMIC HISTORY OF AN EARLY HISTORIC-AGE BISON, CARSON CITY, NEVADA, USA

ABSTRACT We report the occurrence of abundant dipteran puparia of Phormia regina, the black blow fly, in association with an early historic-age bison skeleton excavated near Carson City, Nevada. Cut marks on some of the bones indicate that the bison was butchered and probably skinned by humans. Radiocarbon dating provides two possible age intervals for the death of the bison: (1) latest seventeenth to early eighteenth century or (2) early nineteenth to early twentieth century; we consider the more recent age to be more plausible. The purpose of this study is to explore how the presence of puparia of this well-studied, necrophagous fly species can be used to help constrain the season of death and inform the interpretation of the taphonomic history of the bison. The life cycle of P. regina requires a minimum of 8.8 days within a temperature range of 14°C to 35°C, so the bison carcass must have been exposed to the air for at least that long within that temperature range. However, of the thousands of recovered puparia, 35% remain closed and did not produce adult flies; of this cohort, only a tiny percentage exhibit small exit holes attributable to parasitoid wasps. Cold temperatures, and not parasitoid wasps, are the most probable cause of the high pupal mortality. Climate data for the region, along with P. regina temperature constraints and streamflow and flooding records for the Carson River watershed, indicate that the bison died and was colonized by black blow flies in the spring, when night-time temperatures were low. A short time later the skinned and butchered skeleton was buried by floodplain sediments. Blow fly puparia can contribute useful information for the taphonomic analysis of vertebrate fossil sites.

Natural and Synthetic Pyrethrins Act as Feeding Deterrents against the Black Blowfly, Phormia regina (Meigen).

Simple SummaryPyrethrum is a botanical insecticide derived from pyrethrum flowers. Feeding deterrence caused by pyrethrum has been noted for several insect species. However, it is unclear whether the deterrent property results from a single component or from a combination of the six insecticidal active ingredients, known as pyrethrins. Here, we determined the feeding deterrence of natural pyrethrins and their two main components (pyrethrins I and II) on the blowfly, Phormia regina, in a dual-choice feeding assay. We found that natural pyrethrins and synthetic pyrethrins I/II in sucrose solution induced feeding deterrence at a concentration 16 times lower than the lowest concentration at which insecticidal action occurs. These results demonstrate that pyrethrins act as a feeding deterrent at sub-lethal concentrations. At the deterring concentration, feeding bouts were interrupted by intensive grooming of the proboscis, suggesting that pyrethrins acted instantly on the oral gustatory system of flies. The potent feeding deterrence of pyrethrins may provide effective protection for pyrethrum plants by rapidly deterring insects from feeding before insecticidal activities occur.Pyrethrum is a botanical insecticide derived from pyrethrum flowers. Feeding deterrence caused by pyrethrum has been reported in several sucking insects; however, there is no account of the cause of deterrence—whether from a single component or the combination of six active ingredients, called pyrethrins. We determined the feeding deterrence of natural pyrethrins, their two main components (pyrethrins I and II), and pyrethroid insecticides on the blowfly, Phormia regina. In a dual-choice feeding assay that minimized tarsal contact with food sources but allowed feeding through proboscises, natural pyrethrins, synthetic pyrethrins I/II, and allethrin were observed to induce deterrence at a concentration 16 times lower than the lowest concentration at which the knockdown rate increased. Feeding bouts were interrupted by intensive grooming of the proboscis at the deterring concentration, but no such grooming was observed to occur while feeding on the unpalatable tastants—NaCl, quinine, and tartaric acid. The underlying mode of action for the feeding deterrence of pyrethrins at sub-lethal concentrations probably occurs on the fly oral gustatory system, while differing from that of unpalatable tastants. The potent feeding deterrence of pyrethrins may provide effective protection for pyrethrum plants by rapidly deterring insects from feeding, before insecticidal activities occur.

Exogenous and endogenous microbiomes of wild-caught Phormia regina (Diptera: Calliphoridae) flies from a suburban farm by 16S rRNA gene sequencing

The black blow fly, Phormia regina (Meigen) (Diptera: Calliphoridae) is one of the most abundant carrion flies in North America. Calliphorids are important in agriculture and animal production, veterinary sciences, forensics and medical entomology. While the role of flies in the epidemiology of human and animal diseases is an active area of research, little is known about the microorganisms associated with these insects. We examined the diversity of wild-caught black blow fly endogenous (internal body) and exogenous (external body) microbial communities using 16S rRNA gene sequencing. Overall, 27 phyla, 171 families and 533 genera were detected, and diversity was significantly higher (P < 0.05) on external body surfaces. At the genus level, Dysgonomonas, Ignatzschineria, Acinetobacter, Vagococcus, Myroides, and Wohlfahrtiimonas were predominant. Cloning and sequencing of nearly full-length fragments of the 16S rRNA gene showed that some of the species identified are known to be pathogenic to humans, animals, and plants. Myroides odoratimimus and Acinetobacter radioresistens are well-known, multi-drug resistant bacteria. These results provide a snapshot of the microbial communities harbored by adult black blow flies and call for more comprehensive studies to better characterize the role these flies may play in the transmission of pathogenic microorganisms.

The fly factor phenomenon is mediated by interkingdom signaling between bacterial symbionts and their blow fly hosts.

We tested the recent hypothesis that the "fly factor" phenomenon (food currently or previously fed on by flies attracts more flies than the same type of food kept inaccessible to flies) is mediated by bacterial symbionts deposited with feces or regurgitated by feeding flies. We allowed laboratory-reared black blow flies, Phormia regina (Meigen), to feed and defecate on bacterial Luria-Bertani medium solidified with agar, and isolated seven morphologically distinct bacterial colonies. We identified these using matrix-assisted laser desorption/ionization mass spectrometry and sequencing of the 16S rRNA gene. In two-choice laboratory experiments, traps baited with cultures of Proteus mirabilis Hauser, Morganella morganii subsp. sibonii Jensen, or Serratia marcescens Bizio, captured significantly more flies than corresponding control jars baited with tryptic soy agar only. A mixture of seven bacterial strains as a trap bait was more attractive to flies than a single bacterial isolate (M. m. sibonii). In a field experiment, traps baited with agar cultures of P. mirabilis and M. m. sibonii in combination captured significantly more flies than traps baited with either bacterial isolate alone or the agar control. As evident by gas chromatography-mass spectrometry, the odor profiles of bacterial isolates differ, which may explain the additive effect of bacteria to the attractiveness of bacterial trap baits. As "generalist bacteria," P. mirabilis and M. m. sibonii growing on animal protein (beef liver) or plant protein (tofu) are similarly effective in attracting flies. Bacteria-derived airborne semiochemicals appear to mediate foraging by flies and to inform their feeding and oviposition decisions.

Chemical Assay for the Detection of Vertebrate Fecal Metabolites in Adult Blow Flies (Diptera: Calliphoridae).

Filth flies are commonly implicated in pathogen transmission routes due to their affinity for vertebrate waste and their synanthropic associations. However, solidifying the link between flies and infected feces in the wild can be difficult, as interpretations made solely from microbial culturing or sequencing methods may represent an incomplete picture of pathogen acquisition. We present an analytical assay using high performance liquid chromatography tandem mass spectrometry (HPLC MS/MS) to detect vertebrate fecal metabolites (urobilinoids) in adult blow fly guts. Proof of concept experiments consisted of controlled feeding in which flies were grouped into three treatments (unfed, exposure to beef liver tissue, and exposure to canine feces; N = 20/treatment) using the black blow fly Phormia regina Meigen (Diptera: Calliphoridae). It was revealed that only feces-related samples exhibited peaks with an m/z of 591 and MS/MS spectra consistent with urobilinoids. These peaks were not seen for beef liver tissue, flies exposed to beef liver tissue, or unfed flies. Samples taken directly from beef liver tissue and from feces of several animals were also tested. To test this assay in wild flies, 216 flies were additionally analyzed to determine whether they had ingested vertebrate feces. About 13% of the wild flies exhibited these same peaks, providing a baseline measure of blow flies collected in urban and residential areas consuming feces from the environment. Overall, this assay can be used for P. regina collected in an applied setting and its integration with microbial culturing and sequencing methods will help to improve its use.

Arthropods Associated with Bovine Carrion in Central Oklahoma

Abstract. The major arthropod taxa visiting carrion in central Oklahoma were enumerated qualitatively. Immature and adult arthropods were collected from beef cadavers throughout decomposition during fall/winter and spring/summer seasons. Arthropods were identified to the most specific taxonomic level possible, and more than 35 taxa of Diptera and Coleoptera commonly associated with carrion were confirmed. As expected, Calliphoridae were the most abundant Diptera during both seasons, followed by Sarcophagidae, Muscidae, and Ulidiidae. Most species of blow flies in fall/winter were the black blow fly, Phormia regina (Meigen), and hairy maggot blow fly, Chrysomya rufifaces (Macquart). P. regina and Cochliomyia macellaria (Fabricius) were the major species present during the spring/summer season. The coleopteran most commonly collected both seasons was the hairy rove beetle, Creophilius maxillosus (Linnaeus), in the family Staphylinidae. Beetles in the families Dermestidae and Trogidae also were collected.

Filth Fly Transmission ofEscherichia coliO157:H7 andSalmonella entericato Lettuce,Lactuca sativa

Filth flies have been implicated in the dispersal of human disease pathogens; however, the epidemiological parameters of the transmission of human pathogens from flies to plants are largely undescribed. The capacity of the black blow fly, Phormia regina Meigen, to acquire and subsequently deposit bacteria onto baby lettuce leaves was compared with that of the house fly, Musca domestica (L.). Adult P. regina and M. domestica were exposed to green fluorescent protein-tagged Escherichia coli O157:H7- or Salmonella enterica-inoculated manure and then allowed access to the lettuce plants. Bacteria on the plants and flies were assessed by plating and polymerase chain reaction. Although blow flies acquired significantly more E. coli O157:H7 than house flies, there was no significant difference between the deposition of bacteria on lettuce by the two fly species. In contrast, there was no significant difference in the acquisition of S. enterica by the two fly species. However, blow flies deposited more S. enterica onto lettuce than house flies. To more accurately assess transmission parameters, flies were given timed exposure and inoculation periods of 10 and 30 s. Blow flies acquired more E. coli O157:H7 than S. enterica in the both time periods. Flies exposed to manure for 30 s were then tested for deposition by forcing the flies to walk on lettuce leaves. Blow flies deposited comparable amounts of E. coli O157:H7 and S. enterica. Although house flies have historically been implicated in the transmission of human pathogens to food, the data presented suggest that blow flies are more efficient vectors of E. coli O157:H7 and S. enterica to leafy greens than house flies.

Serotonin Inhibits Protein Feeding in the Blow Fly, Phormia regina (Meigen)

Serotonin is an important signaling molecule involved in the control of feeding in flies and other animals. In this study, a potential neurohemal release site for serotonin and the effects of exogenous serotonin on protein feeding were examined in the black blow fly, Phormia regina. A dense network of varicose neural processes exhibiting serotonin-like immunoreactivity was identified on the dorsal region of the thoracico-abdominal ganglion in P. regina. This dorsal region of the central nervous system is a likely site for the release of serotonin into the hemolymph. Circulating serotonin may have multiple systemic effects on fly physiology, including modulating or regulating feeding related processes and diuresis. Injections of exogenous serotonin reduced protein meal size in female flies at all doses and at all time points examined within a 24 h period relative to control and saline injected flies. Similar results were observed in serotonin-injected males at 35 min post injection. The injection of 50 μg serotonin resulted in the greatest amount of protein feeding inhibition. Serotonin injected flies also experienced greater weight loss than control or saline-injected flies during the 24 h post-injection period, possibly due to increased diuresis.

Insect satiety: Sulfakinin localization and the effect of drosulfakinin on protein and carbohydrate ingestion in the blow fly, Phormia regina (Diptera: Calliphoridae)

Sulfakinins, which are satiety factors in invertebrates, have previously been shown to inhibit feeding in the German cockroach and desert locust. This study examines the occurrence of sulfakinin immunoreactivity and the role of sulfakinin as a feeding satiety factor in the black blow fly, Phormia regina. Specifically, this study examines the effect of sulfakinin on two of the blow fly's nutrient requirements (i.e., carbohydrates and proteins). We observed sulfakinin immunoreactive cells in the brains of both male and female flies. We found that drosulfakinin I (DrmSKI, FDD Y[SO 3H] GHMRFa) significantly inhibited carbohydrate feeding by 44% at the most effective dose (10 nmol) in female flies. Statistically, there was no significant effect on males; however, injections of 10 nmol DrmSKI reduced carbohydrate feeding by 34% compared to the sham. Drosulfakinin had no effect on protein feeding and no significant inhibition was detected in females or males. The results of this study lend further support to the idea that carbohydrate and protein feeding are regulated by separate control mechanisms, especially in Calliphoridae.

Immunological Localization of Mosquito Ovary Ecdysteroidogenic Hormone I and Fruit Fly Insulin Receptor in Adult &lt;I&gt;Phormia regina&lt;/I&gt; (Diptera: Calliphoridae)

The neurohormone ovary ecdysteroidogenic hormone I (OEH I), originally identified from Aedes aegypti (L.), has an effect on mosquito ecdysteroidogenesis. Would there be a similar presence and function in other adult flies, e.g., black blow fly, Phormia regina (Meigen)? One pair of median neurosecretory cells (MNCOEH I) and one pair of lateral neurosecretory cells (LNCOEH I) were immunopositive to OEH I antiserum in the brain of sugar-fed and liver-fed female P. regina. In addition, two pairs of neurosecretory cells (SOGNCOEH I) positive to this antiserum were found in the suboesophageal ganglion only in adult female P. regina at 16, 20, 23, 28, 36, and 72 h after a liver meal. In addition to the positive pairs of MNCOEH I and LNCOEH I in females, there were four more pairs of positive neurons (MSNFNCOEH I) in the region between the MNCOEH I and the esophageal foramen in P. regina males, but not in females. The presence of OEH I in male flies needs further attention as to the function of this hormone in male dipterans. We also found substances positive to Drosophila melanogaster Meigen insulin receptor antiserum located in the corpus cardiacum and the nerves going to the corpus allatum of adult female P. regina at 6 and 30 h after the liver meal, but not to the brain.

Effects of plant-derived compounds on larvae of a blow fly species that causes secondary myiases: laboratory studies.

No-choice and binary-choice bioassays were used to test the effect of a range of plant-derived insecticides on the behaviour of larvae of the black blow fly Phormia regina (Meigen). Azadirachtin (100 and 10 ppm), pyrethrum extract (10 ppm) and the naphthoquinone BTG 505 (1000 ppm) acted as deterrents for P. regina larvae. The mortality of larvae was reduced in instances where they moved away from a treated diet. Larvae given a choice were heavier, when compared with larvae reared exclusively on diets containing either azadirachtin (100 or 10 ppm) or the naphthoquinones, BTG 504 (1000 ppm) or BTG 505 (1000 ppm). The deterrent effect of azadirachtin, pyrethrum and the naphthoquinone BTG 505 together with their larvicidal activity, could be utilized in prophylaxis against myiasis infections due to larvae of Phormia regina.

The development of the black blow fly, Phormia regina (Meigen)

The black blow fly, Phormia regina (Meigen) is a primary species commonly utilized to indicate a postmortem interval, or more appropriately a “time since colonization”. Due to the importance of this species as a secondary myiasis producer in livestock operations, and more recently as a time since death indicator in the field of forensic entomology, a considerable amount of data on its growth and development has been generated. However, the developmental time as reported by these studies varies greatly, and current more detailed data is needed for use in medicocriminal entomology. Hourly developmental data is presented under constant temperatures of 10, 15, 20, 25, 30, 35 and 40°C, and cyclic temperatures of 10–15, 15–25, 25–35 and 35–45°C. This study is in agreement with the results reported by Kamal [Comparative study of thirteen species of sarcosaprophagous Calliphoridae and Sarcophagidae (Diptera). I. Bionomics, Ann. Entomol. Soc. Am. 51 (1958) 261] and Melvin [Incubation period of eggs of certain musciod flies at different constant temperatures, Ann. Entomol. Soc. Am. 27 (1934) 406] only at temperatures of 25°C and below. Bishopp [Flies which cause myiasis in man and animals: some aspects of the problem, J. Econ. Entomol. 8 (1915) 317] reported a shorter developmental duration for larval stages than what was produced with our laboratory rearings.

Relationship Between Size and Mating Success in the Blow Fly &lt;I&gt;Phormia regina&lt;/I&gt; (Diptera: Calliphoridae)

Size variation occurs in natural populations of the black blow fly, Phormia regina (Meigen); head width varied from 2.25 and 1.89 mm to 3.68 and 3.72 mm for males and females, respectively. Using these values as a guide, small flies were produced in the laboratory by early removal of larvae from the rearing medium. Mating experiments, pairing flies of various size categories, showed that when small males mated with large females the percentage of successfully inseminated females was significantly reduced compared with the other combinations (42% versus 73–89%). Previous work led us to suspect that small males might have small aedeagi and therefore have a problem in successful sperm transfer when they mate with large females. Measurement of aedeagus size and head width showed that in wild flies the size of the aedeagus is positively correlated with body size.

Regulation of mating behavior by nutrition and the corpus allatum in both male and female Phormia regina (Meigen)

Both dietary protein and the corpus allatum (CA) were required for normal mating behavior in both male and female black blow fly, Phormia regina (Meigen). Nutrition (protein diet) activated the CA in both sexes. More than 10 mg of dietary liver was required for each male to result in 80% insemination of females, while 20 mg of liver was required for each female to allow 78% of females to become inseminated. Between 10 to 15 mg of protein meal (i.e., liver) was required to activate sexual receptivity in 71% of the females, while between 15 to 20 mg of liver was needed to support full oöcyte development in 70% of the females (Yin, C.-M., Zou, B.-X., Li, M.-F., Stoffolano, J.G., Jr. 1994. Discovery of a midgut peptide hormone which activates the endocrine cascade leading to oogenesis in Phormia regina Meigen. Journal of Insect Physiology, 40, 283–292). Allatectomy suppressed mating behavior more than 2-fold in both sexes. Topical application of 10 μg of S-methoprene (a juvenile hormone analogue) at 12 h after the onset of liver feeding restored sexual activity of both allatectomized males and females. Incidence of successful insemination increased as the oöcyte development progressed. Ovariectomy suppressed sexual receptivity more than 3-fold in liver-fed females. Thus, in addition to nutrition and the CA, ovaries and their developmentaal status can also affect the sexual receptivity in female P. regina.

The effects of injection of amphetamine on female insemination in the black blow fly, Phormia regina (Diptera: Calliphoridae)

Abstract. An earlier study on the blowfly Phormia regina (Meigen) demonstrated that the injection of amphetamine (12 μg) depletes biogenic amine levels in the CNS. In the present study, P. regina females were injected with amphetamine (12 μg), each female was placed with three males and insemination success was determined. Amphetamine inhibited female insemination by 43.3% at 2–90 min post‐injection and by 70% at 10–60 min post‐injection. At 180–270 min post‐injection, there was no significant inhibition of female insemination. This study indicates a possible role in insects for the biogenic amines in female insemination.

Control of pupal fat body disappearance in the female black blow fly, Phormia regina (Meigen) by the brain and the corpus allatum

Neck-ligation, brain implantation, allatectomy, methoprene treatment, and ovariectomy indicated that the disappearance of pupal fat body cells in newly emerged adult female blow fly, Phormia regina, is controlled by the brain and the corpus allatum (CA). Absence of brain or CA greatly lowered the rate of fat body cell disappearance (i.e. death). Dependency on the CA decreased from 0 to 36 h post-emregence, indicating that the CA was active during the earlier part of this timespan. Methoprene treatment enhanced pupal fat body cell disappearance in allatectomized females. Brain implantation restored the rate of pupal fat body cell disappearance in neck-ligated flies. Brains from day 1 sugar-fed flies proved to be more effective than those from day 2 sugar-fed flies, indicating that there may be a window after adult emergence that allows the brain to act directly or indirectly on the death of pupal fat body cells. Ovariectomy did not alter the rate of pupal fat body cell death in test animals. Dying pupal fat body cells were smaller in size, less dense (i.e. did not sink in saline like normal pupal fat body cells), and stickier (i.e. attached to other tissues tighter) than the healthy cells. A possible role played by ecdysteroids is also discussed.

Dispersal and Distribution of Calliphoridae (Diptera) Immatures from Animal Carcasses in Southern Louisiana

Spatial distributions of calliphorid pupae (based on adult emergence) from swine carcasses located in woods and pasture were determined during 4 seasons in southern Louisiana. Adult emergence was substantially greater in pastures than woods for all seasons except winter. Secondary screwworm, Cochliomyia macellaria (F.), was the predominant calliphorid species collected ( > 95%) during summer and fall; whereas, black blow fly, Phormia regina (Meigen), was the primary species (86%) during spring. The highest densities of calliphorid pupae per square meter were located within 0.9 m of the animal carcasses in the woods and the pasture (summer); however, the number of calliphorid pupae per square meter during the summer was substantially greater in the pastures than in the woods (2,370/m2 compared with 77/m2). Postfeeding calliphorid larvae preferred to disperse toward the southeastern quadrant during summer and spring and toward the southeastern to southwestern quadrants in fall. The winter evaluation period was the most prolonged (85 d), had the least number of emergent adults (only 6-11% of the populations that emerged in the other seasons), and consisted of predominantly, Phaenicia sericata (Meigen).

Changes in Oviposition Behavior of the Ectoparasitoids Nasonia vitripennis and Muscidifurax zaraptor (Hymenoptera: Pteromalidae) When Using Different Species of Fly Hosts, Prior Oviposition Experience, and Allospecific Competition

Oviposition behavior was used to determine if handling time was a reliable indicator of host suitability for oviposition by the ectoparasitic wasps, Nasonia vitripennis (Walker) and Muscidifurax zaraptor Kogan & Legner, on pupae of Sarcophaga bullata Parker, the black blow fly, Phormia regina (Meigen), and the house fly, Musca domestica L. Oviposition behavior of both parasitoids was composed of a series of stereotypic and interdependent events or phases. The duration of behavioral events preceding puparium penetration was influenced by host species, indicating that host recognition depends on some external cue associated with the fly puparium. Total handling time was stereotypic regardless of host species or the number of eggs laid on a host, suggesting that the amount of time invested by N. vitripennis and M. zaraptor in oviposition does not predict clutch size or host preference. When N. vitripennis and M. zaraptor were held together at low host densities, females of N. vitripennis reacted aggressively and interfered with all oviposition attempts by M. zaraptor . Aggression by N. vitripennis toward M. zaraptor subsided when host density was increased, presumably because this was a less competitive situation.