Mechanical barrier for preventing climbing by lesser mealworm (Coleoptera: Tenebrionidae) and hide beetle (Coleoptera: Dermestidae) larvae in poultry houses.

Mechanical barriers consisting of bands of polyethylene terepthalate resin attached to wooden posts by latex caulk adhesive and staples were 100% effective in preventing passage of dispersing lesser mealworm, Alphitobius diaperinus (Panzer), larvae in the laboratory. Barriers continued to be 100% effective after beinig held in a caged layer poultry house for 3 mo. Polyethylene terepthalate barriers installed on support posts in a pullet house in Brooker, FL, were >92% effective against natural populations of lesser mealworm larvae 6 mo after installation. The barriers also were >94% effective against natural populations of larvae of the hide beetle, Dermestes maculatus DeGeer, when fly populations were low. Fecal spot depositions by house flies in excess of 31 cumulative fly spots per square centimeter on spot cards reduced the effectiveness of the barriers to 79-90%, and barrier efficacy was reduced to 40-56% when fly spots covered >80% of the surface of the plastic. Washing the barriers with water to remove fly spots restored their effectiveness against hide beetle larvae to >99%.     

Condensed tannins inhibit house fly (Diptera: Muscidae) development in livestock manure

Reducing chemical use for suppressing internal and external parasites of livestock is essential for protecting environmental health. Although plant condensed tannins are known to suppress gastro-intestinal parasites in small ruminants, no research on the effects of tannins on external arthropod populations such as the house fly, Musca domestica L., have been conducted. We examined the impact of plant material containing condensed tannins on house fly development. Prairie acacia (Acacia angustissima (Mill.), Kuntze variety hirta (Nutt.) B.L. Rob.) herbage, panicled tick-clover (Desmodium paniculatum (L.) DC.) herbage, and quebracho (Shinopsis balansae Engl.) extracts were introduced at rates of 1, 3 or 5% condensed tannins/kg beef cattle, dairy cattle, and goat manure, respectively. In a second experiment, we also introduce purified catechin at 1 or 3% of dairy manure dry matter and measured its impact on house fly development. For the house flies used in these experiments, the following was recorded: percent fly emergence (PFE), average daily gain (ADG), and average fly weight (AFW). No effects (P>0.05) in house fly development were measured in the caprine manure. Prairie acacia (20.9% condensed tannins) had no effect on house flies developing in either bovine manures. Tick clover (4.9% condensed tannins) had a negative effect on all three quantifiable variables of house fly development in the bovine manures, whereas quebracho extract (64.0% condensed tannins) at the 3 and 5% rate reduced fly emergence in beef manure and average daily gain in dairy manure. The application of purified catechin at 3%, but not 1%, reduced fly PFE, ADG, and AFW.     

Parasites that attack stable fly and house fly (Diptera: Muscidae) puparia during the winter on dairies in northwestern Florida

Throughout the winter and early spring months, stable fly, Stomoxys calcitrans (L.), and house fly, Musca domestica L., puparia were collected from silage, hay, and manure from six dairies in northwestern Florida and evaluated for parasitism. Of the puparia producing flies or parasites, 23% of the stable flies and 46% of the house flies were parasitized. The predominant parasite observed attacking muscoid flies (76% for stable flies and 58% for house flies) was Spalangia cameroni Perkins. Muscidifurax sp. was recovered from 11 and 36% of the stable fly and house fly pupae, respectively. Other parasite species encountered were Spalangia endius Walker and S. nigroaenea Curtis. Significantly more parasitized fly pupae were collected from silage than from hay residues or manure. Winter and early spring parasite populations in northwestern Florida appear to be present as long as viable fly pupae are available to support the developing parasites.     

Stable fly, house fly (Diptera: Muscidae), and other nuisance fly development in poultry litter associated with horticultural crop production

Poultry litter usage in horticultural crop production is a contributor to nuisance fly populations, in particular stable flies (Stomoxys calcitrans L.) and house flies (Musca domestica L.). Extrapolation of adult emergence data suggests that approximately 1.5 million house flies and 0.2 million stable flies are emerging on average from every hectare of poultry litter applied as a preplant fertilizer for vegetable production in Perth, Western Australia. To a lesser extent, sideband applications to established crops may allow for the development of 0.5 million house flies and 45,000 stable flies per hectare. However, up to 1 million house flies, 0.45 million lesser house flies, Fannia cannicularis L., and 11,000 stable flies per hectare may be produced from surface dressings of poultry litter associated with turf production. Other nuisance flies present in poultry litter included the false stable fly, Muscina stabulans (Fallén), bluebodied blowfly, Calliphora dubia Hardy, black carrion fly, Hydrotaea rostrata Robineau-Desvoidy, Australian sheep blowfly, Lucilia cuprina Wiedemann, and flesh flies (Sarcophagidae). Only house flies developed in poultry litter for the first 4 d after application in the field. Stable flies were not present in poultry litter until 4-7 d after application, and were the only fly species developing in litter > 9 d after application.     

Evaluation of Beauveria bassiana applications against adult house fly, Musca domestica, in commercial caged-layer poultry facilities in New York state

Applications of a commercially produced Beauveria bassiana product, balEnce, were compared with pyrethrin treatments for the control of adult house flies in New York high-rise, caged-layer poultry facilities. An integrated fly management program, which included the release of house fly pupal hymenopteran parasitoids, was used at all facilities. Adult house fly populations were lower in B. bassiana-treated facilities during the spray and post-spray periods, as recorded on spot cards. Concurrently, the numbers of house fly larvae recovered in B. bassiana-treated facilities were less than one-half that of the pyrethrin-treated facilities. House fly pupal parasitism levels were low, but similar under both treatment regimes. The numbers of adult and larval Carcinops pumilio, a predatory beetle, recovered from B. bassiana-treated facilities were 43 and 66% greater than from the pyrethrin-treated facilities, respectively.     

Distributions of lesser mealworm (Coleoptera: Tenebrionidae) in litter of a compacted earth floor broiler house in subtropical Queensland, Australia

Distributions of lesser mealworm, Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae), in litter of a compacted earth floor broiler house in southeastern Queensland, Australia, were studied over two flocks. Larvae were the predominant stage recorded. Significantly low densities occurred in open locations and under drinker cups where chickens had complete access, whereas high densities were found under feed pans and along house edges where chicken access was restricted. For each flock, lesser mealworm numbers increased at all locations over the first 14 d, especially under feed pans and along house edges, peaking at approximately 26 d and then declining over the final 28 d. A life stage profile per flock was devised that consisted of the following: beetles emerge from the earth floor at the beginning of each flock, and females lay eggs, producing larvae that peak in numbers at approximately 3 wk; after a further 3 to 4 wk, larvae leave litter to pupate in the earth floor, and beetles then emerge by the end of the flock time. Removing old litter from the brooder section at the end of a flock did not greatly reduce mealworm numbers over the subsequent flock, but it seemed to prevent numbers increasing, while an increase in numbers in the grow-out section was recorded after reusing litter. Areas under feed pans and along house edges accounted for 5% of the total house area, but approximately half the estimated total number of lesser mealworms in the broiler house occurred in these locations. The results of this study will be used to determine optimal deployment of site-specific treatments for lesser mealworm control.     

Susceptibility of spinosad in Musca domestica (Diptera: Muscidae) field populations

The toxicity of spinosad was determined in one susceptible and five insecticide-resistant laboratory strains of house fly, Musca domestica L. Spinosad was relatively slow-acting, but highly toxic to house flies. In a feeding bioassay, spinosad LC50 at 72 h was 0.51 microg of spinosad per gram of sugar, making it 6.3- and 3.5-fold more toxic to house flies compared with azamethiphos and methomyl, respectively. In topical application bioassay, the LD50 at 48 h of spinosad in susceptible house flies was 40 ng per 20 mg of house fly, making spinosad less toxic than the pyrethroid bioresmethrin synergized by piperonyl butoxide and the organophosphate dimethoate. The insecticide-resistant laboratory strains had resistance factors to spinosad at LC50 in feeding bioassay from 1.5 to 5.5 and at LD50 in topical application bioassay from 2.5 to 4.7, indicating that in house fly cross-resistance to the major insecticide classes will not initially be of major concern for the use of spinosad for house fly control. The toxicity of spinosad was also evaluated against 31 field populations of house flies collected from livestock farms across Denmark. The field populations were 2.2- to 7.5-fold resistant to spinosad at 72 h in feeding bioassay, but based on steep slopes in the bioassay and the limited variation of spinosad toxicity against the various field populations, we consider the field populations to be spinosad-susceptible. We propose a diagnostic dose of 12 microg of spinosad per gram of sugar in feeding bioassay with impregnated sugar for determination of resistant house flies, which is 10x the LC95 of the susceptible strain WHO and approximately = 2x the LD95 of the field populations. Spinosad showed no substantial cross-resistance to the pyrethroid bioresmethrin synergized by piperonyl butoxide, the anticholinesterases dimethoate, azamethiphos, methomyl, and spinosad in house fly field populations.     

Disease dynamics and persistence of Musca domestica salivary gland hypertrophy virus infections in laboratory house fly (Musca domestica) populations

Past surveys of feral house fly populations have shown that Musca domestica salivary gland hypertrophy virus (MdSGHV) has a worldwide distribution, with an average prevalence varying between 0.5% and 10%. How this adult-specific virus persists in nature is unknown. In the present study, experiments were conducted to examine short-term transmission efficiency and long-term persistence of symptomatic MdSGHV infections in confined house fly populations. Average rates of disease transmission from virus-infected to healthy flies in small populations of 50 or 100 flies ranged from 3% to 24% and did not vary between three tested geographical strains that originated from different continents. Introduction of an initial proportion of 40% infected flies into fly populations did not result in epizootics. Instead, long-term observations demonstrated that MdSGHV infection levels declined over time, resulting in a 10% infection rate after passing through 10 filial generations. In all experiments, induced disease rates were significantly higher in male flies than in female flies and might be explained by male-specific behaviors that increased contact with viremic flies and/or virus-contaminated surfaces.     

Knockdown and mortality comparisons among spinosad-, imidacloprid-, and methomyl-containing baits against susceptible Musca domestica (Diptera: Muscidae) under laboratory conditions

The activity of spinosad, imidacloprid, and methomyl baits and technical actives were assessed against susceptible house flies, Musca domestica L. (Diptera: Muscidae). In a feeding assay, imidacloprid affected flies more rapidly than methomyl or spinosad, but spinosad was 2.7 times more potent than methomyl and 8 times more potent than imidacloprid. The profile of technical actives correlated with their respective fly bait formulations in laboratory assays. Although having the most rapid onset of activity in laboratory tests, up to 50% of flies remained alive after exposure to imidacloprid bait. In contrast, <5% of flies survived 24-h exposure to spinosad or methomyl baits. High temperature reduced the knockdown activity of imidacloprid bait and slowed the speed of kill for spinosad and methomyl baits over a 24-h exposure period. Spinosad and methomyl baits were also superior to imidacloprid when applied to the floors of environmentally controlled rooms at label recommended rates, providing good fly control for up to 21 d. The fact that a significant percentage of flies exposed to imidacloprid were rapidly knocked down but subsequently remained alive in all of the assays suggested that flies were recovering from initial exposure to this compound. Given its favorable safety profile, a high degree of initial and residual activity comparable with methomyl and lack of cross-resistance to other chemistries, spinosad bait may be a valuable component of house fly control programs to help control or delay the emergence of resistant populations.     

Factors affecting localized abundance and distribution of lesser mealworm in earth-floor broiler houses in subtropical Australia

Factors that influence the localized abundance and distribution of lesser mealworm, Alphitobius diaperinus (Panzer), in litter of two compacted earth-floor broiler houses in subtropical Australia were studied using various experimental manipulations. Numbers of lesser mealworms substantially increased inside caged areas and under uncaged empty feed pans placed in open areas of the houses. These populations were found to be localized and independent of chicken-feed, manure, and high beetle populations that normally occur under existing feed pans. Substantial horizontal movement of larvae to under feed pans was recorded. Placing metal barriers around these pans significantly restricted this movement. In almost all treatments, lesser mealworms typically peaked in numbers during the middle of the flock time. This temporal pattern of abundance also was observed under pans within barriers, where relatively low insect numbers occurred, but it was not observed in uncaged open areas (where chickens had complete access). It is likely that larvae do not establish in open areas, but fluctuate in numbers as they either move to refuges away from chickens or suffer high rates of mortality. In these refuges, larvae peak in numbers and then leave the litter environment to pupate in the earth floor before the end of the flock time. This behavior might be exploited for management of lesser mealworm by targeting applications of control agents.     

Influence of biotic and abiotic factors on the persistence of a Beauveria bassiana biopesticide in laboratory and high-rise poultry house settings

Recent studies suggest the potential for use of oil formulations of the entomopathogenic fungus, Beauveria bassiana, as residual sprays for control of house flies in poultry production facilities. The current study investigated the influence of biotic and abiotic factors on biopesticide persistence. We found that flies physically removed and deactivated conidia, with higher fly densities and greater cumulative exposure hastening the decline. Nonetheless, very low densities of viable conidia were still able to cause rapid mortality, suggesting the potential for relatively long re-treatment intervals as fly populations are controlled. Considering abiotic factors, we found that fungal spray treatments remained viable for up to 13 weeks under laboratory conditions. Periodic exposure of flies to the spray residue showed high levels of mortality, with very little decline in mortality rate over time. Equivalent treatments placed in a commercial poultry house showed much more rapid decline. One trial at the end of summer showed conidia to remain viable up to seven weeks. However, repeats during the winter months revealed decay in 1–2 weeks, with fly mortality rates influenced accordingly. The exact reasons for the more rapid decay remain unclear but could be linked to high concentrations of ammonia in the basement areas, especially during winter when ventilation is minimal. The combined data suggest the potential for adaptive treatment regimes with weekly spray intervals in conditions with very high fly populations and/or high ammonia levels, and potentially monthly spray intervals when fly populations and ammonia levels are reduced.     

Azadirachtin as a larvicide against the horn fly, stable fly, and house fly (Diptera: Muscidae)

Effects of azadirachtin, a triterpenoid extracted from neem seed, Azadirachta indica A. Juss., were similar to those of insect growth regulators against the immature stages of the born fly, Haematobia irritans (L.), the stable fly, Stomoxys calcitrans (L.), and the house fly, Musca domestica L. When an ethanolic extract of ground seed was blended into cow manure, LC50 and LC90's for larval horn flies were 0.096 and 0.133 ppm azadirachtin, respectively. An emulsifiable concentrate (EC) had an LC50 for larval horn flies of 0.151 ppm and an LC90 of 0.268 ppm. For larval stable flies, the EC formulation had an LC50 of 7.7 ppm and an LC90 of 18.7 ppm azadirachtin in manure. Against larval house flies, the LC50 and LC90 were 10.5 and 20.2 ppm, respectively. When the EC formulation was administered orally to cattle at a rate of greater than or equal to 0.03 mg azadirachtin per kg of body weight per day or when ground neem seed was given as a daily supplement of greater than or equal to 10 mg seed per kg body weight, horn fly development in the manure was almost completely inhibited. In contrast, ground seed mixed in cattle feed at the rate of 100-400 mg seed per kg of body weight per day caused less than 50% inhibition of stable flies in the manure.     

Metabolic mechanisms only partially explain resistance to pyrethroids in Australian broiler house populations of lesser mealworm (Coleoptera: Tenebrionidae)

The susceptibility of six Australian broiler house populations and an insecticide susceptible population of lesser mealworm, Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae), to cyfluthrin, beta-cyfluthrin, gamma-cyhalothrin, and deltamethrin was investigated. One broiler house population had equivalent susceptibility to the susceptible to beta-cyfluthrin and beta-cyhalothrin, with higher susceptibility to cyfluthrin and deltamethrin. The remaining five populations demonstrated strong resistance to cyfluthrin (19-37-fold), the insecticide used most widely for management of A. diaperinus in Australia. Each cyfluthrin-resistant population demonstrated reduced susceptibility to beta-cyfluthrin (resistance ratios were 8-17-fold), deltamethrin (2.5-8-fold), and gamma-cyhalothrin (6-12-fold) compared with the laboratory population, but cross-resistance patterns varied considerably between populations. Adding piperonyl butoxide (PBO) had no effect on the susceptibility of the susceptible population to any of the insecticides, but it increased the susceptibility of each of the five cyfluthrin-resistant populations: to cyfluthrin (synergism ratio range, 1.9-5.0-fold), beta-cyfluthrin (1.6-4.1-fold), and y-cyhalothrin (1.7-2.0-fold). PBO had a more variable effect on susceptibility to deltamethrin, with three of the cyfluthrin-resistant populations being more susceptible to deltamethrin in the presence of PBO, but susceptibility of the remaining two populations was unaffected by adding PBO (synergism ratio range, 0.9-2.5-fold). Overall, the addition of PBO to the four pyrethroids had variable effects on their susceptibility. This variability indicated the presence of other resistance mechanisms in beetle populations apart from metabolic resistance. In addition, the relative importance of metabolic resistance in each beetle population varied widely between pyrethroids. Thus, it cannot be assumed that PBO will reliably synergize pyrethroids against cyfluthrin-resistant lesser mealworm populations when using it to mitigate insecticide resistance.     

Laboratory studies on the biology ofSpalangia nigra [Hym.: Pteromalidae]

At 21 °C,Spalangia nigra Latreille (Hymenoptera: Pteromalidae) averaged 29.3 days between exposure and emergence of 1^st progeny from host house flies,Musca domestica L. (Diptera: Muscidae). At 27 °C, the average developmental time to 1^st emergence was reduced to 26.6 days, and a majority of adult wasps emerged from host house fly puparia between 29 and 40 days postoviposition. The sex ratio of progeny ranged from 1.4 to 1.8 female-to-male, but all progeny of virgin females were male. Male wasps lived from 6.8–15 and females 11–17.8 days at 27 °C; honey as a food source increased longevity. No significant differences in parasitism byS. nigra were associated with host house fly pupal densities ranging from 1 to 200 pupae per female-male pair of wasps, but average percent parasitism decreased at host densities greater than 50. House fly pupae exposed to parasitism at ages ranging from 4 to 96 h did not differ in subsequent production of adult flies.S. nigra did not demonstrate preference for house flies or stable flies,Stomoxys calcitrans (L.) (Diptera: Muscidae) as hosts. The results of these studies indicate thatS. nigra may contribute significantly to previously unexplained mortality of house flies and stable flies.      

Biological control of house flies Musca domestica and stable flies Stomoxys calcitrans(Diptera: Muscidae) by means of inundative releases of Spalangia cameroni(Hymenoptera: Pteromalidae)

The efficacy of the pupal parasitoid Spalangia cameroni Perkins as a biological control agent was tested against house flies Musca domestica Linnaeus and stable flies Stomoxys calcitrans (Linnaeus) in one dairy cattle and two pig installations in Denmark. Weekly releases of S. cameroni from April through to September-October 1999 and 2000 resulted in significant suppressions of house fly populations to below nuisance level, whereas no effect on stable flies was found. Parasitism was significantly higher in the release years compared to the control years, but was below 25% averaged over the fly season for each farm. A statistical model based on a functional relationship between the innate capacity of increase of the two fly species and three explanatory variables (air temperature, fly density and parasitism) provided a fairly good fit to data with the abundances of house flies and stable flies explained mostly by temperature, but intra- and interspecific competition, and parasitism had a significant effect as well. Overall, the model was capable of explaining 14% and 6.6% of the total variation in data for house fly and stable fly, respectively. Spalangia cameroni was the predominant parasitoid to emerge from exposed house fly pupae, but from mid summer onwards Muscidifurax raptor Girault & Sanders (Hymenoptera: Pteromalidae) was also quite common. The study indicated that biological control of house flies can be an efficient alternative to chemical control.     

Genetics of house flies. Variability studies with North Dakota, Texas, and Florida populations.

Genetic data were used to compare the structure of native house fly populations collected in North Dakota, Texas, and Florida. Recombination studies with mutant markers on chromosomes 3 and 4 indicated a lack of inversion polymorphism among the three populations in those areas of the genetic map studied. Significant differences were observed among flies from the three regions with regard to the frequency of 1) females that produced only male progeny, and 2) male-determining 3rd chromosomes (IIIm chromosomes). However, the North Dakota and Texas flies were more similar to each other than to the Florida flies since populations from the two former areas possessed a low frequency of both male-producing females and IIIm chromosomes; in contrast, the Florida population was void of females that produced males only and a high percentage if not all Florida males appeared to possess the IIIm male-determining mechanism. Tests for recessive lethal 3rd chromosomes showed that there was no significant difference in the frequency of lethal factors recovered from the North Dakota and Texas flies; the presence of IIIm chromosomes in Florida males precluded the recovery of lethal factors from this population by the method employed. The data suggest that house fly strains to be employed in genetic control programs should 1) originate from target control areas to avoid possible behavioral differences existing among flies from different locales, 2) be initiated with as many flies as possible to provide a background for the maintenance of variability, and 3) be renewed periodically with field-collected material since the genotype may be capable of rapid reorganization in response to laboratory selection pressures.     

Effects of horn fly and house fly (Diptera: Muscidae) larvae on the development of parasitic nematodes in bovine dung

Laboratory studies were conducted to determine the effects of horn fly, Haematobia irritans (L.), and house fly, Musca domestica L., larvae on the development of a mixed population of parasitic nematodes in compressed and crumbled bovine dung. Fresh dung (100 g per sample) from a single calf passing trichostrongyle type eggs was infested with 150 horn fly or 150 house fly eggs. After 14-15 d, more horn flies and house flies had emerged from the compressed dung than from the crumbled dung, but more third stage parasitic nematode larvae were recovered from the crumbled dung containing either fly species than from dung containing no flies.     

Seasonal abundance of resident parasitoids and predatory flies and corresponding soybean aphid densities, with comments on classical biological control of soybean aphid in the Midwest

Seasonal abundance of resident parasitoids and predatory flies, and corresponding soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), densities were assessed in soybean fields from 2003 to 2006 at two locations in lower Michigan. Six parasitoid and nine predatory fly species were detected in 4 yr by using potted plants infested with soybean aphid placed in soybean fields. The parasitoid Lysiphlebus testaceipes Cresson (Hymenoptera: Braconidae) and the predatory flies Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae), and Allograpta obliqua Say (Diptera: Syrphidae) were most numerous. Generally, L. testaceipes was more abundant late in the soybean growing season, but it also occurred during soybean vegetative growth; A. obliqua was more abundant during vegetative growth; and A. aphidimyza was common throughout the season. Soybean plants were visually inspected to estimate densities of soybean aphid, mummified aphids, and immature predatory flies. From 2003 to 2006, parasitism rates were inversely correlated with aphid density: percentage of parasitism was always very low (< or = 0.1%) at high aphid densities (> 100 aphids per plant), and higher parasitism, up to 17%, was observed at very low aphid densities (< 1 aphid per plant). Populations of immature predatory flies, particularly A. aphidimyza, generally increased in soybean fields with increasing soybean aphid populations, but aphids always outnumbered immature flies by 100-21,000-fold when flies were detected. Rearing field-collected aphid in 2006 substantiated that parasitism varied widely, with parasitism in most cases < 10%. Based on findings of low parasitism and predation, positive response to changing aphid densities by predatory flies but not parasitoids, early season abundance primarily of predatory flies, and past findings on these taxa's diversity and abundance, we discuss the potential use of exotic parasitoids and predatory flies to enhance soybean aphid biological control.     

Muscovy ducks as an adjunct for the control of the house fly (Diptera: Muscidae)

In laboratory trials, Muscovy ducks, Cairina moschata L., removed adult house flies, Musca domestica L., at least 30 times faster than commercial bait cards, coiled fly paper rolls, fly sheets, or fly traps. The LT90 for ducks in 0.24-m3 cages with 100 flies was 0.6 h compared to 15.3 h for the most effective commercial device. Ducks survived for at least 12 wk in pens with calves, without injury or feed supplement. Ducks ingested a mean of 25 house flies per 15-min observation period when populations were low to moderate. The economics and advantages of Muscovy ducks as part of a management program for house fly control are discussed.     

Commercial and naturally occurring fly parasitoids (Hymenoptera: Pteromalidae) as biological control agents of stable flies and house flies (Diptera: Muscidae) on California dairies

Filth fly parasites reared by commercial insectaries were released on two dairies (MO, DG) in southern California to determine their effect on populations of house flies, Musca domestica L., and stable flies, Stomoxys calcitrans (L.). Spalangia endius Walker, Muscidifurax raptorellus Kogan and Legner, and Muscidifurax zaraptor Kogan and Legner were released on the MO dairy from 1985 to 1987 in varying quantities. Parasitism by Muscidifurax zaraptor on the MO dairy was significantly higher (P less than 0.05) from the field-collected stable fly (4.4%) and house fly (12.5%) pupae, compared with a control dairy (0.1%, stable fly; 1.3%, house fly). Muscidifurax zaraptor, released from April through October during 1987 on the DG dairy (350,000 per month), was not recovered in a significantly higher proportion from either fly species relative to the corresponding control dairy. No specimens of Muscidifurax raptorellus were recovered from the MO dairy. Parasite treatments had no apparent effect on adult populations of either fly species or on overall parasitism rate of field-collected stable fly (16.8%, MO; 17.2%, DG) and house fly (23.3%, MO; 20.9%, DG) pupae. Spalangia spp. were the predominant parasites recovered from field-collected stable fly and house fly pupae on all four dairies. Sentinel house fly pupae placed in fly-breeding sites on both release dairies were parasitized at a significantly higher rate, as compared with sentinel pupae on control dairies. The generic composition of parasites emerging from sentinel house fly pupae was 20.6% Spalangia spp. and 73.2% Muscidifurax spp., whereas in field-collected house fly pupae, Spalangia spp. and Muscidifurax spp. constituted 74.3 and 19.6% of the parasites, respectively.