Dispersal Of Flies Research Articles

DISPERSION OF FRUIT FLIES (DIPTERA: TEPHRITIDAE) AT HIGH AND LOW DENSITIES AND CONSEQUENCES OF MISMATCHING DISPERSIONS OF WILD AND STERILE FLIES

Abstract Both wild and released (sterile) Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) and wild Bactrocera papayae (Drew and Hancock) in Australia had patchy distributions and comparisons with predictions of the negative binomial model indicated that the degree of clumping was sometimes very high, particularly at low densities during eradication. An increase of mean recapture rate of sterile B. tryoni on either of 2 trap arrays was not accompanied by a reduction in its coefficient of variation and when recapture rates were high, the percentage of traps catching zero decreased only slightly with increase in recapture rate, indicating that it is not practicable to decrease the heterogeneity of dispersion of sterile flies by increasing the number released. There was often a mismatch between the dispersion patterns of the wild and sterile flies, and the implications of this for the efficiency of the sterile insect technique (SIT) were investigated with a simulation study with the observed degrees of mi...

Seasonality of Old World screwworm myiasis in the Mesopotamia valley in Iraq

Following the first recorded introduction of the Old World screwworm fly (OWS), Chrysomya bezziana Villeneuve (Diptera: Calliphoridae), into the Mesopotamia valley in Iraq in September 1996, cases of livestock myiasis caused by OWS developed a distinctly seasonal pattern. The annual cycle of clinical OWS cases is explained here on the basis of environmental variables that affect the different life-cycle stages of C. bezziana. This analysis suggests that low temperatures restricted pupal development during the winter, whereas the dispersal of adult flies was constrained by hot/dry summer conditions. A restricted number of OWS foci persisted throughout the year. In these foci, pupal development was fastest during the autumn months. In autumn, rapid multiplication, lasting several OWS generations, allowed subsequent adult fly dispersal across the valley floor during the winter. Hence, the monthly incidence of clinical OWS cases in livestock peaked during December-January and was lowest during July-August. In addition to temperature and humidity, vegetation cover played a role in OWS distribution. Hence the majority of OWS cases were clustered in the medium density type of vegetation [normalized difference vegetation index (NDVI) values of 0.2-0.4] along the main watercourses in the marshy Mesopotamia valley. Although sheep were the host most commonly infested by C. bezziana, local sheep density was not found to be a major factor in disease spread. Satellite imagery and the application of Geographical Information System (GIS) tools were found to be valuable in understanding the distribution of OWS in relation to vegetation and watercourses. The presence of screwworm in Iraq, at the perimeter of the intercontinental OWS distribution, may give rise to major seasonal flare-ups.

DISPERSAL OF THE FIRE ANT DECAPITATING FLY, PSEUDACTEON CURVATUS (DIPTERA: PHORIDAE) IN NORTHEAST MISSISSIPPI

Pseudacteon curvatus Borgmeier is one of three species of phorid decapitating flies currently approved for release in the U.S. for the suppression of the red, black, and hybrid imported fire ants, Solenopsis invicta Buren, S. richteri Forel, and S. invicta x richteri, respectively. Two biotypes of P curvatus are established in the U.S. The Las Flores, Argentina biotype prefers black and hybrid imported fire ants (Porter & Briano 2000) and is established at sites in Alabama, Mississippi, and Tennessee (Graham et al. 2003; Thead et al. 2003; Vogt & Streett 2003; Vail et al. 2004; Ward et al. 2004). The Formosa, Argentina biotype prefers red imported fire ants and is established at sites in Florida (R. J. Vasquez & S. D. Porter, pers. comm.). This study reports dispersal of flies of the Las Flores biotype, first released in spring, 2002 in a grazed pasture (Knox site) in Clay Co., MS (3.25 ha, 33040'05.87N, 88034'48.02W) (Fig. 1) (Vogt & Streett 2003). By Sept. 2002, flies had established on a mixture of black and hybrid imported fire ants and had spread up to 600 m from the original release site (Vogt & Streett 2003). Additional releases, with the same protocol, were made during spring 2002 and 2003 in a grazed pasture (Prima site) in Clay Co., MS, about 8.8 km and 149.70 SE of the Knox site (Fig. 1). Fly presence was confirmed at both sites during 2003 (J. T. V. & L. G. T., unpubl. data). Observations were made outside the release sites on 23 dates from May-Sept. 2004, between 09:25 and 15:45 hours at 134 active fire ant mounds. Sampling areas were randomly selected and located on roadsides that were bordered by forests with overhanging vegetation or by grazed pastures. The presence of P curvatus was determined by making a round depression (about 4-5 cm wide and 5-10 cm deep) in black and hybrid imported fire ant mounds. Hovering flies were counted within and around the depression. Ants were macerated and dropped into the depression to release semiochemicals that attract the flies (Porter et al. 2004). All sampled areas were georeferenced. Mounds were observed for up to 35 min. If flies were found in an area, we moved and sampled farther from the release sites. An area was re-sampled later unless flies were found farther from the release sites along a similar compass bearing. Average air temperature during sampling was 29.9 ? 2.6?C (?SD), with a mean relative humidity of 66.3 18.5%, and a mean wind speed of 1.2 ? 1.45 km/h.

Establishment and dispersal of the fire ant decapitating fly Pseudacteon tricuspis in North Florida

The decapitating fly Pseudacteon tricuspis Borgmeier was released at eight sites in North Florida between the summer of 1997 and the fall of 1999 as a self-sustaining biocontrol agent of the red imported fire ant, Solenopsis invicta Buren. Several releases used parasitized fire ant workers while most involved adult flies released over disturbed ant mounds. Establishment and dispersal of fly populations were monitored by disturbing about 10 fire ant mounds at each site and then inspecting them closely for hovering flies over a period of about 30 min. Overwintering populations of flies were successfully established at 6 of 8 release sites. Over several years, fly populations at these sites increased to levels as high or higher than those normally seen in their South American homeland. By the fall of 1999, flies had expanded out 1–6 km from five release sites and occupied about 125 km 2 . By the fall of 2000 the five initial release sites plus one new site had fused into one large area about 70 km in diameter. The flies had expanded out an additional 16–29 km and occupied about 3300 km 2 . By the fall of 2001 the flies had expanded out an additional 10–30 km and occupied approximately 8100 km 2 . Fly dispersal was not related to wind patterns in the Gainesville area. Based on the above rates of dispersal and an establishment rate of 66%, we estimate that a state the size of Florida would require 5–10 releases spaced over a 3-year period to cover the state in 6–9 years. 2003 Elsevier Inc. All rights reserved.

Geographic pattern of genetic variation in the European globeflower Trollius europaeus L. (Ranunculaceae) inferred from amplified fragment length polymorphism markers.

The distribution of genetic variation and the phylogenetic relationships between 18 populations of the arctic-alpine plant Trollius europaeus were analysed in three main regions (Alps, Pyrenees and Fennoscandia) by using dominant AFLP markers. Analysis of molecular variance revealed that most of the genetic variability was found within populations (64%), although variation among regions (17%) and among populations within regions (19%) was highly significant (P < 0.001). Accordingly, the global fixation index FST averaged over loci was high (0.39). The among-population differentiation indicates restricted gene flow, congruent with limited dispersal of specific globeflower's pollinating flies (Chiastocheta spp.). Within-population diversity levels were significantly higher in the Alps (mean Nei's expected heterozygosity HE = 0.229) than in the Pyrenees (HE= 0.197) or in Fennoscandia (HE = 0.158). This finding is congruent with the species-richness of the associated flies, which is maximum in the Alps. We discuss the processes involved in shaping observed patterns of genetic diversity within and among T. europaeus populations. Genetic drift is the major factor acting on the small Pyrenean populations at the southern edge of T. europaeus distribution, while large Fennoscandian populations result probably from a founder effect followed by demographic expansion. The Alpine populations represent moderately fragmented relics of large southern ancestral populations. The patterns of genetic variability observed in the host plant support the hypothesis of sympatric speciation in associated flies, rather than recurrent allopatric speciations.

Dispersal and effects of barriers on the movement of the New Zealand hover fly Melanostoma fasciatum (Dipt., Syrphidae) on cultivated land

Abstract: Dispersal within agricultural fields and the effects of different barriers on between‐field movement of the New Zealand hover fly Melanostoma fasciatum were studied using ingested pollen as markers. Hover flies did not generally disperse more than 20 m from the pollen source. Gravid females had no significant wind‐directed movement pattern whereas males significantly flew downwind. Flies tended to avoid flying over barren land: a dirt track, an asphalt road or a ploughed field all seemed to hamper hover fly dispersal equally. The implications for spatial arrangement of the flowering strips to enhance the biocontrol potential of hover flies are discussed.

Identification of blood meals of Phlebotomus perniciosus (Diptera: Psychodidae) in Spain by a competitive enzyme-linked immunosorbent assay biotin/avidin method.

Blood meals from Phlebotomus perniciosus Newstead, collected in four different places in Spain, were identified to determine host-selection patterns. Blood meals were tested using a competitive enzyme-linked immunosorbent assay (ELISA) biotin-avidin method. Results indicate that this species is an opportunist that feeds on those animals to which it has easiest access. However, some preferences were indicated, because the insect never fed on chickens and frequently fed on sheep at sites where both sheep and goats were present. At some sites, the number of sand flies feeding on dogs was higher than expected, based on the proportion of dogs to man. Nevertheless, differences in host behavior, dispersal of engorged sand flies, and their exo- or endophilic habits make it difficult to assign specific host preferences.

Parasite‐Induced Mortality in Mycophagous Drosophila

Reductions in host survival due to parasitism have immediate effects on the population dynamics of both parasites and hosts. Despite its potential importance, parasite—induced host mortality has rarely been studied experimentally under natural conditions. We conducted a series of field releases of Drosophila putrida and Drosophila neotestacea that had been parasitized in the laboratory by the nematode Howardula aoronymphium. In 11 out of 12 independent tests, the proportion of flies that were parasitized declined significantly through time, suggesting that parasites cause host mortality. Independent tests showed that parasites have no detectable effect on dispersal of flies and that motherworms do not disappear from surviving hosts, thus supporting the conclusion that parasites cause significant host mortality in the field. Host mortality rate increased with worm burden, significantly so in four independent tests. This is a possible mechanism for density—dependent regulation of these parasite populations. Parasite—induced mortality was considerably greater among flies that had been released in the field than among those kept concurrently in the laboratory. Thus, laboratory or clinical traits may not provide an accurate assessment of parasite virulence in natural populations. These results support a central assumption of many recent ecological and evolutionary models, i.e., that parasites are harmful to their hosts under natural conditions.

Stable Fly (Diptera: Muscidae) Insecticide Resistance in Kansas Cattle Feedlots

Insecticide resistance to two organophosphates, dichlorovos and stirofos, and a pyrethroid, permethrin, was detected in stable flies, Stomoxys calcitrans (L.), collected from eight cattle feedlots in southwestern Kansas. The frequency of resistant stable flies ranged from 2 to 100% depending on population and insecticide tested. No resistance was detected to the chlorinated hydrocarbon methoxychlor. Generally, the prevalence of resistance in each fly population in decreasing order was dichlorvos > stirofos > permethrin. Resistant stable flies were found in some feedlots where insecticide use was absent or minimal (i.e., application once per year or less) and was suspected to have resulted from localized dispersal of insecticide-resistant flies from nearby feedlots.

Three‐dimensional dispersion of drosophilid flies in a cool temperate forest of northern Japan

AbstractThree‐dimensional dispersion of drosophilid flies was studied within a secondary broad‐leaved forest in relation to forest structure. The survey area included the forest margin and old canopy gaps and varied in the foliage height profile from place to place. Using multivariate analyses on the data of drosophilid dispersion, five microhabitats which were different from one another for drosophilids were recognized: (i) canopy layer; (ii) middle layer; (iii) floor layer of forest interior; (iv) upper layer of forest margin; and (v) herbaceous layer of forest margin and gap. The height of living space of canopy species was remarkably lowered at the forest margin. The forest edge was richer in both numbers of individuals and species than the forest interior from the overlap of the grassland and the forest canopy subcommunities and the addition of invaders from other habitats. However, no ‘edge’ species, which were mostly restricted to or spend most of their time in ecotones, were found. It is hypothesized that the above‐ground forest structure consists fundamentally of three zones: (i) the canopy; (ii) the floor; and (iii) the edge. A significant positive correlation was found between the foliage height diversity and the degree of vertical habitat segregation among drosophilid species. The patchiness of vegetation structure influential to the three‐dimensional dispersion in a forest drosophilid community was estimated to be on the scale of 110–450 m2. This scale of subjective habitat patchiness or ‘ecological neighbourhood’ corresponds well with the most prevalent size of canopy gaps occurring in various forests.

Factors affecting seasonal dispersal of the tsetse flies Glossina pallidipes and G. longipennis (Diptera: Glossinidae) at Nguruman, south-west Kenya

AbstractSeasonal changes in the distribution of the tsetse flies, Glossina pallidipes Austen and G. longipennis Corti, along a transect from riverine thickets out into open plains were monitored along with tsetse density, climatic factors, vegetation and host abundance. Dispersal of tsetse into open country was quantified using the mean spread. During and after the rains both species extended their distribution out into open country up to at least 3.5 km from riverine thicket areas. The mean spread of G. longipennis was greater than that of G. pallidipes. The spread of males and females was very similar, as was the spread of different age categories of parous females, but nulliparous flies and females carrying second instar larvae were under-represented in samples from open areas. This seasonal dispersal can be accounted for by random diffusion with an average root mean square displacement of about 175 m per day. Observed degrees of spread were best correlated with humidity conditions prior to sampling, but multiple regression models suggested that host abundance, vegetation as measured by Normalised Difference Vegetation Indices (NDVI), and, in the case of G. longipennis, tsetse density, were also factors in determining the degree of spread. The significance of these findings in relation to tsetse control is discussed.

Analyses of the Dispersal of Sterile Mediterranean Fruit Flies (Diptera: Tephritidae) Released from a Point Source

One of the most common methods of studying insect dispersal is the recapture of members of a population released at a single time from a single location. This procedure especially lends itself to quantitative analysis. Quantitative studies of the dispersal of insects released in this manner may be divided into the following three general categories: (1)statistical studies in which certain dispersal parameters such as average distance flown are estimated without assuming any particular model; (2) curve fits to an empirical model having a predetermined functional form, such as an exponential; and (3) fits to a fundamental biological model such as a diffusion-based model. Each of these methods of analysis serves a purpose. For example, the first and second methods are often useful in developing control strategies for invasive pest species, whereas the third is useful for understanding the biological process driving dispersal. The article presents a quantitative analysis of the dispersal of irradiated Mediterranean fruit flies, Ceratitis capitata (Wiedemann) in a macadamia nut orchard near Hilo, Hawaii. All three of the methods described above are used in the analysis. The article had three primary objectives. The first was to review and contrast the three methods of analyzing data on the dispersal of insects from a point source. The second was to illustrate how these methods may be applied to insect recapture data using the example of released Mediterranean fruit flies. The third was to use the quantitative analysis to compare the results of the present release study with other studies on related species and to draw general conclusions about the nature of the dispersal of released sterile Mediterranean fruit flies. Results indicated that a population of irradiated flies released from a single point dispersed in a manner that may be modeled as if the population consisted of two subpopulations, one of which was dispersing in a diffusionlike pattern and the other was not dispersing, and that the fraction of the population in the nondispersing subpopulation increased. The dispersal pattern appeared to be little influenced by prevailing wind and to remain centered near the release point. The pattern had almost complete circular symmetry.

Temporal, Climatic, and Physiological Mediation of Dispersal in the Horn Fly, Haematobia irritans (L.) (Diptera: Muscidae)

The temporal, climatic, and physiological factors mediating dispersal of a naturally occurring population of horn flies, Haematobia irritans (L.), were examined. Resident (nondispersing) horn fly populations increased from late spring through early fall and decreased in late October. Fly-free herds 0.4 km from the source herd were reinfested from the source population at a significantly slower rate in June (4.6 flies per side per animal per day) than in August or October (7.1 flies per side per animal per day). Given that the greatest increase in the resident population occurred in June, it would appear that horn fly dispersal is not density-dependent. However, the rate of reinfestation was greatest in cattle treated with a larvicidal bolus and a daily application of a short-residual insecticide, indicating that reinfestation may be a function of competition for hosts, with the most rapid colonization occurring when few horn flies are present. The rate of reinfestation did not differ as a consequence of bolus treatment, suggesting that newly emerged flies rapidly disperse or replace older, dispersing adults. Physiological age-grading showed that the age structure of the dispersing flies did not differ significantly from that of the resident population. Moreover, all age classes were included in the dispersing population, with the largest proportion being parous adults. The dispersing population was primarily female, which emerged earlier and in greater numbers than males.

Survival and Dispersal of Horse Flies (Diptera: Tabanidae) Feeding on Cattle Sprayed with a Sublethal Dose of Fenvalerate

A mark-recapture study was undertaken to determine the fate of tabanids feeding on cattle sprayed with a sublethal dose of fenvalerate. A total of 15,465 tabanids belonging to at least 17 species and five genera were marked individually; flies were divided equally between control and fenvalerate-sprayed bullocks. Data were summarized for the four most abundant species: Tabanus fuscicostatus Hine, T. lineola F., T. pallidescens Philip, and T. wilsoni Pechuman. The recapture rate of tabanids was 6.6% for the control and 3.4% for the fenvalerate treatment. When the proportion of flies predicted to die or be unaffected after feeding on the sprayed bullocks was considered, an estimated 75% of the flies exposed to a sublethal knockdown dose survived and attempted to seek a subsequent host. Dispersal of tabanids was not affected by treatment. The recapture data after the first day of marking was significantly delayed for tabanids that fed on fenvalerate-sprayed bullocks.

Factors influencing dispersion of larval black flies (Diptera: Simuliidae): effects of the presence of an invertebrate predator

Laboratory experiments were conducted to study aggregative responses of larvae of Simulium vittatum Zetterstedt on a ceramic tile to activity of a predatory perlid stonefly, Claassenia sabulosa (Banks). Nearest-neighbor analysis indicated that larvae became aggregated regardless of whether a predator was present. There was no significant difference in either number of larvae remaining on the tile or degree of black fly aggregation between trials that lacked a predator and those in which a stonefly could travel over the tile. Direct contact between predator and prey caused single simuliid larvae to release from the substrate, but daytime black fly behaviour was otherwise unaffected by stonefly activity.

Appetitive dispersal of sterile fruit flies: aspects of the methodology and analysis of trapping studies

AbstractSterile Anastrepha ludens and Ceratitis capitata (Dipt., Tephritidae) were released in two mango orchards under the tropical conditions of Chiapas, Mexico. Distributions of flies caught in McPhail traps at regular distances from the release point varied widely between experiments and days after release. The density‐distance regressions displayed similarities to a previous study of Drosophila pseudoobscura (Dobzhansky and Wright 1943). It is suggested that the routine pooling of different day's results and from various experiments leads to misleading conclusions. Laboratory flight propensity studies of A. ludens showed notable changes in flight length over the first few days of life; distributions of flight times displayed similarities to field density‐distance distributions. Individual flies differed significantly in their mean flight times when measured repeatedly over periods of 9 to 10 days; in the case of females there was a 30‐fold range between the shortest and longest flier. A simulation experiment which attempted to explain field distributions in terms of random movement with a variable diffusion constant, failed to adequately reproduce field curve shapes. The relevance of field release studies is discussed.ZusammenfassungZum Dispersionsflug steriler Fruchtfliegen: Aspekte zur Methodik und Analyse von FallenfanguntersuchungenSterile Anastrepha ludens und Ceratitis capitata (Dipt., Tephritidae) wurden in einer Mangoplantage im tropischen Chiapas, Mexiko, freigelassen. Die Verteilung der Fruchtfliegen, die mittels McPhail‐Fallen in gleichmäßiger Entfernung vom Freilassungsort gefangen wurden, variierte erheblich zwischen den verschiedenen Versuchen und zwischen der Anzahl Tage nach Freilassung. Die Relation zwischen Dichte und Entfernung zeigte Ähnlichkeiten zu einer früheren Untersuchung über Drosophila pseudoobsura (Dobzhansky und Wright 1943). Es wird angenommen, daß die routinemäßige Zusammenfassung von verschiedenen Tagesfängen und von unterschiedlichen Versuchen zu falschen Schlußfolgerungen führte. In Laboruntersuchungen konnte nachgewiesen werden, daß bei A. ludens in den ersten Lebenstagen deutliche Unterschiede in der Flugdauer bestehen; die Verteilung der Flugzeiten hatte Ähnlichkeit mit der im Freiland testgestellten Verteilung in bezug auf Dichte und Entfernung. Die einzelnen Fruchtfliegen wiesen hinsichtlich der mittleren Flugzeit signifikante Unterschiede auf, wenn die Messungen wiederholt in einer Zeitspanne von 9 bis 10 Tagen durchgeführt wurden; bei Weibchen wurden bis zu 30fache Unterschiede zwischen der kürzesten und der längsten Flugzeit festgestellt. In einem Simulationsversuch, der die Verteilung im Freiland durch zufällige Bewegungen mit einer variablen Diffusionskonstanten nachvollziehen sollte, konnte die im Freiland beobachtete Verbreitungskurve nicht erzielt werden. Die Bedeutung der Untersuchungen von Freilandversuchen wird diskutiert.

Observations on Dispersal of Sterile Screwworms (Diptera: Calliphoridae) in Relation to a Clear-Cut Utility Right of Way in Southern Mexico

Dispersal of ground-released sterile screwworm flies, Cochliomyia hominivorax (Coquerel), in relation to a clear-cut utility right of way was studied near Tuxtla Gutierrez, Chiapas, Mexico, during October and November 1984. Collection data from wind-oriented traps suspended below trees along the edge and along transects 150 m into the woods along both sides of the right of way indicated male and female flies exhibited the same dispersal patterns. Chi-square analysis of these data indicated that the right of way was not important as a flight corridor for screwworm flies. All traps caught flies, indicating dispersal in all directions from the release site. Man-made topographic features such as utility right of ways probably do not affect adult screwworm dispersal through wooded areas. Wind appeared to be a factor in dispersal patterns in that traps downwind collected significantly more flies than traps upwind

Factors Influencing Dispersion of Larval Black Flies (Diptera:Simuliidae): Effects of Current Velocity and Food Concentration

Positioning and feeding of Simulium vittatum larvae were studied on a ceramic tile in a flume at three current velocities and two food concentrations. Locomotory activity and drift decreased with increasing current but were independent of food. Using nearest neighbor analysis, we detected significant aggregation at 15 cm∙s−1 that resulted from accumulation of individuals at upstream boundaries. Larvae also aggregated at 43 cm∙s−1, but independently of tile margins. Dispersion was random at 24 cm∙s−1. Gross pattern formation results from individual responses to prevailing flow conditions independently of food concentration. Relative ingestion rates decreased with increasing food concentration, larval size, velocity, and incidence of parasitism. Individual larvae avoided locations directly upstream or downstream of a nearest neighbor at 15 and 24 cm∙s−1 when food concentration was low (0.5 mg∙L−1), but not at higher food levels (3.5 mg∙L−1). These trends were reversed at high velocity. Larvae immediately downstream of neighbors had reduced ingestion rates. Larvae selected positions lateral to others under low food conditions. These animals had greater ingestion rates than did individuals in other locations. Results are consistent with hypotheses of flow disruption and/or food preemption by upstream larvae and of mutually induced beneficial flow between laterally adjacent individuals. Positioning within aggregations reflects complex behavior among conspecifics that varies with the interaction of flow and food concentration.

Effects of Resource Density on Mycophagous Fly Dispersal and Community Structure

I censused adult flies visiting 1-m2 plots containing high or low densities of a natural host-mushroom analog (Agaricus bisporus). I also conducted a mark-recapture experiment with visiting drosophilids. More individuals and species were captured in high density plots than low density plots, and species used high density plots more frequently. Also, mycophagous drosophilids localized activity in rich resource areas and dispersed more readily from poor resource areas. These patterns are consistent with the 'trapping effect' predicted for dense resource patches in the resource concentration hypothesis. I also collected mushrooms from plots and censused flies that emerged. In contrast to the resource concentration hypothesis, there was no evidence of the establishment of a dominance hierarchy in response to increased resource density. Single mushrooms in the high density plots produced more diverse, species-rich communities, with less dominance and greater resource overlap, than single mushrooms in low density plots. Persistent patches of mushrooms may accumulate species, but the ephemerality of individual resource units may preclude the establishment of large populations and maintain communities in a high diversity state.

Horn fly (Diptera: Muscidae) dispersal among cattle herds.

Dispersal of wild horn flies, Haematobia irritans (L.), from existing populations to fly-free cattle was studied in Louisiana and Kansas. Rates at which horn fliesinfested fly-free cattle placed sequentially, for 10-d intervals, at 0.05, 0.8, and 1.7 km from a source population, did not differ significantly and averaged 18 flies per side per animal per d. Regardless of distance from the source, fly-free cattle were infested with pest populations as great as those present on the source herd within 6 d. For a 13-d period, the rate at which horn flies infested herds placed simultaneously at 0.05 and 0.8 km from a source population did not differ significantly, indicating that the nearer population did not function to trap flies dispersing from the source population. A herd placed behind a border (0.5 km) of trees, 1.7 km from the source, showed a significantly decreased rate of infestation of five fliesper side per animal per day. However, combined factors of long distances, trap animals, and physical barriers did not prevent infestation of cattle by dispersing horn flies.