Infection of the western treehole mosquito,Aedes sierrensis (Diptera: Culicidae), with Lambornella clarki (Ciliophora: Tetrahymenidae)

Lambornella clarki was a common parasite of Aedes sierrensis immatures collected from treeholes in Mendocino County, California, in 1982–1983. The ciliate was not found in mosquitoes from treeholes with water having the most extreme values of electrical conductivity (<0.23 and >1.74 mmhos/cm) and pH (<6.5 and >7.7). Infection rates for individual monthly samples from L. clarki-positive treeholes ranged from 1 to 75%; 67% of all infections were observed in 4th-instar larvae. Infection with pathogens and parasites such as L. clarki, Ascogregarina clarki, Octomyomermis troglodytis, and unidentified bacteria and fungi, appeared responsible for high mortality rates (21–76%). Parasitism with L. clarki did not always result in death of the mosquito host; 7% of adults emerging from samples held in the laboratory were found to be infected. Ciliates were restricted to the host hemocoel except in older females where they invaded the ovaries, resulting in parasitic castration. This phenomenon may be associated with parasite dispersal.     

Vertical oviposition and Lambornella clarki (Ciliophora: Tetrahymenidae) dispersal by Aedes sierrensis (Diptera: Culicidae) in California

Vertical stratification of treeholes used by mosquitoes may reflect resource quality or result from interspecific competition. Mosquitoes able to monopolize treeholes with optimal resources may be over‐represented in the community. Aedes sierrensis, which is well adapted for the Mediterranean climate of California, has evolved in the absence of interspecific competition, so oviposition should reflect resource quality to a large extent. Artificial oviposition traps mounted at four canopy heights facing north or south on trees in a mixed‐oak forest at four elevations of the Pacific Coastal Range were used to assess vertical ovipositional preferences by the western treehole mosquito. Natural dispersal of the ciliated protozoan parasite Lambornella clarki was similarly monitored. Gravid Ae. sierrensis showed no vertical stratification during egg laying in traps. Lambornella clarki were naturally dispersed at relatively low frequency into traps and persisted unless eliminated by larval predation. Aedes sierrensis is not currently constrained into occupying a subset of treeholes. However, invasion of its native range by competitive species may alter oviposition patterns.     

Fine Structure and Life Cycle of Lankesteria clarki sp. n. (Sporozoa: Eugregarinida) Parasitic in the Mosquito Aedes sierrensis (Ludlow)

Lankesteria clarki sp. n. from the treehole mosquito, Aedes sierrensis (Ludlow) in California, is described. This species can be separated from the closely related ones, L. culicis (Ross) and L. barretti Vávra, by the totally intracellular nature of the trophozoite, the shape of the gamont, the position of the gamont nucleus and the structure and position of the residual body in the spore. Morphologic and ultrastructural investigations of gametogenesis and sporogony were conducted and the host relationship discussed. Lankesteria clarki was found in mosquitoes collected from all treeholes examined near Novato in Marin County, California and an examination of over 6,000 larvae of A. sierrensis showed an infection rate of 27.5%. The parasite is capable of destroying the midgut epithelial cells during its trophic stage and Malpighian tubule cells during gametogenesis and sporogony.     

Parasitism of newly-hatched Aedes sierrensis (Diptera: Culicidae) larvae by Lambornella clarki (Ciliophora: Tetrahymenidae) following habitat flooding

Host-parasite interactions between Lambornella clarki (Ciliophora: Tetrahymenidae) and its natural host, Aedes sierrensis (Diptera: Culicidae), were studied in newly flooded treeholes in northern California between 1986 and 1989. First instar host larvae hatched within 1 to 4 hr of flooding, while free-living trophonts of L. clarki appeared between 7 and 24 hr. As early as 24 hr after flooding, ciliates initiated the first parasite cycle by forming cuticular cysts on first instar larvae; by 64 hr, cysts were observed on larvae collected from all positive holes during all years. While larvae with as many as 12 cysts were observed, most supported only 1 cyst, and successful infections were established by the entry of a single ciliate into the host's hemocoel. Among treehole populations, the proportion of larvae with L. clarki cysts ranged from 2 to 100% at 48 hr indicating that enzootics and epizootics develop rapidly in newly flooded treeholes. Average attack rates from all holes by year ranged between 17.0 and 44.4%. Ciliates began entering hosts 48 to 72 hr after flooding, but some larvae escaped parasitization by molting to the second instar before ciliates penetrated the cuticle. In some treeholes, opportunistic microorganisms entered larvae with the invading ciliates and killed both the host and parasite.     

Effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni (Lepidoptera: Noctuidae) and the parasite, Hyposoter exiguae (hymenoptera: Ichneumonidae)

The effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni and the parasite, Hyposoter exiguae, was investigated to determine if the virus could invade and multiply in the tissues of the parasites, if parasites which emerged from virus-infected T. ni larvae had normal emergence, fecundity, and longevity, and if the parasite could serve as a vector for the virus. Light microscopy revealed particles which appeared to be polyhedra within the lumen of the midgut of parasite larvae from virus-infected hosts. Transmission electron microscopy confirmed the presence of polyhedra and free virions within the midgut of the larvae. Polyhedra or free virions were never found within any parasite tissues. Parasite larvae within hosts exposed to virus before parasitization perished when their hosts died of virus infection. Parasite larvae in hosts exposed to virus after parasitization completed their development before their hosts died of virus infection. The proportion of parasites which survived increased as the time between host parasitization and host virus exposure increased. Parasite larvae which developed in hosts exposed to the virus soon after parasitization spent significantly less time in their hosts than did parasites which developed in noninfected hosts. There was no significant difference in time spent in the pupal stage, percent adult emergence, adult longevity with and without food and water, and fecundity of parasites which developed in virus-infected hosts and those which developed in noninfected hosts. Female parasites laid as many eggs in virus-infected hosts as they did in noninfected hosts. Sixty percent of the female parasites which oviposited in virus-infected hosts vectored infective doses of virus to an average of 6% of the healthy hosts subsequently exposed to them. None of the healthy host larvae exposed to male parasites which had been exposed to virus-infected host larvae became infected with the virus. Forty percent of the female parasites which developed in virus-infected hosts transmitted infective doses of the virus to an average of 65% of the healthy host larvae exposed to them. Ninety percent of the male parasites which developed in virus-infected hosts transferred infective doses of the virus to an average of 21% of the healthy host larvae exposed to them.     

A parasite's modification of host behavior reduces predation on its host

Parasite modification of host behavior is common, and the literature is dominated by demonstrations of enhanced predation on parasitized prey resulting in transmission of parasites to their next host. We present a case in which predation on parasitized prey is reduced. Despite theoretical modeling suggesting that this phenomenon should be common, it has been reported in only a few host–parasite–predator systems. Using a system of gregarine endosymbionts in host mosquitoes, we designed experiments to compare the vulnerability of parasitized and unparasitized mosquito larvae to predation by obligate predatory mosquito larvae and then compared behavioral features known to change in the presence of predatory cues. We exposed Aedes triseriatus larvae to the parasite Ascogregarina barretti and the predator Toxohrynchites rutilus and assessed larval mortality rate under each treatment condition. Further, we assessed behavioral differences in larvae due to infection and predation stimuli by recording larvae and scoring behaviors and positions within microcosms. Infection with gregarines reduced cohort mortality in the presence of the predator, but the parasite did not affect mortality alone. Further, infection by parasites altered behavior such that infected hosts thrashed less frequently than uninfected hosts and were found more frequently on or in a refuge within the microcosm. By reducing predation on their host, gregarines may be acting as mutualists in the presence of predation on their hosts. These results illustrate a higher‐order interaction, in which a relationship between a species pair (host–endosymbiont or predator–prey) is altered by the presence of a third species.     

A long-term study of non-native-heartworm transmission among coyotes in a Mediterranean ecosystem

In Mediterranean ecosystems, abiotic factors are known to affect vertebrate population dynamics, but little is known about how these factors affect population dynamics of parasites. We conducted a 9‐year investigation of the roles of temperature, precipitation, and vector abundance as determinants of transmission of the non‐native canine heartworm (Dirofilaria immitis), a dangerous parasite of pets, among coyotes (Canis latrans), an important reservoir, in north‐coastal California. Dates of heartworm transmission and total annual transmission were determined, respectively, from lengths and numbers of heartworms found in known‐age coyotes. Vector host‐seeking activity was assessed through weekly mosquito trapping. Within years, heartworm transmission occurred only when cumulative temperatures were sufficient to allow larval heartworms to develop to the infective stage (as predicted by an existing degree‐day model), and when suitable vectors were available. Most (95%) heartworms infected their hosts between 1 July and 14 September. The onset of transmission periods always occurred after the peak in vector host‐seeking activity and varied annually. Transmission periods ended before temperatures became limiting due to absence of vectors. The timing of host‐seeking activity of the primary vector species, Ochlerotatus sierrensis, also was correlated with the onset of warming temperatures such that parasite and vector phenology were synchronized. For this reason (partly), the variation in timing of seasonal warming had no detectable effect on total annual transmission. Abundance of host‐seeking Oc. sierrensis was positively correlated with annual precipitation, and annual heartworm transmission was positively correlated with abundance of host‐seeking Oc. sierrensis. Annual transmission also was positively correlated with abundance of a less numerous vector species, Anopheles punctipennis, and was directly correlated with precipitation. This study demonstrates that multiannual variability in temperature, which affects seasonality of transmission, has little effect on annual transmission, but that precipitation is a driving force determining annual transmission. These findings imply that in California, and possibly other Mediterranean climate zones, it is especially important to preventively treat pets in summers following high‐rainfall winters.     

Cellular immune competence of a Drosophila mutant with neoplastic hematopoietic organs

In the Tum^l mutant of Drosophila melanogaster, the larval hematopoietic organs undergo neoplastic changes and release into circulation large numbers of blood cells. The lamellocytes, and to a lesser extent the plasmatocytes from which they are derived, are the cells that encapsulate various endogenous tissues and form melanotic tumors. The mutation is temperature sensitive, with maximum gene expression manifested at 29°C. The ability of Tum^l larvae to encapsulate eggs of the wasp parasite Leptopilina heterotoma is dependent not only on temperature, with host larvae much more immune reactive at 29°C than at lower temperatures (15° or 21°C), but also on the interval of time following infection when temperature shift experiments are performed. When the shift of parasitized larvae from 21° to 29°C is delayed by 18 hr the hosts are not as immune reactive as those shifted immediately after infection. Since Tum^l larvae are potentially highly immune reactive at the time of infection (with sufficient numbers of lamellocytes in circulation to encapsulate parasites), the low degree of immune competence in hosts shifted to 29°C after 18 hr or maintained at lower temperatures suggests that the increased capacity of blood cells to react against foreign surfaces is dependent on the cells acquiring new or altered recognition and adherence properties at 29°C. The 18-hr delay may provide the parasite with an opportunity to interfere with the acquisition of these specific cellular alterations. Differential hemocyte counts from parasitized larvae show abnormally low lamellocyte counts in susceptible hosts, indicating that successfully developing parasites interfere with the differentiation of hemocytes.     

Diversity,consistency, and seasonality in parasite assemblages of two sympatric marine fish Pagrus pagrus (Linnaeus, 1758) and Pagellus bogaraveo (Brünnich, 1768) (Perciformes: Sparidae) off the coast of Algeria in the western Mediterranean Sea

Various host characteristics (i. e., feeding habits, geographic distribution) and habitat characteristics (i.e., seasonality) influence the structure of parasite assemblages. To compare the parasite assemblages of hosts representatives of two genera of the same fish family, simultaneously occupying a geographic region, and to examine if seasonal variations influence parasite occurrence and abundance, we examined the parasite assemblages of two sympatric marine fish, Pagrus pagrus (n = 308) and Pagellus bogaraveo (n = 315) off the coast of Algeria in the western Mediterranean. Specimens were collected during summer and autumn over three consecutive years (2014–2016). Parasite assemblages were high in species richness and abundance. We compiled an inventory of 40 parasite taxa, including ectoparasitic monogeneans and crustaceans, and endoparasitic trematodes, cestodes, acanthocephalans, and nematodes. Endoparasite taxa primarily consisted of adult gastro-intestinal parasites and long lived larval helminths. Information on the parasite community structure and seasonal variations in parasite populations of these two hosts from the Mediterranean is here provided. Observed patterns of composition, diversity, dominance, and similarity indicate an overall consistency in assemblage structure. Although each host species harbored distinct parasite communities, they shared a high proportion of parasite species suggesting similar use of a common local pool of parasites. However, most shared species did not contribute to structuring the assemblages. Seasonal patterns in parasite abundance were observed for both hosts, with peak prevalence, abundance, and diversity in autumn. Results suggest that, regardless of a common pool of parasites being available to sympatric species, several ecological filters over time, led to distinct, independent variations in the parasite assemblages in each species.     

Does timing matter? How priority effects influence the outcome of parasite interactions within hosts

In nature, hosts are exposed to an assemblage of parasite species that collectively form a complex community within the host. To date, however, our understanding of how within-host–parasite communities assemble and interact remains limited. Using a larval amphibian host (Pacific chorus frog, Pseudacris regilla) and two common trematode parasites (Ribeiroia ondatrae and Echinostoma trivolvis), we experimentally examined how the sequence of host exposure influenced parasite interactions within hosts. While there was no evidence that the parasites interacted when hosts were exposed to both parasites simultaneously, we detected evidence of both intraspecific and interspecific competition when exposures were temporally staggered. However, the strength and outcome of these priority effects depended on the sequence of addition, even after accounting for the fact that parasites added early in host development were more likely to encyst compared to parasites added later. Ribeiroia infection success was reduced by 14 % when Echinostoma was added prior to Ribeiroia, whereas no such effect was noted for Echinostoma when Ribeiroia was added first. Using a novel fluorescent-labeling technique that allowed us to track Ribeiroia infections from different exposure events, we also discovered that, similar to the interspecific interactions, early encysting parasites reduced the encystment success of later arriving parasites by 41 %, which could be mediated by host immune responses and/or competition for space. These results suggest that parasite identity interacts with host immune responses to mediate parasite interactions within the host, such that priority effects may play an important role in structuring parasite communities within hosts. This knowledge can be used to assess host–parasite interactions within natural communities in which environmental conditions can lead to heterogeneity in the timing and composition of host exposure to parasites.     

Pathogenesis of infection by the hyphomycetous fungus,Tolypocladium cylindrosporum inAedes sierrensis andCulex tarsalis [Dip.: Culicidae]

The mode of infection and cycle of development ofTolypocladium cylindrosporum Gams was examined inAedes sierrensis andCulex tarsalis. Larvae were found to be infected through the external cuticle, the pharynx and the midgut. Blastospores and conidia were both infective although for equal numerical concentrations blastospores proved more virulent causing high mortality within the first 48 h after inoculation (80 % for L2 larvae exposed to 5×10⁵ spores/ml), while conidia generally took 7–10 days to produce the same results. Sporulation did not occur on submerged cadavers. Conidia were produced only on floating cadavers in contact with air. Conidial production on floating 4th instar larvae was found to average 1.8×10⁷ conidia/larva. Invasion of the haemocoele and fairly extensive growth of the fungus almost invariably occurred before larvae were killed. This was particularly true forAedes sierrensis larvae. Details are presented of growth within the host and post-mortem penetration of the fungus out of the cadaver. AdultA. sierrensis sprayed with a conidial suspension proved susceptible to infection with 100 % mortality being recorded at 10 days. Infections originated in the thorax, suggesting, the integument or possibly the thoracic spiracles to be the most probable site of infection.     

Do Parasitic Trematode Cercariae Demonstrate a Preference for Susceptible Host Species?

Many parasites are motile and exhibit behavioural preferences for certain host species. Because hosts can vary in their susceptibility to infections, parasites might benefit from preferentially detecting and infecting the most susceptible host, but this mechanistic hypothesis for host-choice has rarely been tested. We evaluated whether cercariae (larval trematode parasites) prefer the most susceptible host species by simultaneously presenting cercariae with four species of tadpole hosts. Cercariae consistently preferred hosts in the following order: Anaxyrus ( = Bufo) terrestris (southern toad), Hyla squirella (squirrel tree frog), Lithobates ( = Rana) sphenocephala (southern leopard frog), and Osteopilus septentrionalis (Cuban tree frog). These host species varied in susceptibility to cercariae in an order similar to their attractiveness with a correlation that approached significance. Host attractiveness to parasites also varied consistently and significantly among individuals within a host species. If heritable, this individual-level host variation would represent the raw material upon which selection could act, which could promote a Red Queen “arms race” between host cues and parasite detection of those cues. If, in general, motile parasites prefer to infect the most susceptible host species, this phenomenon could explain aggregated distributions of parasites among hosts and contribute to parasite transmission rates and the evolution of virulence. Parasite preferences for hosts belie the common assumption of disease models that parasites seek and infect hosts at random.     

Factor Affecting Sex Ratios of a Mermithid Parasite of Mosquitoes

The ratio of male to female Reesimermis nielseni Tsai and Grundmann, a nematode parasite of mosquito larvae, increased as the number of parasites per host increased. Hosts with a single nematode produced 9% males compared with essentially 100% males in hosts with more than 7 parasites; hosts with 3 nematodes produced about equal numbers of males and females. Males of R. nielseni generally emerged before females because of the earlier death of multiple-infected mosquitoes. The species of the host mosquito influenced the sex ratio, but the size of a specific host at the time of invasion did not. Host diet also had a noticeable influence on the sex ratio of the nematode: singly infected hosts from a starved population produced 92% males compared with 13% in the normally fed group. The importance of these factors in the mass rearing of R. nielseni is discussed.     

Biology of Microplitis kewleyi (Hymenoptera: Braconidae): A parasite of Agrotis ipsilon (Lepidoptera: Noctuidae) larvae

Microplitis kewleyi Muesebeck is a gregarious internal parasite of larvae of the black cutworm Agrotis ipsilon (Hufnagel). Studies of the biology of the parasite revealed that there was an inverse relationship between host instar and parasite preference. Duration of development from egg to pupa ranged from 18 days at 27°C to 68.7 days at 16°C. Development from egg to pupa took 13.5–21.6 days when fourth and first instar host larvae, respectively, were parasitized. A larger number of parasites emerged from hosts parasitized in the fourth instar (22.4) than the first instar (11.5). Parasite pupation occurred when the host was in the fifth/sixth instar, depending on the instar parasitized. Thirty‐nine per cent of host larvae exposed as first instars to parasites died before parasite emergence. This decreased to 0% for host larvae exposed as fourth instars. The sex ratio was 1:1.2 (M:F). Thirty‐seven per cent of hosts exposed diurnally were stung, compared to 24% exposed nocturnally. Mean daily progeny was highest (12) on the first day, decreasing to zero after 20 days. Percent host parasitism was also highest on the first day (35%) decreasing to nearly 0% after 18 days. There appear to be three parasite larval instars. Host larvae often remained alive after parasite emergence.     

Shared geographic histories and dispersal contribute to congruent phylogenies between amphipods and their microsporidian parasites at regional and global scales

In parasites that strongly rely on a host for dispersal, geographic barriers that act on the host will simultaneously influence parasite distribution as well. If their association persists over macroevolutionary time it may result in congruent phylogenetic and phylogeographic patterns due to shared geographic histories. Here, we investigated the level of congruent evolutionary history at a regional and global scale in a highly specialised parasite taxon infecting hosts with limited dispersal abilities: the microsporidians Dictyocoela spp. and their amphipod hosts. Dictyocoela can be transmitted both vertically and horizontally and is the most common microsporidian genus occurring in amphipods in Eurasia. However, little is known about its distribution elsewhere. We started by conducting molecular screening to detect microsporidian parasites in endemic amphipod species in New Zealand; based on phylogenetic analyses, we identified nine species‐level microsporidian taxa including six belonging to Dictyocoela. With a distance‐based cophylogenetic analysis at the regional scale, we identified overall congruent phylogenies between Paracalliope, the most common New Zealand freshwater amphipod taxon, and their Dictyocoela parasites. Also, hosts and parasites showed similar phylogeographic patterns suggesting shared biogeographic histories. Similarly, at a global scale, phylogenies of amphipod hosts and their Dictyocoela parasites showed broadly congruent phylogenies. The observed patterns may have resulted from covicariance and/or codispersal, suggesting that the intimate association between amphipods and Dictyocoela may have persisted over macroevolutionary time. We highlight that shared biogeographic histories could play a role in the codiversification of hosts and parasites at a macroevolutionary scale.     

Development and fine structure of Pleistophora sp. (Cnidospora: Microsporida) in the mosquito, Aedes sierrensis

The development and fine structure of a Pleistophora sp. from larvae of Aedes sierrensis was investigated. The usual site of infection was the posterior midgut epithelium, and small uni- and binucleate forms were the first stages observed. There was no evidence of autogamy or the production of uninucleate sporonts. Binucleate sporonts produced prosporoblasts with two and four nuclei. Cellularization of the prosporoblasts resulted in sporoblasts which matured into spores. Heavily infected larvae have less fat body than healthy hosts and usually die when infected in the first or second instar. The pathogen could remain chronic and be carried into the adult host. Although natural infections were low, the pathogen could serve as a biological control agent.     

Condition status and parasite infection of Neogobius kessleri and N. melanostomus (Gobiidae) in their native and non-native area of distribution of the Danube River

The success of introduced species is often facilitated by escape from the effects of natural predators and parasites. Introduced species can profit from this favourable situation, attaining higher population densities and greater individual sizes in novel areas. In this study, somatic condition and parasite infection were compared between native and non-native populations of Neogobius kessleri Günther; introduced only within the interconnected Danube and Rhine River system, and N. melanostomus (Pallas); widely introduced throughout several river systems in Europe and North America. Higher values of Fulton’s condition factor were observed in non-native populations of both goby species. Neogobius melanostomus attained higher gonadosomatic index values in non-native populations, indicating potential increased investment in reproduction in its new area. A lower splenosomatic index was observed in non-native populations, especially in N. melanostomus. Parasite infracommunity richness and mean abundance were higher in N. kessleri in both native and non-native populations, suggesting higher susceptibility of N. kessleri to these parasites. Non-native populations of both hosts showed higher infra-community richness as a result of acquiring parasites native to the new area, but lower parasite abundance. Differences in success of the introduction and establishment in new areas between the two fish species may be associated with a relatively low parasite infection rate and a higher gonadosomatic index in non-native populations of N. melanostomus in comparison to N. kessleri.     

Long-term dynamics of a predator used for biological control and decoupling from mosquito prey in a subtropical treehole ecosystem

We identified, staged and counted the immature stages of mosquitoes from 1,826 censuses (with replacement) of the aquatic contents of ten treeholes surveyed every 2 weeks between 1978 and 1993. These time series were used to examine the population dynamics and effect on prey of the predatory mosquito Toxorhynchites rutilus. The mean annual frequency of occurrence of T. rutilus ranged from 0.02 to 0.67 among holes, and no fourth instars were recovered during a 30-month dry period. Oviposition and pupation by this species were recorded in all months, but most commonly in the spring. Overwintering larvae of the predator increased in weight during the prolonged fourth instar that preceded pupation in the spring. Time series analyses showed that the presence of a fourth instar T. rutilus significantly reduced the abundances of late-stage Aedes triseriatus mosquitoes. Pupal numbers of this prey species were more negatively affected by T. rutilus than were numbers of fourth instar A. triseriatus. Long-term declines in mean annual abundance of A. triseriatus prey during 16 years of observations on two holes were not correlated with increases in the mean annual frequencies of T. rutilus. Local extinctions of the aquatic stages of A. triseriatus within treeholes were common, but in most holes not significantly associated with the presence of T. rutilus, suggesting that predation does not routinely drive mosquito prey locally extinct in this ecosystem. The decoupling of T. rutilus and A. triseriatus, as revealed through these complete and long-term censuses, is contrasted with other reports of generalist predators causing extinctions of mosquito prey. Discrepancies among reported outcomes probably result from differences in duration of sampling periods and statistical procedures along with real differences in the intensity of predation among systems and sites. Received: 14 August 1996 / Accepted: 24 February 1997     

Spatial and genetic structure of directly-transmitted parasites reflects the distribution of their specific amphibian hosts

Parasite distributions depend on the local environment in which host infection occurs, and the surrounding landscape over which hosts move and transport their parasites. Although host and landscape effects on parasite prevalence and spatial distribution are difficult to observe directly, estimation of such relationships is necessary for understanding the spread of infections and parasite–habitat associations. Although parasite distributions are necessarily nested within host distributions, direct environmental influences on local infection or parasite effects on host dispersal could lead to distinct landscape or habitat relationships relative to their hosts. Our aim was to determine parasite spatial structure across a contiguous prairie by statistical modeling of parasite–landscape relationships combined with analysis of population genetic structure. We sampled northern leopard frogs (Lithobates pipiens) and wood frogs (L. sylvaticus) for host-specific lung nematodes (Rhabdias ranae and R. bakeri; respectively) across the Sheyenne National Grassland in southeastern North Dakota and developed primers for 13 microsatellite loci for Rhabdias. The two Rhabdias species exhibited different correlations with landscape characteristics that conformed with that of their hosts, indicating transmission is driven by host ecology, probably density, and not directly by the environment. There was evidence for localized, patchy spatial genetic structure, but no broader-scale geographic patterns, indicating no barriers to host and parasite dispersal. Nematodes cohabitating in an individual frog were most genetically similar. Worms within the same wetland were also genetically similar, indicating localized transmission and resulting wetland-scale patchiness are not completely obscured by broad-scale host–parasite dispersal. Beyond individual wetlands, we found no evidence of genetic isolation-by-distance or patchiness at the landscape-scale.