Low Host Density Research Articles

Reproductive Biology and Rearing Improvements of Apanteles opuntiarum, Potential Biocontrol Agent of the Argentine Cactus Moth, Cactoblastis cactorum.

The cactus moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) is native to South America and has been used as a biocontrol agent of Opuntia (Cactaceae) in Australia and South Africa. Its invasion in North America has raised concerns for the native Opuntia in the USA and Mexico. We investigated the reproductive biology and rearing procedures of a host-specific potential biocontrol agent, Apanteles opuntiarum Martínez and Berta (Hymenoptera: Braconidae). Given the gregarious nature of the parasitoid larvae, we studied the morphology of the immature stages and evaluated evidence of polyembryony and superparasitism. We also investigated the effects of host exposure arena and host density on parasitism rates and wasp production. The morphological descriptions provide a basis for comparison with other species. Early larval instars of A. opuntiarum are similar to other microgastrine immature stages. However, the mature larva exhibits placoid sensilla in the epistomal region, a unique character not previously reported. We provide evidence that A. opuntiarum eggs are not polyembryonic; females frequently superparasitize and have an oviposition preference for larvae parasitized 1-2 d previously. Exposing larvae of C. cactorum to wasps while inside the cactus resulted in lower parasitism and fewer offspring from each host than exposing larvae in the arena without the cactus. Parasitism and mortality rates were higher at lower host densities, possibly due to reduced host group defensive behavior. These results suggest that preference for superparasitism, host defensive behavior, and interactions with the cactus may play an important role in per-host wasp production under laboratory conditions.

Rabies and the Arctic Fox (Vulpes lagopus): A Review.

The Arctic fox (Vulpes lagopus) is the primary infection reservoir of Arctic rabies, the dynamics of which are poorly understood and subject to significant spatiotemporal variation. Although rabies presence has been documented in the region since the mid-19th century, there is currently no evidence of rabies impacting Arctic fox population size. Under the influence of climate change in a rapidly evolving Arctic ecosystem, alterations in transmission dynamics are predicted, with implications for this species. Concurrently, the World Health Organization leads the United Against Rabies collective in the aim of elimination of dog-mediated rabies by 2030, and although efforts have justifiably been directed to tropical regions, elimination will require a good understanding of rabies in the Arctic. Therefore, this review aimed to provide an overview of current Arctic rabies understanding, while identifying the key knowledge gaps. The review covered spatiotemporal trends in rabies populations, population dynamics of the host species, and current theories about Arctic rabies persistence. It is still unclear how Arctic rabies can persist under low host densities, which has led to several hypotheses in recent years. Creation of high animal density "hotspots" caused by heterogenic fox distribution and multispecies congregations in response to food availability, extensive Arctic fox migration patterns, and the potential evolution to a less lethal variant of rabies may all be part of the explanation. Evidence for these theories by using recent genetic and modeling studies was evaluated within the review. There is currently insufficient evidence about the efficacy and feasibility of vaccines against Arctic rabies. Key knowledge gaps need addressing to enable future control campaigns.

Sympatry of Amblyomma eburneum and Amblyomma variegatum on African buffaloes and prevalence of pathogens in ticks

The Amblyomma genus is represented on the African continent by 24 species, out of which 17 are known to occur in different ecological niches of southern Africa. Amblyomma, known for their aggressive hunting behaviour and aptitude as pathogen vectors, are of main concern to travellers, mainly in rural and conservation areas of Africa. In this study, we highlight the overlapping distribution of Amblyomma eburneum and Amblyomma variegatum found on African buffaloes (Syncerus caffer) at Coutada 11, Central Mozambique. In total, 1,039 Amblyomma ticks were collected and morphologically identified using taxonomic keys, and genomic DNA was extracted. They were subjected to reverse line blotting for pathogen identification followed by molecular analysis (COI sequencing) of both tick species. Pathogens such as Ehrlichia ruminantium, Anaplasma centrale, Theileria sp., Babesia sp. and Rickettsia africae were detected, of which R. africae is zoonotic. Ehrlichia ruminantium, R. africae, Theileria mutans and Theileria velifera are well-established pathogens transmitted by Amblyomma ticks; however, Anaplasma spp. and Babesia spp. are not, suggesting residual parasite DNA in the bloodmeal. Little is mentioned in the literature about A. eburneum, including its role as a vector and reservoir for pathogens. In Mozambique A. eburneum is currently restricted to wildlife but the spread of the tick may be observed given the climate change that is occurring. The infection rates for the pathogens in both Amblyomma tick species were lower than expected, but this may be due to the low host density in the forest niche and the innate immunity of these hosts. With the propensity of ticks of the Amblyomma genus to form parapatric distributions, the mechanisms that allows for the overlapping distribution of these two Amblyomma species while maintaining tick species identity is of great interest.

Vole outbreaks may induce a tularemia disease pit that prevents Iberian hare population recovery in NW Spain

Iberian hare populations have suffered severe declines during recent decades in Spain. Between 1970 and 1990s, a rapid increase in irrigation crop surface in NW Spain (Castilla-y-León region) was followed by a common vole massive range expansion and complete colonization of lowland irrigated agricultural landscapes from mountainous habitats. The subsequent large cyclic fluctuations in abundance of colonizing common voles have contributed to a periodic amplification of Francisellatularensis, the etiological agent that causes human tularemia outbreaks in the region. Tularemia is a fatal disease to lagomorphs, so we hypothesize that vole outbreaks would lead to disease spill over to Iberian hares, increasing prevalence of tularemia and declines among hare populations. Here we report on the possible effects that vole abundance fluctuations and concomitant tularemia outbreaks had on Iberian hare populations in NW Spain. We analysed hare hunting bag data for the region, which has been recurrently affected by vole outbreaks between 1996 and 2019. We also compiled data on F.tularensis prevalence in Iberian hares reported by the regional government between 2007 and 2016. Our results suggest that common vole outbreaks may limit the recovery of hare populations by amplifying and spreading tularemia in the environment. The recurrent rodent-driven outbreaks of tularemia in the region may result in a "disease pit" to Iberian hares: at low host densities, the rate of population growth in hares is lower than the rate at which disease-induced mortality increases with increased rodent host density, therefore, keeping hare populations on a low-density equilibrium. We highlight future research needs to clarify tularemia transmission pathways between voles and hares and confirm a disease pit process.

INFLUENCE OF HOST PLANTS ON THE AGE-SPECIFICLIFE HISTORY TRAITS OF PARASITOID TRICHOGRAMMA CHILONIS (ISHII) A BIO-CONTROL AGENT OF THE LEPIDOPTERAN PEST HELICOVERPA ARMIGERA (LEPIDOPTERA: NOCTUIDAE) EGG DENSITIES

The pod borer, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), is considered to be the most important pest in several crops such as chick pea (Cicer arietinum L.), Pigeon pea (Cajanus cajan L.) and Tomato (Lycopersicum esculentum L.) in North-East Uttar Pradesh, India. Extensive and indiscriminate use of chemical insecticides against pest caused severe effect on both soil health and environment, therefore effective bio-control agent (BCA), is generally perceived as providing both long-lasting insect pest control and negligible damage to the environment and non-target organisms. The aim of the present study is to control the pest population of H. armigera at Gorakhpur district by using effective bio-control agent. The age-specific life-table statistics of the parasitoid were observed at five different host densities (10, 25, 50, 100 and 200 eggs/female/day). It was found that the values of age-specific survival rate (lx) of T. chilonis increased progressively when more and more host eggs exposed to the female parasitoid throughout their life-span. It was highest at 200 host-density for all the host plants. The age-specific net fecundity rate (mx) was found to be host densities dependent. It appears that at low host density egg laying tendency of the female compel her to lay eggs in a previously parasitised egg. The longevity of the female were significantly (P < 0.001) correlated with host density. The data obtained from the present study will used in the development of management strategies for the control of H. armigera on host crops.

Two Better Than One? Potential Effects of Intraguild Predation on the Biological Control of Ceratitis capitata (Diptera: Tephritidae) by the Parasitoid Aganaspis daci (Hymenoptera: Figitidae) and the Predator Pseudoophonus rufipes (Coleoptera: Carabidae)

The use of more than one species to manage a single insect pest is a common practice among biological control programs. However, the beneficial effects of natural enemies are not always additive, which in many cases may be attributed to interspecific interactions such as intraguild predation (IGP). Herein, we investigated the potential IGP between two relevant natural enemies of the Medfly (Ceratitis capitata), the predator Pseudoophonus rufipes and the parasitoid Aganaspis daci, as well as the possible implications of this phenomenon in their efficiency as biocontrol agents. To this end, we assessed their functional responses and different demographic parameters when acting alone and together against C. capitata under laboratory conditions. Coexistence led to a switch in the functional response of both species, from type III to type II in A. daci and the opposite in P. rufipes. Regarding demographic parameters, coexistence resulted in higher parasitoidism and population reduction by A. daci only at low host densities, probably due to competition pressure. In the same circumstances, P. rufipes reduced its predatory activity, rejecting those larvae that were presumably parasitized and causing negligible IGP. At high Medfly densities, A. daci efficiency decreased, and the reduced encounter probability enhanced the predatory activity by P. rufipes. As a result of these trends, Medfly population reduction reached almost 100% at all densities, which suggests an additive effect of both natural enemies and recommends combined releases of these agents as a strategy for the control of the Medfly.

Biological control of the stink bug Nezara viridula (Heteroptera: Pentatomidae) by two parasitoids and their interaction in non-crop habitats: a simulation model.

Non-cultivated areas are resting, overwintering, feeding, and/or reproducing habitats for insects, and also places from where crop areas are colonized; thus, they are essential for understanding the biological control programs in agroecosystems. We developed a simulation model for a non-cultivated area of Buenos Aires province (Argentina), and we analyzed the control of Nezara viridula achieved by the action of two parasitoids: the oophagous Trissolcus basalis and the tachinid Trichopoda giacomellii, which attack older nymphs and adults. The model is a discrete time, deterministic, phenomenological, spatially homogeneous with a 1-week time interval simulation model, based on the age-structure and/or stage-structure of N. viridula and its two parasitoids. The host-parasitoid interactions were combined with a degree-day model affecting development times of T. giacomellii pupae and T. basalis pre-imaginal stages. The simultaneous attack of both parasitoid species enables the persistence of the system at low host densities, mediated by the functional response of the parasitoids, identified as population regulation factors. However, if only one parasitoid exists (i.e., only T. basalis or only T. giacomellii) the interaction N. viridula-parasitoid persisted but at higher density of N. viridula. These results explain the successful biological control of N. viridula after the introduction of T. basalis in the 1980s, when T. giacomellii was the only parasitoid present, unable to control N. viridula. Our model shows an indirect competition when both parasitoids are present: the attack of one of them diminished the potential number of hosts available to the other parasitoid species. In the field this interaction is obscured by the hibernation period which acted as a reset mechanism affecting the density and age/stage structure of all three populations. Our model was supported by field observations, and never exhibited the extinction of any of the parasitoids from the interaction.

A new model formulation for host depletion in parasitoids

A new model of host depletion in the functional response of parasitoids is presented. Most existing models describe the dynamics of prey consumption as a sequential-memoryless/Markovian process of searching for, handling, and consuming prey, but the feeding interaction between a parasitoid and its host is different. One of the most important characteristics is that the host is not consumed, and remains in the arena. On the other hand, functional response models of solitary parasitoids assume that each host can accommodate an infinite number of oviposition events without being killed or becoming unsuitable as a host without the parasitoid noticing it. As a consequence, host depletion is never considered in parasitoid functional response models. To solve that issue, we extended the traditional random predator and random parasitoid functional response models of Rogers into a compartmental model formulated with ordinary differential equations, to group hosts with a different number of attacks to simulate how the parasitoid spreads the attacks between already parasitized and non-parasitized hosts. This model was further modified to simulate non-sequential memoryless search, handling and oviposition. We studied the behaviour of these models to understand how a variation of their parameters led to different shapes of functional response curves and statistical distributions of the number of parasitoid eggs laid per host. The results showed that the expected curves differ from those predicted by the simpler Holling's disk equation (type II functional response), random parasitoid models (no depletion), and the random predator model (proportional depletion). Finally, the non-sequential-memoryless models predict straight curves resembling a type I functional response at low host densities.

Effects of enemy exclusion on biodiversity-productivity relationships in a subtropical forest experiment.

Interspecific niche complementarity is a key mechanism posited to explain positive species richness–productivity relationships in plant communities. However, the exact nature of the niche dimensions that plant species partition remains poorly known.Species may partition abiotic resources that limit their growth, but species may also be specialized with respect to their set of biotic interactions with other trophic levels, in particular with enemies including pathogens and consumers. The lower host densities present in more species‐diverse plant communities may therefore result in smaller populations of specialized enemies, and in a smaller associated negative feedback these enemies exert on plant productivity.To test whether such host density‐dependent effects of enemies drive diversity–productivity relationships in young forest stands, we experimentally manipulated leaf fungal pathogens and insect herbivores in a large subtropical forest biodiversity–ecosystem functioning experiment in China (BEF‐China).We found that fungicide spraying of tree canopies removed the positive tree‐species richness–productivity relationship present in untreated control plots. The tree species that contributed the most to this effect were the ones with the highest fungicide‐induced growth increase in monoculture. Insecticide application did not cause comparable effects. Synthesis. Our findings suggest that tree species diversity may not only promote productivity by interspecific resource‐niche partitioning but also by trophic niche partitioning. Most likely, partitioning occurred with respect to enemies such as pathogenic fungi. Alternatively, similar effects on tree growth would have occurred if fungicide had eliminated positive effects of a higher diversity of beneficial fungi (e.g. mycorrhizal symbionts) that may have occurred in mixed tree species communities.

Propagation of an RNA Bacteriophage at Low Host Density Leads to a More Efficient Virus Entry

The successful spread of a virus in a specific ecological niche is largely determined by host availability. The lower the host density, the longer the virus spends in the external environment between successive infections, thus increasing its probability of degradation due to physical and chemical variables, which ultimately could lead to its extinction. Nevertheless, the high error rate of viral replication, particularly in the case of RNA viruses, can lead to the emergence and subsequent selection of mutants that increase their probability of transmission under unfavorable conditions. This fact could cause some containment measures, such as those based on restriction of contacts, to have unexpected consequences that it is important to analyze. Whereas it is quite difficult to implement this kind of studies during the spread of real epidemics, evolution experiments carried out with controlled variables in the lab can be very useful to unveil regularities in virus behavior that allow to anticipate difficulties. In this work we have carried out an evolution experiment in which the bacteriophage Qβ, a virus with an RNA genome, has been propagated at different host densities under conditions that prevent the selection of defense mechanisms in the bacteria. Our results show that there is a minimal host concentration that separates sustained propagation from extinction. After a certain number of generations, all lineages propagated at suboptimal host concentration selected a mutation in the minor capsid protein whose phenotypic effect was to enhance the entry of the virus into the cell. Although it is difficult to extrapolate our findings to more complex situations, they show the need to carry out an exhaustive monitoring of viral evolution when measures based on confinements or physical barriers that limit transmission are applied.

Population growth of Gyrodactylus kobayashii in goldfish (Carassius auratus) associated with host density.

Host density is a key regulatory factor in parasite transmission. The goldfish (Carassius auratus)-Gyrodactylus kobayashii model was used to investigate effects of host density on population growth of gyrodactylids. A donor fish infected by five gravid gyrodactylids was mixed with 11 parasite-free goldfish at five host densities. There was a significant positive correlation between host density and mean abundance of G. kobayashii throughout the 58-day experiment. During early infection (days 15–24), mean abundance in medium high (0.5 fish L−1) and high host density groups (1 and 2 fish L−1) was significantly higher than that in the low host density groups (0.125 and 0.25 fish L−1). At high host density, prevalence increased more rapidly, and the peak prevalence was higher. Fitting of an exponential growth model showed that the population growth rate of the parasite increased with host density. A hypothesis was proposed that higher host density contributed to increased reinfection of detached gyrodactylids. A reinfection experiment was designed to test this hypothesis. Both mean abundance and prevalence at a host density of 1 fish L−1 were significantly higher than those at 0.25 fish L−1 on days 1 and 3, which suggested that more reinfections of G. kobyashii occurred at the higher host density. Density-dependent transmission during the early infection was an important contributor of population growth of G. kobayashii, as well as density-dependent reinfection of the detached gyrodactylids.

Abundance of Ganoderma sp. in Europe and SW Asia: modelling the pathogen infection levels in local trees using the proxy of airborne fungal spore concentrations

Ganoderma comprises a common bracket fungal genus that causes basal stem rot in deciduous and coniferous trees and palms, thus having a large economic impact on forestry production. We estimated pathogen abundance using long-term, daily spore concentration data collected in five biogeographic regions in Europe and SW Asia. We hypothesized that pathogen abundance in the air depends on the density of potential hosts (trees) in the surrounding area, and that its spores originate locally. We tested this hypothesis by (1) calculating tree cover density, (2) assessing the impact of local meteorological variables on spore concentration, (3) computing back trajectories, (4) developing random forest models predicting daily spore concentration. The area covered by trees was calculated based on Tree Density Datasets within a 30 km radius from sampling sites. Variations in daily and seasonal spore concentrations were cross-examined between sites using a selection of statistical tools including HYSPLIT and random forest models.Our results showed that spore concentrations were higher in Northern and Central Europe than in South Europe and SW Asia. High and unusually high spore concentrations (> 90th and > 98th percentile, respectively) were partially associated with long distance transported spores: at least 33% of Ganoderma spores recorded in Madeira during days with high concentrations originated from the Iberian Peninsula located >900 km away. Random forest models developed on local meteorological data performed better in sites where the contribution of long distance transported spores was lower. We found that high concentrations were recorded in sites with low host density (Leicester, Worcester), and low concentrations in Kastamonu with high host density. This suggests that south European and SW Asian forests may be less severely affected by Ganoderma. This study highlights the effectiveness of monitoring airborne Ganoderma spore concentrations as a tool for assessing local Ganoderma pathogen infection levels.

Vaccination and monitoring strategies for epidemic prevention and detection in the Channel Island fox (Urocyon littoralis).

Disease transmission and epidemic prevention are top conservation concerns for wildlife managers, especially for small, isolated populations. Previous studies have shown that the course of an epidemic within a heterogeneous host population is strongly influenced by whether pathogens are introduced to regions of relatively high or low host densities. This raises the question of how disease monitoring and vaccination programs are influenced by spatial heterogeneity in host distributions. We addressed this question by modeling vaccination and monitoring strategies for the Channel Island fox (Urocyon littoralis), which has a history of substantial population decline due to introduced disease. We simulated various strategies to detect and prevent epidemics of rabies and canine distemper using a spatially explicit model, which was parameterized from field studies. Increasing sentinel monitoring frequency, and to a lesser degree, the number of monitored sentinels from 50 to 150 radio collared animals, reduced the time to epidemic detection and percentage of the fox population infected at the time of detection for both pathogens. Fox density at the location of pathogen introduction had little influence on the time to detection, but a large influence on how many foxes had become infected by the detection day, especially when sentinels were monitored relatively infrequently. The efficacy of different vaccination strategies was heavily influenced by local host density at the site of pathogen entry. Generally, creating a vaccine firewall far away from the site of pathogen entry was the least effective strategy. A firewall close to the site of pathogen entry was generally more effective than a random distribution of vaccinated animals when pathogens entered regions of high host density, but not when pathogens entered regions of low host density. These results highlight the importance of considering host densities at likely locations of pathogen invasion when designing disease management plans.

Influence of adult diet on fitness and reproductive traits of the egg parasitoid Anagrus virlai (Hymenoptera: Mymaridae), a potential biocontrol agent against the corn leafhopper

AbstractDalbulus maidis is the most important leafhopper pest of maize in the Americas. Anagrus virlai is an egg parasitoid commonly associated with the corn leafhopper. We evaluated whether the performance of A. virlai is dependent on different diets provided during 24 hr or throughout adult female lifetime. Additionally, functional response of A. virlai on D. maidis eggs using maize leaves containing honeydew plus honey was described. A. virlai is a mostly pro‐ovigenic autogenic species whose females are able to parasitize eggs immediately after emergence. We found that wasps oviposit the majority of their eggs in the first day of their adult life. Realized lifetime fecundity and lifetime potential fecundity did not differ significantly among treatments, but longevity and egg production increased when honey was added to diet. Data were consistent with an intermediate functional response between type II and III, but closer to type II, indicating a high parasitism rate at low host densities and a decrease in the oviposition rate at high host densities, due to a possible egg limitation. Our results suggest that carbohydrate food sources (honeydew and honey) might not be the factor limiting reproductive success during the first 24 hr. Food supply, however, might influence egg maturation and survivorship of wasps, thus potentially enhancing biological pest control when hosts are scarce in the course of the first few days of adulthood.

Landscape epidemiology of ash dieback

Abstract Ash dieback is induced by Hymenoscyphus fraxineus, an invasive pathogenic fungus. It is causing severe damage to European ash populations. However, the local environment, such as climate or site conditions, is known to affect ash dieback. We studied the landscape epidemiology of the disease on a 22 km2 area in north‐eastern France at two stages of the invasion process using Bayesian spatio‐temporal models fitted with integrated nested Laplace approximation (INLA). Several features characterizing disease severity, crown dieback, frequency of collar canker and density of infected leaf debris in the litter were determined on a regular grid over a 3.5 × 6.5 km area. We first analysed the effect of landscape features on the disease establishment stage in 2012, 2 years after the first report of the disease in the area, and then on further disease development, in 2016–2018. Landscape features had little impact on the disease at the establishment stage, but strongly determined its further development. Local fragmentation of tree cover was the most important factor, with trees that are isolated or in hedges far less affected than trees in a forest environment. We showed that they were subjected to different microclimates, with higher crown temperatures unfavourable to pathogen development. Low host density strongly reduced disease development. The presence of large ash populations in the vicinity affected local disease severity up to several hundred metres. Synthesis. We showed that the landscape characteristics strongly affect the development and spread of ash dieback. The disease is far less severe in forest conditions when ash density is low or in open canopies such as hedges and isolated trees. Ash trees are often in these types of landscapes, which should strongly limit the overall impact of ash dieback.

Systematic review of modelling assumptions and empirical evidence: Does parasite transmission increase nonlinearly with host density?

AbstractHost–parasite dynamics are impacted by the relationship between host density and parasite transmission, and thus, all epidemiological models contain a central transmission–density function. Recent theoretical work demonstrates that this central parasite transmission function might be best represented by a nonlinear continuum from one linear extreme to another: density‐dependent transmission at low host densities to density‐independent transmission at high host densities. But how often are nonlinear transmission functions used, and when are they better at describing transmission in real host–parasite systems?To quantify existing modelling practices, we systematically reviewed seven representative ecology journals, finding 262 studies containing host–parasite models that contained linear and/or nonlinear transmission functions. We also reviewed the literature to find 28 experimental and observational studies that compared multiple transmission functions in real host–parasite systems, and tallied which functions were best supported in those systems. Finally, we created a flexible model simulation tool to explore and quantify the bias in model parameter estimates that is created when using an inaccurate transmission function.We found that most experimental and observational studies reported that nonlinear transmission–density functions outperformed simple linear transmission–density functions, supporting recent theoretical work. In contrast, most studies containing host–parasite models assumed that host density was constant and/or used a single, linear transmission function to explain how transmission rates changed with density. Using the wrong linear function and/or using a linear function when the underlying transmission–density relationship is even slightly nonlinear can substantially bias model parameter estimates, as demonstrated by our simulations over a broad parameter space.Some modelling studies may be using linear functions in host–parasite systems where nonlinear functions are more appropriate. If true, these models would yield substantially biased parameter estimates. To avoid such biases that compromise ecological understanding and prediction, we recommend that future studies compare multiple transmission functions, including nonlinear options, whenever possible.

Amphibian chytrid prevalence on boreal toads in SE Alaska and NW British Columbia: tests of habitat, life stages, and temporal trends.

Tracking and understanding variation in pathogens such as Batrachochytrium dendrobatidis (Bd), the agent of amphibian chytridiomycosis which has caused population declines globally, is a priority for many land managers. However, relatively little sampling of amphibian communities has occurred at high latitudes. We used skin swabs collected during 2005-2017 from boreal toads Anaxyrus boreas (n = 248), in southeast Alaska (USA; primarily in and near Klondike Gold Rush National Historical Park [KLGO]) and northwest British Columbia (Canada) to determine how Bd prevalence varied across life stages, habitat characteristics, local species richness, and time. Across all years, Bd prevalence peaked in June and was >3 times greater for adult toads (37.5%) vs. juveniles and metamorphs (11.2%). Bd prevalence for toads in the KLGO area, where other amphibian species are rare or absent, was highest from river habitats (55.0%), followed by human-modified upland wetlands (32.3%) and natural upland wetlands (12.7%)-the same rank-order these habitats are used for toad breeding. None of the 12 Columbia spotted frogs Rana luteiventris or 2 wood frogs R. sylvatica from the study area tested Bd-positive, although all were from an area of low host density where Bd has not been detected. Prevalence of Bd on toads in the KLGO area decreased during 2005-2015. This trend from a largely single-species system may be encouraging or concerning, depending on how Bd is affecting vital rates, and emphasizes the need to understand effects of pathogens before translating disease prevalence into management actions.

Interspecific competition among aphid parasitoids: molecular approaches reveal preferential exploitation of parasitized hosts

When a guild of species exploit the same limited resources, interspecific competition induces the exclusion of inferior competitors, in which case, interspecific recognition mechanisms are needed. Here, we address resource partitioning and interspecific competition among three main solitary parasitoid species attacking the same host resource, the aphid Sitobion avenae in wheat fields. Optimal host acceptance models predict that parasitoid species should prefer attacking unparasitized hosts when they are available in order to maximize their fitness, as already parasitized hosts are less valuable for laying eggs, especially for inferior competitors. Therefore, we expected the level of competition (multiparasitism) in the field to increase at low host density. By using a combination of taxonomical (determination) and molecular (PCR-based) approaches, we assessed the species of all parasitoid adults and immature stages within aphid hosts. Our results demonstrate that, early in the season, the multiparasitism rates were low, whereas they were high in the mid-late season, corresponding to an aphid density decrease over time. Moreover, parasitoid species could not have been exploiting host resources randomly and the better competitor, Aphidius ervi, seemed to be foraging preferentially on hosts already parasitized by the inferior competitor A. rhopalosiphi, even when unparasitized hosts were still available. This could be due to differences in their host detection capability, as species with a narrow host range may be better at detecting their hosts in comparison with species with a greater host range, such as A. ervi, with a greater host range within the guild. Our study suggests differences in the host exploitation of two prevalent parasitoid species through the main period of aphid colonization, which still allowed the coexistence of a third inferior competitor (A. rhopalosiphi) within the assemblage, in spite of some negative interactions (multiparasitism) and redundancies.

Density dependence and persistence of Morogoro arenavirus transmission in a fluctuating population of its reservoir host.

A key aim in wildlife disease ecology is to understand how host and parasite characteristics influence parasite transmission and persistence. Variation in host population density can have strong impacts on transmission and outbreaks, and theory predicts particular transmission-density patterns depending on how parasites are transmitted between individuals. Here, we present the results of a study on the dynamics of Morogoro arenavirus in a population of multimammate mice (Mastomys natalensis). This widespread African rodent, which is also the reservoir host of Lassa arenavirus in West Africa, is known for its strong seasonal density fluctuations driven by food availability. We investigated to what degree virus transmission changes with host population density and how the virus might be able to persist during periods of low host density. A seven-year capture-mark-recapture study was conducted in Tanzania where rodents were trapped monthly and screened for the presence of antibodies against Morogoro virus. Observed seasonal seroprevalence patterns were compared with those generated by mathematical transmission models to test different hypotheses regarding the degree of density dependence and the role of chronically infected individuals. We observed that Morogoro virus seroprevalence correlates positively with host density with a lag of 1-4months. Model results suggest that the observed seasonal seroprevalence dynamics can be best explained by a combination of vertical and horizontal transmission and that a small number of animals need to be infected chronically to ensure viral persistence. Transmission dynamics and viral persistence were best explained by the existence of both acutely and chronically infected individuals and by seasonally changing transmission rates. Due to the presence of chronically infected rodents, rodent control is unlikely to be a feasible approach for eliminating arenaviruses such as Lassa virus from Mastomys populations.

Impacts of heterogeneous host densities and contact rates on pathogen transmission in the Channel Island fox (Urocyon littoralis)

Diseases threaten wildlife populations worldwide and have caused severe declines resulting in host species being listed as threatened or endangered. The risk of a widespread epidemic is especially high when pathogens are introduced to naïve host populations, often leading to high morbidity and mortality. Prevention and control of these epidemics is based on knowledge of what drives pathogen transmission among hosts. Previous disease outbreaks suggest the spread of directly transmitted pathogens is determined by host contact rates and local host density. While theoretical models of disease spread typically assume a constant host density, most wildlife populations occur at a variety of densities across the landscape. We explored how spatial heterogeneity in host density influences pathogen spread by simulating the introduction and spread of rabies and canine distemper in a spatially heterogeneous population of Channel Island foxes (Urocyon littoralis), coupling fox density and contact rates with probabilities of viral transmission. For both diseases, the outcome of pathogen introductions varied widely among simulation iterations and depended on the density of hosts at the site of pathogen introduction. Introductions into areas of higher fox densities resulted in more rapid pathogen transmission and greater impact on the host population than if the pathogen was introduced at lower densities. Both pathogens were extirpated in a substantial fraction of iterations. Rabies was over five times more likely to go locally extinct when introduced at low host density sites than at high host-density sites, leaving an average of >99% of foxes uninfected. Canine distemper went extinct in >98% of iterations regardless of introduction site, but only after >90% of foxes had become infected. Our results highlight the difficulty in predicting the course of an epidemic, in part due to complex interactions between pathogen biology and host behavior, exacerbated by the spatial variation of most host populations.