Effect Of Host Density Research Articles

The prevalence of avianPlasmodiumis higher in undisturbed tropical forests of Cameroon

Abstract:Habitat fragmentation and deforestation are thought to disrupt host–parasite interactions and increase the risk of epizootic outbreaks in wild vertebrates. A total of 220 individuals from three species of African rain-forest bird (Andropadus latirostris,Andropadus virens,Cyanomitra obscura), captured in two pristine and two agroforests in Cameroon, were screened for the presence of avian haemosporidian parasites (species ofPlasmodiumandHaemoproteus) to test whether habitat differences were associated with differences in the prevalence of infectious diseases in natural populations. Thirteen mitochondrial lineages, including 11Plasmodiumand twoHaemoproteuslineages were identified. Whereas levels ofHaemoproteusspp. infections were too low to permit analysis, the prevalence of infections withPlasmodiumspp. reached significantly greater levels in undisturbed mature forests. Importantly however, the significant association between forest type and parasite prevalence was independent of host density effects, suggesting that the association did not reflect changes in host species composition and abundance between forest types. Our results illustrate how characterizing land-cover differences, and hence changes, may be a prerequisite to understanding and predicting patterns of parasite infections in natural populations of rain-forest birds.

Host density predicts presence of cuckoo parasitism in reed warblers

In some hosts of avian brood parasites, several populations apparently escape parasitism, while others are parasitized. Many migratory specialist brood parasites like common cuckoos, Cuculus canorus, experience a short breeding season, and in order to maintain local parasite populations host densities should be sufficiently high to allow efficient nest search. However, no studies have investigated the possible effect of host density on presence of cuckoo parasitism among populations of a single host species. Here, we investigated possible predictors of common cuckoo parasitism in 16 populations of reed warblers, Acrocephalus scirpaceus, across Europe. In more detail, we quantified the effect of host density, number of host breeding pairs, habitat type, mean distance to nearest cuckoo vantage point, predation rate and latitude on the presence of cuckoo parasitism while controlling for geographical distance among study populations. Host density was a powerful predictor of parasitism. We also found a less pronounced effect of habitat type on occurrence of parasitism, while the other variables did not explain why cuckoos utilize some reed warbler populations and not others. This is the first study focusing on patterns of common cuckoo-host interactions within a specific host species on a large geographic scale. The results indicate that if host density is below a specific threshold, cuckoo parasitism is absent regardless of the state of other potentially confounding variables.

Host density predicts presence of cuckoo parasitism in reed warblers

In some hosts of avian brood parasites, several populations apparently escape parasitism, while others are parasitized. Many migratory specialist brood parasites like common cuckoos, Cuculus canorus, experience a short breeding season, and in order to maintain local parasite populations host densities should be sufficiently high to allow efficient nest search. However, no studies have investigated the possible effect of host density on presence of cuckoo parasitism among populations of a single host species. Here, we investigated possible predictors of common cuckoo parasitism in 16 populations of reed warblers, Acrocephalus scirpaceus, across Europe. In more detail, we quantified the effect of host density, number of host breeding pairs, habitat type, mean distance to nearest cuckoo vantage point, predation rate and latitude on the presence of cuckoo parasitism while controlling for geographical distance among study populations. Host density was a powerful predictor of parasitism. We also found a less pronounced effect of habitat type on occurrence of parasitism, while the other variables did not explain why cuckoos utilize some reed warbler populations and not others. This is the first study focusing on patterns of common cuckoo‐host interactions within a specific host species on a large geographic scale. The results indicate that if host density is below a specific threshold, cuckoo parasitism is absent regardless of the state of other potentially confounding variables.

Effects of tick population dynamics and host densities on the persistence of tick-borne infections

The transmission and the persistence of tick-borne infections are strongly influenced by the densities and the structure of host populations. By extending previous models and analysis, in this paper we analyse how the persistence of ticks and pathogens, is affected by the dynamics of tick populations, and by their host densities. The effect of host densities on infection persistence is explored through the analysis and simulation of a series of models that include different assumptions on tick–host dynamics and consider different routes of infection transmission. Ticks are assumed to feed on two types of host species which vary in their reservoir competence. Too low densities of competent hosts (i.e., hosts where transmission can occur) do not sustain the infection cycle, while too high densities of incompetent hosts may dilute the competent hosts so much to make infection persistence impossible. A dilution effect may occur also for competent hosts as a consequence of reduced tick to host ratio; this is possible only if the regulation of tick populations is such that tick density does not increase linearly with host densities.

Selective information use in parasitoid wasps

Abstract Foraging animals frequently have to decide where to search, what host or prey to accept, and when to leave a patch. To achieve optimal patch exploitation rates, foragers can base their decisions on information about the quality of the current patch and on an estimate of the quality of the entire habitat. The latter has rarely been studied to date. We have analysed the patch-leaving behaviour of the parasitic wasp Asobara tabida Nees (Hymenoptera: Braconidae), a solitary parasitoid of Drosophila larvae, as a function of previous experience. Females were confronted with patches containing different numbers of larvae, which were either unparasitised, or parasitised by a conspecific or larvae of different host species. Our results show that females of A. tabida were able to recognise the different qualities of the patches that we offered but that they used information about host density, host species or parasitism status of the host to different degrees during subsequent patch visits. Encounters with an unsuitable host species had a great impact on future patch-leaving decisions while the effect of host density was relatively weak. If wasps had encountered previously parasitised hosts no response could be detected during subsequent visits. We argue that the various information sources provided are of different value for the searching female and that this might explain the observed pattern of selective information use.

Host–Parasitoid Interactions in a Transgenic Landscape: Spatial Proximity Effects of Host Density

Widespread management practices such as transgenic insecticidal crops influence the distribution and density of targeted pest species across the agricultural landscape. Natural enemies must cope with this altered distribution, and their response potentially influences the rate of resistance evolution in the pest. The purpose of this study was to examine spatial patterns of parasitism by the specialist parasitoid Macrocentrus grandii Goidanich (Hymenoptera: Braconidae) in response to the density of its host, the European corn borer (Ostrinia nubilalis Hubner [Lepidoptera: Crambidae]). When we manipulated host distribution and observed resulting patterns of wasp density and parasitism, we found that the smallest host aggregations had the lowest parasitism, but only when not associated with larger host aggregations. A subsequent field experiment confirmed that proximity to large host aggregations increased parasitism in small host aggregations. Theory indicates that such positive density-dependent parasitism should accelerate the evolution of toxin resistance in pest species, but our study suggests that close spatial proximity between insecticidal crops and refuges may help equalize M. grandii parasitism and that simple management techniques such as in-field refuges could potentially increase the complementarity of transgenic and biological control of European corn borer in this system. Further research is needed, however, before extrapolating the results of our small-scale study to field-level patterns and concluding that M. grandii will necessarily hasten resistance evolution in the European corn borer.

Effects of host density on furunculosis epidemics determined by the simple SIR model

To determine the transmission dynamics of furunculosis, Chinook salmon, Oncorhynchus tshawytscha held at various densities were challenged with a single infectious fish. Data from the in vivo experiments were compared to predicted values from a susceptible–infected–removed (SIR) model. Seven different densities were tested including 9.13, 4.56, 0.72, 0.36, 0.19, 0.06 and 0.03 fish/L and each produced different transmission coefficients ( β) of 0.01, 0.019, 0.0051, 0.0076, 0.0001, 0.0005 and 0.00, respectively. Furunculosis related mortality rates decreased as density of the host decreased. Mortality rates at the highest fish densities where disease specific mortality (DSM) was observed were 0.42, 0.44, 0.18 and 0.02 infected animals per day. The natural mortality rate also followed a similar pattern with 0.005, 0.005, 0.001, 0.005, 0.0001, 0.000 and 0.000 surviving animals per day. The results indicated that the transmission coefficient and the DSM of furunculosis was dependent on host density. It is possible that fish behavioral changes at the lower densities caused the reproductive rate of disease to be persistent even at very low densities. In summary, the simple SIR model suggested that low mortality does not always mean low prevalence of furunculosis in a given population. This is the first study evaluating host density as a risk factor for fish diseases using experimental furunculosis data.

Foraging Behavior of Field Populations of Diadegma Spp. (Hymenoptera: lchneumonidae):Testing for Density-Dependence at Two Spatial Scales

The foraging behavior of populations of Diadegma spp. (Hymenoptera: Ichneumonidae) attacking the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), was studied in the field. The effect of host density on percentage parasitism was investigated at two spatial scales: that of the individual plant, as well as a “cluster of plants” at low, medium and high densities. Using binoculars, parasitoid searching was observed on host plants at different host densities over an 8-h period. Behavior was compared to that predicted under the Patch Selection Theory. Percentage parasitism was independent of host density at both spatial scales. However, the behavioral studies showed parasitoid aggregation behavior at higher host densities. The population displayed an imperfect preference for higher host densities in the first 4 h of foraging. In the second 4-h period, no preference was observed.

Species Coexistence and Pathogens with Frequency‐Dependent Transmission

Pathogens that infect multiple hosts are commonly transmitted by vectors, and their transmission rate is often thought to depend on the proportion of hosts or vectors infected (i.e., frequency dependence). A model of a two-host, one-pathogen system with frequency-dependent transmission is used to investigate how sharing a pathogen with an alternative host influences pathogen-mediated extinction. The results show that if there is frequency-dependent transmission, a host can be rescued from pathogen-mediated extinction by the presence of a second host with which it shares a pathogen. The study provides an important conceptual counterexample to the idea that shared pathogens necessarily result in apparent competition by showing that shared pathogens can mediate apparent mutualism. We distinguish two types of dilution effect (pathogen reduction with increasing host diversity), each resulting from different underlying pathogen transmission processes and host density effects. These results have important consequences for understanding the role of pathogens in species interactions and in maintaining host species diversity.

Patch Time Allocation by the Parasitoid Diadegma semiclausum (Hymenoptera: Ichneumonidae). II. Effects of Host Density and Distribution

This study investigated the effects of host density and distribution on the patch-leaving behavior of Diadegma semiclausum (Hymenoptera: Ichneumonidae), a solitary endoparasitoid of larval Plutella xylostella (Lepidoptera: Plutellidae). Individual female wasps were released onto an experimental plant infested with host larvae at different densities and distributions, and were allowed to freely leave for an alternative host plant placed upwind of the experimental plant in a wind tunnel. The influence of host density and distribution, as well as within-patch foraging experience, on the parasitoid’s patch-leaving tendency was analyzed by means of the proportional hazards model. This study aimed to test the predictions of a number of patch-leaving models, including the Marginal Value Theorem, “rules of thumb”, and incremental or countdown mechanisms. The parasitoid’s patch-leaving tendency decreased with increased host density, more clustered host distribution, and unsuccessful host encounter as a result of host defense, but increased with successful oviposition. None of the simple rules of thumb such as fixed time, fixed number of hosts parasitized, or fixed giving-up time was employed by this parasitoid. The results agreed with the general predictions of the Marginal Value Theorem that patch residence time and numbers of ovipositions by the parasitoid increased with increasing host density. The decreasing influence of oviposition on the parasitoid’s patch-leaving tendency, regardless of host density or distribution, was consistent with the prediction of a countdown mechanism.

Parasite prevalence and richness in sympatric colobines: effects of host density

Factors that influence proximity and the number and duration of contacts among individuals can influence parasite transmission among hosts, and thus parasite prevalence and species richness are expected to increase with increasing host density. To examine this prediction we took advantage of a unique situation. Following the clearing of a forest fragment that supported red colobus (Piliocolobus tephrosceles) and black-and-white colobus (Colobus guereza), the animals moved into a neighboring fragment that we had been monitoring for a number of years and for which we had described the primate parasite community. After the animals immigrated into the fragment, the colobus populations more than doubled and colobus density became almost twice that found in Kibale National Park, Uganda. Despite this increase in host density, the richness of the parasite community did not increase. However, in both colobus species the prevalence of Trichuris sp., the only commonly occurring gastrointestinal parasite, increased. Over the next 5 years the prevalence and intensity of infection of Trichuris sp. in red colobus declined and their population numbers slowly increased. In contrast, the prevalence and intensity of infection of Trichuris sp. increased in black-and-white colobus and remained high following the immigration, and their population size declined. While Trichuris sp. infections are typically asymptomatic, we consider it a possibility that they contributed to the decline of the black-and-white colobus, and that the red colobus may be serving as a reservoir for Trichuris, thereby increasing the infection risk for black-and-white colobus.

Is the prevalence of Taenia taeniaeformis in Microtus arvalis dependent on population density?

Populations of common voles Microtus arvalis were studied as hosts of the tapeworm Taenia taeniaeformis during a 14-yr survey. They were monitored in spring, summer, and autumn in a pastoral ecosystem in eastern France. A total of 7,574 voles were sampled during 2 multiannual population fluctuations. A third fluctuation was sampled during the increase phase only. Overall prevalence was lowest in summer (0.6%), increased by 3 times in autumn (1.5%) and a further 5 times in spring (7.8%). Analysis of prevalence, based on 7,384 voles, by multiple logistic regression revealed that extrinsic factors such as season and intrinsic factors such as host age and host density have a combined effect. In the longer term, host age and host density were positively associated with prevalence in summer. Host density was strongly associated with autumn prevalence. There was no association between the fluctuation phase and prevalence. The study shows that a long timescale (here a multiannual survey) is necessary to demonstrate the positive effect of host density on the prevalence of this indirectly transmitted parasite. The demonstration of this relationship depends on the rodents being intermediate rather than definitive hosts.

Social Organization and Parasite Risk in Mammals: Integrating Theory and Empirical Studies

▪ Abstract Mammals are exposed to a diverse array of parasites and infectious diseases, many of which affect host survival and reproduction. Species that live in dense populations, large social groups, or with promiscuous mating systems may be especially vulnerable to infectious diseases owing to the close proximity and higher contact rates among individuals. We review the effects of host density and social contacts on parasite spread and the importance of promiscuity and mating structure for the spread and evolution of sexually transmitted diseases. Host social organization and mating system should influence not only parasite diversity and prevalence but may also determine the fitness advantages of different transmission strategies to parasites. Because host behavior and immune defenses may have evolved to reduce the spread and pathogenicity of infectious diseases, we also consider selective pressures that parasites may exert on host social and mating behavior and the evolutionary responses of hosts at both the immunological and behavioral levels. In examining these issues, we relate modeling results to observations from wild populations, highlighting the similarities and differences among theoretical and empirical approaches. Finally, the epidemiological consequences of host sociality are very relevant to the practical issues of conserving mammalian biodiversity and understanding the interactions between extinction risk and infectious diseases.

Effectiveness of different species/strains of entomopathogenic nematodes for control of western flower thrips (Frankliniella occidentalis) at various concentrations, host densities, and temperatures

Abstract Entomopathogenic nematode (EPN) species/strains (Rhabditida: Steinernematidae and Heterorhabditidae) were tested at a concentration of 200 infective juveniles (IJs) per cm 2 against mixed soil-dwelling life stages (i.e., second-instar larvae, prepupae, and pupae) of the western flower thrips (WFT) Frankliniella occidentalis (Thysanoptera: Thripidae), in a plant growing substrate under laboratory conditions. The different EPN species/strains resulted in WFT corrected mortality (CM) values ranging between 2.6 and 60%. In general, Heterorhabditis spp. were more virulent than Steinernema spp. Increasing concentrations of selected EPN species/strains led to an increase in thrips CM, with significant differences among species/strains. Heterorhabditis indica LN2 caused higher CM than the other strains, with 30 and 90% as highest CM at the lowest and highest concentration tested, i.e., 100 and 1000 IJs cm −2 , respectively. In an experiment with different WFT densities, CM caused by Steinernema bicornutum negatively correlated with host density. However, with H. indica LN2, CM increased, though not always significantly, with increasing host densities. Generally, the effect of host density on efficacy of EPNs depended on concentrations. When tested over a range of temperatures, 25 °C was the optimal temperature for both H. indica LN2 and S. bicornutum . H. indica LN2, tropical in origin, and S. bicornutum , isolated from a more temperate environment, performed better at higher and lower temperatures, respectively.

Effect of host density on egg dispersion and the sex ratio of progeny of Bracon hebetor (Hymenoptera: Braconidae)

Egg dispersion and the sex ratio of progeny were studied in relation to the host density of the parasitoid, Bracon hebetor Say, infesting larvae of Plodia interpunctella (Hubner). Females appeared to allocate eggs in relation to host density to avoid laying more eggs than could complete development on a host. The dispersion pattern of the parasitoid ovipositions among hosts was influenced by host density. Multiple visitations and ovipositions by females on hosts caused a highly aggregated pattern at low-host densities. Hatch rate of eggs decreased as the number of eggs on a host increased. Females seemed to regulate progeny sex ratio (male/total) based on the number of eggs on the hosts and the clutch size of the hosts they encountered. However, the overall progeny sex ratio remained at approximately 0.5 regardless of host density, probably because the allocation of eggs was related to host density.

The Effect of Host Density on Ectoparasite Distribution: An Example of a Rodent Parasitized by Fleas

The Effect of Host Density on Ectoparasite Distribution: An Example of a Rodent Parasitized by Fleas

Development of Cotesia orobenae Forbes (Hymenoptera: Braconidae) in its Host, Evergestis rimosalis (Guenée) (Lepidoptera: Pyralidae)

Temperature and host density effects on the development of Cotesia orobenae Forbes (Hymenoptera: Braconidae), and its parasitism of the cross-striped cabbageworm, Evergestis rimosalis (Guenée) (Lepidoptera: Pyralidae), were investigated. Time for adult parasitoid emergence from cocoon masses of C. orobenae maintained at 15, 20, 25, 30 and 35°C and a photoperiod of 15:9 (L: D) was inversely related to temperature. Parasitoid emergence exceeded 86% for all temperatures except that at 35°C. Development of C. orobenae on early-instar hosts was best at 20 and 25°C. Although the shortest developmental time was observed at 35°C, survival of both parasitoid and host was low at this temperature. At 15°C, the parasitoid was not very active, and developmental time was the longest among the temperatures tested. Percent of host larvae parasitized was significantly higher at 20 and 25°C than the other temperatures tested. Females were short-lived and oviposited an average of 118 and 136 eggs at 20 and 25°C, respectively. Females died after 24 h of being placed in the oviposition chamber at 30 and 35°C. At 15°C they lived longer, but did not parasitize host larvae provided. Virgin females produced only male progeny. Mated females produced slightly more female offspring than males. Sex ratios of progeny from females paired with single or multiple males were not different. Percentage parasitization of host larvae between treatments differed for the first day, but not the second day. Fecundity of C. orobenae exposed to five host densities (5, 10, 15, 20, and 25 per parasitoid) showed significant differences in percent parasitization among the treatments. Percentage parasitization was highest with five and 10 hosts per parasitoid. The highest number of parasitoid cocoons recovered per cocoon mass was obtained with 10 hosts. The data indicate that optimal temperature for C. orobenae oviposition, development, and parasitism of its host is between 20 to 25°C.

Effect of host density, temperature, and photoperiod on the fitness of Anagyrus kamali, a parasitoid of the hibiscus mealybug Maconellicoccus hirsutus

AbstractThe peformance of the parasitoid Anagyrus kamali Moursi [Hymenoptera: Encyrtidae], as a function of host density, temperature, and photoperiod was investigated with the objective to optimize a mass‐rearing system in the context of a biological control program. The number of hosts parasitized at densities varying from 2–100 hibiscus mealybug (HMB), Maconellicoccus hirsutus Green [Homoptera: Pseudococcidae], corresponded to a type II‐III functional response in fixed‐time conditions and a type III in variable‐time conditions. Twenty‐six percent of the oviposited eggs led to progeny emergence with a sex ratio of 0.49±0.102 (M/F), regardless of host density. Fecundity and oviposition period under six abiotic combinations (i.e., two temperatures (26±2 °C and 32±2 °C) and three photoperiods (L0:D24, L12:D12, L24:D0)) were measured. Lifetime fecundity and reproductive life were significantly affected by temperature and photoperiod conditions. Optimum female parasitoid lifetime fecundity was attained at 26±2 °C, L0:D24 with an average of 116.1±17.43 eggs. At 32±2 °C, L24:D0 and L12:D12, an average of 79.4±34.57 and 85.8±35.81 eggs were laid, respectively. Reproductive longevity was maximal at 26±2 °C, L0:D24 with 12±4.85 days of oviposition. Because the parasite A. kamali can be reared optimally without light, this may save tremendous energy costs.

Sex Ratio of Campoletis Chlorideae Uchida in Response to Helicoverpa Armigera (Hübner) Density

Experiments were conducted to elucidate the effect of host density on sex ratio of parasitoid offspring, using Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) and its endoparasitoid Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae). The sex ratio of Campoletis chlorideae was female-biased showing a linear decrease with increasing parasitoid density at fixed host densities. However, with more hosts available, the number of parasitoids emerging increased, stabilising after a density of 32 hosts per parasitoid. To obtain a female-biased sex ratio, a low density of parasitoids should be released at a recommended site.

Host frequency and density effects on powdery mildew and yield in mixtures of barley cultivars

The effects of frequency and density of susceptible plants on barley powdery mildew epidemics were studied in a combined set of addition and replacement series of field trials. In the addition series, plant densities in pure stands of three cultivars, Rambo, Rodos and Grosso (susceptible, moderately resistant and immune, respectively) were varied six‐fold. In the replacement series, the three possible two‐way mixtures were analysed at different frequencies but at a density corresponding to the maximum pure stand density. Disease and yield were assessed on a per‐plant basis. In the pure stands, tillering reduced the range of densities from six‐fold to between three‐ and four‐fold, while in the mixtures, frequencies changed only slightly over time, indicating that competitive interactions among the cultivars were roughly equal. Yield per plant decreased logarithmically with increasing density as expected. However, yield per seed head was not correlated with the final number of heads per plot, indicating low competition among heads even at the highest density. Disease in susceptible pure stands increased strongly with decreasing density in 1994 and to a lesser degree in 1995. These differences could have been caused by differences in plant nutritional status and consequent epidemiological effects. Disease reduction on the susceptible cultivars in mixtures varied between 33% and 71% among years. Depending on the length and strength of the epidemic, the effects of host density and frequency on disease severity varied substantially among years.