Populations Of Host Species Research Articles

A single natural nucleotide mutation alters bacterial pathogen host tropism.

The capacity of microbial pathogens to alter their host-tropism leading to epidemics in distinct host-species populations is a global public and veterinary health concern. In order to investigate the molecular basis of a bacterial host-switching event in a tractable host-species, we traced the evolutionary trajectory of the common rabbit clone of Staphylococcus aureus. We report that it evolved through a likely human-to-rabbit host jump over 40 years ago, and that only a single natural nucleotide mutation was required and sufficient to convert a human-specific S. aureus strain into one which could infect rabbits. Related mutations were identified at the same locus in other rabbit strains of distinct clonal origin, consistent with convergent evolution. This first report of a single mutation that was sufficient to alter the host-tropism of a micro-organism during its evolution highlights the capacity of some pathogens to readily expand into novel host-species populations.

Hantaviruses in Finnish soricomorphs: Evidence for two distinct hantaviruses carried by Sorex araneus suggesting ancient host-switch

Hantaviruses are emerging viruses carried by rodents, soricomorphs (shrews and moles) and bats. In Finland, Puumala virus (PUUV) was for years the only hantavirus detected. In 2009, however, Seewis virus (SWSV) was reported from archival common shrew (Sorex araneus) samples collected in 1982 in Finland. To elucidate the diversity of hantaviruses in soricomorphs in Finland, 180 individuals were screened, representing seven species captured from 2001 to 2012: hantavirus RNA was screened using RT-PCR, and hantaviral antigen using immunoblotting with polyclonal antibodies raised against truncated SWSV nucleocapsid protein. The overall hantavirus RNA prevalence was 14% (26/180), antigen could be demonstrated in 9 of 20 SWSV RT-PCR positive common shrews. Genetic analyses revealed that four soricomorph-borne hantaviruses circulate in Finland, including Boginia virus (BOGV) in water shrew (Neomys fodiens) and Asikkala virus (ASIV) in pygmy shrew (Sorex minutus). Interestingly, on two study sites, common shrews harbored strains of two different hantaviruses: Seewis virus and a new distinct, genetically distant (identity 57% at amino acid level) virus (Altai-like virus) which clusters together with viruses in the basal phylogroup I of hantaviruses with 62–67% identity at amino acid level. This is the first evidence of coexistence of two clearly distinct hantavirus species circulating simultaneously in one host species population. The findings suggest an ancient host-switching event from a yet unknown host to S. araneus. In addition, phylogenetic analyses of partial S and M segment sequences showed that SWSV in Finland represents a unique genotype in Europe.

New data on gastrointestinal helminths in shags (Phalacrocorax verrucosus) at Kerguelen Archipelago

To date, the knowledge of the helminth communities of Antarctic birds is scarce or fragmented. Knowledge about diseases and parasites is crucial for understanding and managing ecosystems, particularly in isolated areas where host species are more sensitive to new diseases or parasite infections. It has been showed that variations in rate of parasitism may occur between populations of host species. Two major non-exclusive hypotheses have been proposed to explain such variability: exposure to parasitism and, perhaps more important, life history strategies of hosts. We studied the helminth community of the Kerguelen Shag Phalacrocorax verrucosus, an endemic seabird species of the Kerguelen Archipelago. We provide new data on the helminths infecting this species from partial or complete digestive tracts of two birds. Two nematodes (Contracaecum rudolphii s.l. and Ingliseria cirrohamata) were found free or attached to the wall of the proventriculus of birds, while the acanthocephalan Corynosoma sp. and the cestode species Tetrabothrius sp. occurred in the intestine of the shags. The genus Tetrabothrius is reported for the first time in Kerguelen Shags and in this area. The analysis of stomach contents from 41 live Kerguelen Shag individuals revealed infection by Contracaecum nematodes. The proportion of infected birds differed between colonies, possibly in relation to differential exposure to infected fish hosts.

Emergency rabies control in a community of two high-density hosts

BackgroundRabies is a fatal viral disease that potentially can affect all mammals. Terrestrial rabies is not present in the United Kingdom and has been eliminated from Western Europe. Nevertheless the possibility remains that rabies could be introduced to England, where it would find two potentially suitable hosts, red foxes and badgers. With the aim to analyse the spread and emergency control of rabies in this two species host community, a simulation model was constructed. Different control strategies involving anti-rabies vaccination and population culling were developed, considering control application rates, spatial extent and timing. These strategies were evaluated for efficacy and feasibility to control rabies in hypothetical rural areas in the South of England immediately after a disease outbreak.ResultsThe model confirmed that both fox and badger populations, separately, were competent hosts for the spread of rabies. Realistic vaccination levels were not sufficient to control rabies in high-density badger populations. The combined species community was a very strong rabies host. However, disease spread within species appeared to be more important than cross-species infection. Thus, the drivers of epidemiology depend on the potential of separate host species to sustain the disease. To control a rabies outbreak in the two species, both species had to be targeted. Realistic and robust control strategies involved vaccination of foxes and badgers, but also required badger culling. Although fox and badger populations in the UK are exceptionally dense, an outbreak of rabies can be controlled with a higher than 90% chance, if control response is quick and follows a strict regime. This requires surveillance and forceful and repeated control campaigns. In contrast, an uncontrolled rabies outbreak in the South of England would quickly develop into a strong epizootic involving tens of thousands of rabid foxes and badgers.ConclusionsIf populations of both host species are sufficiently large, epizootics are driven by within-species transmission, while cross-species-infection appears to be of minor importance. Thus, the disease control strategy has to target both host populations.

Polymorphic Integrations of an Endogenous Gammaretrovirus in the Mule Deer Genome

Endogenous retroviruses constitute a significant genomic fraction in all mammalian species. Typically they are evolutionarily old and fixed in the host species population. Here we report on a novel endogenous gammaretrovirus (CrERVγ; for cervid endogenous gammaretrovirus) in the mule deer (Odocoileus hemionus) that is insertionally polymorphic among individuals from the same geographical location, suggesting that it has a more recent evolutionary origin. Using PCR-based methods, we identified seven CrERVγ proviruses and demonstrated that they show various levels of insertional polymorphism in mule deer individuals. One CrERVγ provirus was detected in all mule deer sampled but was absent from white-tailed deer, indicating that this virus originally integrated after the split of the two species, which occurred approximately one million years ago. There are, on average, 100 CrERVγ copies in the mule deer genome based on quantitative PCR analysis. A CrERVγ provirus was sequenced and contained intact open reading frames (ORFs) for three virus genes. Transcripts were identified covering the entire provirus. CrERVγ forms a distinct branch of the gammaretrovirus phylogeny, with the closest relatives of CrERVγ being endogenous gammaretroviruses from sheep and pig. We demonstrated that white-tailed deer (Odocoileus virginianus) and elk (Cervus canadensis) DNA contain proviruses that are closely related to mule deer CrERVγ in a conserved region of pol; more distantly related sequences can be identified in the genome of another member of the Cervidae, the muntjac (Muntiacus muntjak). The discovery of a novel transcriptionally active and insertionally polymorphic retrovirus in mammals could provide a useful model system to study the dynamic interaction between the host genome and an invading retrovirus.

Cytoplasmic incompatability in Drosophila melanogaster due to different Wolbachia genotypes

Wolbachia is an endosymbiont prevalent among different arthropoda and in some Nematoda species. The bacteria ads across the populations of host species via induction of reproductive abnormalities: cytoplasmic incompatability, parthenogenesis, feminization, and male killing. we estimated the level of cytoplasmic incompatability in Drosophila melanogaster caused by three most prevalent Wolbachia genotypes, namely wmel, wmelCS и wmelCS2. wmel and wmelCS genotypes were shown to cause mild cytoplasmic incompatibility ( 10 %), while wmelCS Wolbachia were unable to do so. Possible mechanisms of sustainability of Wolbachia in the populations of Drosophila melanogaster are discussed.table border="0" cellpadding="3" cellspacing="0" tbody /tbody/table

Sympatric speciation and radiative evolution of socially parasitic ants - Heretic hypotheses and their factual background

According to current hypotheses the main types of social parasitism among ants, namely slavery, temporary parasitism, and inquilinism, arose from such features as predation on other ants, or territorial behavior, both presumed precursors of slavemaking, and polygyny, a presumed precursor of temporary parasitism and inquilinism. The latter is believed also to represent a final instar in several evolutionary pathways leading from slavery, temporary parasitism, and xenobiosis to this permanently parasitic, workerless condition. Speciation, the origin of parasitic species from their usually closely related host species, is suggested to occur due to temporary geographic isolation and subsequent transition of one of the newly formed daughter species to parasitism in the nests of the other. Evidence is presented suggesting that the main types of social parasitism originated independently of each other. 15 ant genera are parasitized exclusively by inquilines, Eve other genera exclusively by temporary parasites. Only four groups of non-parasitic ant species (Formica, Tet-ramorium, Leptothorax subgenera Leptothorax and Myrafant) have parasites of several types each. Within these roups, however, there is little evidence of evolutionary transitions from one type to another. The few exceptions, mainly workerless species of the genera Epimyrma and Chalepoxenus, represent parasites which clearly derive from slave-making congeners, but differ from ordinary inquilines in that they eliminate the host colony queens like their actively dulotic ancestors. The new hypothesis suggests that all forms of interspecific true social parasitism (excluding xenobiosis) orginated from a common “preparasitic” stage, a subpopulation of reproductives in polygynous colonies and species, with diverging sexual behavior (near-nest mating vs. swarming) and caste ratios (production of more sexuals vs. workers). Arguments for sympatric speciation are compiled. Various features of the ancestral, and then host species (colony sizes, population density and structure, transition from polygyny to monoyny, etc.), and of the “preparasite” (production of few, or no workers, etc.) may shape the developing parasite to become a slave-maker, inquiline, or temporary parasite. These features usually leave open only one, or in a few genera, several options. The different types of parasitism within one host species group thus may have developed in a radiative manner from the common, preparasitic stage, which explains that independent colony foundation is a common feature of all true social parasites among ants.

Helminth species diversity of mammals: parasite species richness is a host species attribute

Studies investigating parasite diversity have shown substantial geographical variation in parasite species richness. Most of these studies have, however, adopted a local scale approach, which may have masked more general patterns. Recent studies have shown that ectoparasite species richness in mammals seems highly repeatable among populations of the same mammal host species at a regional scale. In light of these new studies we have reinvestigated the case of parasitic helminths by using a large data set of parasites from mammal populations in 3 continents. We collected homogeneous data and demonstrated that helminth species richness is highly repeatable in mammals at a regional scale. Our results highlight the strong influence of host identity in parasite species richness and call for future research linking helminth species found in a given host to its ecology, immune defences and potential energetic trade-offs.

Host range expansion of an introduced insect pest through multiple colonizations of specialized clones

Asexuality confers demographic advantages to invasive taxa, but generally limits adaptive potential for colonizing of new habitats. Therefore, pre-existing adaptations and habitat tolerance are essential in the success of asexual invaders. We investigated these key factors of invasiveness by assessing reproductive modes and host-plant adaptations in the pea aphid, Acyrthosiphon pisum, a pest recently introduced into Chile. The pea aphid encompasses lineages differing in their reproductive mode, ranging from obligatory cyclical parthenogenesis to fully asexual reproduction. This species also shows variation in host use, with distinct biotypes specialized on different species of legumes as well as more polyphagous populations. In central Chile, microsatellite genotyping of pea aphids sampled on five crops and wild legumes revealed three main clonal genotypes, which showed striking associations with particular host plants rather than sampling locations. Phenotypic analyses confirmed their strong host specialization and demonstrated parthenogenesis as their sole reproductive mode. The genetic relatedness of these clonal genotypes with corresponding host-specialized populations from the Old World indicated that each clone descended from a particular Eurasian biotype, which involved at least three successful introduction events followed by spread on different crops. This study illustrates that multiple introductions of highly specialized clones, rather than local evolution in resource use and/or selection of generalist genotypes, can explain the demographic success of a strictly asexual invader.

Searching for general patterns in parasite ecology: host identity versus environmental influence on gamasid mite assemblages in small mammals

The abundance and diversity of parasites vary among different populations of host species. In some host-parasite associations, much of the variation seems to depend on the identity of the host species, whereas in other cases it is better explained by local environmental conditions. The few parasite taxa investigated to date make it difficult to discern any general pattern governing large-scale variation in abundance or diversity. Here, we test whether the abundance and diversity of gamasid mites parasitic on small mammals across different regions of the Palaearctic are determined mainly by host identity or by parameters of the abiotic environment. Using data from 42 host species from 26 distinct regions, we found that mite abundances on different populations of the same host species were more similar to each other than expected by chance, and varied significantly among host species, with half of the variance among samples explained by differences between host species. A similar but less pronounced pattern was observed for mite diversity, measured both as species richness and as the taxonomic distinctness of mite species within an assemblage. Strong environmental effects were also observed, with local temperature and precipitation correlating with mite abundance and species richness, respectively, across populations of the same host species, for many of the host species examined. These results are compared to those obtained for other groups of parasites, notably fleas, and discussed in light of attempts to find general rules governing the geographical variation in the abundance and diversity of parasite assemblages.

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.

Population structure and the co‐evolution between social parasites and their hosts

Co-evolutionary trajectories of host-parasite interactions are strongly affected by the antagonists' evolutionary potential, which in turn depends on population sizes as well as levels of recombination, mutation, and gene flow. Under similar selection pressures, the opponent with the higher evolutionary rate is expected to lead the co-evolutionary arms race and to develop local adaptations. Here, we use mitochondrial DNA sequence data and microsatellite markers to assess the amount of genetic variability and levels of gene flow in two host-parasite systems, each consisting of an ant social parasite--the European slavemaker Harpagoxenus sublaevis and the North American slavemaker Protomognathus americanus--and its two main host species. Our population genetic analyses revealed limited gene flow between individual populations of both host and parasite species, allowing for a geographic mosaic of co-evolution. In a between-system comparison, we found less genetic variability and more pronounced structure in Europe, where previous behavioural studies demonstrated strong local adaptation. Within the European host-parasite system, the larger host species Leptothorax acervorum exhibited higher levels of both genetic variability and gene flow, and previous field data showed that it is less affected by the social parasite H. sublaevis than the smaller host Leptothorax muscorum, which has genetically depleted and isolated populations. In North America, the parasite P. americanus showed higher levels of gene flow between sites, but overall less genetic diversity than its hyper-variable main host species, Temnothorax longispinosus. Interestingly, recent ecological and chemical studies demonstrated adaptation of P. americanus to local host populations, indicating the importance of migration in co-evolutionary interactions.

Hypotheses for the richness of gall insects: a review considering the intraespecific, interespecific and community levels

In the literature there are several hypotheses that try to explain the patterns of global, regional and local richness of insects associated to plants. These hypotheses can be recognized in different ecological levels, explaining variations in the number of insects associated to individuals or populations of same host species, to different host species, and to different communities. However, the patterns of richness found in each of these levels are result of distinct ecological processes, thus each hypothesis has particularities related to its application depending on the processes considered and on the characteristics of the insect taxa. The objective of this review is to expose the hypotheses that have been used to explain the richness patterns of insects associated to plants, identifying the validity and the use status of each hypothesis in the intraespecific, interespecific, and community levels to gall insect – plant systems. In this review are explicitly enunciated 13 hypotheses considering the levels of ecological organization. This way of view represents a progress because it makes possible an objective analysis of what has been made in the field and a clear visualization of the gaps in the literature. Few hypotheses were enough tested in all levels, mostly in the intraespecific and interespecific levels. Thus it is evident that it is early to reach a general synthesis that explains the patterns of gall insects’ richness in all levels. This review expects, when enunciating the hypotheses in the different levels, to collaborate to the development of the field pointing directions to future studies. Key words: gall insect, richness patters, hypothesis.

Selecting the drought stressed: effects of plant stress on intraspecific and within‐plant herbivory patterns of the leaf‐cutting ant Atta colombica

Summary 1Leaf-cutting ants (LCA) are highly polyphagous but at the same time highly selective dominant herbivores of neotropical rainforests. While a whole range of leaf properties has been identified to influence interspecific host choice, the mechanisms underlying intraspecific and intra-individual herbivory patterns remain obscure. Here, we evaluate the plant-stress hypothesis by analysing whether and how drought stress of food plants positively affects LCA food plant selection. 2In bioassays with whole plants of Piper marginatum, Atta colombica workers harvested more than twice the leaf area of stressed than of vigorous control plants. Within individual plants, the attractiveness of a given leaf increased with its stress level. 3Drought stress induced an accumulation of proline and non-structural carbohydrates in the plant tissue by a factor of 3·5 and 2, respectively, accompanied by a decrease in the leaf water content of about 35%. Moreover, samples taken from preferred leaves within a plant contained more osmolytes than those representing the whole plant. 4Ants were shown to detect and prefer these osmolytes in bioassays conducted with leaf discs that had been experimentally enriched with proline and/or sucrose. We therefore propose a mechanism that links the preference of LCA to drought stress via the osmolyte concentrations within the leaves. 5Our results support the plant-stress hypothesis, confirming that stressed plants and plant parts are more attractive to LCA. This may account for intraspecific and intra-individual herbivory patterns of LCA, thus influencing populations of host species through the discrimination of drought-sensitive individuals.

Disease dynamics, host specificity and pathogen persistence in isolated host populations

In situations where populations of related host species grow sympatrically but isolated from other populations of either host, cross‐species disease transmission can have large influences on disease dynamic and patterns of pathogen persistence. This study examined host–pathogen dynamics and the degree of host‐specificity in the anther‐smut fungus Microbotryum violaceum at a site where two of its host species, Lychnis alpina and Silene rupestris, grew in adjacent patches which were located at large distances from the nearest intra‐specific host and pathogen populations. In a field‐study carried out over a four‐year period in the sympatric populations, the two host species exhibited broadly parallel patterns of mortality and recruitment across years, although higher rates of mortality and infection, and larger fluctuations in numbers of plants, were seen in the S. rupestris population. However, when the frequency of disease in the populations was taken into account, disease transmission rates were relatively similar in the two hosts. In a reciprocal cross‐inoculation experiment, there were no differences in infectivity between the two fungal populations on progeny of the two hosts from the sympatric populations indicating that cross‐species disease transmission is possible. Patterns of infectivity on plants from the sympatric populations and allopatric populations of each host indicated the occurrence of a host shift in M. violaceum from L. alpina onto S. rupestris at the sympatric site. Results from a population model, parameterised with the demographic data from the two hosts, demonstrated that some cross‐species disease transmission could increase the frequency of disease and lower the threshold level of pathogen persistence in populations. The population model also suggests that high levels of juvenile infection, as observed in both hosts, may increase the probability of pathogen persistence in local populations, especially in the more short‐lived S. rupestris.

Transmission ecology of Echinococcus multilocularis: What are the ranges of parasite stability among various host communities in China?

A striking feature of the transmission ecology of Echinococcus multilocularis in China is the diversity of hosts that contribute to the parasite cycle. Considering the population dynamics of key reservoir intermediate hosts and the ratio of their preferred habitat in a landscape (ROMPA) is essential to understanding transmission, but the numerous communities in which the parasite cycles and the extent of those communities is currently far from being fully documented. On the Tibetan plateau grassland management influences intermediate host species populations and the sheer size of the area could be a major contributing factor to sustaining transmission in the region. In Southern Gansu and Southern Ningxia the processes of deforestation have temporarily provided optimal habitat for key reservoir intermediate hosts of E. multilocularis, resulting in high transmission and human disease, however currently the parasite may be extinct locally. Faced with this pattern of potentially transient transmission in a diversity of communities the question of the dispersal potential of the parasite arises. The reforestation program currently active across much of Western China has the potential to give rise to a massive increase in habitat favourable to suitable intermediate hosts and emergence or re-emergence of the zoonosis alveolar echinococcosis in many areas. This potential epidemic could be mitigated by both natural and human induced parasite dispersal mechanisms including fox migration and the sale of infected dogs originating from stable endemic foci on the Tibetan plateau. However, currently the degree of genetic exchange between discrete transmission foci is unknown and it is expected that genetic techniques could provide crucial information regarding this important question.

Abundance and distribution of fleas on desert rodents: linking Taylor's power law to ecological specialization and epidemiology

We investigated variation in the abundance-prevalence relationships of fleas among 17 different flea-host associations as well as among different species of hosts and fleas in the Negev desert. We explored variation in the value of exponent of Taylor's power relationship with changes in flea community size and flea specialization (host specificity and seasonal pattern of activity). We tested if a simple epidemiological model can reproduce the pattern of the abundance-prevalence relationship. We confirmed aggregated distribution of fleas within a population of host species as well as across a whole host community and the existence of a positive relationship between local flea abundance and their prevalence. Prevalence, mean abundance and variance of abundance were significantly higher in host specific than host opportunistic fleas. When ecological specialization was considered, based on a seasonal pattern of activity, these parameters were higher in year-round-active than seasonal fleas. The degree of flea specialization and flea community richness affected the pattern of the relationship between mean abundance and its variance. Power law slopes decreased with increasing richness of flea community. A simple epidemiological model based on mean flea abundance and degree of aggregation, corrected for host sample size, can predict the observed pattern of prevalence. In some cases, observed flea prevalence was higher than that predicted from the epidemiological model. The discrepancy of the observed prevalence from that predicted by the model can be explained by either a relatively low negative effect of flea parasitism on a host (at least, in terms of pathology) or strong resistance of a host to flea parasitism or both.

Testing the Generality of a Trophic-cascade Model for Plague

Climate may affect the dynamics of infectious diseases by shifting pathogen, vector, or host species abundance, population dynamics, or community interactions. Black-tailed prairie dogs (Cynomysludovicianus) are highly susceptible to plague, yet little is known about factors that influence the dynamics of plague epizootics in prairie dogs. We investigated temporal patterns of plague occurrence in black-tailed prairie dogs to assess the generality of links between climate and plague occurrence found in previous analyses of human plague cases. We examined long-term data on climate and plague occurrence in prairie dog colonies within two study areas. Multiple regression analyses revealed that plague occurrence in prairie dogs was not associated with climatic variables in our Colorado study area. In contrast, plague occurrence was strongly associated with climatic variables in our Montana study area. The models with most support included a positive association with precipitation in April–July of the previous year, in addition to a positive association with the number of “warm” days and a negative association with the number of “hot” days in the same year as reported plague events. We conclude that the timing and magnitude of precipitation and temperature may affect plague occurrence in some geographic areas. The best climatic predictors of plague occurrence in prairie dogs within our Montana study area are quite similar to the best climatic predictors of human plague cases in the southwestern United States. This correspondence across regions and species suggests support for a (temperature-modulated) trophic-cascade model for plague, including climatic effects on rodent abundance, flea abundance, and pathogen transmission, at least in regions that experience strong climatic signals.

Genetic variation can promote system persistence in an experimental host‐parasitoid system

AbstractWe tested experimentally the effects of genetic variation in host population on the host‐parasitoid system persistence. The experimental systems consisted of one parasitoid wasp species (Heterospilus prosopidis), one bean weevil species (Callosobruchus chinensis), and one bean species, of which only the host species (bean weevil) was genetically manipulated. As control treatments with low genetic heterogeneity in the host population, we used two bean weevil strains (Kyoto and Niigata strains) which have several contrasting ecological traits. For the high genetic heterogeneity treatment, hybrid bean weevils which were generated by crossbreeding the strains Kyoto and Niigata were used. In the multiple‐generation experiments, all three treatments had different patterns of extinction. The control treatment with the Niigata strain was very prone to system extinction, whereas the treatment with hybrids showed coexistence of constituent species in almost all replicates and also showed stabilized population dynamics. The other control treatment, using the Kyoto strain, showed intermediate proneness. To interpret the results of multiple‐generation experiments, we conducted several short‐term experiments. The different persistence patterns between the two control treatments were explained by the shapes of the host‐finding abilities of wasps and the growth rate of the bean weevils. The mean values of many ecological traits of hybrid lines were not different from those of the Kyoto strain, but their variability increased. These outcomes corresponded well to the prediction of models by Doebeli [J Theor Biol (1997) 188:109–120]. We discuss the mechanisms in which the variability in host species population was effective for the prolonged system persistence.

Dispersal and persistence of mass released Trichogramma brassicae (Hymenoptera: Trichogrammatidae) in non-target habitats

Field studies were carried out to evaluate whether inundative releases of Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae) against the European corn borer, Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), can have negative effects on the indigenous arthropod fauna in adjacent non-target habitats. Emigration of released T. brassicae from maize fields into non-target habitats was monitored, and its persistence in non-target habitats was assessed throughout the season. For this purpose, sticky traps were installed on metal sticks along transects from the inside of T. brassicae treated maize fields into sown wildflower strips and a natural common reed stand. In addition, cards with host eggs were used to monitor Trichogramma within potential overwintering sites. Although the highest numbers of released parasitoids were trapped within maize, a considerable amount of T. brassicae moved into non-target habitats. We found a significant transient increase of T. brassicae in both wildflower strips and reeds and no consistent decrease in numbers of trapped wasps up to 40 m from the maize field borders, when monitored directly following mass releases. Indigenous Trichogramma species were present both in the wildflower strip and reeds, and T. brassicae represented only a minor part of the overall Trichogramma population therein, except for a few days after release. However, a few T. brassicae persisted in non-target habitats throughout the season and were still present prior to releases in the subsequent year. These low numbers are not expected to seriously affect populations of native Trichogramma or non-target host species.