Effects of Density, Age and Host Cues on the Dispersal of Heterorhabditis megidis

Agar plate assays were used to assess the effect of density, incubation time and age of nematodes and the presence of insect hosts on the dispersal of infective juveniles (IJs) of Heterorhabditis megidis (strain NLH-E87.3). IJs dispersed faster and further at high densities than at low densities. Dispersal was also influenced by the age of the IJs. Individuals stored for a period of 1.5 and 4.5 weeks showed to be more active than those stored for 2.5 and 3.5 weeks. The presence of a host insect enhanced the dispersion of nematodes. The increasing in the incubation period showed that IJs responded positively to host cues from Galleria mellonella but poorly to cues from Otiorhynchus sulcatus larvae.     

Dynamics of carbon dioxide release from insects infected with entomopathogenic nematodes

The quality of an insect as a host to an entomopathogenic nematode infective juvenile depends in part on whether or not the insect is already infected and on the stage of that infection. Previous research has shown that nematode response to hosts can change after infection and that, for uninfected hosts, CO(2) can be an important cue used by infective stage juveniles during attraction. We hypothesized that CO(2) production from an insect changes after it is infected, and that these changes could influence nematode infection decisions. Changes in CO(2) released by two insect species (Galleria mellonella and Tenebrio molitor) after infection by one of four nematode species (Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri, or Steinernema riobrave) were measured. Measurements were taken every 2h from time of initial exposure to nematodes up to 224 h after infection. Dead (freeze-killed) and live uninfected insects were used as controls. Infected G. mellonella showed two distinct peaks of CO(2) production: one between 20 and 30 h and the other between 70 and 115 h after exposure to the nematodes. Peaks were up to two times higher than levels produced by uninfected insects. Infected T. molitor showed only one peak between 25 and 50h. We found differences in peak height and timing among nematode and insect species combinations. The influence of these changes in CO(2) production on IJ attraction and infection behavior remains to be determined.     

Infectivity, distribution, and persistence of the entomopathogenic nematode Steinernema carpocapsae all strain (Rhabditida: Steinernematidae) applied by sprinklers or boom sprayer to dry-pick cranberries.

We evaluated infectivity, distribution, and persistence of commercially produced Steinernema carpocapsae (Weiser) All strain applied through solid set sprinkler irrigation or boom sprayer to 2 dry-pick cranberry farms on peat soil in British Columbia in 1993. Most infectivity assays used Galleria mellonella (L.) larvae. When possible, larvae of the target pest, Otiorynchus sulcatas (F.) were used as assay organisms. Nematodes in almost all samples of nematode suspensions diluted from shipping containers, from spray tanks, or collected in cups after passage through application equipment were infective to G. mellonella larvae. When O. sulcatus larvae were used as assay organisms, 93% (n = 14) of assays from the spray tank and 67% (n = 12) of assays after application showed infectivity. In the spring, sprinklers delivered nematodes to only 15 of 20 sample points on the 0.2-ha plot; delivery by the boom sprayer was better but 2 of 20 points on the 0.2-ha plot received approximately twice as many nematodes as the other points. In the fall, nematode delivery by both systems was more even. However, the average number of nematodes per milliliter of sprayed water collected from the 20 samples on each farm after each application did not correspond to the rates of nematodes applied. Persistence of nematodes in the soil was encouraging, but percentage of infectivity was lower than expected. After application in the spring, assays using G. mellonella larvae showed the presence of infective nematodes in soil samples (0-5 and 5-10 cm deep) on each sampling day (0, 3, 7, and 25) after application by boom sprayer, and on days 0, 3, and 7 after application through sprinklers. In the fall, G. mellonella assays showed infective nematodes in soil samples on each sampling day (0, 3, 7, and 25) after application by boom sprayer, and on days 0, 3, 7, 35, 60, 135, and 250 after application through sprinklers. In the spring, when assays lasted 4 d, percentage of infectivity rose to a maximum of 45% on the 3rd d after application by boom sprayer and declined thereafter. In the fall, when assays lasted 10 d, percentage of infectivity rose to a maximum of 58% on the 7th d after application through sprinklers and remained between 20 and 58% until day 135, declining thereafter; infectivity after boom application remained between 37 and 45% on days 3 and 7, and began to decline on day 25. Nematode infectivity was not compromised in peat soil, muck, or silty clay loam, but infectivity in loam (that may have contained nematicide residues) was very low. We suggest that the inconsistent control of O. sulcatus by S. carpocapsae on British Columbia cranberry farms may be partially explained by problems associated with application and factors related to nematode entry into the soil.     

Influence of Insect Larvae and Plant Roots on the Host-finding Behaviour of Heterorhabditis megidis

We studied the host-finding and dispersion behaviour of Heterorhabditis megidis (strain NLHE 87.3) in the presence of Galleria mellonella or Otiorhynchus sulcatus larvae and strawberry roots. In large Petri dishes (19 cm diameter) filled with moist sand (8% w/w), and incubated at 15°C over 24 h, infective juveniles (IJs) responded positively to the presence of G. mellonella , to roots of a single strawberry plant and to O. sulcatus larvae in direct contact with roots of a single strawberry plant. A neutral or negative response was observed when IJs were presented with only O. sulcatus larvae or a combination of several strawberry plants with O. sulcatus larvae, either in contact or not in contact with the roots. IJs responded strongly to the combination of plant roots and feeding larvae indicating that the tritrophic interaction formed by IJs - O. sulcatus larvae - strawberry plants may be an infochemical-mediated interaction.     

Parasite co-infection and interaction as drivers of host heterogeneity

We examined the hypothesis that the interaction between concomitant infecting parasites modifies host susceptibility, parasite intensity and the pattern of parasite distribution within the host population. We used a 26 year time series of three common parasites in a natural population of rabbits: two gastrointestinal nematodes (Trichostrongylus retortaeformis and Graphidium strigosum) and the immunosuppressive myxoma virus. The frequency distribution of nematodes in the host population and the relationship between host age and nematode intensity were explored in rabbits with either single or dual nematode infections and rabbits infected with the nematodes and myxoma virus. The aggregation of T. retortaeformis and G. strigosum among the rabbits varied with the nature of the co-infection both in male and female hosts. The two nematodes exhibited different age-intensity profiles: G. strigosum intensity increased exponentially with host age while T. retortaeformis intensity exhibited a convex shape. The presence of a secondary infection did not change the age-intensity profile for G. strigosum but for T. retortaeformis co-infection (either both nematodes or myxoma-nematodes) resulted in significantly greater intensities in adult hosts. Results suggest that multi-species infections contributed to aggregation of parasites in the host population and to seasonal variation in intensity, but also enhanced differences in parasitism between sexes. This effect was apparent for T. retortaeformis, which appears to elicit a strong acquired immune response but not for G. strigosum which does not produce any evident immune reaction. We concluded that concomitant infections mediated by host immunity are important in modifying host susceptibility and influencing heterogeneity amongst individual hosts.     

Facultative scavenging as a survival strategy of entomopathogenic nematodes

Entomopathogenic nematodes cannot be considered only as parasitic organisms. With dead Galleria mellonella larvae, we demonstrated that these nematodes use scavenging as an alternative survival strategy. We consider scavenging as the ability of entomopathogenic nematodes to penetrate, develop and produce offspring in insects which have been killed by causes other than the nematode-bacteria complex. Six Steinernema and two Heterorhabditis species scavenged but there were differences among them in terms of frequency of colonisation and in the time after death of G. mellonella larvae that cadavers were penetrated. The extremes of this behaviour were represented by Steinernema glaseri which was able to colonise cadavers which had been freeze-killed 240 h earlier and Heterorhabditis indica which only colonised cadavers which had been killed up to 72 h earlier. Also, using an olfactometer, we demonstrated that entomopathogenic nematodes were attracted to G. mellonella cadavers.     

Pathogenicity, development, and reproduction of Heterorhabditis bacteriophora and Steinernema carpocapsae under axenic in vivo conditions

Galleria mellonella larvae cultured axenically were treated with axenic dauer juveniles of Heterorhabditis bacteriophora and Steinernema carpocapsae. After 3 days S. carpocapsae had killed all insects, with 9.4 +/- 4.3 nematodes per larva. H. bacteriophora were unable to kill G. mellonella, although 13.3 +/- 6.4 nematodes per Galleria were found in the hemocoel. Invading nematodes of both strains recovered from the dauer stage. H. bacteriophora developed into hermaphrodites with eggs and J1 in the uterus and in the hemolymph of the living insects. Development beyond the J1 stage was not recorded. An injection of supernatants from different Photorhabdus luminescens cultures killed the insects but could not provide nutrients to support a further development. Only the injection of bacterial cells supported production of dauers in the axenic insects. Axenic S. carpocapsae developed to adults and produced offspring. After 3 weeks an average of 5275 nematodes per larva were counted, of which 6.7% were dauer juveniles, 39.2% other juvenile stages, 11.9% males, and 42.2% females. Compared to in vivo reproduction in the presence of the symbiotic bacterium Xenorhabdus nematophilus the dauer juvenile yields were low. Even after 5 weeks the percentage of dauer juveniles did not surpass 10%.     

Improved Control of Otiorhynchus sulcatus at 9°C by Cold-stored Heterorhabditis megidis UK211

The effect of storage temperature (9 and 20°C) on North West European Heterorhabditis megidis isolate UK211 for control of Otiorhynchus sulcatus larvae at 9°C is assessed. O. sulcatus mortality increased from -5.3% (corrected mortality) using freshly produced nematodes, to 27.1% using nematodes that had been cold-stored for 12 weeks. The number of nematodes invading the insect larvae increased almost 27-fold. Nematode storage at 9°C for 11 to 12 weeks weeks resulted in significantly higher O. sulcatus mortality (41%) than storage at 20°C for 2 to 3 weeks (12%). Thus, cold storage does enhance nematode infectivity for O. sulcatus larvae.     

Entomopathogenic nematodes to control black vine weevil (Coleoptera: Curculionidae) on strawberry.

We investigated the potential of heterorhabditid nematodes to control larvae of the black vine weevil, Otiorhynchus sulcatus (F.), in 2 field experiments in commercial strawberry plantings. In both experiments, nematodes were applied directly onto the straw mulch, or onto the soil after temporary removal of the mulch. Heterorhabditis marelatus Lui & Berry (Rhabditida: Heterorhabditidae) reduced numbers of weevil larvae and the percentage of plants infested in both experiments, irrespective of straw removal. In the 1st field experiment, a sponge-packed H. marelatus formulation produced lower numbers of O. sulcatus larvae per strawberry plant (mean O. sulcatus larvae per plant = 0.7) and proportion of infested plants (42%) compared with a vermiculite formulation (mean O. sulcatus larvae per plant = 1.8, proportion infested plants 67%) and an untreated control (mean O. sulcatus larvae per plant = 1.9, proportion infested plants 75%). In the first 2 wk after application, more H. marelatus were found in soil samples collected from plots treated with sponge-packed nematodes, than from plots treated with vermiculite-formulated nematodes. In the 2nd field experiment, sponge-packed formulations of H. bacteriophora Poinar (Rhabditida: Heterorhabditidae) and H. marelatus were tested. H. marelatus caused a reduction in both numbers of weevil larvae (mean O. sulcatus larvae per plant = 0.1) and proportion of infested plants (9%) but H. bacteriophora did not (mean O. sulcatus larvae per plant = 0.45, proportion infested plants 34%). More H. bacteriophora were recovered from soil samples than H. marelatus during the first 7 d of this experiment. However, laboratory studies revealed no difference in the persistence of these 2 nematodes in sand.     

Positive correlation between virulence of Pseudomonas aeruginosa mutants in mice and insects

Strain PA14, a human clinical isolate of Pseudomonas aeruginosa, is pathogenic in mice and insects (Galleria mellonella). Analysis of 32 different PA14 mutants in these two hosts showed a novel positive correlation in the virulence patterns. Thus, G. mellonella is a good model system for identifying mammalian virulence factors of P. aeruginosa.     

Effects and implications of antibiotic treatment on Wolbachia-infected vine weevil (Coleoptera: Curculionidae)

1 The parthenogenetic weevil Otiorhynchus sulcatus was collected from five geographical locations in the U.S.A. All weevils from each location were infected by Wolbachia belonging to supergroup B. 2 We hypothesized that treatment with tetracycline, which has been used to clear Wolbachia infection, would influence the reproduction of Wolbachia‐infected parthenogenetic O. sulcatus females. Tetracycline treatment of preovipositional O. sulcatus females specifically inhibited egg hatching but had no effect relative to controls on any other physiological trait. 3 Treatment with gentamicin, which reportedly has no effect on Wolbachia infection but is indistinguishable from tetracycline in inhibiting protein synthesis of other bacterial genera, did not influence egg hatching. 4 These findings strongly suggest that the inhibition of egg development results from the antibiotic effect on Wolbachia rather than by direct toxic effects on O. sulcatus physiology. 5 We speculate that Wolbachia may be necessary for normal development of O. sulcatus eggs and discuss the implications of these findings for O. sulcatus ecology.     

拟双角斯氏线虫D43品系鞘蛋白对大蜡螟幼虫的免疫抑制作用

侵染期的拟双角斯氏线虫Steinernema ceratophorum D43品系体外都包裹着一个透明的体鞘。为探明体鞘对线虫侵染力的影响, 了解鞘蛋白（sheath proteins, SPs）对大蜡螟Galleria mellonella 幼虫的免疫抑制作用, 本研究通过化学方法使拟双角斯氏线虫D43脱鞘, 以对寄主的致死率和侵入点数量为指标, 与包鞘线虫比较对大蜡螟幼虫的侵染力; 采用乙醇提取的方法获得线虫鞘蛋白, 利用双向电泳和质谱技术对鞘蛋白进行鉴定分析; 从血细胞数量和酚氧化酶活力两个方面评价鞘蛋白对大蜡螟幼虫免疫反应的抑制作用。结果表明： 0.5%次氯酸钠处理20 min可以保证95%以上的线虫存活和脱鞘。与包鞘线虫相比, 脱鞘线虫对大蜡螟幼虫的致死率显著降低, 致死时间延后, 节间膜侵入点数量显著减少。以35%乙醇提取的鞘蛋白提取物可鉴定出6种鞘蛋白, 其中一个被鉴定为丝氨酸蛋白酶。此外, 血腔注射鞘蛋白提取物可导致试虫血细胞数量明显降低, 酚氧化酶活力受到显著抑制。由此说明, 体鞘对拟双角斯氏线虫D43的侵染力具有显著影响, 鞘蛋白在抑制寄主昆虫免疫反应中发挥重要作用。     

Interaction of microbial populations in Steinernema (Steinernematidae,Nematoda) infected Galleria mellonella larvae

Infection of Galleria mellonella larvae with the entomopathogenic nematodes Steinernema feltiae (A21 and R strains) and Steinernema glaseri (Dongrae) resulted in several species of bacteria, including the respective bacterial symbiont, Xenorhabdus spp., growing in the infected insect cadavers. These other bacteria were Enterococcus in all three nematode infections studied and Acinetobacter in the S. feltiae infections. The respective populations of these bacteria changed with time. Following infection of G. mellonella larvae with any one of the Steinernema sp., only Enterococcus bacteria were detected initially in the dead larvae. Between 30 and 50h post-infection Xenorhabdus bacteria were detected and concurrent with this Enterococcus population declined to zero. This was probably due to secondary metabolites with antibacterial properties that were produced by Xenorhabdus. In the S. feltiae (both R and A21 strains) infections a third bacterium, Acinetobacter, appeared at about 130h (in S. feltiae A21 infections) or 100h (in S. feltiae R infections) and increased in population size to approximately that of Xenorhabdus. It was demonstrated that Enterococcus, orginating from the G. mellonella digestive tract, was sensitive to the organically soluble antimicrobials produced by Xenorhabdus but Acinetobacter, which was carried by the nematode, was not.     

Entomopathogenic Nematode Infectivity Assay: Comparison of Penetration Rate into Different Hosts

Penetration rate (the percentage of the initial infective juvenile inoculum that invades an insect host) was tested as an indicator of entomopathogenic nematode infectivity. Several host-parasite-substrate combinations were evaluated for penetration rate. Four steinernematids, Steinernema carpocapsae, S. glaseri, S. feltiae, S. riobravis and two strains of Heterorhabditis bacteriophora were tested in a contact bioassay against the wax moth, Galleria mellonella, the yellow meal worm, Tenebrio molitor, the beet armyworm, Spodoptera exigua, the black cutworm, Agrotis ipsilon, and the European corn borer, Ostrinia nubilalis. The insect larvae were confined individually in sand and filter paper arenas and exposed to 200 infective juveniles. After incubation, dead insects were dissected in order to count the nematodes penetrated. The data were analyzed for the effects of nematode strain and substrate on penetration rate. The bioassay substrate had a variable effect depending on the insect species. The nematode effect was highly significant for all insects tested. The penetration rate therefore allowed comparisons among nematode strains invading a host. Nematode ranking for infectivity differed according to the insect tested.     

Relationship between the successful infection by entomopathogenic nematodes and the host immune response

Reproduction of entomopathogenic nematodes requires that they escape recognition by a host's immune system or that they have mechanisms to escape encapsulation and melanization. We investigated the immune responses of larvae for the greater wax moth (Galleria mellonella), tobacco hornworm (Manduca sexta), Japanese beetle (Popillia japonica), northern masked chafer (Cyclocephala borealis), oriental beetle (Exomala orientalis) and adult house crickets (Acheta domesticus), challenged with infective juveniles from different species and strains of entomopathogenic nematodes. The in vivo immune responses of hosts were correlated with nematode specificity and survival found by infection assays. In P. japonica, 45% of injected infective juveniles from Steinernema glaseri NC strain survived; whereas the hemocytes from the beetle strongly encapsulated and melanized the Heterorhabditis bacteriophora HP88 strain, S. glaseri FL strain, Steinernema scarabaei and Steinernema feltiae. Overall, H. bacteriophora was intensively melanized in resistant insect species (E. orientalis, P. japonica and C. borealis) and had the least ability to escape the host immune response. Steinernema glaseri NC strain suppressed the immune responses in susceptible hosts (M. sexta, E. orientalis and P. japonica), whereas S. glaseri FL strain was less successful. Using an in vitro assay, we found that hemocytes from G. mellonella, P. japonica, M. sexta and A. domestica recognized both nematode species quickly. However, many S. glaseri in M. sexta and H. bacteriophora in G. mellonella escaped from hemocyte encapsulation by 24h. These data indicate that, while host recognition underlies some of the differences between resistant and susceptible host species, escape from encapsulation following recognition can also allow successful infection. Co-injected surface-coat proteins from S. glaseri did not protect H. bacteriophora in M. sexta but did protect H. bacteriophora in E. orientalis larva; therefore, surface coat proteins do not universally convey host susceptibility. Comparisons of surface coat proteins by native and SDS-PAGE demonstrated different protein compositions between H. bacteriophora and S. glaseri and between the two strains of S. glaseri.     

Host range, specificity, and virulence of Steinernema feltiae, Steinernema rarum, and Heterorhabditis bacteriophora (Steinernematidae and Heterorhabditidae) from Argentina

Infections were carried out in the laboratory to determine the host range, specificity, and virulence of Steinernema rarum, S. feltiae, and Heterorhabditis bacteriophora that were isolated from different regions of Argentina. All insect orders showed a remarkable susceptibility to the three nematode species, showing mortality values higher than 66%, except for Anoplura. The mortality of the insects of agronomic interest was more than 90%. As for insects of sanitary importance, S. feltiae and S. rarum caused 60% of mortality, whereas H. bacteriophora caused 80%. The results fluctuated when considering the buccal apparatus function of the insects. Nematodes completed their cycle in anoplurans, orthopterans, lepidopterans, and hymenopterans, while the development varied in coleopterans, homopterans, hemipterans, and dipterans. S. rarum developed completely both in immature and adult, while S. feltiae and H. bacteriophora developed preferably in immature. The three isolates are capable of parasitizing a wide host range under laboratory conditions; H. bacteriophora is more virulent than the evaluated Steinernema spp. against Galleria mellonella larvae; the pathogenicity and specificity depend on the bioecological characteristics of nematodes and hosts.     

Correlation between virulence of Candida albicans mutants in mice and Galleria mellonella larvae

Candida albicans is a dimorphic human pathogen in which the yeast to hyphal switch may be an important factor in virulence in mammals. This pathogen has recently been shown to also kill insects such as the Greater Wax Moth Galleria mellonella when injected into the haemocoel of the insect larvae. We have investigated the effect of previously characterised C. albicans mutations that influence the yeast to hyphal transition on virulence in G. mellonella larvae. There is a good correlation between the virulence of these mutants in the insect host and the virulence measured through systemic infection of mice. Although the predominant cellular species detected in G. mellonella infections is the yeast form of C. albicans, mutations that influence the hyphal transition also reduce pathogenicity in the insect. The correlation with virulence measured in the mouse infection system suggests that Galleria may provide a convenient and inexpensive model for the in vivo screening of mutants of C. albicans.     

Rapid age-related changes in infection behavior of entomopathogenic nematodes

Nonfeeding infective juvenile (IJ) entomopathogenic nematodes (EPNs) are used as biological agents to control soil-dwelling insects, but poor storage stability remains an obstacle to their widespread acceptance by distributors and growers as well as a frustration to researchers. Age is one factor contributing to variability in EPN efficacy. We hypothesized that age effects on the infectiousness of IJs would be evident within the length of time necessary for IJs to infect a host. The penetration behavior of "young" (<1-wk-old) and "old" (2- to 4-wk-old) Heterorhabditis bacteriophora (GPS 11 strain), Steinernema carpocapsae (All strain), and Steinernema feltiae (UK strain) IJs was evaluated during 5 "exposure periods" to the larvae of the wax moth, Galleria mellonella. Individual larvae were exposed to nematode-infested soil for exposure periods of 4, 8, 16, 32, and 64 hr. Cadavers were dissected after 72 hr, and the IJs that penetrated the larvae were counted. Larval mortality did not differ significantly between 72- and 144-hr "observation periods," or points at which larval mortality was noted, for any age class or species. However, age and species effects were noted in G. mellonella mortality and nematode penetration during shorter time periods. Initial mortality caused by S. carpocapsae and H. bacteriophora IJs declined with nematode age but increased with S. feltiae IJ age. Young S. carpocapsae IJs penetrated G. mellonella larvae at higher rates than old members of the species (27-45% vs. 1-4%). Conversely, old S. feltiae IJs had higher penetration rates than young IJs (approximately 8 to 57% vs. 4 to approximately 31%), whereas H. bacteriophora IJs had very low penetration rates regardless of age (3-5.6%). Our results show that the effect of age on IJ infectiousness can be detected in IJs aged only 2 wk by a 4-hr exposure period to G. mellonella. These results have important implications for storage and application of EPNs and suggest the possibility of shortening the time required to detect nematodes in the soil.     

Interaction of calyx fluid and venom from Microplitis croceipes (Braconidae) on developmental disruption of the natural host,Heliocoverpa zea,and two atypical hosts,Galleria mellonella and Spodoptera exigua

Polydnaviruses of many braconid and ichneumonid endoparasitoids play an important role in the successful parasitism of their hosts. The host's development is altered and its immune response is also suppressed. In this study, we compared the effects of calyx fluid and venom on the development of the natural host, Helicoverpa zea, and two atypical hosts that the parasitoid does not normally attack in nature, Galleria mellonella and Spodoptera exigua. The levels of calyx fluid and\or venom injected was 0.05, 0.1 and 0.2 female equivalents (FE)/larva. In H. zea, calyx fluid significantly reduced larval growth on day 5 post injection. Venom alone did not affect larval growth but it synergized the action of calyx fluid by reducing growth earlier and for a longer period after injection. Other effects of calyx fluid on the host, either alone or in combination with venom, were an increase in developmental period, and a reduction in percent emergence and weight of adult moths. The percentage of H. zea larvae that pupated was not affected by calyx fluid or venom. In Galleria mellonella, venom alone reduced larval growth comparable to calyx fluid and both tissues induced the effects on day 1 post injection. Other effects caused by calyx fluid or venom alone or the combination were a reduction in percent pupation and emergence, and the average adult weight. In S. exigua, high mortality occurred when 4th instar larvae were injected. Although the injection of larger fifth instars reduced overall mortality, the sham-injected larvae only gained weight during the first 24 hours after injection (from day 0 to day 1). However, adults were produced at all doses of calyx fluid or venom. The effects of the virus on development in this species were a prolongation of the larval stage and reduction of adult weight by calyx fluid in combination with venom. In conclusion, injections of calyx fluid and venom of Microplitis croceipes can differentially affect the growth and development of its natural host H. zea, and atypical host, G. mellonella, but only a minimal effect was observed in S. exigua.     

Patterns of parasite aggregation in the wild European rabbit (Oryctolagus cuniculus)

Understanding the factors controlling the distribution of parasites within their host population is fundamental to the wider understanding of parasite epidemiology and ecology. To explore changes in parasite aggregation, Taylor's power law was used to examine the distributions of five gut helminths of the wild rabbit. Aggregation was found to be a dynamic process that varied with year, season, host sex, age class, and myxomatosis. Yearly and seasonal changes are thought, in the main, to be the result of variations in weather conditions acting upon infectious stages (or intermediate hosts). Evidence in support of this was the comparatively low degree of fluctuation in the aggregation of the pinworm, Passalurus ambiguus, as the infectious stage of this parasite is likely to be less susceptible to environmental variation. Host age had a marked effect on the level of aggregation of all parasites, but this effect varied between parasite species. P. ambiguus, Trichostrongylus retortaeformis and Cittotaenia denticulata aggregation were lower in adult than juvenile rabbits whilst Graphidium strigosum and Mosgovoyia pectinata aggregation tended to increase with age. Host immunity is thought to be responsible for these differences. Differences in aggregation for different parasites were also seen when the rabbit population was split into males and females. Myxomatosis had a marked effect on helminth distribution with substantially less aggregation in rabbits showing clinical signs of the disease.