Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

Food stoichiometry affects the outcome of Daphnia–parasite interaction

Phosphorus (P) is an essential nutrient for growth in consumers. P‐limitation and parasite infection comprise one of the most common stressor pairs consumers confront in nature. We conducted a life‐table study using a Daphnia–microsporidian parasite model, feeding uninfected or infected Daphnia with either P‐sufficient or P‐limited algae, and assessed the impact of the two stressors on life‐history traits of the host. Both infection and P‐limitation negatively affected some life‐history traits tested. However, under P‐limitation, infected animals had higher juvenile growth rate as compared with uninfected animals. All P‐limited individuals died before maturation, regardless of infection. The numbers of spore clusters of the microsporidian parasite did not differ in P‐limited or P‐sufficient hosts. P‐limitation, but not infection, decreased body phosphorus content and ingestion rates of Daphnia tested in separate experiments. As parasite spore production did not suffer even under extreme P‐limitation, our results suggest that parasite was less limited by P than the host. We discuss possible interpretations concerning the stoichiometrical demands of parasite and suggest that our results are explained by parasite‐driven changes in carbon (C) allocation of the hosts. We conclude that the impact of nutrient starvation and parasite infection on consumers depends not only on the stoichiometric demands of host but also those of the parasite.     

Studies on the Physiology of Host-parasite Relationship in Orobanche. I. Respiratory Metabolism of Host and Parasite

The respiratory rate of the roots of mustard (Brassica cam-pestris L.) and tomato (Lycopersicum esculentum Mill.) serving as hosts for the total root parasites Orobanche aegyptiaca Pers. and O.cernua Loefll. was measured using Warburg manometric technique. At the same time determinations were made of the respiration of the apical, basal and root regions of the parasites. The effects of sodium fluoride, malonic acid, sodium azide and DNP (2,4-dinitrophenol) on the rate of respiration of the host roots as well as of the parasites were studied. The Orobanche infection results in a marked increase in the respiratory rate near the host-parasite contact region. The damaging effect of infection seems to be due mainly to a continuous flow of water, minerals and metabolites from host to parasite. The haustorial invasion creates an obstruction in the translocation of metabolites. The respiration rate is lower in Orobanche than in the host, which might be related to its slower growth rate, inefficient oxidative processes and an escaping of certain energy-requiring interconversion processes. Roots of O. aegyptiaca are more well-developed and have higher rate of respiration. They can absorb more water and minerals from the soil. This fact might be connected with the specificity of the two species. NaF and malonic acid inhibit the respiration to a similar extent in healthy and infected roots. This indicates that the pathway of respiration does not change materially after infection. The EMP and Krebs cycle seem to operate at a lower intensity in Orobanche, which is proved by the lower inhibition of the respiration as compared to in the host. Azide causes a stronger reduction of the respiration in infected than in healthy roots. It would imply that the infection stimulates the activity of metal containing oxidases. The weaker inhibition of the respiration in Orobanche tissues indicates a mediation of other enzymes in the oxidation processes than in the host. The respiration is less stimulated by DNP in infected than in healthy roots. Contrary to the general effect of DNP, this substance decreases the O₂ uptake in the parasite tissues. This fact may be explained by the occurrence of exceptionally high amounts of endogenous phenolic compounds and an insufficient production of ATP in the parasite.     

Host–parasite interactions and vectors in the barn swallow in relation to climate change

Recent climate change has affected the phenology of numerous species, and such differential changes may affect host–parasite interactions. Using information on vectors (louseflies, mosquitoes, blackflies) and parasites (tropical fowl mite Ornithonyssus bursa, the lousefly Ornithomyia avicularia, a chewing louse Brueelia sp., two species of feather mites Trouessartia crucifera and Trouessartia appendiculata, and two species of blood parasites Leucozytozoon whitworthi and Haemoproteus prognei) of the barn swallow Hirundo rustica collected during 1971–2008, I analyzed temporal changes in emergence and abundance, relationships with climatic conditions, and changes in the fitness impact of parasites on their hosts. Temperature and rainfall during the summer breeding season of the host increased during the study. The intensity of infestation by mites decreased, but increased for the lousefly during 1982–2008. The prevalence of two species of blood parasites increased during 1988–2008. The timing of first mass emergence of mosquitoes and blackflies advanced. These temporal changes in phenology and abundance of parasites and vectors could be linked to changes in temperature, but less so to changes in precipitation. Parasites had fitness consequences for hosts because intensity of the mite and the chewing louse was significantly associated with delayed breeding of the host, while a greater abundance of feather mites was associated with earlier breeding. Reproductive success of the host decreased with increasing abundance of the chewing louse. The temporal decrease in mite abundance was associated with advanced breeding of the host, while the increase in abundance of the lousefly was associated with earlier breeding. Virulence by the tropical fowl mite decreased with increasing temperature, independent of confounding factors. These findings suggest that climate change affects parasite species differently, hence altering the composition of the parasite community, and that climate change causes changes in the virulence of parasites. Because the changing phenology of different species of parasites had both positive and negative effects on their hosts, and because the abundance of some parasites increased, while that of other decreased, there was no consistent temporal change in host fitness during 1971–2008.     

Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients

Although elevated CO₂ may affect various forms of ecological interactions, the effect of elevated CO₂ on interactions between parasitic plants and their hosts has received little attention. We examined the effect of elevated CO₂ (590 μl l⁻¹) at two nutrient (NPK) levels on the interactions of the facultative root hemiparasite Rhinanthus alectorolophus with two of its hosts, the grass Lolium perenne and the legume Medicago sativa. To study possible effects on parasite mediation of competition between hosts, the parasite was grown with each host separately and with both hosts simultaneously. In addition, all combinations of hosts were grown without the parasite. Both the parasite and the host plants responded to elevated CO₂ with increased growth, but only at high nutrient levels. The CO₂ response of the hemiparasite was stronger than that of the hosts, but depended on the host species available. With L. perenne and M. sativa simultaneously available as hosts, the biomass of the parasite grown at elevated CO₂ was 5.7 times that of parasites grown at ambient CO₂. Nitrogen concentration in the parasites was not influenced by the treatments and was not related to parasite biomass. The presence of the parasite strongly reduced both the biomass of the hosts and total productivity of the system. This effect was much stronger at low than at high nutrient levels, but was not influenced by CO₂ level. Elevated CO₂ did not influence the competitive balance between the two different hosts grown in mixture. The results of this study support the hypothesis that hemiparasites may influence community structure and suggest that these effects are robust to changes in CO₂ concentration. Received: 17 August 1998 / Accepted: 3 March 1999     

Parasite‐induced Changes in the Anti‐predator Behavior of a Cricket Intermediate Host

Many parasites with complex life cycles are known to modify their host phenotype to enhance transmission from the intermediate host to the definitive host. Several earlier studies explored these effects in acanthocephalan and trematode parasites, especially in aquatic ecosystems; however, much less is known about parasite‐mediated alterations of host behavior in terrestrial systems involving nematodes. Here, we address this gap by investigating a trophically transmitted nematode (Pterygodermatites peromysci) that uses a camel cricket (Ceuthophilus pallidipes) as the intermediate host before transmission to the final host, the white‐footed mouse (Peromyscus leucopus). In a laboratory experiment, we quantified the anti‐predatory responses of the cricket intermediate host using simulated predator cues. Results showed a decrease in jumping performance among infected crickets as compared with uninfected crickets, specifically in terms of frequency of jumps and jumping distance. Additionally, the relationship between parasite load and frequency of jumps is negatively correlated with the intensity of infection. These behavioral modifications are likely to increase vulnerability to predation by the definitive host. An analysis of the age‐intensity pattern of infection in natural cricket populations appears to support this hypothesis: parasites accumulate with age, peak at an intermediate age class before the intensity of infection decreases in older age groups. We suggest that older, heavily infected crickets are preferentially removed from the population by predators because of increased vulnerability. These results show that cricket intermediate hosts infected with P. peromysci have diminished jumping performance, which is likely to impair their anti‐predatory behavior and potentially facilitate parasite transmission.     

Elevated atmospheric concentrations of carbon dioxide reduce monarch tolerance and increase parasite virulence by altering the medicinal properties of milkweeds

Hosts combat their parasites using mechanisms of resistance and tolerance, which together determine parasite virulence. Environmental factors, including diet, mediate the impact of parasites on hosts, with diet providing nutritional and medicinal properties. Here, we present the first evidence that ongoing environmental change decreases host tolerance and increases parasite virulence through a loss of dietary medicinal quality. Monarch butterflies use dietary toxins (cardenolides) to reduce the deleterious impacts of a protozoan parasite. We fed monarch larvae foliage from four milkweed species grown under either elevated or ambient CO₂, and measured changes in resistance, tolerance, and virulence. The most high‐cardenolide milkweed species lost its medicinal properties under elevated CO₂; monarch tolerance to infection decreased, and parasite virulence increased. Declines in medicinal quality were associated with declines in foliar concentrations of lipophilic cardenolides. Our results emphasize that global environmental change may influence parasite–host interactions through changes in the medicinal properties of plants.     

Climate changes influence free‐living stages of soil‐transmitted parasites of European rabbits

Climate warming has been suggested to augment the risk of infectious disease outbreaks by extending the seasonal window for parasite growth and by increasing the rate of transmission. Understanding how this occurs in parasite‐host systems is important for appreciating long‐term and seasonal changes in host exposure to infection and to reduce species extinction caused by diseases. We investigated how free‐living stages of two soil‐transmitted helminths of the European rabbit (Oryctolagus cuniculus) responded to experimental changes in temperature by performing laboratory experiments with environmental chambers and field manipulations using open‐top‐chambers. This study was motivated by our previous observations that air temperature has increased over the last 30 years in our field site and that during this period intensity of infection of Graphidium strigosum but not Trichostrongylus retortaeformis was positively associated with this temperature increase. Laboratory and field experiments showed that both parasites accelerated egg development and increased hatching rate and larval survival in response to accumulating thermal energy. Both parasites behaved similarly when exposed to diverse temperature regimes, decadal trends, and monthly fluctuations, however, T. retortaeformis was more successful than G. strigosum by showing higher rates of egg hatching and larval survival. Across the months, the first day of hatching occurred earlier in warmer conditions suggesting that climate warming can lengthen the period of parasite growth and host exposure to infective stages. Also, T. retortaeformis hatched earlier than G. strigosum. These findings showed that seasonal changes in intensity, frequency, and duration of daily temperature are important causes of variability in egg hatching and larva survival. Overall, this study emphasizes the important role of climate warming and seasonality on the dynamics of free‐living stages in soil‐transmitted helminths and their contribution to enhance host exposure to parasitic infections. Yet, the ability to infect might ultimately depend on how hosts interact with parasites.     

Host specific variation in photosynthesis of an obligate xylem‐tapping mistletoe Dendrophthoe curvata in a Bornean heath forest

Most mistletoe–host ecophysiological studies have paid attention to the influence of parasites on host performance. This paper explored the impact of varying hosts on the photosynthesis of a single mistletoe species. Here, we studied an obligate xylem‐tapping tropical mistletoe (Dendrophthoe curvata (Blume) Miquel) parasitizing four different hosts (Acacia auriculiformis A. Cunn. Ex Benth, Andira inermis (W. Wright) DC., Mangifera indica L. and Vitex pinnata L.) in a homo geneous tropical heath forest patch in Brunei Darussalam. We compared photosynthetic capacity and photosynthesis‐related characteristics of the mistletoe on four different hosts to evaluate the overall impact of hosts on the parasite. Results showed that the mistletoe–host patterns of CO₂ assimilation rates, transpiration rates and water use efficiency varied significantly based on the host. In the D. curvata–Vitex pinnata association, the mistletoe exhibited significantly lower CO₂ assimilation rates but showed no significant variations in transpiration rates and water use efficiency when compared to the host. In D. curvata–Andira inermis and D. curvata–Mangifera indica associations, the mistletoe showed significantly higher photosynthetic rates than the hosts, whereas in the D. curvata–Acacia auriculiformis association, there was no significant difference in photosynthetic rates between the counterparts. Host specificity also significantly influenced some mistletoe photosynthetic parameters such as light saturated photosynthesis, specific leaf area, leaf chlorophyll content, CO₂ assimilation rates, stomatal conductance, transpiration rates and water use efficiency. Different tree hosts intrinsically offer different resources to their obligate mistletoe parasites based on their physiology and environmental parameters. We argue that host‐specific responses have driven these intra‐specific variations in mistletoe physiology. This study provides background for future investigation on potential host‐regulated mechanisms that drive functional changes in host‐dependent mistletoes.     

Relative importance of chemical attractiveness to parasites for susceptibility to trematode infection

While the host immune system is often considered the most important physiological mechanism against parasites, precontact mechanisms determining exposure to parasites may also affect infection dynamics. For instance, chemical cues released by hosts can attract parasite transmission stages. We used the freshwater snail Lymnaea stagnalis and its trematode parasite Echinoparyphium aconiatum to examine the role of host chemical attractiveness, physiological condition, and immune function in determining its susceptibility to infection. We assessed host attractiveness through parasite chemo‐orientation behavior; physiological condition through host body size, food consumption, and respiration rate; and immune function through two immune parameters (phenoloxidase‐like and antibacterial activity of hemolymph) at an individual level. We found that, although snails showed high variation in chemical attractiveness to E. aconiatum cercariae, this did not determine their overall susceptibility to infection. This was because large body size increased attractiveness, but also increased metabolic activity that reduced overall susceptibility. High metabolic rate indicates fast physiological processes, including immune activity. The examined immune traits, however, showed no association with susceptibility to infection. Our results indicate that postcontact mechanisms were more likely to determine snail susceptibility to infection than variation in attractiveness to parasites. These may include localized immune responses in the target tissue of the parasite. The lack of a relationship between food consumption and attractiveness to parasites contradicts earlier findings that show food deprivation reducing snail attractiveness. This suggests that, although variation in resource level over space and time can alter infection dynamics, variation in chemical attractiveness may not contribute to parasite‐induced fitness variation within populations when individuals experience similar environmental conditions.     

Sex‐dependent infection causes nonadditive effects on kissing bug fecundity

The influence of parasites on host reproduction has been widely studied in natural and experimental conditions. Most studies, however, have evaluated the parasite impact on female hosts only, neglecting the contribution of males for host reproduction. This omission is unfortunate as sex‐dependent infection may have important implications for host–parasite associations. Here, we evaluate for the first time the independent and nonindependent effects of gender infection on host reproductive success using the kissing bug Mepraia spinolai and the protozoan Trypanosoma cruzi as model system. We set up four crossing treatments including the following: (1) both genders infected, (2) both genders uninfected, (3) males infected—females uninfected, and (4) males uninfected—females infected, using fecundity measures as response variables. Interactive effects of infection between sexes were prevalent. Uninfected females produced more and heavier eggs when crossed with uninfected than infected males. Uninfected males, in turn, sired more eggs and nymphs when crossed with uninfected than infected females. Unexpectedly, infected males sired more nymphs when crossed with infected than uninfected females. These results can be explained by the effect of parasitism on host body size. As infection reduced size in both genders, infection on one sex only creates body size mismatches and mating constraints that are not present in pairs with the same infection status. Our results indicate the fitness impact of parasitism was contingent on the infection status of genders and mediated by body size. As the fecundity impact of parasitism cannot be estimated independently for each gender, inferences based only on female host infection run the risk of providing biased estimates of parasite‐mediated impact on host reproduction.     

Gyrodactylus salaris infecting allopatric Arctic charr Salvelinus alpinus fry: an experimental study of host survival

Artificially reared Arctic charr Salvelinus alpinus fry (post yolk‐sac stage) from Skibotnelva in northern Norway were exposed to infection with the monogenean ectoparasite Gyrodactylus salaris to determine if the parasite caused mortality in fry. About 95% of the fry became infected within the first week post‐exposure, and at the end of the experiment (77 days) all fry carried the parasite. Mortality rate was significantly higher in the groups of infected fry than in the uninfected control groups, and when the experiment was terminated c. 30% of the infected and 8% of the uninfected fish had died. Gyrodactylus salaris had a pronounced negative effect upon fry survival and caused an additive host mortality. Moreover, the concurrent temporal changes in parasite intensities and aggregation indicated that the parasite‐induced host mortality was density dependent.     

Parasite-induced Host Mortality: Indirect Evidence From a Long-term Study

A long-term field study of a perturbed host–helminth system provides indirect evidence that a long-lived swimbladder nematode, Cystidicola farionis, induces mortality of Arctic charr, Salvelinus alpinus. The prevalence and abundance of this parasite has changed little over the period from 1987 to 1999. The cumulative numbers of L₃-stage larvae steadily increased with increasing host age, indicating a continuous exposure to infection throughout the life of the target fish host. Indirect methods, which used data pooled over years and long-term cohort analyses, indicate that parasite-induced host mortality (PIHM) occurs in hosts older than 10 years. Furthermore, using a short-term cohort method adjusted for worm recruitment, we found indications of PIHM occurrence even in younger age groups. These patterns do not seem to be caused by high parasite mortality rates since dead worms are rarely observed inside the swimbladder. Age-related changes in infection rates or in resistance to infection seem to play only a minor role as there were only slight changes in the preference of charr for feeding on amphipods (which are intermediate hosts) and in the acquisition rate of L₃ larvae in older hosts. Mortality of the most heavily infected hosts is the most probable explanation for the observed patterns.     

Avian malaria infection intensity influences mosquito feeding patterns

Pathogen-induced host phenotypic changes are widespread phenomena that can dramatically influence host–vector interactions. Enhanced vector attraction to infected hosts has been reported in a variety of host–pathogen systems, and has given rise to the parasite manipulation hypothesis whereby pathogens may adaptively modify host phenotypes to increase transmission from host to host. However, host phenotypic changes do not always favour the transmission of pathogens, as random host choice, reduced host attractiveness and even host avoidance after infection have also been reported. Thus, the effects of hosts’ parasitic infections on vector feeding behaviour and on the likelihood of parasite transmission remain unclear. Here, we experimentally tested how host infection status and infection intensity with avian Plasmodium affect mosquito feeding patterns in house sparrows (Passer domesticus). In separate experiments, mosquitoes were allowed to bite pairs containing (i) one infected and one uninfected bird and (ii) two infected birds, one of which treated with the antimalarial drug, primaquine. We found that mosquitoes fed randomly when exposed to both infected and uninfected birds. However, when mosquitoes were exposed only to infected individuals, they preferred to bite the non-treated birds. These results suggest that the malarial parasite load rather than the infection itself plays a key role in mosquito attraction. Our findings partially support the parasite manipulation hypothesis, which probably operates via a reduction in defensive behaviour, and highlights the importance of considering parasite load in studies on host–vector–pathogen interactions.     

Parasite infection alters host stable‐isotope composition under controlled feeding

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Genomic evidence for population‐specific responses to co‐evolving parasites in a New Zealand freshwater snail

Reciprocal co‐evolving interactions between hosts and parasites are a primary source of strong selection that can promote rapid and often population‐ or genotype‐specific evolutionary change. These host–parasite interactions are also a major source of disease. Despite their importance, very little is known about the genomic basis of co‐evolving host–parasite interactions in natural populations, especially in animals. Here, we use gene expression and sequence evolution approaches to take critical steps towards characterizing the genomic basis of interactions between the freshwater snail Potamopyrgus antipodarum and its co‐evolving sterilizing trematode parasite, Microphallus sp., a textbook example of natural coevolution. We found that Microphallus‐infected P. antipodarum exhibit systematic downregulation of genes relative to uninfected P. antipodarum. The specific genes involved in parasite response differ markedly across lakes, consistent with a scenario where population‐level co‐evolution is leading to population‐specific host–parasite interactions and evolutionary trajectories. We also used an F_ST‐based approach to identify a set of loci that represent promising candidates for targets of parasite‐mediated selection across lakes as well as within each lake population. These results constitute the first genomic evidence for population‐specific responses to co‐evolving infection in the P. antipodarum‐Microphallus interaction and provide new insights into the genomic basis of co‐evolutionary interactions in nature.     

Parasite influences on host life history: Echinostoma revolutum parasitism of Lymnaea elodes snails

Using field surveys and experimental infections, we investigated the influence of a trematode parasite on life history traits of adult Lymnaea elodes snails. We found that parasitism significantly affected the growth, fecundity, and survival of host snails. Within five of the six natural L. elodes populations we sampled, shell length of echinostome-infected hosts was significantly greater than for uninfected conspecifics. Furthermore, we show that gigantism occurs among experimentally infected snails due to an accelerated growth rate and size-selective mortality following an Echinostoma revolutum infection. The fecundity of infected snails sharply decreased beginning at 3 weeks post exposure (PE) and all egg production eventually ceased for most hosts by 5–6 weeks PE. Energy constraints, imposed by parasite development, alter the host energy budget. Early in the infection, parasite depletion of host energy reserves reduces host reproduction, but sufficient resources remain to allow accelerated host growth. Mortality was increased among host snails at two distinct stages: shortly after exposure and several weeks after cercariae were first released. We did not observe tissue degradation in snails during the first 4 weeks after exposure to the parasite, but destruction of host tissues was noted among snails dying later in the infection. Received: 5 September 1997 / Accepted: 19 November 1997     

Early host damage in the infection cycle of Bdellovibrio bacteriovorus

The effects of bdellovibrio infection on host permeability and respiration were investigated by measuring respiration rates and the rate of o-nitrophenyl-beta-d-galactopyranoside hydrolysis during the course of single infection cycles of Bdellovibrio bacteriovorus strain 109 growing on Escherichia coli ML 35 (lac i(-)z(+)y(-)). The data show that among the very early consequences of parasite attack on the host are an increase in permeability and a general disruption of respiratory activity of the host, and it is suggested that both phenomena stem from early damage to host membrane. The rapid onset of damage after inception of the cycle and the failure of streptomycin to prevent the damage indicate that complete penetration of the parasite into the host is not a requirement for the observed effects. The data also show that bdellovibrio does not use host energy-generating mechanisms for its growth and suggest that the parasites may have a search mechanism that permits them, to some degree, to distinguish between infected and uninfected hosts.     

The influence of parasitic water mites on the instantaneous death rate of their hosts

Summary The effect of 2 species of water mites on the instantaneous death rate of their hosts was measured on the basis of laboratory experiments. In both parasite-host association — the parasitic water mite Hydryphantes tenuabilis on the aquatic insect host Hydrometra australis and the parasitic water mite Arrenurus pseudotenuicollis on the mosquito Anopheles crucians — the effect of mite load on the instantaneous death rate of the host appeared to be linear. Also, the impact of a single parasite on the host's death rate was apparently related to the ratio of parasite to host body weight. The results of this study are in general agreement with recent theoretical investigations of the regulation of host populations by parasites.