High Host Density Research Articles

The shape of the functional response curve of Dolichogenidea tasmanica (Hymenoptera: Braconidae) is affected by recent experience

Dolichogenidea tasmanica (Hymenoptera: Braconidae) is the most commonly collected parasitoid of light brown apple moth, Epiphyas postvittana (Lepidoptera: Tortricidae; LBAM) in Australia. We studied the functional response of D. tasmanica, and the effect of recent experience on this behavior. The functional response was evaluated in wind tunnels and enclosed cages. In both arenas, D. tasmanica exhibited a sigmoid functional response, but there was no clear tendency for a deceleration in the functional response curve at high host densities as would be expected with a typical type III functional response. Another experiment revealed that recent experience with high host densities increases the proportion of hosts parasitized by D. tasmanica, which explains much of the difference between the observed functional response curve and a typical type III curve. In general the searching behavior of D. tasmanica varies in response to host density in a manner that directly affects its searching rate. Our results have contributed to understanding the behavior of this parasitoid and indicate its capacity to control its host under laboratory conditions. At lower host densities that are characteristic of field populations, D. tasmanica responded in a density-dependent manner that should contribute to suppression of pest populations before they reach economically damaging levels.

Bottom–up regulation of parasite population densities in freshwater ecosystems

Theory predicts the bottom–up coupling of resource and consumer densities, and epidemiological models make the same prediction for host–parasite interactions. Empirical evidence that spatial variation in local host density drives parasite population density remains scarce, however. We test the coupling of consumer (parasite) and resource (host) populations using data from 310 populations of metazoan parasites infecting invertebrates and fish in New Zealand lakes, spanning a range of transmission modes. Both parasite density (no. parasites per m2) and intensity of infection (no. parasites per infected hosts) were quantified for each parasite population, and related to host density, spatial variability in host density and transmission mode (egg ingestion, contact transmission or trophic transmission). The results show that dense and temporally stable host populations are exploited by denser and more stable parasite populations. For parasites with multi‐host cycles, density of the ‘source’ host did not matter: only density of the current host affected parasite density at a given life stage. For contact‐transmitted parasites, intensity of infection decreased with increasing host density. Our results support the strong bottom–up coupling of consumer and resource densities, but also suggest that intraspecific competition among parasites may be weaker when hosts are abundant: high host density promotes greater parasite population density, but also reduces the number of conspecific parasites per individual host.

Oviposition, Superparasitism, and Egg Load in the Solitary Parasitoid Diachasmimorpha longicaudata (Hymenoptera: Braconidae): Response to Host Availability

Parasitoid reproduction is often limited by disposable time for accessing hosts or egg availability. In the case of synovigenic parasitoids (which mature eggs during their lifetime), theory predicts that egg limitation is infrequent because they are able to replenish their egg supply throughout life. However, this theory does not contemplate the effect of superparasitism on egg load. If superparasitism is applied as a strategy for modulating offspring sex ratio, as extra eggs are normally deposited in each host, it is to be expected that females experience egg limitation, especially when host density is high. In this article, the effect of host density on egg load, oviposition, and sex ratio in the tephritid parasitoid Diachasmimorpha longicaudata (Ashmead) was investigated. Specifically we analyzed the rate of oviposition per host and adjustments in offspring sex ratio in response to changes in host density and how these affect egg load. Parasitoids maintained a similar rate of oviposition, independent of host densities, except when host density fluctuated between 25 and 5 hosts per day. Females increased their oviposition rate and parasitism when confronted with 5 hosts, after having experienced a density of 25 hosts. Offspring sex ratio was biased in favor of females at high host density and with increasing prevalence of superparasitism. Results suggest that parasitoids have a strategy for maintaining high egg load in order to mitigate egg limitation, and for adjusting oviposition rate and offspring sex ratio with reference to previous oviposition experience.

Mutual interference in Dolichogenidea tasmanica (Cameron) (Hymenoptera: Braconidae) when foraging for patchily-distributed light brown apple moth

Dolichogenidea tasmanica (Hymenoptera: Braconidae) is a solitary endoparasitoid of light brown apple moth (LBAM) and other tortricids that is native to Australia. The foraging behavior of single and multiple female D. tasmanica in the presence of patchily-distributed hosts was observed in wind tunnels. Results show that D. tasmanica was able to choose those patches which initially offered the highest oviposition rate. Single foragers spent the longest times on patches with higher host densities, but they visited patches with lower host densities sooner in presence of other foragers. The overall rate of parasitism was equalized across patches at the end of the experiment, which indicates that hosts were equally susceptible to attack by D. tasmanica, irrespective of their local density. A decrease in the searching efficiency of individual parasitoids with increasing parasitoid density was evidence of interference among female D. tasmanica. Nonlinear regression indicates that there was a consistent pattern of mutual interference as wasp density increased and the area of discovery and mutual interference coefficients of Hassell and Varley’s (1969) model of parasitoid foraging were estimated as Q=0.45 and m=0.526, respectively. Females chose to visit patches that were not previously visited, which suggests that repellent chemicals were produced by attacked hosts or marking pheromones were deposited by searching females on patches they have visited. This is one factor that can reduce searching efficiency and cause mutual interference among competing D. tasmanica.

Density-dependent immunity and parasitism risk in experimental populations of lizards naturally infested by ixodid ticks.

When effective immune defenses against parasites are costly and resources limited, individuals are expected to alter their investment in immunity in response to the risk of infection. As an ecological factor that can affect both food abundance and parasite exposure, host density can play an important role in host immunity and host-parasite interactions. High levels of intraspecific competition for food and social stress at high host density may diminish immune defenses and increase host susceptibility to parasites. At the same time, for contagious and environmentally transmitted parasites, parasite exposure often increases with host density, whereas in mobile parasites that actively search for hosts, parasite exposure can decrease with host density due to the "encounter-dilution effect." To unravel these multiple and potentially opposing effects of host density on immunity, we manipulated density of the common lizard Zootoca vivipara and measured local inflammation in response to PHA injection and levels of infestation by the tick Ixodes ricinus, a mobile ectoparasite for which we expected an encounter-dilution effect to occur. Local inflammation strongly decreased with lizard density in adults, but not in yearlings. Tick infestation (abundance and prevalence) was negatively correlated with lizard density in both age classes. Using path analyses, we found independent, direct negative density feedbacks on immunity and parasite exposure in adults, supporting the hypothesis of energy constraints and/or physiological stress acting on immunity at high density. In contrast, for yearlings, the best path model showed that density diluted exposure to parasites, which themselves down-regulated immune defenses in lizards. These results highlight the importance of investigating the pathways among host density, host immunity, and parasite infestation, while accounting for relevant individual traits such as age.

Absence of Transgenerational Phenotypic Plasticity in Fecundity in the ParasitoidAnagrus erythroneurae(Hymenoptera: Mymaridae)

To improve biological pest control, we would like to understand the main factors that limit the reproductive success of key biological control agents, including parasitoid wasps. Previous research with proovigenic parasitoids in the genus Anagrus collected from multiple field sites revealed positive correlations between parasitoid fecundity and local host density. In this article, we test whether this variation in fecundity is a reflection of transgenerational phenotypic plasticity. We exposed females in laboratory microcosms to either low or high host densities and then quantified the fecundity of their adult daughters. Mean fecundity of daughters did not differ across host density treatments, arguing against the operation of phenotypic plasticity. Significant variation in mean fecundity across different families instead suggested the possibility that genetic variation may underlie the observed fecundity differences.

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

Host density likely plays a key role in hostparasite interactions, but empirical evi- dence in marine ecosystems remains limited. Classical models predict a positive relationship between host density and parasite infection parameters, but this depends on the parasite transmis- sion mode. Evidence from systems where mobile parasites actively seek hosts suggests that num- bers of parasites per host decrease with increasing host density ('dilution effect'). Copepodids, the infective stage of the salmon louse Lepeophtheirus salmonis, are mobile larvae that display a range of behaviours to detect their salmonid hosts. We hypothesized that high host density would decrease infection intensity, prevalence and degree of aggregation, but not infection success, which reflects parasite performance. We infected multiple groups of Atlantic salmon Salmo salar post-smolts at low (12 fish; 7.9 kg m �3 ) and high (96 fish; 68.5 kg m �3 ) densities, with the same number of L. salmonis copepodids in swimming chambers to enable more realistic swimming behaviours during infection. Infection intensity was 8.4 times higher in the low density treatment, but there were no differences in infection success and degree of aggregation. We observed 100% prevalence in the low density treatment, which was significantly higher than the high density treatment (68%). The dilution effect most likely explained the negative relationship between host density and infection intensity, as the individual risk of being 'attacked' by a parasite decreased as host density increased. Host density is crucial in salmonsea lice infection dynamics, and opportunities may exist within production environments to use the dilution effect of density to improve fish welfare outcomes.

Parasite fitness traits under environmental variation: disentangling the roles of a chytrid's immediate host and external environment.

Parasite environments are heterogeneous at different levels. The first level of variability is the host itself. The second level represents the external environment for the hosts, to which parasites may be exposed during part of their life cycle. Both levels are expected to affect parasite fitness traits. We disentangle the main and interaction effects of variation in the immediate host environment, here the diatom Asterionella formosa (variables host cell volume and host condition through herbicide pre-exposure) and variation in the external environment (variables host density and acute herbicide exposure) on three fitness traits (infection success, development time and reproductive output) of a chytrid parasite. Herbicide exposure only decreased infection success in a low host density environment. This result reinforces the hypothesis that chytrid zoospores use photosynthesis-dependent chemical cues to locate its host. At high host densities, chemotaxis becomes less relevant due to increasing chance contact rates between host and parasite, thereby following the mass-action principle in epidemiology. Theoretical support for this finding is provided by an agent-based simulation model. The immediate host environment (cell volume) substantially affected parasite reproductive output and also interacted with the external herbicide exposed environment. On the contrary, changes in the immediate host environment through herbicide pre-exposure did not increase infection success, though it had subtle effects on zoospore development time and reproductive output. This study shows that both immediate host and external environment as well as their interaction have significant effects on parasite fitness. Disentangling these effects improves our understanding of the processes underlying parasite spread and disease dynamics.

Effects of juvenile host density and food availability on adult immune response, parasite resistance and virulence in a Daphnia-parasite system.

Host density can increase infection rates and reduce host fitness as increasing population density enhances the risk of becoming infected either through increased encounter rate or because host condition may decline. Conceivably, potential hosts could take high host density as a cue to up-regulate their defence systems. However, as host density usually covaries with food availability, it is difficult to examine the importance of host density in isolation. Thus, we performed two full-factorial experiments that varied juvenile densities of Daphnia magna (a freshwater crustacean) and food availability independently. We also included a simulated high-density treatment, where juvenile experimental animals were kept in filtered media that previously maintained Daphnia at high-density. Upon reaching adulthood, we exposed the Daphnia to their sterilizing bacterial parasite, Pasteuria ramosa, and examined how the juvenile treatments influenced the likelihood and severity of infection (Experiment I) and host immune investment (Experiment II). Neither juvenile density nor food treatments affected the likelihood of infection; however, well-fed hosts that were well-fed as juveniles produced more offspring prior to sterilization than their less well-fed counterparts. By contrast, parasite growth was independent of host juvenile resources or host density. Parasite-exposed hosts had a greater number of circulating haemocytes than controls (i.e., there was a cellular immune response), but the magnitude of immune response was not mediated by food availability or host density. These results suggest that density dependent effects on disease arise primarily through correlated changes in food availability: low food could limit parasitism and potentially curtail epidemics by reducing both the host’s and parasite’s reproduction as both depend on the same food.

Egg load dynamics and the risk of egg and time limitation experienced by an aphid parasitoid in the field.

Insect parasitoids and herbivores must balance the risk of egg limitation and time limitation in order to maximize reproductive success. Egg and time limitation are mediated by oviposition and egg maturation rates as well as by starvation risk and other determinants of adult lifespan. Here, we assessed egg load and nutritional state in the soybean aphid parasitoid Binodoxys communis under field conditions to estimate its risk of becoming either egg- or time-limited. The majority of female B. communis showed no signs of egg limitation. Experimental field manipulations of B. communis females suggested that an average of 4–8 eggs were matured per hour over the course of a day. Regardless, egg loads remained constant over the course of the day at approximately 80 eggs, suggesting that egg maturation compensates for oviposition. This is the first case of such “egg load buffering” documented for a parasitoid in the field. Despite this buffering, egg loads dropped slightly with increasing host (aphid) density. This suggests that egg limitation could occur at very high host densities as experienced in outbreak years in some locations in the Midwestern USA. Biochemical analyses of sugar profiles showed that parasitoids fed upon sugar in the field at a remarkably high rate. Time limitation through starvation thus seems to be very low and aphid honeydew is most likely a source of dietary sugar for these parasitoids. This latter supposition is supported by the fact that body sugar levels increase with host (aphid) density. Together, these results suggest that fecundity of B. communis benefits from both dynamic egg maturation strategies and sugar-feeding.

Quantitative parameters and ecological implications of a specialized tritrophic interaction involving a seed-feeding tortricid, Pseudargyrotoza conwagana, a braconid parasitoid, Bracon otiosus, and the wild privet, Ligustrum vulgare

Little is known about tritrophic interactions involving seed-feeding insects, parasitoid wasps, and wild fleshy fruits. Here, we examine relationships between Pseudargyrotoza conwagana (F.) (Lepidoptera: Tortricidae), Bracon otiosus Marshall (Hymenoptera: Braconidae), and the wild privet, Ligustrum vulgare L. (Lamiales: Oleaceae), after collecting fruits in a hedgerow habitat in northwest Spain and rearing insects indoors. No other insect species was detected in this trophic system. Each fruit contained one to four seeds, each infested fruit contained only one seed-feeding tortricid caterpillar, and each parasitized caterpillar was affected by a single braconid individual, i.e., B. otiosus was a solitary parasitoid. Almost half of the wild privet shrubs were infested by P. conwagana , and infestation ranged from 2 to 32% of fruits per infested shrub. The general effect of P.conwagana on wild privet dispersal can be considered low, as the overall rate of seed infestation was low (6% of seeds). The infestation rate was higher in wild privet shrubs with a larger number of seeds per fruit, and tortricid caterpillars that left the fruits successfully ate >80% of seeds. In total, the parasitism rate was moderate (25% of caterpillars), but varied considerably (0‒75%) among shrubs where P. conwagana infestation was detected. Parasitism only occurred in shrubs showing high infestation rates (19‒32% infested fruits), i.e., with high host densities; however, the parasitism rate was density-independent in these shrubs. The wild privets benefited from the action of B. otiosus in two ways: the tortricid caterpillar population was partly eliminated, and the caterpillars were prevented from eating more than one seed per fruit. The B. otiosus sex ratio was very balanced (1 male to 1.18 females). Winter diapause and protandry were prevalent in B. otiosus .

Departure mechanisms for host search on high-density patches by the Meteorus pulchricornis.

Less attention has been paid to the parasitoid–host system in which the host occurs in considerably high density with a hierarchical patch structure in studies on time allocation strategies of parasitoids. This study used the parasitoid Meteorus pulchricornis (Wesmael) (Hymenoptera: Braconidae) and the Oriental leafworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) as the parasitoids–host model system to investigate patch-leaving mechanisms as affected by the high-host density, hierarchical patch structure, and foraging behaviors on both former and current patches. The results showed that three out of eight covariates tested had significant effects on the patch-leaving tendency, including the host density, ovipositor insertion, and host rejection on the current patch. The parasitoid paid more visits to the patch with high-density hosts. While the patch with higher host densities decreased the leaving tendency, the spatial distribution of hosts examined had no effect on the leaving tendency. Both oviposition and host rejection decreased the patch-leaving tendency. The variables associated with the former patch, such as the host density and number of ovipositor insertions, however, did not have an effect on the leaving tendency. Our study suggested that M. pulchricornis females may use an incremental mechanism to exploit high-density patches to the fullest.

Mortality by Parasitization in the Association between <i>Diachasmimorpha tryoni</i> (Hymenoptera: Braconidae) and <i>Ceratitis capitata</i> (Diptera: Tephritidae) under Field-Cage Conditions

Ceratitis capitata (Wiedemann) is one of the major pests currently affecting world fruit production. In Argentina’s northern Citrus-producing regions, C. capitata is actively multiplying in large exotic host fruits, such as Citrus paradisi Macfadyen (grapefruit), Citrus aurantium L. (sour orange) and Citrus sinensis L. (Osbeck) (sweet orange). Faced with this situation, the use of parasitoids as biocontrol agents is currently receiving renewed attention as a new biological tool for controlling pestiferous fruit flies within the Argentinean National Fruit Fly Control and Eradication Program (ProCEM). Consequently, a viable approach to controlling C. capitata involves the use of exotic parasitoids such as Diachasmimorpha tryoni (Cameron). In this study, the effectiveness of D. tryoni females to find and successfully parasitize C. capitata larvae infesting all Citrus species mentioned earlier was assessed. Parasitoids were allowed to forage for 8 h on grapefruits and oranges artificially infested with laboratory-reared C. capitata larvae under natural environmental conditions (field cage). Parasitoid emergence, parasitism, overall effectiveness, and sex ratio of parasitoid offspring were estimated as response variables. The higher effectiveness of D. tryoni females recorded from C. sinensis would be mainly a result of both increased host density per unit of fruit surface area and fruit physical features. The study provides evidence that D. tryoni contributed to C. capitata mortality in all Citrus species assessed. However, the mortality values recorded from C. sinensis, C. aurantium, and C. paradisi did not exceed 10%, 1.5%, and 1.7%, respectively. Nonetheless, D. tryoni might be selected to forage under both high and low host density conditions.

Out-of-Africa migration and Neolithic coexpansion of Mycobacterium tuberculosis with modern humans.

Tuberculosis caused 20% of all human deaths in the Western world between the 17th and 19th centuries, and remains a cause of high mortality in developing countries. In analogy to other crowd diseases, the origin of human tuberculosis has been associated with the Neolithic Demographic Transition, but recent studies point to a much earlier origin. Here we used 259 whole-genome sequences to reconstruct the evolutionary history of the Mycobacterium tuberculosis complex (MTBC). Coalescent analyses indicate that MTBC emerged about 70 thousand years ago, accompanied migrations of anatomically modern humans out of Africa, and expanded as a consequence of increases in human population density during the Neolithic. This long co-evolutionary history is consistent with MTBC displaying characteristics indicative of adaptation to both low- and high host densities.

Human presence increases parasitic load in endangered lion-tailed macaques (Macaca silenus) in its fragmented rainforest habitats in Southern India.

BackgroundUnderstanding changes in the host-parasite relationship due to habitat fragmentation is necessary for better management and conservation of endangered species in fragmented landscapes. Pathogens and parasites can pose severe threat to species in restricted environments such as forest fragments where there is increased contact of wildlife with human and livestock populations. Environmental stress and reduced nutritional level in forest fragments can influence parasite infection and intensity on the native species. In this study, we examine the impact of habitat fragmentation on the prevalence of gastrointestinal parasites in lion-tailed macaques in a fragmented rainforest in Western Ghats.MethodsThe prevalence of different gastrointestinal parasites was estimated from 91 fecal samples collected from 9 lion-tailed macaque groups in nine forest fragments. The parasites were identified up to genus level on the basis of the morphology and coloration of the egg, larva and cyst. The covariates included forest fragment area, group size and the presence/absence of human settlements and livestock in proximity. We used a linear regression model to identify the covariates that significantly influenced the prevalence of different parasite taxa.ResultsNine gastrointestinal parasite taxa were detected in lion-tailed macaque groups. The groups near human settlements had greater prevalence and number of taxa, and these variables also had significant positive correlations with group size. We found that these parameters were also greater in groups near human settlements after controlling for group size. Livestock were present in all five fragments that had human settlements in proximity.ConclusionThe present study suggests that high prevalence and species richness of gastrointestinal parasites in lion-tailed macaque groups are directly related to habitat fragmentation, high anthropogenic activities and high host density. The parasite load partially explains the reason for the decline in immature survival and birth rate in small and isolated rainforest fragments in Anamalai Hills.

Parasite consumption and host interference can inhibit disease spread in dense populations

Disease dynamics hinge on parasite transmission among hosts. However, canonical models for transmission often fit data poorly, limiting predictive ability. One solution involves building mechanistic yet general links between host behaviour and disease spread. To illustrate, we focus on the exposure component of transmission for hosts that consume their parasites, combining experiments, models and field data. Models of transmission that incorporate parasite consumption and foraging interference among hosts vastly outperformed alternatives when fit to experimental data using a zooplankton host (Daphnia dentifera) that consumes spores of a fungus (Metschnikowia bicuspidata). Once plugged into a fully dynamic model, both mechanisms inhibited epidemics overall. Foraging interference further depressed parasite invasion and prevalence at high host density, creating unimodal (hump-shaped) relationships between host density and these indices. These novel results qualitatively matched a unimodal density-prevalence relationship in natural epidemics. Ultimately, a mechanistic approach to transmission can reveal new insights into disease outbreaks.

Different patterns of oviposition learning in two closely related ectoparasitoid wasps with contrasting reproductive strategies

Many parasitoid wasps learn host-associated cues and use them in subsequent host-searching behavior. This associative learning, namely "oviposition learning," has been investigated in many studies. However, few studies have compared multiple species, and no comparative study has previously been conducted on ectoparasitoid species. We compared the effects of oviposition learning on host preference and offspring sex ratio in two closely related ectoparasitoid wasps with contrasting reproductive strategies, Anisopteromalus calandrae (r-strategist) and its sibling species (K-strategist). Using two bruchine hosts, Callosobruchus chinensis and Callosobruchus maculatus larvae infesting the cowpea Vigna unguiculata, oviposition choice experiments were performed at high and low host densities. In both species, no conspicuous effect on the offspring sex ratio was detected, but effects on host preference were found to differ between the species. In A. calandrae, the effects were detected only at high host density, suggesting that oviposition learning plays a role in host discrimination from a short distance but not from a long distance. In the sibling species, those effects were not detected in any of the cases, suggesting the absence of oviposition learning. These results are compatible with those of previous comparative studies of endoparasitoid wasps in that few lifetime oviposition experiences and/or low reward per foraging decision result in low or absent oviposition learning ability. This finding may indicate that ecological traits contributing to learning ability are similar between endoparasitoid and ectoparasitoid wasps. Thus, our species comparison of ectoparasitoids provides another model system for investigating learning and memory dynamics in parasitoid wasps.

Monkeys in the middle: parasite transmission through the social network of a wild primate.

In wildlife populations, group-living is thought to increase the probability of parasite transmission because contact rates increase at high host densities. Physical contact, such as social grooming, is an important component of group structure, but it can also increase the risk of exposure to infection for individuals because it provides a mechanism for transmission of potentially pathogenic organisms. Living in groups can also create variation in susceptibility to infection among individuals because circulating levels of immunosuppressive hormones like glucocorticoids often depend on an individual’s position within the group’s social structure. Yet, little is known about the relative roles of socially mediated exposure versus susceptibility in parasite transmission among free-living animal groups. To address this issue, we investigate the relationship between host dominance hierarchy and nematode parasite transmission among females in a wild group of Japanese macaques (Macaca fuscata yakui). We use social network analysis to describe each individual female’s position within the grooming network in relation to dominance rank and relative levels of infection. Our results suggest that the number of directly-transmitted parasite species infecting each female, and the relative amount of transmission stages that one of these species sheds in faeces, both increase with dominance rank. Female centrality within the network, which shows positive associations with dominance hierarchy, is also positively associated with infection by certain parasite species, suggesting that the measured rank-bias in transmission may reflect variation in exposure rather than susceptibility. This is supported by the lack of a clear relationship between rank and faecal cortisol, as an indicator of stress, in a subset of these females. Thus, socially mediated exposure appears to be important for direct transmission of nematode parasites, lending support to the idea that a classical fitness trade-off inherent to living in groups can exist.

Insect oviposition plasticity in response to host availability: the case of the tephritid fruit fly Bactrocera dorsalis

1. Insect oviposition behaviour is ecologically and physiologically plastic. For tephritid fruit flies, Bactrocera dorsalis Hendel, host availability varies spatially and temporally. Females are expected to adopt adaptive oviposition strategies to maximise lifetime reproductive fitness, including survival. Bactrocera dorsalis oviposition tactics in response to different host availabilities were investigated.2. This study includes three treatments: (i) variable host densities (host density varied according to a fixed cycle from day to day over values of 1, 5, 10 and 20 hosts per cage), (ii) a fixed high host density (20 hosts per cage), and (iii) a fixed low host density (1 host per cage).3. Daily egg‐laying number per female over the course of 27 days was entirely independent of host density and highly dependent on female age. As host availability increased, females accepted significantly more hosts, generally laid small egg clutches, and more broadly distributed the eggs.4. Tephritid fruit flies adaptively adjusted egg clutches in ways that reflected the variability of host availability. Egg‐ and time‐limitation constraints appeared to drive these adjustments. Female egg maturation was triggered by oviposition activity and reflected marked lifetime trade‐offs. Such strategies involved specific time schedules for egg laying.5.This study defined the oviposition plasticity of the tephritid fruit fly. These results have general implications for the behavioural ecology of insect herbivores and parasitoids.

Population Density and Seasonality Effects on Sin Nombre Virus Transmission in North American Deermice (Peromyscus maniculatus) in Outdoor Enclosures

Surveys of wildlife host-pathogen systems often document clear seasonal variation in transmission; conclusions concerning the relationship between host population density and transmission vary. In the field, effects of seasonality and population density on natural disease cycles are challenging to measure independently, but laboratory experiments may poorly reflect what happens in nature. Outdoor manipulative experiments are an alternative that controls for some variables in a relatively natural environment. Using outdoor enclosures, we tested effects of North American deermouse (Peromyscus maniculatus) population density and season on transmission dynamics of Sin Nombre hantavirus. In early summer, mid-summer, late summer, and fall 2007–2008, predetermined numbers of infected and uninfected adult wild deermice were released into enclosures and trapped weekly or bi-weekly. We documented 18 transmission events and observed significant seasonal effects on transmission, wounding frequency, and host breeding condition. Apparent differences in transmission incidence or wounding frequency between high- and low-density treatments were not statistically significant. However, high host density was associated with a lower proportion of males with scrotal testes. Seasonality may have a stronger influence on disease transmission dynamics than host population density, and density effects cannot be considered independent of seasonality.