Ectoparasitic mites exert non-consumptive effects on the larvae of a fruit fly host.

The mere presence of predators or parasites can negatively impact the fitness of prey or hosts. Exposure to predators during an organism's development can have deleterious effects on juvenile survival and the subsequent adult stage. Currently, it is unknown if parasites have analogous impacts on host larval stages and whether these effects carry over into other subsequent life stages. However, parasites may be exerting widespread yet underestimated non-consumptive effects (NCEs). We tested if Drosophila nigrospiracula larvae avoid pupating near mite cues (caged Macrocheles subbadius) in arena experiments, and measured the rate of pupation in arenas with mites and arenas without mites. Larvae disproportionately pupated on the side of arenas that lacked mite cues. Furthermore, fewer larvae successfully pupated in arenas containing mites cues compared to arenas without mite cues. We found that ectoparasitic mites exert NCEs on Drosophila larvae, even though the larval stage is not susceptible to infection. We discuss these results in the context of parasite impacts on host population growth in an infectious world.

Endosymbiotic Male-Killing Spiroplasma Affects the Physiological and Behavioral Ecology of Macrocheles-Drosophila Interactions.

While many arthropod endosymbionts are vertically transmitted, phylogenetic studies reveal repeated introductions of hemolymph-dwelling Spiroplasma into Drosophila. Introductions are often attributed to horizontal transmission via ectoparasite vectors. Here, we test if mites (Macrocheles subbadius) prefer to infect Spiroplasma poulsonii MSRO (Melanogaster sex ratio organism)-infected flies and if MSRO infection impairs fly resistance against secondary mite attack. First, we tested if mites prefer MSRO+ or MSRO- flies using pairwise choice tests across fly ages. We then tested whether mite preferences are explained by changes in fly physiology, specifically increased metabolic rate (measured as CO2 production). We hypothesize that this preference is due in part to MSRO+ flies expressing higher metabolic rates. However, our results showed mite preference depended on an interaction between fly age and MSRO status: mites avoided 14-day-old MSRO+ flies relative to MSRO- flies (31% infection) but preferred MSRO+ flies (64% infection) among 26-day-old flies. Using flowthrough respirometry, we found 14-day-old MSRO+ flies had higher CO2 emissions than MSRO- flies (32% greater), whereas at 26 days old the CO2 production among MSRO+ flies was 20% lower than that of MSRO- flies. Thus, mite preferences for high-metabolic-rate hosts did not explain the infection biases in this study. To assess changes in susceptibility to infection, we measured fly endurance using geotaxis assays. Older flies had lower endurance consistent with fly senescence, and this effect was magnified among MSRO+ flies. Given the biological importance of male-killing Spiroplasma, potential changes in the interactions of hosts and potential vectors could impact the ecology and evolution of host species. IMPORTANCE Male-killing endosymbionts are transmitted from mother to daughter and kill male offspring. Despite these major ecological effects, how these endosymbionts colonize new host species is not always clear. Mites are sometimes hypothesized to transfer these bacteria between hosts/host species. Here, we test if (i) mites prefer to infect flies that harbor Spiroplasma poulisoni MSRO and (ii) flies infected with MSRO are less able to resist mite infection. Our results show that flies infected with MSRO have weaker anti-mite resistance, but the mite preference/aversion for MSRO+ flies varied with fly age. Given the fitness and population impacts of male-killing Spiroplasma, changes in fly-mite interactions have implications for the ecology and evolution of these symbioses.

Scared of the dark? Phototaxis as behavioural immunity in a host-parasite system.

Behavioural immunity describes suites of behaviours hosts use to minimize the risks of infection by parasites/pathogens. Research has focused primarily on the evasion and physical removal of infectious stages, as well as behavioural fever. However, other behaviours affect infection risk while carrying ecologically significant trade-offs. Phototaxis, in particular, has host fitness implications (e.g. altering feeding and thermoregulation) that also impact infection outcomes. In this study, we hypothesized that a fly host, Drosophila nigrospiracula, employs phototaxis as a form of behavioural immunity to reduce the risk of infection. First, we determined that the risk of infection is lower for flies exposed in the light relative to the dark using micro-arena experiments. Because Drosophila vary in ectoparasite resistance based on mating status we examined parasite-mediated phototaxis in mated and unmated females. We found that female flies spent more time in the light side of phototaxis chambers when mites were present than in the absence of mites. Mating marginally decreased female photophobia independently of mite exposure. Female flies moved to lighter, i.e. less infectious, environments when threatened with mites, suggesting phototaxis is a mechanism of behavioural immunity. We discuss how parasite-mediated phototaxis potentially trades-off with host nutrition and thermoregulation.

Relative contributions of parasite consumptive and non-consumptive effects to host population suppression in simulated fly-mite populations.

The "ecology of fear" framework was developed to describe the impacts predators have on potential prey and prey populations, outside of consumption/predation (i.e. non-consumptive effects, NCEs). This framework has recently been extended to symbiotic interactions such as host-parasite associations. Although the NCEs of predators and parasites on their individual victims can be measured experimentally, it is currently not known whether parasites can exert population-level effects on potential hosts through their NCEs. Modelling can be a useful tool for scaling individual-level NCEs to populations to determine impacts on host population growth. In this study, we used previously published data on the consumptive and non-consumptive effects of an ectoparasitic mite (Macrocheles subbadius) on a fruit fly (Drosophila nigrospiracula) to simulate populations experiencing fear (NCEs only), both fear and infection (consumption + NCEs) or neither. Population-level models indicate that NCEs alone were insufficient to reduce population growth. In fact, host populations experiencing NCEs but not infection had slightly larger final populations than unexposed populations (by ~ 550 flies). This result suggests there is compensation (i.e. increased daily reproduction that overcomes shorter lifespans) among exposed flies. By contrast, the consumptive effects of parasites suppressed the growth of simulated host populations, and this deleterious impact grew non-linearly with infection prevalence.

The influence of infection status and parasitism risk on host dispersal and susceptibility to infection in Drosophila nigrospiracula.

For many organisms, habitat avoidance provides the first line of defence against parasitic infection. Changes in infection status can shift the cost-benefit ratio of remaining in a given habitat vs dispersing. The aim of this study was to test the hypothesis that the propensity to disperse in Drosophila nigrospiracula is mediated by current parasite load and the risk of further infection by an ectoparasitic mite (Macrocheles subbadius). An activity monitor was used to assess dispersal propensity among infected and uninfected flies. The activity level of uninfected females increased threefold upon exposure to a mite, whereas the activity among uninfected males increased by 17-fold in the presence of a questing mite. Among infected flies, the risk of further infection also generated a change in activity, but the magnitude of the response was dependent on host sex. Current infection status influenced the probability of acquiring more parasites due to increased susceptibility to infection with mite load. The probability of acquiring additional mites among males increased more rapidly compared to female flies. Current infection status can potentially determine the risk of further infection, the host propensity and ability to disperse, with consequence for hosts and parasites at the individual, population and species level.

Facultative parasites as evolutionary stepping-stones towards parasitic lifestyles.

Parasites and parasitic lifestyles have evolved from free-living organisms multiple times. How such a key evolutionary transition occurred remains puzzling. Facultative parasites represent potential transitional states between free-living and fully parasitic lifestyles because they can be either free-living or parasitic depending on environmental conditions. We suggest that facultative parasites with phenotypically plastic life-history strategies may serve as evolutionary stepping-stones towards obligate parasitism. Pre-adaptations provide a starting point for the transition towards opportunistic or facultative parasitism, but what evolutionary mechanism underlies the transition from facultative to obligate parasitism? In this Opinion Piece, we outline how facultative parasites could evolve towards obligate parasites via genetic assimilation, either alone or in combination with the Baldwin effect. We further describe the key predictions stemming from each of these evolutionary pathways. The importance of genetic assimilation in evolution has been hotly debated. Studies on facultative parasites may not only provide key insights regarding the evolution of parasitism, but also provide ideal systems in which to test evolutionary theory on genetic accommodation.

Phenotypic plasticity more essential to maintaining variation in host-attachment behaviour than evolutionary trade-offs in a facultatively parasitic mite.

A prevailing hypothesis for the evolution of parasitism posits that the fitness benefits gained from parasitic activity results in selection for and fixation of parasitic strategies. Despite the potential fitness advantage of parasitism, facultative parasites continue to exhibit genetic variation in parasitic behaviour in nature. We hypothesized that evolutionary trade-offs associated with parasitic host-attachment behaviour maintain natural variation observed in attachment behaviour. In this study, we used replicate lines of a facultatively parasitic mite, previously selected for increased host-attachment behaviour to test whether increased attachment trades off with mite fecundity and longevity, as well as the phenotypic plasticity of attachment. We also tested for potential correlated changes in mite morphology. To test for context-dependent trade-offs, mite fecundity and longevity were assayed in the presence or absence of a host. Our results show that selected and control mites exhibited similar fecundities, longevities, attachment plasticities and morphologies, which did not provide evidence for life history trade-offs associated with increased attachment. Surprisingly, phenotypic plasticity in attachment was maintained despite directional selection on the trait, which suggests that phenotypic plasticity likely plays an important role in maintaining attachment variation in natural populations of this facultative parasite.

Ecology of fear: environment-dependent parasite avoidance among ovipositing Drosophila.

Habitat avoidance is an anti-parasite behaviour exhibited by at-risk hosts that can minimize exposure to parasites. Because environments are often heterogeneous, host decision-making with regards to habitat use may be affected by the presence of parasites and habitat quality simultaneously. In this study we examine how the ovipositing behaviour of a cactiphilic fruit fly, Drosophila nigrospiracula, is affected by the presence of an ectoparasitic mite, Macrocheles subbadius, in conjunction with other environmental factors - specifically the presence or absence of conspecific eggs and host plant tissue. We hypothesized that the trade-off between site quality and parasite avoidance should favour ovipositing at mite-free sites even if it is of inferior quality. We found that although flies avoided mites in homogeneous environments (86% of eggs at mite-free sites), site quality overwhelmed mite avoidance. Both conspecific eggs (65% of eggs at infested sites with other Drosophila eggs) and host plant tissue (78% of eggs at infested sites with cactus) overpowered mite avoidance. Our results elucidate the context-dependent decision-making of hosts in response to the presence of parasites in variable environments, and suggest how the ecology of fear and associated trade-offs may influence the relative investment in anti-parasite behaviour in susceptible hosts.

Life history of Macrocheles muscaedomesticae (Parasitiformes: Macrochelidae): new insights on life history and evidence of facultative parasitism on Drosophila.

Macrocheles muscaedomesticae is a cosmopolitan macrochelid mite whose populations have likely diverged considering the many locations they inhabit, but most of the work published on this mite species has been on the basis of their association with the house fly, Musca domestica. Here, we studied several aspects of the biology of M. muscaedomesticae associated with drosophilid flies collected in Alberta, Canada. We assessed the degree of divergence of our populations from others, compared their life history to other published populations and experimentally tested whether M. muscaedomesticae feeds on Drosophila hydei hosts by comparing the body mass of mites that attached to hosts to those that did not. There was no strong phylogenetic differentiation among any of the M. muscaedomesticae specimens, suggesting multiple recent introductions of this species to Canada. Compared to other populations, our mites exhibited lower fecundity, which may have been a result of the temperature or nematode-only diet in which they were maintained. Finally, mites that attached to hosts for 4 h weighed significantly more than those that did not. Without direct evidence for host tissue transfer to the mites, it is difficult to determine whether the mites are indeed feeding on their hosts while attached. However, the existing evidence for the costs fly hosts endure at the expense of these mites makes this relationship antagonistic.

Proximity to parasites reduces host fitness independent of infection in a Drosophila-Macrocheles system.

Parasites are known to have direct negative effects on host fitness; however, the indirect effects of parasitism on host fitness sans infection are less well understood. Hosts undergo behavioural and physiological changes when in proximity to parasites. Yet, there is little experimental evidence showing that these changes lead to long-term decreases in host fitness. We aimed to determine if parasite exposure affects host fitness independent of contact, because current approaches to parasite ecology may underestimate the effect of parasites on host populations. We assayed the longevity and reproductive output of Drosophila nigrospiracula exposed or not exposed to ectoparasitic Macrocheles subbadius. In order to preclude contact and infection, mites and flies were permanently separated with a mesh screen. Exposed flies had shorter lives and lower fecundity relative to unexposed flies. Recent work in parasite ecology has argued that parasite-host systems show similar processes as predator-prey systems. Our findings mirror the non-consumptive effects observed in predator-prey systems, in which prey species suffer reduced fitness even if they never come into direct contact with predators. Our results support the perspective that there are analogous effects in parasite-host systems, and suggest new directions for research in both parasite ecology and the ecology of fear.

Longitudinal study of parasite-induced mortality of a long-lived host: the importance of exposure to non-parasitic stressors.

Hosts face mortality from parasitic and environmental stressors, but interactions of parasitism with other stressors are not well understood, particularly for long-lived hosts. We monitored survival of flour beetles (Tribolium confusum) in a longitudinal design incorporating cestode (Hymenolepis diminuta) infection, starvation and exposure to the pesticide diatomaceous earth (DE). We found that cestode cysticercoids exhibit increasing morphological damage and decreasing ability to excyst over time, but were never eliminated from the host. In the presence of even mild environmental stressors, host lifespan was reduced sufficiently that extensive degradation of cysticercoids was never realized. Median host lifespan was 200 days in the absence of stressors, and 3-197 days with parasitism, starvation and/or DE. Early survival of parasitized hosts was higher relative to controls in the presence of intermediate concentrations of DE, but reduced under all other conditions tested. Parasitism increased host mortality in the presence of other stressors at times when parasitism alone did not cause mortality, consistent with an interpretation of synergy. Environmental stressors modified the parasite numbers needed to reveal intensity-dependent host mortality, but only rarely masked intensity dependence. The longitudinal approach produced observations that would have been overlooked or misinterpreted if survival had only been monitored at a single time point.

State-dependent parasitism by a facultative parasite of fruit flies.

Parasites can evolve phenotypically plastic strategies for transmission such that a single genotype can give rise to a range of phenotypes depending on the environmental condition. State-dependent plasticity in particular can arise from individual differences in the parasite's internal state or the condition of the host. Facultative parasites serve as ideal model systems for investigating state-dependent plasticity because individuals can exhibit two life history strategies (free-living or parasitic) depending on the environment. Here, we experimentally show that the ectoparasitic mite Macrocheles subbadius is more likely to parasitize a fruit fly host if the female mite is mated; furthermore, the propensity to infect increased with the level of starvation experienced by the mite. Host condition also played an important role; hosts infected with moderate mite loads were more likely to gain additional infections in pairwise choice tests than uninfected flies. We also found that mites preferentially infected flies subjected to mechanical injury over uninjured flies. These results suggest that a facultative parasite's propensity to infect a host (i.e. switch from a free-living strategy) depends on both the parasite's internal state and host condition. Parasites often live in highly variable and changing environments, an infection strategy that is plastic is likely to be adaptive.

A facultative ectoparasite attains higher reproductive success as a parasite than its free-living conspecifics.

Comparative and phylogenetic studies reveal that parasites evolved from free-living ancestors, but the underlying processes and mechanisms are not well understood. In order for selection to favor the evolution of parasitism, there must be pre-existing adaptations for host exploitation and a fitness advantage favoring the transition to parasitism. Here, we experimentally investigate the differential reproductive success of a facultative parasite, Macrocheles subbadius. Lifetime reproductive success was higher among female mites that fed on fruit fly hemolymph as parasites compared to free-living mites. The per capita lifetime fecundity for parasitic females was 2.4 times that of free-living females. The offspring sex ratio also differed between the two modes of life, with parasitic mites producing more sons and hence a relatively weaker female-biased sex ratio (0.61) than mites that were free-living (0.94). These results suggest that parasitic female mites allocate more resources to egg production, and were limited by sperm rather than food, as was the case with free-living mites. The higher lifetime reproductive success of mites that opportunistically feed on host hemolymph provides the fitness advantage necessary for the evolution of parasitism.

Mite choice generates sex- and size-biased infection in Drosophila hydei.

Heterogeneities in parasite infection among conspecific hosts often manifest as sex- or size-biased infections, which are typically attributed to differential host susceptibility and exposure. Since parasite fitness is often tied to host quality, host preference by parasites is likely to be under strong selection. We test the hypothesis that host preference is sufficient to generate variability in infection rate among conspecifics. Specifically, we ask whether the mite Macrocheles muscaedomesticae is able to discriminate between Drosophila hydei hosts of different sex and size, while explicitly accounting for the potential confounding effects of these two factors. Our results indicate a preference for female hosts, but this preference appears to be driven by size and not sex per se. When differences in body size were controlled for, the sex-biased infection disappeared, while mites presented with the choice of two female flies of disparate sizes were more likely to select the larger host. Across the distribution of fly body weight in this study, mites preferentially attached to flies of intermediate size. This study provides evidence that mite choice for certain host types can play an important role in parasite transmission, even in the absence of differential susceptibility or exposure among hosts.

Mechanisms underlying parasite infection: influence of host body mass and age on chewing louse distribution among brown-headed cowbirds.

Animal populations exhibit various patterns in ectoparasite distribution across different sexes and age classes, and numerous factors can potentially contribute to ectoparasite abundance and distribution. We examined the influence of host body size and age on the distribution of a chewing louse among brown-headed cowbirds. Differences in louse prevalence (males 62.9 ± 2.8%, females 47.5 ± 4.9%) and intensity (males 15.42 ± 1.51, females 9.04 ± 1.69) were primarily driven by differences in host body mass and not host sex. Larger birds had larger louse infra-populations, which likely translated into a lower risk of local extinction, a possible explanation for higher louse prevalence observed among larger birds. Among males, younger individuals showed higher louse prevalence (70.21 ± 4.72%) compared to older males (59.36 ± 3.59). We speculate that this pattern is likely driven by behavioural difference and not body size, with young males spending relatively more time foraging in large groups, increasing their risk of louse transmission. By examining the mechanisms that underlie the sex- and age-biased infections observed in natural populations, we can better identify the hosts most responsible for parasite transmission.

CHEMOTAXIS RESPONSE OF PHASMARHABDITIS CALIFORNICA (FAMILY: RHABDITIDAE) AND PRISTIONCHUS ENTOMOPHAGUS (FAMILY: NEODIPLOGASTERIDAE) TO THE MUCUS OF FOUR SLUG SPECIES.

The chemotaxis responses of soil nematodes have been well studied in bacteriophagic nematodes, plant-parasitic nematodes, entomopathogenic nematodes, and to a lesser extent malacopathogenic nematodes. Free-living stages of parasitic nematodes often use chemotaxis to locate hosts. In this study, we compared the chemotaxis profile of 2 slug-associated nematodes with overlapping host ranges. Phasmarhabditis californica is a facultative parasite that has been shown to express strain-dependent variation in chemoattraction profile. We tested 4 slug species to determine the attraction index of a Canadian strain of Ph. californica and a sympatric necromenic nematode, Pristionchus entomophagus. When tested against a control (distilled water), Ph. californica showed a clear (positive) attraction towards the mucus of slugs Ambigolimax valentianus, Arion rufus, and Arion fasciatus, but not Deroceras reticulatum. However, when given a choice between the mucus of D. reticulatum and Ar. fasciatus in a pairwise test, Ph. californica was strongly attracted to the former. Other pairwise comparisons did not reveal a clear preference for either slug species in the following pairs: D. reticulatum-Ar. rufus, Am. valentianus-Ar. rufus, D. reticulatum-Am. valentianus. The chemotaxis assay for Pr. entomophagus showed an attraction toward D. reticulatum and Ar. fasciatus (tested against controls); the attraction index for Am. valentianus was positive, but this was not statistically significant. In contrast, the attraction index for Ar. rufus was negative, suggesting possible repulsion to the mucus of this slug species. Given that Pr. entomophagus and Ph. californica occupy overlapping habitats, utilize similar hosts, and exhibit similar chemotaxis profiles, there is a potential for direct interaction between these 2 nematodes. Like other members of the genus Pristionchus, Pr. entomophagus may be able to prey upon the co-occurring Ph. californica, such antagonistic interactions could have important implications for the coexistence of these 2 species and Ph. californica in particular as a biocontrol agent against pestiferous slugs.

PHASMARHABDITIS CALIFORNICA (NEMATODA: RHABDITIDAE) HAS REDUCED ESTABLISHMENT SUCCESS AND PROGENY PRODUCTION IN THE PRESENCE OF PRISTIONCHUS ENTOMOPHAGUS (NEMATODA: DIPLOGASTRIDAE).

Phasmarhabditis (syn. Pellioditis) californica is a facultative parasite that has been marketed as a popular biocontrol agent against pestiferous slugs in England, Scotland, and Wales. The necromenic nematode Pristionchus entomophagus has also been recovered from slugs infected with Ph. californica. In this study, we experimentally investigated the outcome of single and mixed applications of Pr. entomophagus and Ph. californica on the slug Deroceras reticulatum (Müller). Host mortality was comparable for single and mixed applications of Ph. californica, with time to death significantly shorter in both treatment groups compared with controls. However, trials with Pr. entomophagus alone did not cause any significant host mortality relative to controls. Compared with the single Ph. californica applications, mixed applications resulted in 67% fewer infective juveniles establishing in the host, and subsequently far fewer infective juveniles were recovered in the next generation. In contrast, the establishment rate and progeny production in Pr. entomophagus were not impacted by the presence of Ph. californica (i.e., mixed applications). Hence, the presence of Pr. entomophagus had a deleterious effect on the establishment success and progeny production of Ph. californica. Our findings reveal an asymmetrical, antagonistic interaction between Ph. californica and Pr. entomophagus and highlight the importance of understanding the ecological relationships between co-occurring species. A decrease in parasite establishment success and progeny production has the potential to directly impact the persistence, sustainability, and efficacy of Ph. californica as a biological control agent.

Linking predator-prey interactions with exposure to a trophically transmitted parasite using PCR-based analyses.

Parasite transmission is determined by the rate of contact between a susceptible host and an infective stage and susceptibility to infection given an exposure event. Attempts to measure levels of variation in exposure in natural populations can be especially challenging. The level of exposure to a major class of parasites, trophically transmitted parasites, can be estimated by investigating the host's feeding behaviour. Since the parasites rely on the ingestion of infective intermediate hosts for transmission, the potential for exposure to infection is inherently linked to the definitive host's feeding ecology. Here, we combined epidemiological data and molecular analyses (polymerase chain reaction) of the diet of the definitive host, the white-footed mouse (Peromyscus leucopus), to investigate temporal and individual heterogeneities in exposure to infection. Our results show that the consumption of cricket intermediate hosts accounted for much of the variation in infection; mice that had consumed crickets were four times more likely to become infected than animals that tested negative for cricket DNA. In particular, pregnant female hosts were three times more likely to consume crickets, which corresponded to a threefold increase in infection compared with nonpregnant females. Interestingly, males in breeding condition had a higher rate of infection even though breeding males were just as likely to test positive for cricket consumption as nonbreeding males. These results suggest that while heterogeneity in host diet served as a strong predictor of exposure risk, differential susceptibility to infection may also play a key role, particularly among male hosts. By combining PCR analyses with epidemiological data, we revealed temporal variation in exposure through prey consumption and identified potentially important individual heterogeneities in parasite transmission.

Network transmission inference: host behavior and parasite life cycle make social networks meaningful in disease ecology.

The process of disease transmission is determined by the interaction of host susceptibility and exposure to parasite infectious stages. Host behavior is an important determinant of the likelihood of exposure to infectious stages but is difficult to measure and often assumed to be homogenous in models of disease spread. We evaluated the importance of precisely defining host contact when using networks that estimate exposure and predict infection prevalence in a replicated, empirical system. In particular, we hypothesized that infection patterns would be predicted only by a contact network that is defined according to host behavior and parasite life cycle. Two competing host contact criteria were used to construct networks defined by parasite life cycle and social contacts. First, parasite-defined contacts were based on shared space with a time delay corresponding to the environmental development time of nematode parasites with a direct fecal-oral life cycle. Second, social contacts were defined by shared space in the same time period. To quantify the competing networks of exposure and infection, we sampled natural populations of the eastern chipmunk (Tamias striatus) and infection of their gastrointestinal helminth community using replicated longitudinal capture-mark-recapture techniques. We predicted that (1) infection with parasites with direct fecal-oral life cycles would be explained by the time delay contact network, but not the social contact network; (2) infection with parasites with trophic life cycles (via a mobile intermediate host; thus, spatially decoupling transmission from host contact) would not be explained by either contact network. The prevalence of fecal-oral life cycle nematode parasites was strongly correlated to the number and strength of network connections from the parasite-defined network (including the time delay), while the prevalence of trophic life cycle parasites was not correlated with any network metrics. We concluded that incorporating the parasite life cycle, relative to the way that exposure is measured, is key to inferring transmission and can be empirically quantified using network techniques. In addition, appropriately defining and measuring contacts according the life history of the parasite and relevant behaviors of the host is a crucial step in applying network analyses to empirical systems.