Variation In Host Density Research Articles

The balance between fitness advantages and costs drives adaptation of bacteriophage Qβ to changes in host density at different temperatures.

Host density is one of the main factors affecting the infective capacity of viruses. When host density is low, it is more difficult for the virus to find a susceptible cell, which increases its probability of being damaged by the physicochemical agents of the environment. Nevertheless, viruses can adapt to variations in host density through different strategies that depend on the particular characteristics of the life cycle of each virus. In a previous work, using the bacteriophage Qβ as an experimental model, we found that when bacterial density was lower than optimal the virus increased its capacity to penetrate into the bacteria through a mutation in the minor capsid protein (A1) that is not described to interact with the cell receptor. Here we show that the adaptive pathway followed by Qβ in the face of similar variations in host density depends on environmental temperature. When the value for this parameter is lower than optimal (30°C), the mutation selected is the same as at the optimal temperature (37°C). However, when temperature increases to 43°C, the mutation selected is located in a different protein (A2), which is involved both in the interaction with the cell receptor and in the process of viral progeny release. The new mutation increases the entry of the phage into the bacteria at the three temperatures assayed. However, it also considerably increases the latent period at 30 and 37°C, which is probably the reason why it is not selected at these temperatures. The conclusion is that the adaptive strategies followed by bacteriophage Qβ, and probably other viruses, in the face of variations in host density depend not only on their advantages at this selective pressure, but also on the fitness costs that particular mutations may present in function of the rest of environmental parameters that influence viral replication and stability.

Invasive brown widow spiders avoid parasitism despite high densities.

Invasive species are sometimes less susceptible to natural enemies compared to native species, but the mechanism is often unclear. Here we tested two potential mechanisms for lower parasitism of invasive species: density-dependent parasitism and preference for human-dominated habitats. We investigated how variation in host density and habitat type affect egg sac parasitismintwo widow spider species (family Theridiidae). We compared parasitism on the egg sac of the brown widow,Latrodectus geometricus, an urban invasive species, and the white widow,Latrodectus pallidus, a species native to Israel. To investigate variation in host and parasitoid density, we measured nearest-neighbor distance between spider webs and parasitism rates in 16 sites, and in a single site monthly throughout a year. In L. pallidus, denser sites were more heavily parasitized (up to 55%) and parasitism rate increased with population density throughout the season. Extremely dense L. geometricus populations, however, had very low rates of parasitism (0-5%). We then conducted an egg sac transplant experiment in human-dominated and natural habitats. We found no parasitism of either species in the human-dominated habitat, compared to 30% parasitism of both species in the natural habitat. In addition, we found evidence for higher predation of L. pallidus than of L. geometricus egg sacs, particularly in the natural habitat. These combined results suggest that the human-dominated habitats inhabited by L. geometricus have a lower abundance of predators and parasites. We conclude that lower parasitism and predation in human-dominated habitats could contribute to the invasion success of L. geometricus.

Long‐term trends and seasonal variation in host density, temperature, and nutrients differentially affect chytrid fungi parasitising lake phytoplankton

Abstract Parasites are generally considered the most commonly occurring type of consumers, yet their biomass and population dynamics are rarely quantified at community level. Here, we used 12 years of weekly or fortnightly monitoring data (518 time points) to determine the occurrence of chytrids, fungal parasites of phytoplankton, to assess their seasonality and long‐term (seasonally‐detrended) dynamics in the pelagic plankton community of a temperate, eutrophic, and polymictic lake. Chytrid infections were observed in c. 75% of all samples with recurrent infections in multiple host taxa. Infection prevalence was highest in spring, but infections occurred throughout the entire year with an average of 2.3 host taxa infected per time point (ranging from 0 to 10 host taxa) and an average infection prevalence of 2.78% (ranging from 0% to 47.35%). Infected host biomass equalled that of the carnivorous zooplankton and decreased over time, while infection prevalence remained unchanged. Seasonal infection prevalence increased with phytoplankton biomass, but decreased with increasing temperature and phosphorus concentrations, reflecting that peak prevalence occurred in spring when temperature and phosphorus concentrations were relatively low. In contrast, seasonally‐detrended prevalence increased with temperature, but decreased with increasing phosphorus concentrations. Chytrids are a common component of the pelagic plankton community with sizeable biomass and removing an—at times—substantial proportion of the primary production, challenging the long‐standing underrepresentation of parasites in ecological studies. Chytrids responded differentially to seasonal variation and long‐term trends in host density, water temperature and nutrient availability, highlighting the need to disentangle seasonal signals from long‐term changes.

Negative density-dependent parasitism in a group-living carnivore.

Animals living at high population densities commonly experience greater exposure to disease, leading to increased parasite burdens. However, social animals can benefit immunologically and hygienically from cooperation, and individuals may alter their socio-spatial behaviour in response to infection, both of which could counteract density-related increases in exposure. Consequently, the costs and benefits of sociality for disease are often uncertain. Here, we use a long-term study of a wild European badger population (Meles meles) to investigate how within-population variation in host density determines infection with multiple parasites. Four out of five parasite taxa exhibited consistent spatial hotspots of infection, which peaked among badgers living in areas of low local population density. Combined movement, survival, spatial and social network analyses revealed that parasite avoidance was the likely cause of this negative density dependence, with possible roles for localized mortality, encounter-dilution effects, and micronutrient-enhanced immunity. These findings demonstrate that animals can organize their societies in space to minimize parasite infection, with important implications for badger behavioural ecology and for the control of badger-associated diseases.

Host-seeking efficiency can explain population dynamics of the tsetse fly Glossina morsitans morsitans in response to host density decline.

Females of all blood-feeding arthropod vectors must find and feed on a host in order to produce offspring. For tsetse—vectors of the trypanosomes that cause human and animal African trypanosomiasis—the problem is more extreme, since both sexes feed solely on blood. Host location is thus essential both for survival and reproduction. Host population density should therefore be an important driver of population dynamics for haematophagous insects, and particularly for tsetse, but the role of host density is poorly understood. We investigate the issue using data on changes in numbers of tsetse (Glossina morsitans morsitans Westwood) caught during a host elimination experiment in Zimbabwe in the 1960s. During the experiment, numbers of flies caught declined by 95%. We aimed to assess whether models including starvation-dependent mortality could explain observed changes in tsetse numbers as host density declined. An ordinary differential equation model, including starvation-dependent mortality, captured the initial dynamics of the observed tsetse population. However, whereas small numbers of tsetse were caught throughout the host elimination exercise, the modelled population went extinct. Results of a spatially explicit agent-based model suggest that this discrepancy could be explained by immigration of tsetse into the experimental plot. Variation in host density, as a result of natural and anthropogenic factors, may influence tsetse population dynamics in space and time. This has implications for Trypanosoma brucei rhodesiense transmission. Increased tsetse mortality as a consequence of low host density may decrease trypanosome transmission, but hungrier flies may be more inclined to bite humans, thereby increasing the risk of transmission to humans. Our model provides a way of exploring the role of host density on tsetse population dynamics and could be incorporated into models of trypanosome transmission dynamics to better understand how spatio-temporal variation in host density impacts trypanosome prevalence in mammalian hosts.

A hierarchical multi‐scale analysis of the spatial relationship between parasitism and host density in urban habitats

AbstractStudies on spatial density dependence in parasitism have paid scarce attention to how changes in host density at different hierarchical scales could influence parasitism in an herbivore at a particular scale. Here, we evaluated if rates of parasitism per leaf (by the whole parasitic complex and by dominant species) of the specialist leaf miner Liriomyza commelinae (Diptera: Agromyzidae) respond to variations in host density at the leaf, plant patch and site levels in an urban setting. We used multi‐level Bayesian models that incorporate the spatial hierarchy occurring in this system, as well as habitat factors previously found to have an effect on the L. commelinae parasitoid community in an urban context (patch size, patch isolation and urbanization level). According to the fitted model, overall parasitism rates decreased with increasing number of mines per leaf, being independent of host‐density variations at patch and site level. Patch structure was found to have a strong effect on parasitism rates per leaf. The analysis of parasitism by parasitoid species separately showed consistent results with the response at community level. These results suggest that parasitism of the parasitoid community here studied would be sensitive to hierarchical cues related to the host at the leaf level and to the host habitat at the patch level.

Invasion rate of deer ked depends on spatiotemporal variation in host density

Invasive parasites are of great global concern. Understanding the factors influencing the spread of invading pest species is a first step in developing effective countermeasures. Growing empirical evidence suggests that spread rates are essentially influenced by spatiotemporal dynamics of host-parasite interactions, yet approaches modelling spread rate have typically assumed static environmental conditions. We analysed invasion history of the deer ked (Lipoptena cervi) in Finland with a diffusion-reaction model, which assumed either the movement rate, the population growth rate, or both rates may depend on spatial and temporal distribution of moose (Alces alces), the main host of deer ked. We fitted the model to the data in a Bayesian framework, and used the Bayesian information criterion to show that accounting for the variation in local moose density improved the model's ability to describe the pattern of the invasion. The highest ranked model predicted higher movement rate and growth rate of deer ked with increasing moose density. Our results suggest that the historic increase in host density has facilitated the spread of the deer ked. Our approach illustrates how information about the ecology of an invasive species can be extracted from the spatial pattern of spread even with rather limited data.

Host Density Drives Spatial Variation in Parasitism of the Alfalfa Weevil,Hypera postica, Across Dryland and Irrigated Alfalfa Cropping Systems

Classical biological control against the alfalfa weevil, Hypera postica (Gyllenhal), a destructive pest of alfalfa (Medicago sativa L.), has resulted in the establishment of nine parasitoid species in the United States. Despite widespread redistribution of a number of species, there remains little postrelease data on their establishment and potential effectiveness in many regions. I surveyed parasitoids associated with alfalfa weevil larvae across 30 or more sites in eastern Montana and western North Dakota over 2 yr. Replicate sites were sampled in two habitat types that differ in their physical characteristics, flood-irrigated and dryland alfalfa fields. Irrigated systems are more productive but also more intensively disturbed habitats because of increased harvest frequency and repeated flooding. Given evidence that both habitat disturbance and herbivore density, which often increases with productivity, can influence parasitoid dynamics, I predicted that parasitism levels, the relative importance of different species, or both, would differ across these two system types. Of four larval parasitoid species released previously or recovered in the region, two were found in this study, Bathyplectes curculionis (Thomson) and Oomyzus incertus (Ratzenberg), with average levels of parasitism across habitat types and years of 37.2 and 3.5%, respectively. Parasitism levels differed between habitat types, but the effect was driven by concomitant differences in host densities that were higher in irrigated than dryland fields. Parasitoid responses to host density varied across years and species. B. curculionis exhibited positive density dependence in parasitism across sites in 2009 and negative density dependence in 2010 when host densities were higher regionally. In contrast, O. incertus exhibited positive density dependence in 2010. Our results suggest that these species may be differentially effective at different host densities. Thus, variation in host density could represent an important axis along which parasitoids exhibit spatial complementarity in function.

Influence of Multiple Factors on Insect Colonization of Heterogeneous Landscapes: A Review and Case Study with Periodical Cicadas (Homoptera: Cicadidae)

The literature on herbivorous insects in heterogeneous habitats has addressed insect population responses to patch size, distance from source populations, habitat edges, and variation in host stem density. Studies typically conclude that insect colonists respond positively to the area of host plant patches, but there is little consensus on how insects respond to variation in host density at the patch level. Although observed variation likely results partially from differences in study species and focal habitats, deviations from expectations also may reflect the importance of unmeasured habitat parameters. In this study of colonization by cicada Magicicada cassinii (Fisher) in an experimentally fragmented old field, we simultaneously examined landscape variation in patch size, distance from the sources of colonization, and local host plant stem density (at a within-patch scale) and also considered edge effects. Per stem colonization was positively related to proximity to the population source and habitat patch size but negatively related to local host stem density. The effect of edge was nonsignificant. When coarser scale estimates of local stem density (calculated by averaging the figures for all quadrats within patches) were used in analyses, fewer significant main effects were found and sometimes interactions occurred. Our study highlights the importance of including all potential explanatory variables in analyses, with what we feel is a novel observation that the explicit consideration of fine-scale, within-patch variation in local stem density can be important to the interpretation of insect dispersal and colonization.

Age‐dependent clutch size in a koinobiont parasitoid

Abstract. 1. The Lack clutch size theory predicts how many eggs a female should lay to maximise her fitness gain per clutch. However, for parasitoids that lay multiple clutches it can overestimate optimal clutch size because it does not take into account the future reproductive success of the parasitoid.2. From egg‐limitation and time‐limitation models, it is theoretically expected that (i) clutch size decreases with age if host encounter rate is constant, and (ii) clutch size should increase with host deprivation and hence with age in host‐deprived individuals.3. Clutch sizes produced by ageing females of the koinobiont gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) that were provided daily with hosts, and of females ageing with different periods of host deprivation were measured.4. Contrary to expectations, during the first 2 weeks, clutch size did not change with the age of the female parasitoid, neither with nor without increasing host‐deprivation time.5. After the age of 2 weeks, clutch size decreased for parasitoids that parasitised hosts daily. The decrease was accompanied by a strong decrease in available eggs. However, a similar decrease occurred in host‐deprived parasitoids that did not experience egg depletion, suggesting that egg limitation was not the only factor causing the decrease in clutch size.6. For koinobiont parasitoids like M. tristis that have low natural host encounter rates and short oviposition times, the costs of reproduction due to egg limitation, time limitation, or other factors are relatively small, if the natural lifespan is relatively short.7. Koinobiont parasitoid species that in natural situations experience little variation in host density and host quality might not have strongly evolved the ability to adjust clutch size.

Reproduction now or later: optimal host‐handling strategies in the whitefly parasitoidEncarsia formosa

We developed a dynamic state variable model for studying optimal host‐handling strategies in the whitefly parasitoidEncarsia formosaGahan (Hymenoptera: Aphelinidae). We assumed that (a) the function of host feeding is to gain nutrients that can be matured into eggs, (b) oögenesis is continuous and egg load dependent, (c) parasitoid survival is exponentially distributed and (d) parasitoids encounter hosts randomly, are autogenous and have unlimited access to non‐host food sources to obtain energy for maintenance and activity. The most important prediction of the model is that host feeding is maladaptive under field conditions of low host density (0.015 cm−2) and short parasitoid life expectancy (maximum reproductive period of 7 d). Nutrients from the immature stage that can be matured into eggs are sufficient to prevent egg limitation. Both host density and parasitoid life expectancy have a positive effect on the optimal host‐feeding ratio. Parasitoids that make random decisions gain on average only 35% (0.015 hosts cm−2) to 60% (1.5 hosts cm−2) of the lifetime reproductive success of parasitoids that make optimal decisions, independent of their life expectancy. Parameters that have a large impact on lifetime reproductive success and therefore drive natural selection are parasitoid life expectancy and the survival probability of deposited eggs (independent of host density), the number of host encounters per day (when host density is low) and the egg maturation rate and number of host types (when host density is high). Explaining the evolution of host‐feeding behaviour under field conditions requires field data showing that life expectancy in the field is not as short as we assumed, or may require incorporation of variation in host density. Incorporating variation in walking speed, parasitised host types or egg resorption is not expected to provide an explanation for the evolution of host‐feeding behaviour under field conditions.

Behavioural plasticity in relation to egg and time limitation: the case of two fly species in the genus Anastrepha (Diptera: Tephritidae)

Reproductive opportunities in insects that deposit their eggs in discrete resource patches are frequently limited because the availability of oviposition substrates is often spatially and temporally restricted. Such environmental variability leads individuals to confront time‐ or egg‐limitation constraints. Additionally, species with different oviposition strategies (i.e. single egg layers vs clutch layers) commonly deal with different structural and ecological characteristics of larval host plants. To test the hypothesis that oviposition strategies such as laying eggs singly or in batches (clutches) are related to these constraints (i.e. egg vs time limitation), we compared the lifetime oviposition patterns of two closely related sympatric species of Anastrepha (Diptera: Tephritidae) with different oviposition strategies. We exposed five cohorts of A. obliqua and A. ludens females, over the course of their adult lifetimes, to three conditions of “habitat quality” (measured as host density per cage): unpredictable habitat quality (host density varied randomly from day to day between 1, 5, 15, 30 and 60 hosts/cage), low habitat quality (fixed density of one host/cage) and high habitat quality (fixed density of 60 hosts/cage). Responses to host density conditions were strikingly different in the two species. (1) Frequency of host visits and oviposition events increased in A. obliqua but not in A. ludens when host densities increased. (2) Anastrepha ludens females accepted low quality hosts (i.e. fruits on which eggs had already been laid and were therefore partially covered with host marking pheromone) significantly more often than A. obliqua females did. (3) Females of A. obliqua adjusted their oviposition activity to variations in host density, whereas A. ludens females exhibited a constant oviposition pattern (i.e. did not respond to variations in host density). Based on the above, it is likely that in A. obliqua oviposition is governed by egg‐limitation and in A. ludens by time‐limitation constraints. We discuss the relationship between the oviposition strategies of each fly species and the fruiting phenology and density of their native host plants. We also address the possible influence of oogenesis modality and parasitism by braconid wasps in shaping oviposition behaviour in these insects.

Dynamics of Conjunctivitis and Mycoplasma Gallisepticum Infections in House Finches

Abstract Conjunctivitis, an infectious disease caused by Mycoplasma gallisepticum (MG), has produced a significant decline in eastern House Finches (Carpodacus mexicanus) of North America. In this paper, we present findings from two complementary studies designed to clarify annual and seasonal trends of MG infections in House Finches from the northeastern United States. The first was a field study of House Finches common to urban and residential habitat from Mercer County, New Jersey. We documented conjunctivitis in 11% (188/1,651) of the birds examined. Conjunctivitis prevalence in House Finches ranged from 0 to 43% per month, and exhibited marked seasonal fluctuation (elevations during fall and winter months and lower disease prevalence during the breeding season). There was excellent intermethod agreement on disease prevalence when measured by either presence of physical signs (conjunctivitis) or MG infection (kappa = 0.75). During the peak of the breeding season (April through June), conjunctivitis was present in a greater proportion of males lacking a cloacal protuberance than males with a cloacal protuberance (P < 0.01), but was similar between breeding and nonbreeding females. The second study, a volunteer survey, revealed the proportion of northeastern U.S. monitoring sites with at least one diseased House Finch each month ranged from a peak of 59% (August 1995) to a minimum of 12% (July 1999). Subsequent to the epidemic peak of disease in 1995, a series of recurring cycles occurred, with elevations in those proportions noted in late fall and winter and minima during the breeding season. Mycoplasmal conjunctivitis now appears endemic among House Finches of that region and demonstrates dynamics consistent with annual variation in host density.

The effect of spatial scale on interactions between two weevils and their parasitoid

Summary1. The effect of spatial scale on the interactions between three hymenopteran parasitoids and their weevil hosts was investigated. The parasitoid Mesopolobus incultus (Walker) parasitised Gymnetron pascuorum Gyll.; the parasitoids Entodon sparetus (Walker) and Bracon sp. parasitised Mecinus pyraster Herbst. Both of these weevils develop inside the seedhead of Plantago lanceolata L. but occupy different niches. Seedheads were sampled annually from 162 plants at each of two experimental sites consisting of a series of habitat patches of two distinct sizes. Data were analysed from three site‐years.2. Parasitoid densities at each site‐year were closely related to the abundance of their respective weevil hosts. The overall proportion of hosts parasitised was more variable for M. incultus than for E. sparetus and Bracon sp.3. Changes in spatial scale affected the variability of parasitoid densities. For M. incultus, there was generally a greater degree of additional heterogeneity for all increases of scale; for E. sparetus, this was true only at the largest scales; for Bracon sp., all components of variance were negative.4. The rate of parasitism was related to host density in different ways at different spatial scales. Mesopolobus incultus exhibited inverse density dependence at the finest (seedhead) scale, direct density dependence at the intermediate (plant) scale, and density independence at the large (habitat area 729 m2) scale. Entodon sparetus showed no response to variation in host density at any spatial scale. Bracon sp. showed direct density dependence only at the intermediate and largest scales.5. Parasitoids E. sparetus and Bracon sp. seemed able to detect more than one M. pyraster individual in seedheads with multiple host occupancy; a greater incidence of conspecific parasitoids than expected emerged from such seedheads.

Influence of host density on the reproductive strategy of Cephalonomia stephanoderis, a parasitoid of the coffee berry borer

AbstractThe effect of the density of the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae) on reproductive and host‐feeding behaviours of the parasitoid Cephalonomia stephanoderis (Hymenoptera: Bethylidae) was evaluated under laboratory conditions. The number of hosts used for oviposition was density‐dependent at low host density. Beyond a density of six hosts/day, the oviposition rate reached a maximum of 1.2 eggs/day due to egg limitation. Cephalonomia stephanoderis females responded to increasing host availability with a linear increase in host feeding. Overall, parasitoids killed more coffee berry borers by feeding and paralysis than by parasitism. At low host density, the pre‐oviposition phase was extended, oogenesis was delayed, more males were produced, and host feeding occasionally occurred concurrently with oviposition. We suggest that the efficacy of C. stephanoderis as a biological control agent depends on seasonal variations in host density. Inoculative vs inundative releases in coffee plantations are discussed in relation to the abundance of the coffee berry borer during the fructification and interharvest periods.

Parasitoids of the goldenrod gall moth: effects of scale on spatial density dependence

A parasitoid assemblage consisting of four hymenopteran species caused larval and pupal mortality of the host Gnorimoschema gallaeosolidaginis. In the absence of discrete host patches, the relationship between densities of hosts and parasitized hosts was examined on scales of 0.25, 1, 4, 16 and 25 m2 within a 400 m2 plot and between ten 16 m2 plots within a 1·3 km2 area. Within the 400 m2 plot, positive density dependence was observed on scales of 1, 4 and 16 m2. Rates of parasitism were found to be density independent across the ten 16 m2 plots. The nature of parasitoid and host spatial patterns were examined at the 400 m2 plot by using Goodall's (1974) paired-quadrat variance function, and by fitting the observed distribution of galls and parasitized galls to the Poisson and negative binomial on three scales. Postive density dependence at the 400 m2 plot occurred in the context of a host density gradient and may indicate aggregation of search time by parasitoids. However, significant responses on scales of 4 and 16 m2 may only reflect the sum of parasitoid responses on lower scales because of this gradient. Predominance of significant responses on a 1 m2 scale may be related to the high degree of variation in host density, and limits to active searching range of parasitoids.

Aggregation by Foraging Insect Parasitoids in Response to Local Variations in Host Density: Determining the Dimensions of a Host Patch

SUMMARY (2) We attempted to demonstrate with simulations that current analyses based upon B, the slope of the linear regression of percentage parasitism on host density, are flawed. B is inappropriate due to its dependence upon the variance of host density measurements, and, therefore, upon the nature of the underlying host distribution. (3) An alternative analysis is proposed, based upon the correlation coefficient of the same linear regression, calculated over a series of increasing quadrat sizes. (4) The proposed analysis was applied to field data of the foraging behaviour of Argochrysis armilla (Hymenoptera: Chrysididae), a parasitoid of the solitary, groundnesting wasp, Ammophila dysmica (Hymenoptera: Sphecidae). A. armilla aggregated in areas of high host nest density both during and after nest excavation. Parasitoids responded maximally to host patchiness over areas of 3-50 m2. (5) Attempts to analyse parasitoid foraging behaviour indirectly through the resultant patterns of parasitism are criticized, due to (i) the tendency of such indirect analyses to ignore temporal variation in host distribution, (ii) the tenuous link between patterns of foraging and patterns of parasitism, and (iii) the inability of studies of parasitism to distinguish active, behaviourally mediated aggregation from passive, demographically mediated aggregation.

Patch-exploitation Efficiency in a Torymid Parasite (Hymenoptera: Torymidae) of a Gall Midge (Diptera: Cecidomyiidae)

Galls of Rhopalomyia californica Felt contain from 1 to >50 larvae per gall and can be viewed as discrete host patches in space and time. Analysis of >1,800 galls from three adjacent field sites in northern California revealed that Torymus koebelei (Huber) parasitized hosts in a sufficient number of galls to permit an analysis of spatial density dependence for 10 sample dates. Average parasitization per exploited gall was consistently low (<25%), and response to spatial variation in host density was either density independent (five cases) or inversely density dependent (five cases). For all galls combined (exploited + nonexploited), average parasitization never exceeded 12%; response to spatial variation in host density was density independent in six cases and directly density dependent in four. Virtually all responses to spatial variation in host density (both data sets) qualified as “density vague.” Results indicate that T. koebelei is relatively inefficient in exploiting host patches and that it is not capable of consistently regulating or controlling the midge population. It is suggested that patch-exploitation patterns may be of some value in selecting parasites for use in biological control, particularly in augmentative release programs.

EFFECT OF HOST DISTRIBUTION ON PARASITOIDISM OF HOUSE-FLY (DIPTERA: MUSCIDAE) PUPAE BY SPALANGIA SPP. AND MUSCIDIFURAX RAPTOR (HYMENOPTERA: PTEROMALIDAE)

AbstractThe solitary, pupal parasitoids Spalangia spp. Muscidifurax raptor oviposited supernumerary eggs on house-fly pupae exposed at 3 poultry farms in north central Florida. The percentage total parasitoidism at the 3 sites was 23.7%, 33.6%, and 77.5%, while the percentage of the hosts containing supernumerary parasitoids was 6.2%, 10.7%, and 55.8%, respectively. The average number of parasitoids found per parasitoidized pupa ranged from 1.45 to 2.96. The parasitoids did not show a density-dependent response to spatial variation in host density. Among aggregates of hosts that contained at least 1 parasitoid, percentage total parasitoidism either decreased as host density increased or was unrelated to host density. When total parasitoidism was low, females tended to avoid already parasitoidized hosts. When total parasitoidism was high, more of the hosts contained multiple parasitoids than a single parasitoid.

Inferring Parasitoid Searching Behavior from Host Mortality Data: A Note of Caution

In a recent paper, Chesson (Environ. Entomol. 11: 531–537, 1982) proposed a model based on the binomial distribution which, when applied to field measurements of host mortality, can be used to obtain post hoc estimates of the probability of encounter of host-containing patches, and the conditional probability of parasitism (given encounter) for hosts present in such patches. Chesson recommends that these estimates be used to draw inferences concerning the behavior of searching parasitoids-in particular, to determine whether individuals of a given parasitoid species respond to between-patch variations in host density during search. Here, we examine the model and suggest that it may produce misleading behavioral conclusions if interpreted in the way its author recommends. Chesson's critique of an earlier study by Morrison, Lewis, and Nordlund (Environ. Entomol. 9:79–85, 1980) is also discussed.