Increasing Host Density Research Articles

EFFECTS OF TEMPERATURE, RELATIVE HUMIDITY AND SOME BIOLOGICAL ASPECTS ON PARASITOIDS PERFORMANCE AGSINST OLEANDER APHID, APHIS NERII BOYER DE FONSCOLOMBE (HOMOPTERA, APHIDIDAE)

The study was performed to declare the effects of temperature and relative humidity on the population density of Aphis nerii, and the parasitoids, Diaeretiella rapae and Aphidius sp. at two districts, Zagazig and Mansoura (Sharkia and Dakahlia Governorates, respectively) throughout two seasons, 2005-2006 and 2006-2007 on Difla plants. The results indicated a highly significant positive correlation between temperature and population density of A. nerii in the second seasons at the two districts, while relative humidity showed negative significant at Mansoura district in the last season. On the other hand, the data cleared highly significant correlation between temperature and D. rapae during the two seasons at the two districts. Results indicated a highly negative significant between temperature and Aphidius sp at Mansoura district during two seasons. A significant negative correlation was obtained between the two parasitoids, D. rapae & Aphidius sp and A. nerii during two seasons at two districts. The parasitoid, D. rapae accepted the stages of oleander aphid, A. nerii, with average number of mummies per female parasitoid 64.2. Results indicated insignificant effect on incubation period of eggs, larval and pupal durations between the two parasitoids, D. rapae and Aphidius sp., but the total developmental period of two parasitoids had a significant effect on A. nerii, the averages were 19.38±0.57 and 22.09±0.54 with D. rapae and Aphidius sp., respectively. The work showed that leaf -arrival time decreased with increasing host density. Meanwhile, the first sting-time, numbers of oviposition and mummies increased with increasing host density.

Effects of host density, host age, temperature and gamma irradiation on the mass production of Nesolynx thymus (Hymenoptera: Eulophidae), an endoparasitoid of Uzi fly, Exorista sorbillans (Diptera: Tachinidae)

The Uzifly, Exorista sorbillans Weidemann, is an endoparasitoid of the the silkworm moth, Bombyx mori L., and can impact commercial sericulture. Effects of Uzifly age, density, temperature and gamma radiation (60Co) on the mass-production of the hyperparasitoid Nysolynx thymus (Girault) were investigated. There was a direct relationship between progeny production and an increase in number of host puparia presented, and a significant decline in the number of progeny with increased host-age. Maximum progeny production was obtained by maintaining a 1:5 to 1:8 parasitoid-to-host ratio for all the age groups. Two to 4-day-old host puparia were most suitable for obtaining the maximum progeny production of N. thymus for all the host-densities. The intrinsic rate of increase (r m day−1) increased with the increased host density for all the host-age groups. The values of net reproductive rate (R 0) and gross reproductive rate (GRR) increased for all the density levels and host age groups. On the other hand, the values for the doubling time (D) and finite capacity of increase (λ) gradually decreased with increased host density for all the host age groups. Host density and host-age significantly influenced the sex-ratio of progeny in N. thymus. Higher proportions of females were observed at higher host density levels and for younger host age groups. The progeny production and sex ratio of the parasitoid varied significantly with temperature. The maximum mean number of progeny was recorded at 25°C, while the minimum was at 30°C. The trend of progeny production at different temperatures was on the order 25 > 20 > 30°C. The highest values for the net reproductive rate (R 0) and GRR for the progeny production were recorded at 25°C compared to 20 and 30°C. Both the values for doubling time of capacity (D) and finite capacity (λ) increased with the trend of 25, 30 and 20°C. The highest value for the intrinsic rate of increase (r m day−1) of progeny production was recorded at 25°C, while the lowest value was at 20°C. The sex-ratios were always female-biased at all the temperatures. Temperature had a significant effect on the longevity of adult N. thymus. The longevity of the adults decreased with an increase of temperature for both sexes. The highest rate of parasitism was observed at 20°C followed by 25 and 30°C. More than 95% parasitism was observed at all temperatures. Gamma irradiation significantly increased the progeny production of N. thymus when reared either on early or late irradiated host puparia, particularly in the parental generation, but irradiated early host pupae were more suitable for mass production of N. thymus than the irradiated late pupae. The sex ratio of parasitoids developing from gamma irradiated host pupae varied significantly. Higher proportions of females were observed for all the dose and host-age groups.

STUDI LALAT PENGOROK DAUN LIRIOMYZA SPP. PADA PERTANAMAN BAWANG DAUN, DAN PARASITOID OPIUS CHROMATOMYIAE BELOKOBYLSKIJ & WHARTON (HYMENOPTERA: BRACONIDAE)

Studies on Leafminer Liriomyza spp. in Green Onion Fields, and Parasitoid Opius chromatomyiae Belokobylskij & Wharton (Hymenoptera: Braconidae). Field studies were conducted to determine population abundance of leafminers and their parasitoids in green onion fields in Puncak, West Java. In addition to that, laboratory studies were carried out to determine demographic parameter of Opius chromatomyiae as well as response of parasitoid to increasing host density. Results revealed that green onions were infested by two species of leafminers, Liriomyza huidobrensis and Liriomyza chinensis. Leafminer flies emerged from Erwor leaves (54.5) were significantly higher than those of RP leaves (18.65) (P = 0.0005). However, number of leafminer flies caught on sticky traps was not statistically different (P = 0.297). Two species of parasitoid, Hemiptarsenus varicornis and O. chromatomyiae, were associated with leafminers in green onion fields. Higher number of parasitoids emerged from Erwor leaves (13.68) as compared to RP (6.90) (P =0.0007 ). However, level of parasitization were 24.36% on Erwor and 28.45% on RP, and was not significantly different (P = 0.387). Laboratory studies indicated that net reproduction (Ro) of O. chromatomyiae was 28.55, generation time (T) 15.96 days, intrinsic growth rate 0.21, and total of reproductive value 223.64. The stable age distribution of parasitoid were 37.93% eggs, 24.92% larvae, 20.36% pupae and 16.78% adults. The parasitoid showed functional response type II to increasing host density, with a = 0.08 and Th = 2.58.

Host density and human activities mediate increased parasite prevalence and richness in primates threatened by habitat loss and fragmentation

1. Habitat loss and fragmentation are the principal causes of the loss of biological diversity. In addition, parasitic diseases are an emerging threat to many animals. Nevertheless, relatively few studies have tested how habitat loss and fragmentation influence the prevalence and richness of parasites in animals. 2. Several studies of nonhuman primates have shown that measures of human activity and forest fragmentation correlate with parasitism in primates. However, these studies have not tested for the ecological mechanism(s) by which human activities or forest fragmentation influence the prevalence and richness of parasites. 3. We tested the hypothesis that increased host density due to forest fragmentation and loss mediates increases in the prevalence and richness of gastrointestinal parasites in two forest primates, the Tana River red colobus (Procolobus rufomitratus, Peters 1879) and mangabey (Cercocebus galeritus galeritus, Peters 1879). We focused on population density because epidemiological theory states that host density is a key determinant of the prevalence and richness of directly transmitted parasites in animals. 4. The Tana River red colobus and mangabey are endemic to a highly fragmented forest ecosystem in eastern Kenya where habitat changes are caused by a growing human population increasingly dependent on forest resources and on clearing forest for cultivation. 5. We found that the prevalence of parasites in the two monkeys was very high compared to primates elsewhere. Density of monkeys was positively associated with forest area and disturbance in forests. In turn, the prevalence and richness of parasites was significantly associated with monkey density, and attributes indicative of human disturbance in forests. 6. We also found significant differences in the patterns of parasitism between the colobus and the mangabey possibly attributable to differences in their behavioural ecology. Colobus are arboreal folivores while mangabeys are terrestrial habitat generalists.

Egg Distributions of Insect Parasitoids: Modelling and Analysis of Temporal Data with Host Density Dependence

A simple numerical procedure is presented for the problem of estimating the parameters of models for the distribution of eggs oviposited in a host. The modelling is extended to incorporate both host density and time dependence to produce a remarkably parsimonious structure with only seven parameters to describe a data set of over 3,000 observations. This is further refined using a mixed model to accommodate several large outliers. Both models show that the level of superparasitism declines with increasing host density, and the rate declines over time. It is proposed that the differing behaviours represented by the mixed model may reflect a balance between behavioural strategies of different selective benefit.

Influência do aumento do número de pupas hospedeiras de Cochliomyia macellaria (Diptera, Calliphoridae) no desenvolvimento do parasitóide Nasonia vitripennis (Hymenoptera, Pteromalidae) em laboratório

O desenvolvimento pós-embrionário, a produtividade, a taxa de parasitismo e a razão sexual de Nasonia vitripennis (Walker, 1836) criada em pupa de Cochliomyia macellaria (Fabricius, 1775) foram estudados. Densidades diferentes de hospedeiro foram usadas (proporções 1:1, 1:2, 1:3, 1:4 e 1:5) com tempo de exposição de 72 horas, em cada uma delas. Fêmeas nulíparas originárias da colônia estoque foram individualizadas em tubos de teste cobertos com algodão hidrófobo e contendo as pupas hospedeiras. Cada tratamento constituiu-se de 10 repetições. Depois da exposição, as vespas foram descartadas e as pupas hospedeiras foram individualizadas em tubos de teste até a emergência dos adultos de C. macellaria ou N. vitripennis. Amostras das pupas hospedeiras não expostas ao parasitismo e recebendo o mesmo tratamento experimental foram usadas como controle. Os parasitóides mostraram um desenvolvimento mais lento em relação ao aumento da densidade de hospedeiros. Houve uma diminuição na produção de parasitóides por hospedeiro em densidades elevadas. A razão sexual tendeu para um desvio para nascimento de fêmeas com o aumento da densidade do hospedeiro. Os índices de parasitismo mostraram uma diminuição quando o parasitóide foi exposto a mais de duas pupas hospedeiras.

Effects of associational resistance and host density on woodland insect herbivores

1. Specialist herbivores often become less abundant per unit of host tissue as host density increases (resource dilution). They usually become less abundant when non-host species are mixed with their host plants (associational resistance). Most studies of these trends have involved herbaceous host plants and have not examined both trends for the same herbivores. 2. Three hypotheses were tested for the response of insect specialists to host plant density: resource concentration, plant apparency and resource dilution. Two hypotheses were tested for the response of herbivores to non-host plants: associational resistance and plant apparency. 3. From 1992 to 2007, I examined the responses of three monophagous insect herbivores to the densities of their host, Pinus edulis, and of two non-hosts, Pinus ponderosa and Juniperus spp. 4. Herbivore loads increased with host density, though the correlations were weak and often variable between generations. These results were consistent with the resource concentration and plant apparency hypotheses, but not with resource dilution. 5. Herbivore loads decreased as non-host density increased, consistent with the associational resistance hypothesis. This and other studies have shown that associational resistance is important in many types of plant communities. 6. The absence of resource dilution on woodland trees contrasted with studies of herbaceous host plants. Further comparisons of woody and herbaceous host plants are needed to elucidate the reasons for this difference.

Spatial patterns of Trybliographa rapae parasitism of Delia radicum larvae in oilseed rape and cauliflower

Abstract: Trybliographa rapae (Westwood) is an important parasitoid of Delia radicum (L.). Parasitism of D. radicum larvae by T. rapae in relation to host density on canola (oilseed rape) and cauliflower roots was examined at 10 field sites in Germany and Switzerland. For roots with host larvae, the proportion of roots with one or more parasitized hosts increased with increasing host density. However, for these infested roots, the parasitism of individual larvae was not consistently related to host density. When considering only roots on which there were parasitized larvae and the opportunity for multiple attacks, the proportion of larvae that were parasitized decreased with increasing host density in the field locations, and in a cage study under controlled conditions. A model of patch‐finding and number of attacks by female parasitoids suggests that patch‐finding is density‐dependent, but that low attack rate and interference effects limit numbers of attacks to three or less per visit to a host patch; the reduced number of attacks per visit leads to the inverse relationship of larval parasitism with host density in the host patches visited. The interplay of the density‐dependent and inversely density‐dependent processes appears to be responsible for the inconsistency of density dependence of overall larval parasitism in this and previous studies. In the laboratory, adult female T. rapae parasitized hosts at ≤4 cm deep in soil, but not at 6 cm deep. From the depth distribution of larval feeding sites in the field, we infer that between 4% and 20% of Delia larvae may be in a physical refuge from T. rapae parasitism, which may have a stabilizing influence on the host–parasitoid interaction.

Functional response and progeny production of the Madeira mealybug parasitoid, Anagyrus sp. nov. nr. sinope: The effect of host stage preference

Anagyrus sp. nov. nr. sinope Noyes & Menezes (Hymenoptera: Encyrtidae) is a koinobiont parasitoid with an oviposition preference for the third-instar and pre-reproductive adult female Madeira mealybug, Phenacoccus madeirensis Green (Hemiptera: Pseudococcidae). In this study, we report that host stage preference significantly influenced the parasitoid’s functional response and progeny production. The parasitoids exhibited a type II functional response when offered either second-instar or pre-reproductive adult female P. madeirensis at densities of 2–70 mealybugs per patch. The number of mealybugs parasitized within a 24-h period increased with host density up to the estimated maximum of 13 adults and 28 second-instar nymphs. The total number of progeny produced was higher from adult mealybugs and increased with host density until threshold densities (20 adults or 30 second-instar nymphs per patch) were reached. The maximum number of progeny produced in both host stages over a 24-h period was limited by the daily egg load, which was on average 23. Brood size was consistently higher in adult mealybugs and at higher host densities. The proportion of males decreased with an increase in host density and was higher when emerging from the adult mealybugs. Brood size and sex ratio of Anagyrus sp. nov. nr. sinope were similar at higher host densities, suggesting that there was a critical density after which the parasitoids only deposited a fixed brood size or sex ratio.

Are ectoparasite communities structured? Species co‐occurrence, temporal variation and null models

1. We studied temporal variation in the structure of flea communities on small mammalian hosts from eastern Slovakia using null models. We asked (a) whether flea co-occurrences in infracommunities (in the individual hosts) in different hosts as well as in the component communities (in the host species) demonstrate a non-random pattern; (b) whether this pattern is indicative of either positive or negative flea species interactions; (c) whether this pattern varies temporally; and (d) whether the expression of this pattern is related to population size of either fleas or hosts or both. 2. We constructed a presence/absence matrix of flea species for each temporal sample of a host species and calculated four metrics of co-occurrence, namely the C-score, the number of checkerboard species pairs, the number of species combinations and the variance ratio (V-ratio). Then we compared these metrics with the respective indices calculated for 5000 null matrices that were assembled randomly using two algorithms, namely fixed-fixed (FF) and fixed-equiprobable (FE). 3. Most co-occurrence metrics calculated for real data did not differ significantly from the metrics calculated for simulated matrices using the FF algorithm. However, the indices observed for 42 of 75 presence/absence matrices differed significantly from the null expectations for the FE models. Non-randomness was detected mainly by the C-score and V-ratio metrics. In all cases, the direction of non-randomness was the same, namely the aggregation, not competition, of flea species in host individuals and host species. 4. The inclusion or exclusion of the uninfested hosts in the FE models did not affect the results for individual host species. However, exclusion of the uninfested host species led to the acceptance of the null hypothesis for only six of 13 temporal samples of the component flea communities for which non-randomness was detected when the uninfested hosts were included in the analysis. 5. In most host species, the absolute values of the standardized size effect of both the C-score and V-ratio increased with an increase in host density and a concomitant decrease in flea abundance and prevalence. 6. Results of this study demonstrated that (a) flea assemblages on small mammalian hosts were structured at some times, whereas they appeared to be randomly assembled at other times; (b) whenever non-randomness of flea co-occurrences was detected, it suggested aggregation but never segregation of flea species in host individuals or populations; and (c) the expression of structure in flea assemblages depended on the level of density of both fleas and hosts.

HOST PREFERENCE OF THE COMMON VAMPIRE BAT (DESMODUS ROTUNDUS; CHIROPTERA) ASSESSED BY STABLE ISOTOPES

Abstract We examined the host selection of common vampire bats (Desmodus rotundus) with stable carbon isotopes in an area that offered 2 isotopically contrasting food sources, either introduced livestock living on C4 plants or native rainforest mammals living on C3 plants. We predicted that vampire bats would have carbon isotope ratios (δ13C) similar to those of livestock if they select exclusively cattle. The δ13C of vampire bats averaged −10.3 ± 2.6‰ (mean ± SD) and was almost identical to that of livestock (X = −12.9 ± 1.6‰). Thus, both vampire bats and livestock are clearly members of the C4 food web. Typical rainforest mammals such as the fruit-eating bat Carollia perspicillata had a δ13C of −24.4 ± 0.6‰, which identified them as members of the C3 food web. The stable carbon isotope signature of local vampire bats implies a high degree of preference for cattle. We suggest that the population expansion of D. rotundus is only indirectly linked to increasing host densities and is more directly related ...

Biological characteristics of Evania appendigaster (L.) (Hymenoptera: Evaniidae) in different densities of Periplaneta americana (L.) oothecae (Blattodea: Blattidae)

Abstract Evania appendigaster (L.) is a solitary endoparasitoid of oothecae of the American cockroach, Periplaneta americana (L.). In this work we report our studies on the parasitism of P. americana oothecae by E. appendigaster and the biological characteristics of this egg parasite, as it is affected by host density and mating. Cockroach oothecae were presented for individual females at different densities (one, two, three, four, five and six oothecae) during 2 days of exposition time. Reproduction was determined to be biparental and arrhenotokous. Host density did not influence sex ratio and longevity. The greatest reproductive efficiency of the females occurred in the first 14 days of their lifetime. At higher densities low parasitism rates were accompanied by a high percentage of dead oothecae. The percentage of parasitism decreases with increase in host density.

Patch Time Allocation by the Parasitoid Diadegma semiclausum (Hymenoptera: Ichneumonidae). II. Effects of Host Density and Distribution

This study investigated the effects of host density and distribution on the patch-leaving behavior of Diadegma semiclausum (Hymenoptera: Ichneumonidae), a solitary endoparasitoid of larval Plutella xylostella (Lepidoptera: Plutellidae). Individual female wasps were released onto an experimental plant infested with host larvae at different densities and distributions, and were allowed to freely leave for an alternative host plant placed upwind of the experimental plant in a wind tunnel. The influence of host density and distribution, as well as within-patch foraging experience, on the parasitoid’s patch-leaving tendency was analyzed by means of the proportional hazards model. This study aimed to test the predictions of a number of patch-leaving models, including the Marginal Value Theorem, “rules of thumb”, and incremental or countdown mechanisms. The parasitoid’s patch-leaving tendency decreased with increased host density, more clustered host distribution, and unsuccessful host encounter as a result of host defense, but increased with successful oviposition. None of the simple rules of thumb such as fixed time, fixed number of hosts parasitized, or fixed giving-up time was employed by this parasitoid. The results agreed with the general predictions of the Marginal Value Theorem that patch residence time and numbers of ovipositions by the parasitoid increased with increasing host density. The decreasing influence of oviposition on the parasitoid’s patch-leaving tendency, regardless of host density or distribution, was consistent with the prediction of a countdown mechanism.

Parasite prevalence and richness in sympatric colobines: effects of host density

Factors that influence proximity and the number and duration of contacts among individuals can influence parasite transmission among hosts, and thus parasite prevalence and species richness are expected to increase with increasing host density. To examine this prediction we took advantage of a unique situation. Following the clearing of a forest fragment that supported red colobus (Piliocolobus tephrosceles) and black-and-white colobus (Colobus guereza), the animals moved into a neighboring fragment that we had been monitoring for a number of years and for which we had described the primate parasite community. After the animals immigrated into the fragment, the colobus populations more than doubled and colobus density became almost twice that found in Kibale National Park, Uganda. Despite this increase in host density, the richness of the parasite community did not increase. However, in both colobus species the prevalence of Trichuris sp., the only commonly occurring gastrointestinal parasite, increased. Over the next 5 years the prevalence and intensity of infection of Trichuris sp. in red colobus declined and their population numbers slowly increased. In contrast, the prevalence and intensity of infection of Trichuris sp. increased in black-and-white colobus and remained high following the immigration, and their population size declined. While Trichuris sp. infections are typically asymptomatic, we consider it a possibility that they contributed to the decline of the black-and-white colobus, and that the red colobus may be serving as a reservoir for Trichuris, thereby increasing the infection risk for black-and-white colobus.

Host density influences parasitism of the armyworm Pseudaletia unipuncta in agricultural landscapes

Summary Several studies have shown positive responses of parasitism to either host density or landscape complexity. However, no experiments have manipulated host density in landscapes of differing complexity. Here we report the results of a field experiment conducted to determine how host density and agricultural landscape structure jointly affect parasitism and parasitoid diversity of Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae). Parasitism was assessed by experimentally adding P. unipuncta sentinel larvae at low (1 larvae/plant) and high (3 larvae/plant) densities to detect parasitism in commercial cornfields located in a complex and a simple agricultural landscape. The braconid wasps Glyptapanteles militaris (Walsh) and Meteorus spp. accounted for 98.4% of the observed parasitism. Landscape structure did not influence parasitism (80.2% on average) and contrary to expectations, showed a trend towards increased parasitoid richness and diversity in the simple landscape. Increasing host density revealed a trend of increasing parasitoid richness and diversity, and differentially affected parasitism at the parasitoid specific level. G. militaris parasitized a significantly greater proportion of hosts at low host density, while the opposite occurred for Meteorus spp. (primarily M. communis ). These offsetting responses of parasitoids resulted in the lack of an overall host density effect on parasitism. The differential response of these parasitoids to host density is discussed in relation to differences in morphological and life history characteristics. Our results suggest that the specific composition of parasitoid assemblages could significantly alter parasitism at different host densities independently of landscape structural complexity.

Relative abundance of Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and its parasitoids, and the impact of augmentative release of Eretmocerus spp. (Hymenoptera: Aphelinidae) on the population dynamics of the pest in Burkina Faso (West Africa)

Whitefly infestations and parasitism were monitored year-round in overlapping cotton crops sown on three dates in Burkina Faso. The relative abundance of B. tabaci (Gennadius) and its parasitoids, Eretmocerus spp. and Encarsia spp., was recorded in control and insecticide-sprayed plots. Low B. tabaci populations developed during the first half of the rainy season. Pest populations increased when rainfall was ending, and the levels reached were higher in insecticide-treated plots (48 nymphs/leaf) than in control plots (25 nymphs/leaf). Parasitism reached 88.7% in control plots, and 53.7% in insecticide-treated plots. Eretmocerus spp. nymphs were more abundant than Encarsia spp. in both treated and control plots. A positive and significant curvilinear relationship was observed where % parasitism, on a linear scale, rose to a plateau with logarithmic increase in host density. In general % parasitism was correlated with the abundance of pest populations except in March and April where parasitism increased while B. tabaci populations decreased. In a separate experiment, adult Eretmocerus spp. were released into caged cotton plants to study the impact of augmentative releases of the parasites on the population dynamics of the pest. Pest densities increased from 1.47 nymphs/leaf to 39.4 nymphs/leaf in the control, but were reduced to 0.8 and 0.6 nymphs/leaf in the cages where, respectively, 4 and 8 parasitoids were released per plant. It appears that parasitism is an important factor reducing B. tabaci populations during and after the cotton-growing season, and that Eretmocerus spp. are promising biological control candidates against the pest in cotton.

Effectiveness of different species/strains of entomopathogenic nematodes for control of western flower thrips (Frankliniella occidentalis) at various concentrations, host densities, and temperatures

Abstract Entomopathogenic nematode (EPN) species/strains (Rhabditida: Steinernematidae and Heterorhabditidae) were tested at a concentration of 200 infective juveniles (IJs) per cm 2 against mixed soil-dwelling life stages (i.e., second-instar larvae, prepupae, and pupae) of the western flower thrips (WFT) Frankliniella occidentalis (Thysanoptera: Thripidae), in a plant growing substrate under laboratory conditions. The different EPN species/strains resulted in WFT corrected mortality (CM) values ranging between 2.6 and 60%. In general, Heterorhabditis spp. were more virulent than Steinernema spp. Increasing concentrations of selected EPN species/strains led to an increase in thrips CM, with significant differences among species/strains. Heterorhabditis indica LN2 caused higher CM than the other strains, with 30 and 90% as highest CM at the lowest and highest concentration tested, i.e., 100 and 1000 IJs cm −2 , respectively. In an experiment with different WFT densities, CM caused by Steinernema bicornutum negatively correlated with host density. However, with H. indica LN2, CM increased, though not always significantly, with increasing host densities. Generally, the effect of host density on efficacy of EPNs depended on concentrations. When tested over a range of temperatures, 25 °C was the optimal temperature for both H. indica LN2 and S. bicornutum . H. indica LN2, tropical in origin, and S. bicornutum , isolated from a more temperate environment, performed better at higher and lower temperatures, respectively.

Host–parasite interaction between branchiurans (Crustacea: Argulidae) and piranhas (Osteichthyes: Serrasalminae) in the Pantanal wetland of Brazil

Ecological studies of host–parasite interactions in the tropics are generally restricted to descriptive taxonomic aspects. The present study had as its objective identification of the metazoan ectoparasites of piranhas Pygocentrus nattereri, Serrasalmus spilopleura and S. marginatus in lentic and lotic environments in the Pantanal region, Brazil. We collected the samples in the Miranda River basin and in three ponds. We caught fishes using hook and line. We found that branchiuran crustaceans infested about 30% of the piranhas. We found four branchiuran species: Dolops carvalhoi, Argulus elongatus, Argulus aff. juparanaensis and Dipterolpetis hirundo, which represent new findings in host–parasite relationships in the Neotropics. When comparing lotic and lentic environments, we observed higher infestation rates in rivers than in ponds for all the hosts. P. nattereri had the highest prevalence and the intensity rates in both the environments. The body parts of hosts with the highest frequencies of parasitism were the ventral area in P. nattereri, the gullet area in S. spilopleura and ventral areas and external gills in S. marginatus. In relation to the prevalence of the parasites we observed significant differences between the hydrological seasons and the piranha species, nevertheless hosts were more parasitized in the dry season. The species with highest rates of infestation, P. nattereri and S. spilopleura, live in schools, which indicates that the high parasite load may be a direct consequence of the proximity between individuals. We believe that the higher rates of infestation in the dry season result from the increase in host density in drying ponds imitating the conditions found in fish farming. Seasonal changes in water level and temperature can also influence the rates of parasitism.

MATRIX HETEROGENEITY AND HOST–PARASITOID INTERACTIONS IN SPACE

In this study, I experimentally examined how the landscape matrix influenced the movement, oviposition behavior, and spatial distribution of Anagrus columbi, a common egg parasitoid of the planthopper Prokelisia crocea. Both species exist among discrete patches of prairie cordgrass (Spartina pectinata), the sole host plant of P. crocea. Based on out-planted cordgrass pots bearing host eggs (to assess parasitism) or sticky leaves (traps for adult A. columbi), I found that the distribution of adult female A. columbi and pattern of ovipositions within a cordgrass patch were strongly matrix dependent. Female densities were 59% lower on the edge than interior of patches embedded in a mudflat matrix, but were evenly distributed within patches embedded in a matrix consisting of either native grasses or the exotic grass smooth brome (Bromus inermis). In contrast, parasitism was higher in the interior than edge for patches in all three matrix types. The lack of correspondence between A. columbi density and parasitism was attributed to differences in oviposition behavior: A. columbi parasitized 71% more hosts per capita in the interior than edge for patches embedded in nonhost grasses, but equal numbers on the edge and interior of patches embedded in mudflat. Matrix-dependent differences in the within-patch distribution and oviposition behavior of A. columbi can influence the distribution of parasitism risk and host–parasitoid stability at the patch level. Matrix composition also affected the pattern of movement through the matrix and the colonization of nearby cordgrass patches. Anagrus columbi females emigrating from a mudflat-embedded patch were captured at very low, but constant, numbers with distance out into the matrix, suggesting that they were reluctant to enter or remain in the mudflat. In contrast, A. columbi females entering a nonhost grass matrix had numbers that were high near the patch border and then declined exponentially with distance. These patterns of movement were likely responsible for the very different colonization rates for experimental patches embedded in different matrix types and located 3 m from a source patch of A. columbi. Patches embedded in brome were colonized at a rate that was 3.0 and 5.7 times higher than for patches in native grass or mudflat, respectively. Finally, based on a census of cordgrass patches spanning five generations, A. columbi densities and proportion of patches occupied generally increased with increasing host density, patch isolation, and the proportion of the surrounding matrix that was mudflat. Patch size had no effect on the distribution of A. columbi. Overall, these data suggest that cordgrass patches in a nonhost grass matrix, particularly smooth brome, have high connectivity relative to patches in a mudflat matrix. Changes in connectivity due to changes in matrix composition can significantly influence host–parasitoid persistence at the metapopulation level. Corresponding Editor (ad hoc): J. A. Rosenheim.

Trade-offs in group living: transmission and disease resistance in leaf-cutting ants.

Sociality can be associated with significant costs due to the increased risk of disease transmission. However, in some organisms the costs may be offset by benefits due to improvements in defences against parasites. To examine this possible trade-off between infection risk and disease resistance, we used Acromyrmex leaf-cutting ants and the entomopathogenic fungus Metarhizium anisopliae as the model system. Ants exposed to the parasite were found to have substantially improved survival when they were kept with nest-mates, while the cost of being in a group in terms of increased disease transmission was very low. The efficiency of transmission is described by the transmission parameter, which decreased with increasing host density showing that transmission rates are inversely density dependent. Both grooming and antibiotic secretions appeared to be important in resistance against the parasite, with the defences of small workers being particularly effective. The results indicate that leaf-cutting ant colonies may have much greater resistance to disease than would be predicted from the high densities of host individuals within them. Unlike most organisms, group living in these ants may actually be associated with a net benefit in terms of disease dynamics.