Variation In Parasitism Rates Research Articles

Parasitism of the leaf‐cutting ant Atta bisphaerica by phorid flies: biology and seasonal and inter‐colonial parasitism rates

AbstractThe leaf‐cutting ant Atta bisphaerica Forel (Hymenoptera: Formicidae, Attini) forages leaves from monocotyledons in natural environments and is a serious pest of pastures and sugarcane plantations in Brazil. This study investigated aspects of the interactions between A. bisphaerica and dipteran Phoridae parasitoids. The seasonal and inter‐colonial variations in parasitism rates were also investigated. Ants from four A. bisphaerica colonies were sampled, with a total of 91 554 worker ants collected and evaluated. Of these ants, 4.4% were found to have been parasitized by three phorid species: Apocephalus attophilus Borgmeier (relative parasitism rate: 40%), Myrmosicarius grandicornis Borgmeier (31%), and Eibesfeldtphora bragancai (Brown) (28%). The months with the lowest average temperatures, lowest relative humidity, and lowest rainfall (characteristics of the dry season of the region under study) corresponded to the highest parasitism rates, especially by A. attophilus. Apocephalus attophilus and E. bragancai had the highest relative parasitism rates in two of the colonies, 45 and 41%, whereas M. grandicornis was the dominant species in each of the other two colonies, corresponding to approximately 40% of total parasitism. The variation in parasitism rates observed between colonies could be explained, at least in part, by the relationship between the size of the parasitoids and the size of the host ants. Concerning the biological characteristics, A. attophilus had the shortest life cycle, the highest successful emergence rate, and produced 2.5× more parasitoids per host. These characteristics, combined with the ability to cause higher levels of mortality of parasitized worker ants when compared to the two other phorid species, indicates that A. attophilus is an efficient natural enemy of A. bisphaerica and is therefore a candidate for a biocontrol program.

Natural biocontrol of fruit flies in indigenous hosts: A perspective for population control in the agroecosystem

Fruit flies (Diptera Tephritidae) are a major pest to fruit production in Africa. Most efforts to control these pests are focused directly on the orchards where fruit damage has been recorded. However, fruit flies are generally able to develop on alternative indigenous fruits from which they continuously recolonize the orchards, challenging the ability to efficiently control these pests. In the natural and semi-natural environments, parasitic Hymenoptera remain the primary means of keeping populations of fruit flies at a reasonable level, but little is known about the diversity, the distribution and the ability of these parasitoids to effectively control populations. In this study, we explored the diversity and abundance of parasitoids of fruit fly pests in indigenous and exotic cultivated fruits of two separate regions of South Africa: North-East (Limpopo and Mpumalanga provinces) and South-West (Western Cape Province). A total of 16 fruit species were collected and incubated to obtain emergence of fruit flies and their associated parasitoids. Ceratitis capitata (Wiedemann), Ceratitis rosa sensu lato Karsch, Ceratitis cosyra (Walker) and Bactrocera dorsalis (Hendel) were reared from the incubated fruit. In the Western Cape Province, only C. capitata and C. rosa sensu lato were recorded. Our results show an important contrast in the distribution of fruit fly parasitoid species between regions and between indigenous fruit types. Fopius ceratitivorus Wharton and Psyttalia humilis (Silvestri) (Hymenoptera: Braconidae) dominated parasitism in the north-eastern part of the country, while fruit fly parasitoids were almost absent on fruit flies in the Western Cape Province. Parasitism rate of fruit flies ranged from 0 to ca 50% depending on fruit species and locality within a region. The exact drivers of this variation in parasitism rate remain unclear, but smaller fruits generally seem to allow better control of infesting fruit flies by parasitoids than larger ones. The fruits of some indigenous trees (Berchemia discolor (Klotzsch) Hemsl.; Syzygium cordatum Hochst. ex Krauss and Garcinia livingstonei T. Anderson) are hosts of high densities of both fruit flies and their parasitoids. These species are good candidates for the application of augmentoria for the mass production and release of biological control agents.

Do natural enemies explain fluctuations in low-density spruce budworm populations?

Understanding the causal pathways through which forest insect outbreaks are triggered is important for resource managers. However, detailed population dynamics studies are hard to conduct in low-density, pre-outbreak populations because the insects are difficult to sample in sufficient numbers. Using laboratory-raised larvae installed in the field across a 1,000km east-west gradient in Québec (Canada) over an 11-yr period, we examined if parasitism and predation were likely to explain fluctuations in low-density spruce budworm (Choristoneura fumiferana; SBW) populations. Parasitism rates by the two main larval parasitoid species, Elachertus cacoeciae and Tranosema rostrale, peaked during different years. This suggests that temporal fluctuations in overall parasitism were partly buffered by compensatory dynamics among parasitoid species. Still, spatial covariance analyses indicate that the residual interannual variation in parasitism rates was substantial and correlated over large distances (up to 700km). On the other hand, interannual variation in predation rates was not spatially correlated. Piecewise structural equationmodels indicate that temporal variation in parasitism and predation does not influence temporal variation in wild SBW abundance. Spatially, however, SBWs installed in warmer locations tended to show higher parasitism rates, and these higher rates correlated with lower wild SBW population levels. Overall, the results indicate that large-scale drops in parasitism occur and could potentially contribute to SBW population increases. However, during the period covered by this study, other factors such as direct effects of weather on SBW larval development or indirect effects through host tree physiology or phenology were more likely to explain large-scale variation in wild SBW populations.

Preliminary evaluation of the parasitoid wasp, Collyria catoptron, as a potential biological control agent against the wheat stem sawfly, Cephus cinctus, in North America

ABSTRACTThe wheat stem sawfly, Cephus cinctus (Hymenoptera: Cephidae) is the major pest of wheat in the northern plains of North America, with biological control providing a potentially useful management tool. Foreign exploration by the USDA-ARS identified Collyria catoptron (Hymenoptera: Ichneumonidae), a parasitoid wasp of Cephus fumipennis in northern China, as a potential biological control agent. Here we carry out a preliminary evaluation of the potential suitability of C. catoptron, as a candidate agent against C. cinctus. Specifically we: (1) Quantify the spatio-temporal variation in parasitism rates on the native host from collection sites in northern China over six years, and (2) Assess whether C. catoptron will oviposit and complete development in the novel targeted host. Maximum parasitism of the native host in the native range was 38%, exceeding the theoretical threshold for successful biological control. Site occupancy levels were high (83%) suggesting good colonisation abilities. C. catoptron clearly recognised and attacked C. cinctus, with parasitism levels exceeding those observed on the co-evolved host C. fumipennis. However, we found no evidence that C. catoptron can complete development in C. cinctus; no adults were reared from this host in either year of the study. In contrast 50–60% of the parasitoids survived to emerge as adults in C. fumipennis. Thus, C. catoptron is unlikely to be a suitable agent against C. cinctus due to basic host incompatibility. Future work will be directed towards developing conservation biological control approaches using native parasitoid species already present in the USA.

Geographic variation in parasitism rates of two sympatric cuckoo hosts in China.

Rates of brood parasitism vary extensively among host species and populations of a single host species. In this study, we documented and compared parasitism rates of two sympatric hosts, the Oriental Reed Warbler (Acrocephalus orientalis) and the Reed Parrotbill (Paradoxornis heudei), in three populations in China. We found that the Common Cuckoo (Cuculus canorus) is the only parasite using both the Oriental Reed Warbler and Reed Parrotbill as hosts, with a parasitism rate of 22.4%-34.3% and 0%-4.6%, respectively. The multiple parasitism rates were positively correlated with local parasitism rates across three geographic populations of Oriental Reed Warbler, which implies that higher pressure of parasitism lead to higher multiple parasitism rate. Furthermore, only one phenotype of cuckoo eggs was found in the nests of these two host species. Our results lead to two conclusions: (1) The Oriental Reed Warbler should be considered the major host of Common Cuckoo in our study sites; and (2) obligate parasitism on Oriental Reed Warbler by Common Cuckoo is specialized but flexible to some extent, i.e., using Reed Parrotbill as a secondary host. Further studies focusing on egg recognition and rejection behaviour of these two host species should be conducted to test our predictions.

Indirect effects on mutualisms: parasitism of bumble bees and pollination service to plants

Researchers increasingly recognize the important role of mutualisms in structuring communities and view positive interactions in a community context rather than as simple pairwise interactions. Indirect effects, such as those that predators have on lower trophic levels, are a key process in community ecology. However, such top-down indirect effects have rarely been extended to mutualisms. Antagonists of one mutualist have the potential to negatively affect the second mutualist through negative effects on their partner, and the magnitude of such effects should vary with mutualism strength. Bumble bees are ecologically and economically important pollinators that are an ideal system to determine if such indirect effects play an important role in mutualisms. Bumble bees are attacked by an array of parasites and predators, and they interact with a range of plants that vary in their dependence on bumble bees for reproduction. We tested whether variation in parasitism rates by Nosema bombi, Crithidia bombi, and conopid flies correlated with reproduction of greenhouse-raised plants placed in the field. At multiple sites over two years, we studied four plant species that varied in reliance on bumble bees as pollinators. We found a consistent negative relationship between Nosema parasitism and measures of pollination for Trifolium pratense and Solanum carolinense, plant species with high bumble bee visitation, whereas Rudbeckia hirta and Daucus carota, plant species with generalized pollination, experienced no impacts of Nosema. However, both Crithidia and conopids showed inconsistent relationships with pollination service. Although these patterns are correlational, they provide evidence that parasites of bumble bees may have negative indirect effects on plants, and that mutualism strength can moderate the magnitude of such effects.

Low parasitism rates in parthenogenetic bagworm moths do not support the parasitoid hypothesis for sex

The parasite hypothesis for sex is one of the many theories that have been suggested to solve the mystery of the widespread occurrence of sex despite its high short-term costs. It suggests that sexual lineages have an evolutionary advantage over parthenogens because they can frequently generate new genotypes that are temporarily less prone to coevolving parasites. In this study, we looked for further supporting evidence for the parasite hypothesis of sex in an attempt to understand the coexistence of sexual and parthenogenetic bagworm moths (Naryciinae). The bagworm moths and their parasitoids form one of the few natural host-parasite systems where sexual and parthenogenetic hosts are apparently not separated by ecological or geographical barriers. Furthermore, in support of the parasite hypothesis for sex, parthenogenetic presence is negatively correlated with parasitism rate. We specifically tested, by identifying the reproductive mode of the parasitized individuals, whether parasitoids preferentially attack the parthenogens in sites with both sexual and parthenogenetic forms, as predicted by the parasite hypothesis. We collected hosts from sites with different frequencies of parthenogenetic and sexual moths. A DNA barcoding approach was used to determine the reproductive mode of the parasitized hosts. Furthermore, we investigated whether differences in host and parasitoid phenology could provide an alternative explanation for the variation in parasitism rates between parthenogens and sexuals. Our results contradict the prediction of the parasite hypothesis because parthenogenetic bagworm moths were less parasitized than sexuals in sympatric sites. Our findings can be explained by differences in phenology between the parthenogenetic and sexual moths rather than genetic incompatibility between parthenogenetic hosts and parasitoids. The stable coexistence of sexual and parthenogenetic Naryciinae despite the many apparent costs of sex in this system remains a mystery. Our work adds to the list of studies were the assumptions of the parasite hypothesis for sex are not all met.

Parasitoid diversity reduces the variability in pest control services across time on farms

Recent declines in biodiversity have increased interest in the link between biodiversity and the provision and sustainability of ecosystem services across space and time. We mapped the complex network of interactions between herbivores and parasitoids to examine the relationship between parasitoid species richness, functional group diversity and the provision of natural pest control services. Quantitative food webs were constructed for 10 organic and 10 conventional farms. Parasitoid species richness varied from 26 to 58 species and we found a significant positive relationship between parasitoid species richness and temporal stability in parasitism rates. Higher species richness was associated with lower variation in parasitism rate. A functional group analysis showed significantly greater parasitoid species complementarity on organic farms, with on average more species in each functional group. We simulated parasitoid removal to predict whether organic farms experienced greater robustness of parasitism in the face of local extinctions. This analysis showed no consistent differences between the organic and conventional farm pairs in terms of loss of pest control service. Finally, it was found that the different habitats that make up each farm do not contribute equally to parasitoid species diversity, and that hedgerows produced more parasitoid species, significantly more so on organic farms.

Habitat and plant specificity of Trichogramma egg parasitoids—underlying mechanisms and implications

Egg parasitoids of the genus Trichogramma are among the most important and best-studied natural enemies worldwide. Parasitism levels by Trichogramma vary greatly among different habitats, plants or plant structures on which the host eggs are located. Here we summarise the published evidence on mechanisms that may underlie the observed variation in parasitism rates. These mechanisms include plant spacing, plant structure, plant surface structure and chemistry, plant volatiles and plant colour. In addition, plants can affect parasitoid behaviour and activity by providing carbohydrate food sources such as nectar to the adult wasps, and by affecting the nutritional quality of the host eggs for progeny development. Knowledge of plant and habitat factors that affect Trichogramma spp. efficacy has important implications for biological control, and for assessing the risks that mass-released Trichogramma spp. may pose to non-target insects. [KEYWORDS: Biological control ; Conservation biological control ; Food ecology ; Multi-trophic interactions ; Non-target effects ; Plant volatiles ; Trichomes]

Density-dependent foraging behaviors in a parasitoid lead to density-dependent parasitism of its host.

Empirical studies of spatial heterogeneity in parasitism by insect parasitoids have focused largely on patterns, while the many possible underlying mechanisms have been little studied in the field. We conducted experimental and observational studies on Tachinomyia similis (Diptera: Tachinidae) attacking western tussock moths (Orgyia vetusta; Lepidoptera: Lymantriidae) on lupine bushes at Bodega Bay, Calif., USA. We examined several foraging behaviors that have been hypothesized to create density-dependent variation in parasitism rates, including spatial aggregation of parasitoids to high host density, mutual interference among searching parasitoids and decelerating functional responses of the parasitoid. At the spatial scale of individual bushes, we detected both aggregation to a high density and a decelerating functional response. The resulting spatial pattern of parasitism was best fit by two models; one included an effect of parasitoid aggregation and the other included an effect of aggregation and a decelerating functional response. Most of the variation in parasitism was not correlated with density of O. vetusta.

Forest composition, host‐population density, and parasitism of spruce budworm Choristoneura fumiferana eggs by Trichogramma minutum

AbstractThe frequency of egg parasitism in the spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae) by Trichogramma minutum Riley (Hymenoptera: Trichogrammatidae) was recorded in several host populations during the rising phase of an outbreak in the Gatineau Valley (Quebec, Canada) in 1999 and 2000. Some of the observed spruce budworm populations were subjected to experimental aerial applications of Bacillus thuringiensis var. kurstaki during the course of the study. Untreated spruce budworm population densities at the onset of the study were directly related to stand composition (the proportion of non‐host basal area) but unrelated to plant species diversity. Much of the variation in budworm egg mass parasitism rates was accounted for (77.1% of total variance) by three variables: year, spruce budworm egg population density, and proportion of non‐host basal area in the stand. The exploitation of eggs within parasitised egg masses was related to year, B. thuringiensis treatment, egg mass size, and the proportion of egg masses parasitised in the host population (76.9% of variance). The yearly variation in parasitism rate was considerable, and was consistent with climatic conditions, namely heat accumulation and precipitation during the spruce budworm oviposition period. Parasitism by T. minutum on eggs of C. fumiferana was recorded in 16 additional sites in 2000, to confirm the general form of the inverse density dependence of parasitism. Observations recorded by Neilson in the Green River area of New Brunswick (Canada) between 1948 and 1957 are discussed in the context of the present analysis. These results indicate that parasitism by T. minutum may be more predictable than previously believed and may be an important and consistent source of mortality in the several years preceding an outbreak of C. fumiferana, especially in mixed‐wood stands and in warmer, drier portions of the spruce budworm's range.

Temporal and Spatial Variation in a Parasitoid Community Associated with the Herbivores That Feed on Missouri <I>Quercus</I>

Abstract To determine whether the probability of parasitoid attack differed across years, seasons, sites, host plant species, and herbivore feeding modes, leaf-chewing insects (Lepidoptera) that feed on Quercus alba L. and Q. velutina Lam. were collected four times in the Missouri Ozarks (May, June, July, and August–September) in each of the years 1993–1995 and reared in the laboratory. Parasitism rates at a given census were relatively constant from year to year and decreased as the season progressed from May to September: 30% of the herbivores collected in May were parasitized, whereas <15% were attacked in the summer and fall. Similarly, parasitism rates were predictable based on host plant (higher on Q. velutina than on Q. alba) and feeding guild (higher for leaf miners than for external feeders, leaf tiers and leaf webbers, in the summer and fall). Leaf rollers encountered in May had the highest probability of being parasitized. There was also significant variation in parasitism rates among sites. Co...

Rapid decline of host defences in response to reduced cuckoo parasitism: behavioural flexibility of reed warblers in a changing world

On Wicken Fen and nearby watercourses eastern England, parasitism by cuckoos, Cuculus canorus, declined from 26 and 16 of reed warbler (Acrocephalus scirpaceus) nests in 1985 and 1986, respectively...

Influence of Alternate Host Densities on Brown-Headed Cowbird Parasitism Rates in Black-Capped Vireos

Brown-headed Cowbird (Molothrus ater) parasitism is thought to be partly influenced by density of the host species, although tests of host density are relatively rare. We examined parasitism rates relative to the density of individual host species and densities of coexisting host species. We monitored 392 nests among coexisting host species and measured their densities among six habitats on Fort Hood Military Reservation, Texas during 1991-1992 to test the hypothesis that coexisting species affect parasitism rates in the endangered Black-capped Vireo (Vireo atricapillus). Black-capped Vireos and White-eyed Vireos (V. griseus) suffered three to four times higher cowbird parasitism than Northern Cardinals (Cardinalis cardinalis) or Painted Buntings (Passerina ciris). After controlling for removal of female cowbirds, which has been conducted on the study site since 1988, parasitism rates in Black-capped Vireos were positively correlated with cumulative host density in general, and Northern Cardinal density in particular. Only density of Northern Cardinals explained a significant amount of variation in parasitism rates in Black-capped Vireos among sites. We suggest that cowbirds may be attracted to conspicuous species, such as cardinals, and that high densities of such species may negatively affect coexisting species by increasing probabilities of being parasitized. Vireo nests were characterized by less nest concealment, greater canopy cover, and more stems than other species. However, nest site and vegetation characteristics did not differ between parasitized and unparasitized nests for any species, suggesting habitat was unimportant to parasitism.

Structure of the Parasitoid Communities of Grass-Feeding Chalcid Wasps

The parasitoid assemblages associated with 18 species of chalcid wasps feeding on 10 grass species were sampled quantitatively between 1980 and 1992 at 24 sites in Wales and England to examine food web structure, the size and composition of the parasitoid complexes, and structures of the local communities. The complete food web included 87 species organized into five trophic levels. The lower two consumer trophic levels (the herbivores and primary parasitoids) were characterized by extreme host specificity, whereas the top two trophic levels (hyperparasitoids and tertiary parasitoids) comprised more generalized omnivores. The high levels of specialization resulted in a compartmentalized web. The regional species richness of the parasitoid complexes associated with Tetramesa species (Eurytomidae) depends primarily on measures of host abundance. Total species richness was significantly positively correlated with mean log host density (number per m 2 ) and the number of larvae per grass stem, explaining 70.5% of the variance in richness. Similarly, the number of specialist parasitoids was correlated with host density, whereas generalist species richness was most strongly correlated with host density and more weakly with host gall-formation and voltinism. The only determinant of local parasitoid species richness is regional richness (the latter explained 97.3% of the variance in the former). Further, there was no β-diversity in this food web, and every species found on a host species in Britain is found in virtually every local host population. The relatively few cases where fewer parasitoid species were found represent undersampling at particular sites. Limited evidence indicates that similar results apply to host populations on continental Europe. Thus, there is no evidence that the richness of local communities is constrained by species interactions. Variation in parasitism rates both between and within host species cannot be explained by the available variables, so the forces affecting host utilization rates remain unknown. We found that the formation of a gall had no influence on host mortality rates, offering no support for the hypothesis that galling provides protection from parasitoids. The parasitoids of most host species have consistent relative abundances over time and space. We could not identify any ecological correlates of the degree of consistency, but it may be due to the extremely widespread occurrence of all of the parasitoids coupled with their innate host-finding abilities.

The Window of Vulnerability of a Shoot‐Galling Sawfly to Attack by a Parasitoid

We studied the susceptibility of a shoot—galling sawfly, Euura lasiolepis, to attack by the parasitoid Lathrostizus euurae. Each Euura had a window of vulnerability: a limited time period when the parasitoid attacked it. Lathrostizus attacked a limited range of gall sizes and ages during each sample period. Gall age was related to its position on a shoot, and Lathrostizus shifted the gall position it attacked as it tracked vulnerable—aged galls. The parasitoid emergence period matched the window of vulnerability of its host. Lathrostizus attacked and successfully parasitized a low proportion of hosts encountered. A low interval—parasitism rate uniformly distributed among Euura galls formed at different times is consistent with the pattern expected if the window of vulnerability influenced attack. The window of vulnerability was formed by the interaction of gall and sawfly development rates. Two hypothesis that defined the window were supported: (1) the host development hypothesis, that Lathrostizus is a larval parasite and does not attack galls containing eggs and (2) the gall toughness hypothesis, that large, old galls that are tough are not attacked by Lathrostizus. Our data did not support (3) the ovipositor limitation hypothesis, that some galls were too large for the larva inside to be reached by the Lathrostizus ovipositor. We observed that if these galls were soft, they were attacked. Lathrostizus parasitism rates on Euura varied widely among willow clones. Willows differed in the rate of gall toughening, which produced different—sized windows and thus contributed to variation in parasitism rates among Euura populations on different willow clones.

Simulations on the role of the egg parasite, Ooencyrtus kuvanae (Howard), in the population dynamics of the gypsy moth

Computer simulations were run to examine the effects of Ooencrytus kuvanae (Howard) parasitism on gypsy moth populations. Two difference equation models were used, one, a simple theoretical model containing only a few important components, the other, a more comprehensive model with component functions based on field data. The simulations of the first model showed that O. Kuvanae cannot control gypsy moth populations by itself. Egg parasitism was most important during the outbreak and collapse phase of the gypsy moth population cycle, and was of little importance in the endemic phase. The endemic phase was, however, lower and lasted longer with O. kuvanae parasitism than without. Simulations with the second model generally supported these conclusions. The second set of simulations also demonstrated that due to the non-linearity of the density dependence of O. kuvanae parasitism, they gypsy moth populations could be forced into a chaotic regime (unstable, aperiodic population behavior). The variation in parasitism rate from the simulations was compared with that of field populations and found to be essentially the same, indicating that the simulation models are realistic.