Parasitoid Guild Research Articles

Numerical relationships of the Solidago altissima stem gall insect-parasitoid guild food chain.

The field site conditions (soil pH, soil moisture, soil nutrient availability, etc.) and abundances of Solidago altissima (often included in S. canadensis sensu lato), three S. altissima specific stem gall formers, and the parasitepredator guilds for two of the three gall insects were investigated. It was found that S. altissima is tolerant of a wide range of site conditions. Herbivore (stem gall insects) occurrences were positively correlated with plant occurrence, in a linear fashion. However, there was no disproportionate increase in stem gall insect densities with plant density as might be predicted by the resource concentration hypothesis. Parasitoid guilds were exploiting stem gall insect populations over a wide range of occurrence, but were under-utilizing fields of higher herbivore occurrences. Path analysis showed a high degree of predictability in the causal models, with all but 14% of the ball gall parasitoid guild and all but 43% of the elliptical gall parasitoid guild occurrences explained by the direct influences of stem gall insect occurrence and the indirect influences of goldenrod occurrence and site conditions. The numerical relations of this three trophic level system suggest a well-integrated and well-controlled food chain.

Studies on the Natural Enemy Complex of the Holly Leaf-Miner: The Effects of Scale on the Detection of Aggregative Responses and the Implications for Biological Control

We measured holly leaf-miner (Phytomyza ilicis) densities, and mortalities imposed by a guild of specific parasitoids and birds, using nested quadrats varying in size from 0.03 to 1 m2. Mortalities imposed by three species of pupal parasitoids, one of which behaves as a facultative hyperparasitoid, were independent of host density at all spatial scales. The larval parasitoid, Chrysocharis gemma, in contrast, apparently aggregated strongly in regions of high host density, imposing spatially density-dependent mortality. The effect was most obvious at the smallest sampling-scale (0.03 m2) and became progressively weaker as the data were aggregated into larger sampling units. It could not be detected at all in samples of 1 m2. Within the smallest sampling units, aggregative responses (as revealed by host mortality imposed by C. gemma) were strongest in regions of low overall host abundance, and vice-versa. These results are in accord with predictions of parasitoid behaviour based on optimum foraging theory. Predation by birds, mainly blue tits Parus caeruleus, was inversely density-dependent. The possibility that birds may aggregate in response to very large patches of the holly leaf-miner (>1 m2) is discussed. Consistent with recent theoretical predictions, the species imposing spatially density dependent mortality (C. gemma) has been used as an effective biological control agent against the holly leaf-miner in Canada. Pupal parasitoids, which do not impose spatially density-dependent mortality, failed as biocontrol agents. Our results are discussed in relation to two problems, namely that of detecting an aggregative response by natural enemies in the field, and what constitutes a 'patch'. Prey grouped on the scale evoking an aggregative response by predators constitute 'patches'. Patches of the same species will be much larger for large active predators than for small, inactive ones. If the wrong spatial scale is chosen, it may be impossible to find evidence for aggregative responses in the field, even though they are present.

Parasitoids, Patches, and Phenology: Their Possible Role in the Local Extinction of a Cynipid Gall Wasp Population

The cynipid gall wasp Xanthoteras politum (Bassett) and its associated chalcid and inquiline parasitoid community were studied from 1976 to 1978 at a postfire site near Lake Oswego in the New Jersey Pine Barrens. X. politum and two other cynipid species selectively attacked new sucker shoots growing from larger fire—damaged trees. Chalcid and inquiline parasitoid attack was the major mortality factor in the population in all 3 yr. In each successive year a higher frequency of the host population was killed by parasitoid attack, and by 1978 on 76 host galls occurred at the site. Inquiline cynpids were minor parasitoid elements the 1st yr of the gall outbreak, but dominated the parasitoid guild in subsequent seasons. Their dominance is attributed to two major factors. First, they attack galls earlier than other parasitoids, and those galls attacked remained at a smaller size and were not chosen for oviposition by later arriving chalcids. Secondly, inquilines did not emerge in spring and infest other gall species as did the chalcid parasitoids. These inquilines remained in the leaf litter and reinfested the next asexual generation of X. politum which appeared on trees in mid—July. The parastioid community displayed neither strong stability nor consistent composition. The apparent overexploitation of the host population by parasitoids may be to their selective advantage as phenological changes in host plants eventually eliminate X. politum from postfire sites anyway.

Niche Relationships Among Parasitic Insects Occurring in a Temporary Habitat

Pattern in the parasitism of a lepidopteran host, Spodoptera praefica, were assessed through an analysis of niche relationships among the parasitic species. No parasitized hosts were recovered from nondisrupted habitats. However, three species, Chelonus insularis, Apanteles marginiventris, and Hyposoter exiguae, commonly parasitized S. praefica in a disrupted habitat (mowed alfalfa field). Overall, H. exiguae was numerically dominant as well as the most rapid exploiter of hosts following habitat disruption. Species diversity within the parasitoid guild significantly increased with time following each habitat disruption. Significant trends were also observed in species evenness. The niche breadth of C. insularis and H. exiguae varied with time of year and time following habitat disruption while the niche breadth of A. marginiventris remained relatively constant. Niche overlap was generally high between the two larval parasitoids, A. marginiventris and H. exiguae, and low between these species and C. insularis, an egg—larval parasitoid. The values determined for species diversity, niche breadth, and niche overlap suggest that a low degree of temporal stability rather than interspecific competition strongly influenced species composition within the parasitoid guild of S. praefica.

Energetics of the Solidago Canadensis‐stem Gall Insect‐parasitoid Guild Interaction

Energy budgets are developed for two goldenrod stem gall insects: Eurosta solidaginis (Diptera: Tephritidae) and Gnorimoshema gallaesolidaginis (Lepidoptera: Gelechiidae). Energy budgets are also constructed for the parasitoid guilds associated with each of the gall insects. Over 25 kJ or °7% of mean ramet production were used in formation of these falls and support of the insects. If the gall insects were parasitized, only 15—20 kJ were used in gall and insect production. The ecological efficiencies of energy transfer between the gall insects and their parasites on a square—metre basis were 10 and 8% for Eurosta and Gnorimoshema, respectively. These insect energetics are then related to Harnett and Abrahamson's (1979) results for the effects of the two stem gall insects on resource allocation patterns in goldenrod, where gall—bearing ramets compared to non—gall—bearing ramets showed decreases of 45% (Eurosta) and 28% (Gnorimoshema) in ramet porgagule production. The energy lost form the ramet by having a ball gall is greater than is accounted for by energy flow into the gall, and the presence of either gall can have an appreciable impact on plant fitness.

ADAPTIVE STRATEGIES OF LARVAL PARASITOIDS OF THE ALFALFA WEEVIL (COLEOPTERA: CURCULIONIDAE)

AbstractThree species of Hymenoptera comprise the guild of parasitoids utilizing alfalfa weevil, Hypera postica, larvae in Ohio. Where all three coexist, there has been a yearly increase in parasitization by Bathyplectes anurus and concomitant decreases in Bathyplectes curculionis and Tetrastichus incertus. We conclude that B. anurus dominance is due to its precise synchronization with peak host numbers, its high reproductive capacity, competitive superiority, and rapid search and handling. Domination of ephemeral hosts by such a parasitoid is in agreement with the general conclusions of Force, and Ehler and van den Bosch. B. curculionis and T. incertus compensate for imperfect synchrony with multivoltism and vagility (both spp.), aggregation and rapid search (B. curculionis), or random search and gregarious development (T. incertus).

Parasitiods Utilizing the Same Host: Adaptive Nature of Differences in Size and Form

Eleven hymenopterous parasitoid species commonly exploit one sawfly host, Neodiprion swainei. A comparison of the area of forewing, structure of ovary, and length of ovipositor of some of these species is revealing. The three guilds of parasitoids that attck the larval, eonymphal, and cocoon stages of the host, respectively, face different problems in host exploitation. Females of the larval parasitoid guild have large egg production and large wings. They are adapted to exploit a relatively abundant, easily discovered stage of the host, but their progeny are subject to high mortality while in the host. Females of the cocoon parasitoid guild have small wings and low egg production. They seek a scarcer, well—concealed stage of the host, and their progeny sufer less mortality. Members of the eonymphal parasitoid guild show intermediate characteristics. Among the cocoon parasitoids, the species have different ovipositor lengths, presumably allowing them to utilize segments of the host population, either different size classes or hosts that are buried at different depths in the forest litter. Comparison of ovipositor lengths indicates how similar the species can be in size, and therefore larval food exploitation, before competitive displacement prevents coexistence.