Absence Of Parasitoids Research Articles

Variation in resource size distribution around colonies changes ant–parasitoid interactions

The distribution of resources within habitats affects species abundance, richness and composition, but the role of resource distribution in species interactions is rarely studied. In ant communities, changes in resource distribution within habitats may influence behavioral interactions because many ant species are specialized to efficiently harvest a subset of available resources. This study investigates whether interactions between the behaviorally dominant host ant Pheidole diversipilosa and its specialist parasitoid (Phoridae: Apocephalus orthocladus) depend on resource size distribution around the colony. Using in situ foraging arenas to manipulate parasitoid abundance and resource size distribution around colonies, we tested whether variation in resource size distribution allows P. diversipilosa to alter its foraging behavior in ways that lessen the impact of parasitoid attack. P. diversipilosa colonies do not lower the impact of parasitoid attack by increasing the number of workers foraging individually on small and widely dispersed resources. However, the presence of multiple large resources allows colonies to temporarily redistribute soldier ants from resources patrolled by parasitoids to other resources not patrolled by parasitoids, and to maintain soldier abundance at levels found in the absence of parasitoids. These results highlight the importance of placing behavioral interactions within the context of variation in resource distribution.

Factors Affecting Stem Borer Parasitoid Species Diversity and Parasitism in Cultivated and Natural Habitats

The effects of biotic and abiotic factors on stem borer parasitoid diversity, abundance, and parasitism were studied in cultivated and natural habitats in four agroecological zones in Kenya. Comparing habitat types, we found partial support for the "natural enemy" hypothesis, whereby, across all localities, parasitoid diversity was higher in more diverse host plant communities in natural habitats, whereas parasitoid abundance was higher in cultivated habitats. For both habitats, parasitoid richness was mainly influenced by stem borer density and/or its interaction with stem borer richness, whereas parasitoid abundance was mainly affected by stem borer abundance. Parasitoid richness was higher in localities (with bimodal rainfall distribution) with increased spatial and temporal availability of host plants that harbored the borers. Across seasons, parasitoid richness was lower in both cultivated and natural habitats in the driest locality, Mtito Andei. Overall, parasitoid diversity was low in Suam and Mtito Andei, where maize cultivation was practiced on a commercial scale and intense grazing activities persist across seasons, respectively. Across localities, habitats, and seasons, stem borer parasitism was positively correlated with parasitoid richness and abundance. Furthermore, the interaction of rainfall and altitude influenced the presence and absence of parasitoids, and consequently, stem borer parasitism. Parasitism was positively and negatively correlated with temperature in cultivated and natural habitats, respectively. Overall, natural habitats seem to serve as important refugia for sustaining parasitoid diversity, which in turn can affect stem borer parasitism in the cereal cropping system.

GENOTYPIC VARIATION AND THE ROLE OF DEFENSIVE ENDOSYMBIONTS IN AN ALL-PARTHENOGENETIC HOST-PARASITOID INTERACTION

Models of host-parasite coevolution predict pronounced genetic dynamics if resistance and infectivity are genotype-specific or associated with costs, and if selection is fueled by sufficient genetic variation. We addressed these assumptions in the black bean aphid, Aphis fabae, and its parasitoid Lysiphlebus fabarum. Parasitoid genotypes differed in infectivity and host clones exhibited huge variation for susceptibility. This variation occurred at two levels. Clones harboring Hamiltonella defensa, a bacterial endosymbiont known to protect pea aphids against parasitoids, enjoyed greatly reduced susceptibility, yet clones without H. defensa also exhibited significant variation. Although there was no evidence for genotype-specificity in the H. defensa-free clones' interaction with parasitoids, we found such evidence in clones containing the bacterium. This suggests that parasitoid genotypes differ in their ability to overcome H. defensa, resulting in an apparent host x parasitoid genotype interaction that may in fact be due to an underlying symbiont x parasitoid genotype interaction. Aphid susceptibility to parasitoids correlated negatively with fecundity and rate of increase, due to H. defensa-bearing clones being more fecund on average. Hence, possessing symbionts may also be favorable in the absence of parasitoids, which raises the question why H. defensa does not go to fixation and highlights the need to develop new models to understand the dynamics of endosymbiont-mediated coevolution.

Strong parasitoid-mediated selection in experimental populations of aphids.

Clonal diversity in asexual populations may be maintained if different clones are favoured under different environmental conditions. For aphids, parasitoids are an important variable of the biotic environment. To test whether parasitoids can mediate selection among host clones, we used experimental populations consisting of 10 clones of the peach-potato aphid, Myzus persicae, and allowed them to evolve for several generations either without parasitoids or in the presence of two species of parasitoid wasps. In the absence of parasitoids, strong shifts in clonal frequencies occurred, mostly in favour of clones with high rates of increase. The parasitoid Diaeretiella rapae hardly affected aphid densities but changed the outcome of competition by favouring one entirely resistant clone and disfavouring a highly susceptible clone. Aphidius colemani, the more infective parasitoid, strongly reduced aphid densities and dramatically changed host clonal frequencies. The most resistant clone, not a successful clone without parasitoids, became totally dominant. These results highlight the potential of temporal or spatial variation in parasitoid densities to maintain clonal diversity in their aphid hosts.

Intraspecific competition and density dependence in an Ephestia kuehniella–Venturia canescens laboratory system

A model host‐parasitoid system of Ephestia kuehniella and Venturia canescens was used to examine the influence of host and parasitoid density on host and parasitoid life‐history parameters via a two‐way factorial experimental design (5 initial host densities×3 parasitoid densities). In the absence of parasitoids, E. kuehniella experienced scramble‐type competition with reduced growth, diminished adult size and a subsequent fecundity trade‐off for mortality. The mortality that did occur was confined to the late larval and pupal stages. In the presence of parasitoids attacking the late larval stage, competition changed from scramble for food to contest for enemy‐free space, with hosts escaping parasitism being small with low fecundity and reduced egg size, and with parasitoid adult size inversely dependent on host density. Total insect emergence (host+parasitoid), a measure of the influence of host resource competition on survivorship, exhibited a threshold effect as a function of initial host density; the threshold value was increased to a higher initial host density in the presence of parasitoids. Models of host self‐limitation were fitted to the data, with the generalized Beverton‐Holt model that incorporates a threshold effect providing the best fit, and the Ricker model with no threshold providing a very poor fit to the data.

Classical Biological Control of the Spiralling whitefly, Aleurodicus dispersus Russell—An Appraisal

—Native to the Caribbean islands and Central America, the spiralling whitefly, Aleurodicus dispersus Russell (Hemiptera: Aleyrodidae), has spread to North America, South America, Asia, Africa, Australia and several Pacific islands. A total of 62 natural enemies, including 8 parasitoids, 53 predators and one fungal pathogen, are known to attack the whitefly in different countries. Of the parasitoids, the accidentally or deliberately introduced aphelinids Encarsia sp., Encarsia (?) haitiensis Dozier and E. guadeloupae Viggiani provide control of A. dispersus in many countries. The exotic coccinellid predator Nephaspis oculata Wingo also plays an important role in reducing the whitefly's populations. However, the A. dispersus remains a pest in some countries in the absence of parasitoids. This article reviews current information on biological control of A. dispersus in different countries/states, in particular India, the Pacific islands, Hawaii and countries in West Africa.

Effect of cabbage cultivars with varying levels of resistance to aphids on the performance of the parasitoid, Aphidius colemani (Hymenoptera: Braconidae).

The effect of aphid resistance in plant cultivars on parasitism of Myzus persicae (Sulzer) and Brevicoryne brassicae (Linnaeus) by Aphidius colemani Viereck was investigated under laboratory conditions using three cultivars of common cabbage, Brassica oleracea var. capitata. Significantly greater aphid populations were found on cv. Derby Day (green-leaved, susceptible to both aphid species), regardless of the presence or absence of parasitoids, compared with cv. Minicole (green-leaved, partially resistant with antibiosis factors for B. brassicae) or cv. Ruby Ball (red-leaved, partially resistant with antixenosis factors for B. brassicae). Minicole had the greatest proportion of parasitized aphids and Derby Day the least. A significantly lower percentage of emerged parasitoids were recorded on Minicole for both aphid species compared with Derby Day and Ruby Ball. The implication of the results are discussed in relation to the integrated control of aphids.

Evaluation of Inundative Releases of Eretmocerus eremicus and Encarsia formosa Beltsville Strain in Commercial Greenhouses for Control of Bemisia argentifolii (Hemiptera: Aleyrodidae) on Poinsettia Stock Plants

The effectiveness of weekly inundative releases of 3 female Eretmocerus eremicus (evaluated in 2 greenhouses) and 4–7 Encarsia formosa Beltsville strain (evaluated in 2 greenhouses) per plant for control of Bemisia argentifolii Bellows & Perring [=Bemisia tabaci (Gennadius) strain B (Bellows et al. 1994)] (Homoptera: Aleyrodidae) on poinsettia stock plants was determined under commercial growing conditions. We constructed partial life-tables for B. argentifolii in the presence and absence of parasitoids in each of 4 greenhouses to quantify parasitoid impact. Weekly population counts of B. argentifolii were also made. Initial starting densities of live B. argentifolii nymphs and pupae on cuttings used for the stock plant crop ranged from 0.25 to 1.86. Each site where each parasitoid species was tested had 1 relatively low and high initial starting density of B. argentifolii. Under these conditions, E. eremicus effectively controlled B. argentifolii at 2 sites and cuttings were harvested with 0.66–1.49 live nymphs and pupae per leaf without insecticide use. Trials with E. formosa Beltsville strain were terminated early at 2 sites and insecticides were applied before the harvesting of cuttings because this parasitoid failed to control B. argentifolii although initial starting densities of B. argentifolii were similar to those in E. eremicus greenhouses. The cost of using E. eremicus was compared with the cost of the insecticide imidacloprid for B. argentifolii control at 1 site. At a retail cost of $22 per 1,000 parasitoids in 1995, the use of E. eremicus was 44 times more expensive than imidacloprid.

Phorid parasitoids affect foraging activity ofSolenopsis richteriunder different availability of food in Argentina

Summary1. In Argentina, six species ofPseudacteonparasitoids (Phoridae) attackSolenopsis richteri, one of the two species of South American fire ant that are exotic pests in North America.2. The presence of thesePseudacteonspecies significantly reduces the number of ants at food resources in the field, as well as foraging activity generally.3. SomePseudacteonnot only attack ants walking on trails or at feeding sites, but also at mound entrances, inhibiting workers from leaving to forage.4. The average size of foraging ants (which prescribes their suitability as hosts) decreased in the presence of phorids.5. The number of attacking phorids was correlated positively with the number of ants walking towards the food on the trail before the attack.6.Solenopsis richteriworkers responded to manipulations of food size and presence or absence of parasitoids in a risk‐adjusting way, i.e. although more foragers were recruited to larger food items, attacking phorids reduced ant foraging activity by the same factor regardless of the size of the food offered.7. The data suggest thatS. richtericolonies juggle the needs to harvest food efficiently, reduce competition, and avoid excess risks from parasitoids in complex ways.

Biological control ofBemisia argentifolii(Hemiptera: Aleyrodidae) on poinsettia with inundative releases ofEretmocerus eremicus(Hymenoptera: Aphelinidae): does varying the weekly release rate affect control?

AbstractThe effectiveness of varying weekly release rates of the parasitoidEretmocerus eremicusRose & Zolnerowich for control ofBemisia argentifoliiBellows & Perring on poinsettias was determined. Two variable release rate strategies forE. eremicuswere evaluated: a low–high and a high–low release regimen. In the low–high treatment, one female parasitoid was released per plant per week for seven weeks, then the release rate was increased to five female parasitoids per plant per week for the remaining seven weeks of the trial. In the high–low treatment, five female parasitoids per plant per week were released for the first seven weeks, then the release rate was reduced to one female parasitoid per plant per week for the final seven weeks of the trial. Both release rates averaged three female parasitoids per plant per week. Life-tables were made forB. argentifoliiin the presence and absence of parasitoids. In the absence ofE. eremicus, egg to adult survivorship forB. argentifoliiwas 65%. In low–high release greenhouses, average egg to adult survivorship forB. argentifoliiwas 6% and parasitism was 28%. In high–low greenhouses, average egg to adult survivorship forB. argentifoliiwas 5% and parasitism was 16%. Average net reproductive rates forB. argentifoliiin the absence of parasitoids was 17 indicating a rapidly increasing population. Net reproductive rates were 1.46 and 1.24 for low–high and high–low release greenhouses, respectively, indicating substantially reducedB. argentifoliipopulation growth. At week 14 of the trial, densities of live nymphs and pupae were lower in high–low greenhouses when compared to low–high greenhouses. Better whitefly control is achieved with inundative releases ofE. eremicuswhen this natural enemy acts as a predator as opposed to a parasitoid.

Persistence of a parasitoid–host system: refuges and generation cycles?

The elaboration and quantification of the mechanisms lending stability and persistence to biological populations is perhaps the central issue in population ecology, both fundamentally and applied to important issues such as the maintenance of biodiversity, species conservation and the biocontrol of pests. Empirical support for mechanisms with the apparent potential to provide stability or persistence has hitherto been weak. Here, by contrast, long-term, replicated population data show that a demonstrable ‘refuge’ for prey (hosts) from parasitoid attack leads to the persistence of an otherwise unstable parasitoid–host interaction, as predicted by theory. However, the unequivocal demonstration of that refuge, and distinguishing it from a habitat with simply a lower parasitoid attack rate, is not straightforward, emphasizing the dangers of assuming the existence of a refuge too readily. The resultant parasitoid-host dynamics follow host generation length cycles, another prediction hitherto lacking convincing empirical support. Here, the support, while superficially strong, is misleading: host populations exhibit similar cycles even in the absence of parasitoids.