Parasitoid Populations Research Articles

Spatial distribution of flower vs. honeydew resources in cereal fields may affect aphid parasitism

The potential impact of nectar availability for biological pest control is of increasing relevance due to the disappearance of non-crop plants in intensified agricultural landscapes. However, little is known about how floral resources and their spatial distribution in and around crop fields influence cereal aphid parasitoid (Hymenoptera, Aphidiidae) survival and population dynamics. Cereal aphid parasitoids may be less dependent on flowers as they can use honeydew from their aphid hosts as an alternative food resource. However, honeydew appears to be less suitable as a food source for parasitoids when compared to nectar. Here the effects of flower distribution (nectar), aphid abundance (honeydew quantity) and honeydew quality on parasitoid life history parameters were explored using a spatially explicit, individual-based simulation model. In addition, effects of the availability of floral resources in field margins on parasitism levels in winter wheat were investigated in a field study. In the simulation study three flower distributions were compared: (i) flowers restricted to the field margin (most common in agricultural landscapes) or (ii) flowers in strips within the crop, and (iii) random flower distribution in the field. Simulations suggest that nectar feeding increases parasitoid longevity and parasitism at low aphid densities, but only in fields with randomly distributed flowers. In contrast, the spatial distribution of flowers did not affect the parasitoid longevity and parasitism rate at high aphid densities, where parasitoid feeding was restricted to the abundant honeydew resources. In simulations, parasitoid survival and the number of aphids parasitized were also affected by the nutritional quality of the honeydew, with low quality honeydew having a less positive effect on parasitoid longevity and parasitism rate. The field study did not indicate evidence of a significant relationship between flower cover in field margins and parasitism rates in wheat near the margins. This finding is compatible with results from simulations and suggests that honeydew feeding is more prevalent in cereal aphid parasitoids than nectar feeding. In conclusion, nectar may be an important factor for the potential biocontrol success of cereal aphid parasitoids, but most likely only at low aphid densities and especially when flowers are broadly distributed in the field.

Responses of the specialist biological control agent, Aleochara bilineata, to vegetational diversity in canola agroecosystems

Plant biodiversity is known to affect insect populations, both herbivores and their natural enemies, and as a consequence, habitat management through increased plant species composition and abundance can be exploited for sustainable pest management. In agroecosystems where crop monocultures are the routine production practice, plant biodiversity can be increased by maintaining small populations of weeds, with potential beneficial effects arising from concomitant increases in the abundance of predator and parasitoid populations. We manipulated weed populations in both species of canola, Brassica rapa L. and Brassica napus L., to investigate responses of adults of Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae), an important natural enemy of root maggots ( Delia spp., Diptera: Anthomyiidae). Larval root maggots feed on canola taproots, disrupting the flow of water and nutrients within the plants, causing substantial yield reductions. Aleochara bilineata is a predator–parasitoid that attacks all root maggot pre-imaginal life stages. Activity density of A. bilineata increased as monocotyledonous weed biomass declined. Significant preferences between canola species were observed, with A. bilineata associated most frequently with B. rapa compared with B. napus. Our research suggests that improved management of root maggot infestations in canola through enhancement of populations of the A. bilineata predator–parasitoid could be accomplished by reducing weed infestations; however, such recommendations should consider other predators in the system and the role of weeds in reducing root maggot oviposition and damage.

Enhancing biological control of sugarcane shoot borer, Chilo infuscatellus (Lepidoptera: Pyralidae), through use of radiation to improve laboratory rearing and field augmentation of egg and larval parasitoids

Feasibility studies were conducted to evaluate the use of gamma radiation to improve the production and field performance of the egg parasitoid Trichogramma chilonis Ishii and to improve production of the gregarious larval endoparasitoid Cotesia flavipes Cameron for biological control of the sugarcane shoot borer, Chilo infuscatellus Snellen. Sitotroga cerealella (Olivier) eggs were used as hosts for T. chilonis and the suitability of non-irradiated host eggs decreased as the age of the eggs increased, with no success in parasitization of eggs older than 4 days of age. However, irradiation of host eggs using 20–25 Gray (Gy) decreased the age effect and significantly more 2-, 3-, 4- and 6-day-old irradiated eggs were successfully parasitized than non-irradiated eggs. Radiation doses of 20 and 25 Gy were most effective for economical production of T. chilonis. Irradiation of host eggs did not affect the hatch percentage up to a dose of 15 Gy. Hatchability was significantly reduced at higher doses, with negligible hatching at 50 Gy. Irradiation also skewed the sex ratio of S. cerealella in favor of males at higher doses. Radiation at 60–80 Gy improved the suitability of C. infuscatellus larvae for parasitism by C. flavipes, allowing normally unsuitable fourth and fifth instar larvae of C. infuscatellus to be successfully parasitized. The sex ratio of parasitoids reared on irradiated larvae was skewed in favor of females. Irradiation also slowed immature development of C. flavipes and the combination of irradiation and low temperature (10°C) proved effective for prolonged storage of the parasitoids. Pupae of C. flavipes irradiated at 20 Gy could be stored for 2 months at 10° C without apparent loss of quality and deferred emergence by 29–30 days. Provisioning with irradiated supplemental hosts in the field increased overall pest suppression. The infestation by C. infuscatellus was higher in the control sugarcane block, where it remained above the economic threshold level (10% infestation) from April to October. High temperatures and low relative humidity during May to July reduced increases of the parasitoid populations. These findings were used to facilitate the area-wide application of biocontrol agents in a 40,000 hectare area to suppress sugarcane borers to sub-economic levels (<10% infestation). Sugarcane borer damage was 5.9% in treated blocks vs. 19.2% in untreated control blocks.

Spatial Scale and Density Dependence in a Host Parasitoid System: An Arboreal Ant,Azteca instabilis, and ItsPseudacteonPhorid Parasitoid

Much literature is dedicated to the study of density dependence in host-parasitoid systems. The theoretical literature identifies two potential stabilization mechanisms relating the response of the parasitoid to host density. One is a behavioral response that occurs at a local level, and the other is a demographic response that occurs at a larger spatial scale with heterogeneous patches of host concentrations. In a series of field trials at the small spatial scale (tens of meters) and a 10-mo census at the large spatial scale (hundreds of meters), we examined the attack rates of Pseudacteon sp. phorid flies on Azteca instabilis F. Smith ant hosts. At the local scale, we examined phorid attack rates on ants placed at increasing distances from ant nests (0-12 m) and on different densities of ants (1-50 workers) placed near the nests. At the large scale, we placed whole body extracts of A. instabilis in areas with several A. instabilis nests and in areas >100 m from the nearest nest. For all observations, we recorded the time of the first phorid attack and the number of phorid attacks in a defined time period. We found clear density-dependent responses at both scales. Phorid attack rates were highest within 2 m of A. instabilis nests and at ant densities >25. At the larger scale, phorid attack rate was greater in areas with A. instabilis nests, although this result was obscured during the dry season when the population of parasitoids is dramatically reduced. We propose several behavioral and population-level responses that may contribute to the observed results and propose several testable hypotheses. We conclude that, in this system, density dependence does happen through a behavioral mechanism of the phorid actively seeking concentrations of ants, but also, a population level response is likely caused by the significant difference in phorid attack rates in areas far from ant nests compared with areas nearby to nests.

Evaluation of the local population of Eretmocerus mundus (Hymenoptera: Aphelinidae) for biological control of Bemisia tabaci biotype B (Hemiptera: Aleyrodidae) in greenhouse peppers in Argentina

Bemisia tabaci biotype B is a key pest in pepper crops in Argentina. The parasitoid Eretmocerus mundus is frequently found parasitizing this whitefly in greenhouses without pesticide applications. The present studies were carried out with the objective of evaluating control obtained with different rate and number of parasitoid releases under experimental conditions. Release rate: cages with pepper pots were positioned in an experimental greenhouse and randomly assigned to the release rate treatments (0, 1 and 3 pairs of E. mundus/plant/week with a total of three introductions). Number of releases: similar cages were assigned to the number of parasitoid introduction treatments (0, 1, 2 and 3) with the best release rate obtained in the previous trial. In both assays whitefly (adults and nymphs) and parasitoid (parasitized nymphs) population sizes in each cage were monitored weekly for a period of 10 weeks. Results suggested that the introduction of 2 E. mundus/plant/week was enough to suppress host population compared to control treatment (peaks of 7.75 adults and 58.75 nymphs/cage and 643.75 adults and 1598 nymphs/cage, respectively) ( p < 0.05), with 85% of parasitism. E. mundus had to be introduced three times to achieve the best pest control (peaks of 1.17 adults and 20.33 nymphs/cage vs. 55.67 adults and 75 nymphs/cage in control treatment) with 84% of parasitism ( p < 0.05). These results were then validated in a pepper crop under experimental greenhouse conditions. Whitefly population was lower in those greenhouses where E. mundus was released compared to control greenhouses (0.15 adults and 0.71 nymphs/4 leaves and 0.73 adults and 1.64 nymphs/4 leaves, respectively), with a peak of 54% of parasitism ( p < 0.05). We concluded that good suppression of B. tabaci could be achieved using E. mundus under spring conditions in Argentina.

Predicting the time to colonization of the parasitoid Diadegma semiclausum: The importance of the shape of spatial dispersal kernels for biological control

The time at which natural enemies colonize crop fields is an important determinant of their ability to suppress pest populations. This timing depends on the distance between source and sink habitats in the landscape. Here we estimate the time to colonization of sink habitats from a distant source habitat, using empirical mark-capture data of Diadegma semiclausum in Broccoli. The data originated from experiments conducted at two locations and dispersal was quantified by suction sampling before and after a major disturbance. Three dispersal kernels were fitted to the dispersal data: a normal, a negative exponential, and a square root negative exponential kernel. These kernels are characterized by a thin, intermediate and a fat tail, respectively. The dispersal kernels were included in an integro-difference equation model for parasitoid population redistribution to generate estimates of time to colonization of D. semiclausum in sink habitats at distances between 100 and 2000 m from a source. We show that the three dispersal kernels receive similar support from the data, but can produce a wide range of outcomes. The estimated arrival time of 1% of the D. semiclausum population at a distance 2000 m from the source ranges from 12 days to a length of time greatly exceeding the life span of the parasitoid. The square root negative exponential function, having the thickest tail among the tested functions, gave the fastest spread and colonization in three of the four data sets, but it gave the slowest redistribution in the fourth. In all four data sets, the rate of accumulation at the target increased with the mean dispersal distance of the fitted kernel model, irrespective of the fatness of the tail. This study underscores the importance of selecting a proper dispersal kernel for modelling spread and colonization time of organisms, and of the collection of pertinent data that enable kernel estimation and that can discriminate between different kernel shapes.

Compatibility of an entomopathogenic fungus with a predator and a parasitoid in the biological control of greenhouse whitefly

An ongoing debate in biological control consists of whether interference between biological agents can disrupt pest control. This study investigated the outcome of interactions between the entomopathogen Beauveria bassiana with the whitefly predator Dicyphus hesperus and the parasitoid Encarsia formosa, as well as their effect on the control of the greenhouse whitefly Trialeurodes vaporariorum on greenhouse tomato crops. Our objective was to determine whether the generalist B. bassiana would disrupt biological control by interfering with D. hesperus or E. formosa. In experimental greenhouses, whitefly, parasitoid and predator populations were established, and over 27 days, tomato plants were sprayed with three applications of the B. bassiana based product BotaniGard® (5.13×103 conidia/mm2) or water (control). Populations of greenhouse whitefly and biological control organisms were regularly monitored in control and B. bassiana-treated compartments. Overall, 10.6% of all whiteflies in treated compartments were infected, and 0.98% were both infected and parasitized. There were 31.7 and 22.3% fewer immature and adult whiteflies, respectively, on B. bassiana-treated plants relative to controls. Parasitism by E. formosa and predation by D. hesperus occurred at rates of 7.5 and 2.5%, respectively, in B. bassiana-treated compartments, and 5 and 6%, respectively in control compartments. Our study suggests that applications of B. bassiana for short-term biological control of greenhouse whiteflies are compatible with the concurrent use of E. formosa and D. hesperus on greenhouse tomato crops.

톱다리개미허리노린재의 집합페로몬이 노린재검정알벌의 발생에 미치는 영향

The kairomonal activity of the aggregation pheromones of Riptortus clavatus for the egg parasitoid, Gryon japonicum, were investigated in soybean field. The parasitism rate of G. japonicum gradually decreased when the distances between pheromone traps and egg sites were increased from 0 to 15 m. The active distance of pheromone source for parasitoid attraction was estimated to be ca. 15 m. Under field conditions, the parasitoid wasp population peaked during the late August and early September, and the parasitism rate increased when the number of pheromone traps was increased from one to three per . However, insecticide-treated plots, the level of parasitism recorded is relatively low. Sex ratios in field populations of G. japonicum were female-biased.

The impact of parasitoid emergence time on host–parasitoid population dynamics

We investigate the effect of parasitoid phenology on host–parasitoid population cycles. Recent experimental research has shown that parasitized hosts can continue to interact with their unparasitized counterparts through competition. Parasitoid phenology, in particular the timing of emergence from the host, determines the duration of this competition. We construct a discrete-time host–parasitoid model in which within-generation dynamics associated with parasitoid timing is explicitly incorporated. We found that late-emerging parasitoids induce less severe, but more frequent, host outbreaks, independent of the choice of competition model. The competition experienced by the parasitized host reduces the parasitoids’ numerical response to changes in host numbers, preventing the ‘boom-bust’ dynamics associated with more efficient parasitoids. We tested our findings against experimental data for the forest tent caterpillar ( Malacosoma disstria Hübner) system, where a large number of consecutive years at a high host density is synonymous with severe forest damage.

Mitigation of egg limitation in parasitoids: immediate hormonal response and enhanced oogenesis after host use

Synovigenic insects (i.e., insects emerging with few ripe eggs and maturing more eggs during the course of their lifetime) may suffer from transient egg limitation due to the stochastic nature of encounters with patchy hosts and the low availability of ripe eggs at any given time point. Egg limitation also affects the stability of host-parasitoid models. Thus, quantification of the behavioral decisions influencing egg maturation, identification of the underlying physiological mechanisms, and determination of the rate of egg maturation are highly relevant to both parasitoid behavioral ecology and host-parasitoid population dynamics. The aim of this study was to identify, in a highly controlled setting, the physiological processes responsible for egg manufacture after varying host use by a synovigenic parasitoid. We quantified the time course of the reproductive hormonal response and subsequent egg production in the host feeding bruchid parasitoid, Eupelmus vuilleti (Hymenoptera: Eupelmidae) for three treatments: (1) host examination without further host use, (2) host feeding, and (3) host feeding followed by oviposition. We carried out continuous behavioral observations with single hosts, enzyme immunoassays for quantifying ecdysteroids, and ovary dissection. Ecdysone levels increased within two minutes of contact with a host, the fastest hormonal response reported for any insect. Even simple contact with a host, without further host use, triggered an increase in hormone levels, leading to the maturation of a single egg, using body reserves only. Feeding on the host caused a much larger increase in ecdysone levels and was followed by a marked increase in oogenesis. Oviposition had a weak effect on hormone levels, but increased oogenesis. We discuss the mechanisms responsible for these rapid responses, the source of ecdysteroids, and the implications of our results for the population dynamics of host-parasitoid systems and the behavioral ecology of synovigenic species.

Extended larval development time for aphid parasitoids in the presence of plant endosymbionts

Abstract 1. Variation in plant chemistry does not only mediate interactions between plants and herbivores but also those between herbivores and their natural enemies, and plants and natural enemies.2. Endophytic fungi complete their whole life cycle within the host plant’s tissue and are associated with a large diversity of plant species. Endophytes of the genus Neotyphodium alter the chemistry of the host plant by producing herbivore toxic alkaloids.3. Here we asked whether the endophyte‐tolerant aphid species Metopolophium festucae could be defended against its parasitoid Aphidius ervi when feeding on endophyte‐infected plants. In a laboratory experiment, we compared life‐history traits of A. ervi when exposed to hosts on endophyte‐infected or endophyte‐free Lolium perenne.4. The presence of endophytes significantly increased larval and pupal development times, but did not affect the mortality of immature parasitoids or the longevity of the adults. Although the number of parasitoid mummies tended to be reduced on endophyte‐infected plants, the number of emerging parasitoids did not differ significantly between the two treatments.5. This shows that the metabolism of individual aphids feeding on infected plants may be changed and help in the defence against parasitoids. An increase in parasitoid development time should ultimately reduce the population growth of A. ervi. Therefore, endophyte presence may represent an advantage for endophyte‐tolerant aphid species through extended parasitoid development and its effect on parasitoid population dynamics.

Effects of host plant resistance on insect pests and its parasitoid: A case study of wheat-aphid–parasitoid system

Host plant resistance can effectively reduce pest insect populations, but a concern is whether plant resistance could also negatively affect the natural enemies of the insect pests. In this paper the effect of three wheat cultivars on the population of an aphid species, Sitobion avenae (F.) and it’s parasitoids, Aphidius spp., were investigated in the field experiments in 2004 and 2005. Percentage of parasitized aphids at peak sampling date was also recorded. Both the aphid and parasitoid populations varied in the three wheat cultivars at the sampling dates. The density peak of parasitoid population in all the three wheat cultivars was 9–12 days behind the density peak of the aphid population. At the population peaks both the aphid and parasitoid densities were significantly higher in the susceptible than in the resistant cultivars. Furthermore, the accumulative parasitism was significantly higher in the resistant than in the susceptible cultivar, and level of resistance of the wheat cultivars had no effect on aphid mummy weight parasitized by A. spp. These results suggested that the host plant resistance against aphids enhanced the parasitism of aphid species S. avenae (F.) by its parasitoid A. spp. in wheat field.

Effect of spinosad against major insect pests and natural enemies in rice ecosystem.

Field experiments were carried out to evaluate the efficacy of spinosad 45 SC, a biological product from actinomycetes, Saccharopolyspora spinosa @ 45 and 54 g a.i/ha in comparison to three other insecticides, viz., lambdacyhalothrin (12.5 g a.i./ha), chlorpyriphos 50% + cypermethrin 5% @ 344 g a.i./ha and monocrotophos @ 500 g a.i./ha as check against major insect pests of rice at Regional Agricultural Research Station, Pattambi, Kerala Agricultural University for three seasons viz., Kharif 2005, Rabi 2005 and 2006. Pooled analysis of three crop seasons indicated that spinosad @ 54 g a.i./ha was the most effective against rice stem borer, gall midge, leaffolder and whorl maggot. It caused 63 and 49 per cent reduction in dead hearts and white ears, respectively. The lower dosage of spinosad @ 45 g a.i./ha reduced whorl maggot infestation by 34 per cent. Spinosad @ 54 g a.i./ha resulted in 94 per cent reduction in leaffolder and 50 per cent reduction in gall midge infestation. Spinosad treatment also resulted in 14 per cent increase in rice yield. Spinosad caused no significant effect on spider population and was safe to spiders that predominate the predatory fauna in rice. Monocrotophos and lambdacyhaiothrin significantly reduced the spider and larval parasitoid populations in the rice ecosystem.

Wolbachia in two populations of Melittobia digitata Dahms (Hymenoptera: Eulophidae)

We investigated two populations of Melittobia digitata Dahms, a gregarious parasitoid (primarily upon a wide range of solitary bees, wasps, and flies), in search of Wolbachia infection. The first population, from Xalapa, Mexico, was originally collected from and reared on Mexican fruit fly pupae, Anastrepha ludens Loew (Diptera: Tephritidae); the other, from Athens, Georgia, was collected from and reared on prepupae of mud dauber wasps, Trypoxylon politum Say (Hymenoptera: Crabronidae). PCR studies of the ITS2 region corroborated that both parasitoid populations were the same species; this potentially provides a useful molecular taxonomic profile since females of Melittobia species are superficially similar. Amplification of the Wolbachia surface protein gene (wsp) confirmed the presence of this endosymbiont in both populations. Sequencing revealed that the Wolbachia harbored in both populations exhibited a wsp belonging to a unique subgroup (denoted here as Dig) within the B-supergroup of known wsp genes. This new subgroup of wsp may either belong to a different strain of Wolbachia from those previously found to infect Melittobia or may be the result of a recombination event. In either case, known hosts of Wolbachia with a wsp of this subgroup are only distantly related taxonomically. Reasons are advanced as to why Melittobia - an easily reared and managed parasitoid - holds promise as an instructive model organism of Wolbachia infection amenable to the investigation of Wolbachia strains among its diverse hosts.

Dinâmica populacional do psilídeo-de-concha Glycaspis brimblecombei (Moore, 1964) (Hemiptera: Psyllidae) e de seu parasitóide Psyllaephagus bliteus (Hymenoptera: Encyrtidae) em floresta de Eucalyptus camaldulensis

O psilídeo-de-concha, Glycaspis brimblecombei, é uma praga exótica específica de Eucalyptus, que causa desfolha, secamento de ponteiros e ramos, podendo levar as árvores à morte. O parasitóide Psyllaephagus bliteus é indicado como principal agente no controle biológico desta praga. Portanto, o monitoramento adequado de as ambas espécies por um método eficiente de amostragem facilitaria o manejo para uma futura tomada de decisão. Devido à falta de informações referentes ao seu comportamento em plantações extensivas de eucalipto no Brasil, este trabalho teve como objetivo estudar as populações de G. brimblecombei e de P. bliteus em florestas de Eucalyptus camaldulensis, utilizando armadilhas amarelas para determinação de sua flutuação populacional e correlacionado-as com variáveis meteorológicas (temperatura e precipitação). A determinação da flutuação populacional (53 pontos amostrais) foi realizada em talhão de 19ha com E. camaldulensis em Luíz Antônio, SP, em 10avaliações quinzenais, entre janeiro e junho de 2005. As populações de ambas as espécies apresentaram correlação inversamente proporcional em função da temperatura e não há correlação com a precipitação no período avaliado.

Effects of Prior Experience on Host Selection and Host Utilization by Two Populations of &lt;I&gt;Anisopteromalus calandrae&lt;/I&gt; (Hymenoptera: Pteromalidae)

The effect of adult learning through an oviposition and host feeding experience was studied in two populations of Anisopteromalus calandrae (Hymenoptera: Pteromalidae), in choice and no-choice experiments in the laboratory using last instars of rice weevil, Sitophilus oryzae L., and lesser grain borer, Rhyzopertha dominica (F.), as hosts. In choice experiments, 20 wheat kernels infested by rice weevil and 20 by lesser grain borer were placed randomly in petri dish (15 by 100 mm) arenas. In no-choice experiments, 10 host-infested wheat kernels, either with rice weevils or with lesser grain borers, were mixed thoroughly in 500 g of uninfested wheat placed in a 0.945-liter glass jar (75 by 170 mm) arenas. A. calandrae females from a laboratory and field populations that were experienced with host larvae or naïve were introduced singly into each experimental arena and allowed to sting and oviposit for 24 h. An oviposition experience with rice weevil or lesser grain borer had a stronger effect on host preference in choice experiments compared with experiments with naïve wasps. Host-finding and parasitism rates were increased by prior experience with rice weevils in no-choice experiments. A. calandrae females clearly preferred rice weevil larvae for parasitization over larvae of lesser grain borer regardless of parasitoid population or prior experience. Also, A. calandrae females chose rice weevil larvae for producing female progeny and lesser grain borer for male progeny in choice experiments. The implications of the results of this study are discussed in relation to host preference and the selection of parasitoid populations for biological control programs in stored grains.

Laboratory rearing and biological parameters of the eulophidPnigalio agraules, a parasitoid ofCameraria ohridella

Predators as well as parasitoids native to Europe accept the exotic horse chestnut leafminer, Cameraria ohridellaDeschka and Dimic 1986 (Lepidoptera: Gracillariidae), either as prey or as host. However, the influence of these antagonists on the populations of the pest insect is so far very low. Therefore, efforts to develop an integrated pest management system against C. ohridella should include methods which foster the natural enemy complex. In the present study we developed a laboratory rearing method and investigated several biological parameters of Pnigalio agraules (Walker 1839) (Hymenoptera: Eulophidae), a dominant species in the horse chestnut leafminer’s parasitoid complex in many European regions. This native parasitoid satisfies three basic requirements for successful use as a biocontrol agent with regard to C. ohridella. The parasitoid’s fecundity, longevity and the speed of juvenile development by far exceeds that of the leafminer, enabling the parasitoid population to increase faster than that of the host. Furthermore, our results show that the impact of this species on C. ohridella populations has been previously underestimated, because non-reproductive killing (i.e. host-feeding and host-stinging) of the hosts, resulting in considerable larval mortality of the leafminer, has not been quantitatively assessed. However, naturally occurring parasitoid populations have negligible impact on C. ohridella populations. Further studies, including experimental releases of P. agraules, are necessary to understand the constraints limiting the parasitoid’s performance in the field and to assess the potential benefits of releases for the control of C. ohridella.

Population response to resource separation in conservation biological control

Enhancement of resources to improve the impact of natural enemies in agroecosystems is an important component of conservation biological control. As many organisms depend on more than one resource throughout their life cycle it is necessary to take into account the distance separating vital resources and how it may affect natural enemy population development. It is also important to examine possible interactions with the fourth trophic level. In this study life history values for the parasitoid Diaeretiella rapae are incorporated into a mathematical model to explore these issues. In particular, a matrix model is employed to explore the impact of distance of separation between food (nectar in flowering plants) and reproductive (hosts found in the crop field) resources on the primary parasitoid population growth rate. Furthermore, the effects of spatial dissociation of resources on hyperparasitoids are considered. The results suggest that primary parasitoid population dynamics are influenced mainly by limitations in reproduction and by the response of parasitoids to separation of floral resources and host patches. Furthermore, hyperparasitism may cause primary parasitoid populations to decline and undermine biological control, depending on the extent to which hyperparasitoids systematically search for patches of primary parasitoids. The approach provides an initial theoretical framework to natural enemy use of multiple resources.

Assortative mating and the role of phenotypic plasticity in male competition: implications for gene flow among host-associated parasitoid populations

Local adaptation is promoted when habitat or mating preferences reduce gene flow between populations. However, gene flow is not only a function of dispersal but also of the success of migrants in their new habitat. In this study I investigated mating preference in conjunction with phenotypic plasticity using Aphidius parasitoids adapted to different host species. Males actively attempted to assortatively mate, but actual mating outcomes were strongly influenced by the relative size of the adult males. Results are discussed in the context of assortative mating in combination with the success of migrant males in mitigating gene flow between host-associated parasitoid populations.

A landscape view of cereal aphid parasitoid dynamics reveals sensitivity to farm- and region-scale vegetation structure

Functioning of plant-aphid-natural enemy interactions may be associated with the structure and composition of withinfield vegetation, neighborhood fields and field borders, and the regional plant community of cropped and noncropped areas. Farmand region-scale vegetation in the wheat-growing area of the North American Great Plains was hypothesized to effect the abundance of two hymenopteran parasitoids, that differ in physiological and behavioral attributes, of the key pest aphid of wheat, Diuraphis noxia (Mordvilko). The parasitoids had greater sensitivity to farm-scale vegetation (wheat strip rotation with or without spring-sown sunflower) than region-scale vegetation (degree of diversification with other crops and wheat fields converted to conservation grasslands). A two-way factorial design of scale (farm- and region-scale) revealed that parasitoid abundance in grass-dominant (homogeneous) areas especially benefited from adding sunflower to the wheat-fallow strip crop rotation. Considerable sensitivity of the analysis was added when adjusting for seasonality of vegetation, revealing that the region-scale effects were most prominent late season. From a management viewpoint, adding sunflower into the wheat production system, especially in relatively homogeneous vegetation regions, tends to promote local parasitoid populations during the summer when spring-sown plants are maturing and wheat is not in cultivation. Contrasting results for A. albipodus and L. testaceipes were consistent with expectations based on behavioral and physiological attributes of the two aphid parasitoid families they represent. Still, the general management interpretation seems robust for the two parasitoids and has relevance to both farm- and region-scale management schemes that are occurring in the wheat production zone of North American Great Plains.