Reciprocal influences and costs of parasitism on the development of Corcyra cephalonica and its endoparasitoid Venturia canescens

Many endoparasitoids develop successfully within a range of host instars. Parasitoid survival is highest when parasitism is initiated in earlier host instars, due to age-related changes in internal (physiological) host defences. Most studies examining fitness-related costs associated with differences in host instar have concentrated on the parasitoid, ignoring the effects of parasitism on the development of surviving hosts that have encapsulated parasitoid eggs. A laboratory experiment was undertaken examining fitness-related costs associated with encapsulation of Venturia canescens (Hymenoptera: Ichneumonidae) eggs by fifth (L5) instar larvae of Corcyra cephalonica (Lepidoptera: Pyralidae). Growth and development of both host and parasitoid were monitored in C. cephalonica larvae containing 0, 1, 2, or 4 parasitoid eggs. Adult size and fecundity of C. cephalonica did not vary with the number of eggs per host. However, there was a distinct increase in host mortality with egg number, although most parasitoids emerged from hosts containing a single egg. The most dramatic effect on the host was a highly significant increase in development time from parasitism to adult eclosion, with hosts containing 4 parasitoid eggs taking over 2.5 days longer to complete development than unparasitized larvae. The egg-to-adult development time and size of adult V. canescens did not vary with egg number per host, as demonstrated in a previous experiment using a different host (Plodia interpunctella). The results described here show that there are fitness-related costs to the host associated with resistance to parasitism.     

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

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.     

Do distances among host patches and host density affect the distribution of a specialist parasitoid?

The effect of spatial habitat structure and patchiness may differ among species within a multi-trophic system. Theoretical models predict that species at higher trophic levels are more negatively affected by fragmentation than are their hosts or preys. The absence or presence of the higher trophic level, in turn, can affect the population dynamics of lower levels and even the stability of the trophic system as a whole. The present study examines different effects of spatial habitat structure with two field experiments, using as model system the parasitoid Cotesia popularis which is a specialist larval parasitoid of the herbivore Tyria jacobaeae. One experiment examines the colonisation rate of the parasitoid and the percentage parasitism at distances occurring on a natural scale; the other experiment examines the dispersal rate and the percentage parasitism in relation to the density of the herbivore and its host plant. C. popularis was able to reach artificial host populations at distances up to the largest distance created (at least 80 m from the nearest source population). Also, the percentage parasitism did not differ among the distances. The density experiment showed that the total number of herbivores parasitised was higher in patches with a high density of hosts, regardless of the density of the host plant. The percentage parasitism, however, was not related to the density of the host. The density of the host plant did have a (marginally) significant effect on the percentage parasitism, probably indicating that the parasitoid uses the host plant of the herbivore as a cue to find the herbivore itself. In conclusion, the parasitoid was not affected by the spatial habitat structure on spatial scales that are typical of local patches.     

Host nutritional status mediates degree of parasitoid virulence

Parasitic organisms rely on the resources of their hosts to obtain nutrients essential for growth and reproduction. Insect parasitoids constitute an extreme condition since they develop in a single host from which they typically consume all available resources. As a result, the host is killed following parasitism. However, a few intriguing cases of host survival have been reported wherein hosts resume foraging and may even reproduce following parasitoid emergence. Yet, the ultimate and proximate mechanisms responsible for host recovery remain unresolved. We tested the impact of host nutrition on host fate and parasitoid fitness, using the association between Dinocampus coccinellae and the spotted lady beetle Coleomegilla maculata. Under laboratory conditions, we fed parasitized ladybirds on different aphid diets, with or without pollen. In the field, we followed the fate of parasitized ladybirds during seasonal variations in pollen and aphid abundance. We found that ladybirds fed on aphids or a combination of aphids and pollen recovered more frequently from parasitism (from 65 to 81%) than those eating only pollen (48%). Field data suggest that the fate of parasitized ladybirds is also related to food availability. On the other hand, when hosts fed on a combination of aphids and pollen, consequences for parasitoid fitness were often ‘all‐or‐nothing’: parasitoid emergence rate was the lowest of all host nutrition regimes (～50%), but parasitoids that did emerge were larger than individuals emerging from other host nutrition regimes. Laboratory and field results concur to show that host nutritional status during parasitoid development significantly influences both host fate and parasitoid fitness.     

Dynamic effects of parasitism by an endoparasitoid wasp on the development of two host species: implications for host quality and parasitoid fitness

1. The study reported here examined growth and developmental interactions between the gregarious larval koinobiont endoparasitoid Cotesia glomerata (Hymenoptera: Braconidae) and two of its hosts that vary considerably in growth potential: Pieris rapae and the larger P. brassicae (Lepidoptera: Pieridae). At pupation, healthy larvae of P. brassicae are over twice as large, in terms of fresh body mass, as those of P. rapae. 2. Clutch size of C. glomerata was manipulated artificially, and the relationship between parasitoid burden and the maximum weight of the parasitised host (= host–parasitoid complex) was measured. In both hosts, the maximum complex weight was correlated positively with parasitoid burden. Compared with unparasitised hosts, however, the growth of P. rapae was increased at significantly lower parasitoid burdens than in P. brassicae. Emerging wasp size was correlated negatively with parasitoid burden in both host species, whereas development time was less affected. 3. After larval parasitoid egress, the weight of the host carcass increased slightly, but not significantly, with parasitoid burden, although there was a strong correlation between the proportion of host mass consumed by C. glomerata larvae during development and parasitoid burden. 4. Clutch size was generally correlated positively with instar parasitised in both hosts, and greater in P. brassicae than in P. rapae. Sex ratios were much more female biased in L1 and L2 P. rapae than in all other host classes. Adult parasitoid size was correlated inversely with host instar at parasitism, and wasps emerging from P. brassicae were larger, and completed development faster, than conspecifics emerging from P. rapae. 5. The data reveal that parasitism by C. glomerata has profound species‐specific effects on the growth of both host species. Consequently, optimality models in which host quality is often based on host size at parasitism or unparasitised growth potential may have little utility in describing the development of gregarious koinobiont endoparasitoids. The results of this investigation are discussed in relation to the potential effectiveness of gregarious koinobionts in biological control programmes.     

Fecundity and Longevity of Green Vegetable Bug,Nezara viridula, Following Parasitism byTrichopoda giacomellii

The effects of parasitism by the ArgentinianTrichopoda giacomellii(Blanchard) on reproduction and longevity of its host,Nezara viridula(L.) are reported. Parasitoid larvae suppress egg maturation, reducing by 70% the fecundity of mature female hosts during the period of larval development. Egg viability was not affected, but mating frequency was reduced by approximately 50%. When parasitized as newly eclosed adults, 84% of females fail to reproduce. In male hosts, fertility and mating frequency were not affected during the period of larval parasitoid development. In male and reproductively immature female hosts, death was coincident with, or occurred shortly after parasitoid emergence (2–4 days); in mature females, death occurred on average 2 weeks after larval parasitoid emergence. Host mortality occurred as a consequence of tissue damage incurred as the parasitoid larvae emerged from the host. Some individuals survived parasitism though no further reproductive activity (mating or oviposition) occurred. The effectiveness ofT. giacomelliias a biological control agent is discussed in relation to its impact on reproduction and survival of its host and contrasted with the action of otherTrichopodaspecies.     

Effect of expected offspring survival probability on host selection in a solitary parasitoid

Optimal Foraging Theory predicts that parasitoid females should optimize their host selection to maximize their lifetime fitness gain and parasitize the most profitable hosts. In particular, in solitary parasitoids, females should avoid superparasitism, at least when sufficient unparasitized hosts are available. However, when unparasitized hosts are scarce, they should prefer, among already parasitized hosts, those that provide the best survival probability to their progeny, which depends on the age and the developmental stage of the first parasitoid. To test this hypothesis in a solitary ectoparasitoid, Pachycrepoideus vindemmiae Rondani (Hymenoptera: Pteromalidae), we first assessed the survival probability of a second parasitoid according to the time elapsed since initial parasitism. We then analyzed the female selection behavior in patches containing a mixture of hosts parasitized over various time intervals. Our results showed that the older the opponent larva was, the lower the survival probability of the second parasitoid was. However, when the first individual had reached the prepupal stage, both individuals could complete their development. At this stage, the survival probability of the second parasitoid was surprisingly high but such individuals were reduced in size. Our study also showed that host acceptance by females was strongly correlated with the survival probability of their progeny when the first parasitoid was from 0 to less than 10 days‐old. When the first parasitoid had reached the prepupal stage, females usually rejected these hosts, although the survival probability of the offspring was quite high. This discordance between female host selection behavior and progeny survival probability is discussed.     

Developmental interaction between suboptimal instars of Spodoptera littoralis (Lepidoptera:Noctuidae) and its parasitoid Microplitis rufiventris (Hymenoptera:Braconidae)

Relative effects of parasitism by Microplitis rufiventris on the development of the third instar Spodoptera littoralis (preferable, optimal host) with the development of penultimate (5th) and last (6th) instars (suboptimal hosts) were investigated. Newly molted 6th instar hosts were more acceptable for parasitization by the wasp female than older hosts. In singly parasitized 3rd instar hosts, 82.0 +/- 3.9% of the parasitoid eggs developed to full-grown instar wasp larvae. However, parasitoid eggs deposited singly in 73.9 +/- 3.3% of 5th and 100% of 6th instar hosts failed to develop. Superparasitization in the 3rd instar hosts reduced the production of pseudoparasitized larvae and, conversely, all parasitized hosts yielded viable parasitoid offspring. In suboptimal hosts, the development interaction between the parasitoid and its host larvae was highly influenced by the age of hosts at parasitism, load of deposited eggs, and other parasitoid factors. The latter factors, e.g., mainly calyx fluid particles, might be involved in establishing parasitoid eggs in the suboptimal hosts. In the last two host instars, superparasitization significantly increased the number of parasitoid larvae successfully reaching their final instar. Variation in host quality, e.g., physiological status, might be attributed, in part, to the partial breakdown of the solitary habit observed in the earlier instars. More parasitoid eggs developed to mature parasitoid larvae in hosts superparasitized as 6th instar than parasitoid eggs laid in 5th instar hosts. Superparasitization significantly lengthened the developmental period of 5th and 6th host instars and inhibited their development to the pupal stage. Studying parasitoid development in suboptimal instars of its habitual host provided physiological insight, as shown here. The results may have implication for biological control and in vitro mass rearing programs with solitary parasitoids.     

Molecular tracking of individual host use in the Shiny Cowbird – a generalist brood parasite

Generalist parasites exploit multiple host species at the population level, but the individual parasite's strategy may be either itself a generalist or a specialist pattern of host species use. Here, we studied the relationship between host availability and host use in the individual parasitism patterns of the Shiny Cowbird Molothrus bonariensis, a generalist avian obligate brood parasite that parasitizes an extreme range of hosts. Using five microsatellite markers and an 1120‐bp fragment of the mtDNA control region, we reconstructed full‐sibling groups from 359 cowbird eggs and chicks found in nests of the two most frequent hosts in our study area, the Chalk‐browed Mockingbird Mimus saturninus and the House Wren Troglodytes aedon. We were able to infer the laying behavior of 17 different females a posteriori and found that they were mostly faithful to a particular laying area and host species along the entire reproductive season and did not avoid using previously parasitized nests (multiple parasitism) even when other nests were available for parasitism. Moreover, we found females using the same host nest more than once (repeated parasitism), which had not been previously reported for this species. We also found few females parasitizing more than one host species. The use of an alternative host was not related to the main hosts' nest availability. Overall, female shiny cowbirds use a spatially structured and host species specific approach for parasitism, but they do so nonexclusively, resulting in both detectable levels of multiple parasitism and generalism at the level of individual parasites.     

Interactions between an ectoparasitoid and a nucleopolyhedrovirus when simultaneously attacking Spodoptera exigua (Lepidoptera: Noctuidae)

Insect pathogenic viruses and parasitoids represent distinct biological entities that exploit a shared host resource and have similar effects in suppressing host populations. This study explores the interactions between the ectoparasitoid Euplectrus plathypenae (Hymenoptera: Eulophidae) and the Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) in larvae of S. exigua (Lepidoptera: Noctuidae). Parasitoid progeny failed to complete development in hosts that had been infected prior to parasitism. However, infection of S. exigua fourth instars at 48 h post‐parasitism had no significant effects on the survival of parasitoid progeny. Larval and pupal development times of E. plathypenae that survived on virus‐infected S. exigua did not differ significantly from that of parasitoids on healthy hosts. Virus‐induced mortality and the production of occlusion bodies were very similar in parasitized and non‐parasitized S. exigua. The virus was genetically stable over three passages in parasitized and unparasitized hosts. These results suggest that applications of SeMNPV‐based insecticides are unlikely to disrupt pest control exerted by the parasitoid E. plathypenae in biological pest control programs as long as virus applications are timed not to coincide with parasitoid releases.     

Varying degree of physiological integration among host instars and their endoparasitoid affects stress‐induced mortality

In natural populations of insect herbivores, genetic differentiation is likely to occur due to variation in host plant utilization and selection by the local community of organisms with which they interact. In parasitoids, engaging in intimate associations with their host during immature development, local variation may exist in host quality for parasitoid development. We compared the development of a gregarious endoparasitoid, Cotesia glomerata L. (Hymenoptera: Braconidae), collected in The Netherlands, in three strains and three caterpillar instars (L1–L3) of its main host, Pieris brassicae L. (Lepidoptera: Pieridae). Hosts had been collected in The Netherlands and France, and were reared in the laboratory for one generation. We also used an established Dutch laboratory strain that had not been exposed to parasitoids for at least 24 generations. Parasitoid survival to adulthood was inversely correlated with host instar at parasitism. Adult parasitoid body mass was largest when hosts were parasitized as L1 and smallest when hosts were parasitized as L3, whereas egg‐to‐adult development time was quickest on L3 hosts and slowest on L1 hosts. Higher survival and faster development of C. glomerata on French L2 hosts also showed that there is variation in host‐instar‐related suitability. Many L2 and most L3 caterpillars that were parasitized exhibited signs of pathogen infection and perished within a few days of parasitism, whereas this never happened when hosts were parasitized as L1 or in non‐parasitized control caterpillars. Our results reveal that, irrespective of the host strain, L1 hosts are optimally synchronized with C. glomerata development. By contrast, the high precocious mortality of L3 larvae may be due to stress‐induced regulation by the parasitoid in order to ‘force’ its developmental program into synchrony with the developing parasitoid larvae. Our results underscore a potentially important role played by pathogens in mediating herbivore–parasitoid interactions that are host‐instar‐dependent in their expression.     

The effect of host nutrition on growth and development of the parasitoid waspVenturia canescens

We investigated the effect of host (Plodia interpunctella; Lepidoptera: Pyralidae) nutritional status on development of the solitary endoparasitoid,Venturia canescens (Hymenoptera: Ichneumonidae). Parasitoids from 3rd (L3) instars reared on a deficient diet during early parasitism took longer to develop and suffered higher mortality than those reared from hosts fedad libitum although there was not a significant difference in the size of eclosing wasps from the two groups. L5 hosts reared at high density produced smaller parasitoids, which developed more rapidly than those reared from hosts from low density containers, although mortality was higher in the latter. In a separate experiment we starved groups of 10–20 hosts (parasitized as L3) daily beginning on the 4th day after parasitism, to determine the host developmental stage required for successful parasitoid development to eclosion. Parasitoid survivorship increased with length of host access to food, while the egg-to-adult parasitoid development time increased throughout the experiment. Parasitoid size decreased with increasing periods of host starvation. The successful emergence ofVenturia depends uponPlodia reaching the size normally attained in the mid-5th instar, or 50–70% of the mass of healthy late 5th instars. Our results show that when earlier instars are parasitized, host growth is essential for successful parasitoid development to eclosion. Furthermore, they suggest that, for many koinobionts, host suitability may be greatly influenced by feeding rate and food quality.     

The effects of host weight at parasitism on fitness correlates of the gregarious koinobiont parasitoid Microplitis tristis and consequences for food consumption by its host, Hadena bicruris

Gregarious koinobiont parasitoids attacking a range of host sizes have evolved several mechanisms to adapt to variable host resources, including the regulation of host growth, flexibility in larval development rate, and adjustment of clutch size. We investigated whether the first two mechanisms are involved in responses of the specialist gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) to differences in the larval weight and parasitoid load of its host Hadena bicruris Hufn. (Lepidoptera: Noctuidae). In addition, we examined the effects of parasitism on food consumption by the host. Parasitoids were offered caterpillars of different weight from all five instars, and parasitoid fitness correlates, including survival, development time, and cocoon weight, were recorded. Furthermore, several host growth parameters and food consumption of parasitized and unparasitized hosts were measured. Our results show that M. tristis responds to different host weights by regulating host growth and by adjusting larval development rate. In hosts with small weights, development time was increased, but the increase was insufficient to prevent a reduction in cocoon weight, and as a result parasitoids experienced a lower chance of successful eclosion. Cocoon weight was negatively affected by parasitoid load, even though host growth was positively affected by parasitoid load, especially in hosts with small weights. Later instars were more optimal for growth and development of M. tristis than early instars, which might reflect an adaptation to the life‐history of the host, whose early instars are usually concealed and inaccessible for parasitism on its food plant, Silene latifolia Krause (Caryophyllaceae). Parasitism by M. tristis greatly reduced total host food consumption for all instar stages. Whether plants can benefit directly from the attraction of gregarious koinobiont parasitoids of their herbivores is a subject of current debate. Our results indicate that, in this system, the attraction of a gregarious koinobiont parasitoid can directly benefit the plant by reducing the number of seeds destroyed by the herbivore.     

Mating and host density affect host feeding and parasitism in two species of whitefly parasitoids

Abstract The parasitoids in the genera of Encarsia and Eretmocerus (Hymenoptera: Aphelinidae) are important biological control agents of whiteflies, and some of them not only parasitize hosts but also kill them with strong host‐feeding capacity. Two whitefly parasitoid species, Encarsia sophia and Eretmocerus melanoscutus were examined to determine if mating and host density affected their host feeding and parasitism. The whitefly host, Bemisia tabaci, was presented to these two wasp species in densities of 10, 20, 30, 40, 50 and 60 third‐instar nymphs per clip cage. Mated whitefly parasitoid females fed on more hosts than unmated females under a range of host densities (under all six host densities for En. sophia; under the densities of 40 nymphs or more for Er. melanoscutus). Meanwhile, mated females parasitized more whitefly nymphs than unmated females under all host densities for both species. With increase of host density, mated or unmated Er. melanoscutus females killed more hosts by host feeding and parasitism. Mated En. sophia females killed more hosts by host feeding with increase of host density, whereas unmated females did not parasitze whitefly nymphs at all. Our results suggest that only mated female parasitoids with host‐feeding behavior should be released in crop systems to increase their bio‐control efficiency.     

Assessing augmentative releases of parasitoids using the «Recruitment method», with reference toEdovum puttleri,a parasitoid of the colorado potato beetle [Coleoptera: chrysomelidae]

Release of approximately 17,700 experienced adult femaleEdovum puttleri Grissell against 1^st generation Colorado potato beetle eggs in 1987 in a 0.4 ha potato field in S. Deerfield, Massachusetts resulted in only 3.6% parasitism as assessed by direct measurement of host and parasitoid recruitment. Levels of non-viability indicated an additional 2.8% of hosts killed by parasitoid hostfeeding, for an overall impact of 6.4%. Release in 1988 of 126,300 parasitoid against 1^st generation hosts in a 0.4 ha potato (Solanum tuberosum L.) field at the same site produced only slightly higher levels of parasitism (10.6%) and host feeding (2.0%). Release in 1987 of 32,800 wasps against 2^nd generation eggs resulted in only 0.7% parasitism and 0.3% host feeding due to the toxicity of fenvalerate (Pydrin^R) residues from a single application applied for control of the potato leafhopper,Empoasca fabae (Harris). Release in 1988 of 47,400 wasps against the 2^nd host generation in the absence of any pesticide applications resulted in 34.4% parasitism and 16.1% host feeding, for a total impact of 50.5%. Difference in parasitization levels between host generations supports the idea thatE. puttleri adults require an in-field carbohydrate source such as aphid honeydew to reproduce. In Massachusetts, aphid populations in potato typically do not develop until the end of the 1^st larval generation. The recruitment method ofVan Driesche & Bellows (1988) proved to be a satisfactory approach for determining results of augmentative parasitoid releases.      

Mechanisms structuring host–parasitoid networks in a global warming context: a review

1. In natural communities, multiple host and parasitoid species are linked to form complex networks of trophic and non‐trophic interactions. Understanding how these networks will respond to global warming is of wide relevance for agriculture and conservation. 2. This study synthesises the emerging evidence surrounding host–parasitoid networks in the context of global warming. The suite of direct and indirect interaction types within host–parasitoid networks is summarised, as well as their sensitivity to temperature changes. The study also compiles and reviews studies investigating the responses of whole host–parasitoid networks to increasing temperatures or proxy variables. The findings reveal there is limited evidence overall for the prediction that parasitism will be reduced under global warming: approximately equal numbers of studies show elevated and reduced parasitism. 3. Increasingly, endosymbiotic bacteria are recognised as influential mediators of host–parasitoid interactions. These endosymbionts can change how individual species respond to global warming, and their effects can cascade to affect whole host–parasitoid networks. The evidence that symbiotic bacteria are likely to affect the response of host–parasitoid networks to global warming is reviewed. Symbionts can protect hosts from their parasitoids or influence thermal tolerance of their host species. Furthermore, the symbionts themselves can be impacted by global warming. 4. Finally, the study considers the most promising avenues for future research into the mechanisms structuring host–parasitoid networks in the context of global warming. Alongside the increasing availability of modern molecular methods to document the structure of real, species‐rich host–parasitoid networks, the study highlights the utility of manipulative experiments and mathematical models.     

A model for the coevolution of resistance and virulence in coupled host—parasitoid interactions

A coevolutionary model is developed of the interaction between a host and an internal parasitoid, where the outcome of parasitism depends upon the extent to which individual hosts invest in resistance mechanisms and individual parasitoids in countermeasures (virulence). The host and parasitoid are assumed to have coupled population dynamics (of Nicholson–Bailey form) and to be composed of a series of asexual clones with different levels of resistance and virulence. Investment in resistance and virulence mechanisms is assumed to be costly. The model has two main outcomes. First, if resistance is relatively costly compared to virulence, the host may be selected not to invest in resistance mechanisms despite parasitoid investment in virulence, in effect trading off the risks of parasitism against the savings in costs. A number of cases which appear to correspond to this result have been reported. Second, for most other feasible parameter values, an arms race occurs between host and parasitoid, until effective resistance becomes so costly that the host abandons defence. This abandonment is followed by a reduction in parasitoid virulence and the cycle begins again. These cycles may explain reports of persistent additive genetic variation in resistance and virulence, and may also contribute towards population dynamic stability.     

Effects of parasitoid fecundity and host resistance on indirect interactions among hosts sharing a parasitoid

We examine the effects of fecundity‐limited attack rates and resistance of hosts to parasitism on the dynamics of two‐host–one‐parasitoid systems. We focus primarily on the situation where one parasitoid species attacks two host species that differ in their suitability for parasitism. While all eggs allocated to suitable hosts develop into adult parasitoids, some of the eggs allocated to marginal host do not develop. Marginal hosts can therefore act as a sink for parasitoid eggs. Three‐species coexistence is favoured by low levels of parasitoid fecundity and by low levels of suitability of the marginal host. Our model also produces an indirect (+, ?) interaction in which the suitable host can benefit from the presence of the marginal host, but the marginal host suffers from the presence of the suitable host. The mechanism driving the indirect (+, ?) interaction is egg limitation of parasitoids incurred by allocating eggs to marginal hosts.     

The spatial patterns of parasitism of eumenid wasps,Anterhynchium flavomarginatum andOrancistrocerus drewseni by the miltogrammine flyAmobia distorta

1. Spatial patterns of parasitism of eumenid wasps Anterhynchium flavomarginatum and Orancistrocerus drewseni by the miltogrammine fly Amobia distorta were studied in Kyoto, Japan during 1980–1984.
2. In generations of low (<5%) and medium (5–20%) parasitism, percent parasitism per shed (the habitat of the hosts) increased as a function of host density. Conversely, in generations of high (>20%) parasitism, percent parasitism was rather constant over different host densities.
3. The spatial distributions of adult miltogrammine flies among sheds were censused in generations of low and medium parasitism. The frequency of observations of adult miltogrammine flies was higher at sheds of higher host density (aggregative behavioral response), but on the other hand, the adult miltogrammine flies distributed in an underdispersed (or regular) manner in relation to other conspecifics.
4. The spatially density independent relationship between host density and percent parasitism in generations of high parasitism was explained in relation to parasitoid dispersal from patches of high parasitoid density.

     

The developmental strategies and related profitability of an idiobiont ectoparasitoid Sclerodermus pupariae vary with host size

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