Facultative gregarious development in a solitary parasitoid wasp,Dendrocerus carpenteri: larvae may share nutritional resources

Solitary parasitoids generally produce only one offspring per host. Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae) develops as an idiobiont ectoparasitoid on prepupae and pupae of primary aphid parasitoids inside the mummified aphid host. Females normally deposit a single egg but superparasitize when suitable hosts are scarce. We show that facultative gregarious development may occur but is constrained by resource competition between larvae. The probability of more than one offspring surviving increased with the intensity of parasitism; an age difference of ≤9 h between older and younger first instars did not promote gregarious development. Two female parasitoids and, rarely, up to three male parasitoids could develop together. Average body size in terms of dry mass did not differ between singly developing females and the combined mass of two females sharing host resources, but the combined mass of gregarious males was greater than that of their singly developing counterparts. Females were 3× more likely to eclose from single than gregarious mummies. The amount of host resources available per larva declines with increasing clutch size, in turn causing a corresponding reduction of adult size and size‐dependent fitness attributes. We suggest that competition for limiting host supplies may influence the transition from solitary to gregarious development and should be considered in models of clutch size evolution in parasitoid wasps.     

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.     

Fitness consequences of ovicide in a parasitoid wasp

Ovicide, superparasitism and host rejection are alternative reproductive tactics facing female parasitoid wasps encountering an already-parasitized host. Superparasitism is simply the addition of an egg or a clutch of eggs by the secondary parasitoid, but under ovicide the primary clutch is removed or destroyed. Host rejection occurs if the wasp leaves without laying a clutch. The ectoparasitoid Laelius pedatus (Say) (Hymenoptera: Bethylidae) performs ovicide in this situation. Clutch manipulation experiments show that secondary clutches suffer high mortality in competition with primary clutches, which increases with increasing time delay between clutches. Primary clutches however suffer little in competition with secondary clutches, even if there is minimal time delay between clutches. These data suggest that the offspring of ovicidal females are substantially fitter than the offspring of superparasitizing females. Handling time and clutch size do not differ significantly between first (sole) parasitoids and second (ovicidal) parasitoids. The same is true for offspring survival and development time. However, offspring of second females are slightly smaller. This suggests that parasitized and unparasitized hosts are resources of similar quality when ovicide is performed. These data strongly support the predictions of evolutionary models of ovicide. They may also give some insight into the taxonomic distribution of ovicide in parasitoids.     

Clutch-size behavior and coexistence in ephemeral-patch competition models

Systems of patchy, ephemeral resources often support surprisingly diverse assemblages of consumer insects. Aggregation of consumer individuals over the landscape of patches has been suggested as one mechanism that can stabilize competition among consumer species. One mechanism for larval aggregation is the laying of eggs in clutches by females traveling among patches to distribute their total fecundity. We use simulation models to explore the consequences, for coexistence of competitors, of larval aggregation that arises from clutch laying. Contrary to some previous treatments, we find that clutch laying can be strongly stabilizing and under certain conditions can be sufficient to allow competitors to coexist stably. We extend these models by considering clutch size as a variable that responds to the abundance of resource patches. Such a relationship might be expected because females should lay their eggs in fewer but larger clutches when the cost of travel among patches is high (because patches are rare). When females adjust clutch size in response to resource abundance, coexistence can be easiest when resource patches are scarce and most difficult when resources are abundant.     

Influence of Conspecific Presence, Experience, and Host Quality on Oviposition Behavior and Clutch Size Determination in Anastrepha ludens (Diptera: Tephritidae)

Females of some insect species adjust the number of ovipositions and clutch size adaptively depending on conspecific density and probably experience. In a series of three experiments, we examined the effect of the presence of conspecifics, host quality, and oviposition experience on oviposition behavior and clutch size determination by females of the polyphagous fruit fly Anastrepha ludens (Diptera: Tephritidae). In the first experiment, we determined that grouped (eight females per cage) A. ludens females tended to visit and oviposit in more hosts than did solitary females probably as a result of stimulation by the presence of conspecifics. We also determined that females with previous oviposition experience visited and oviposited in more hosts than inexperienced ones. Importantly, when females were grouped, we observed significantly more landings on unoccupied hosts (i.e., devoid of flies) than on occupied ones (i.e., with at least one fly on it). However, oviposition experience, and not female density, was the most important factor affecting clutch size. Naive females deposited larger egg clutches than experienced ones. In the second experiment, we found that oviposition experience and host quality (i.e., clean fruit or fruit covered with a host marking pheromone [HMP] extract), influenced clutch size and the decision of females to defend or not defend the host. Clutch size and number of fights were greater on clean than on HMP-marked hosts. In the third experiment, we observed that host quality (i.e., size) played a significant role with regard to the number of female fights, host marking behavior, and clutch size. Specifically, females fought and dragged their aculeus longer on small- and medium-sized hosts than on large ones. But this behavior varied according to whether females were kept alone or grouped. Clutch size was greatest in the largest hosts. Considering all the above, we believe that the observed increase in ovipositional activity by grouped A. ludens females can be attributed to competition through mutual interference and not social facilitation as has been reported in other tephritid species.     

Age-dependent clutch size in a koinobiont parasitoid

Abstract.  1. The Lack clutch size theory predicts how many eggs a female should lay to maximise her fitness gain per clutch. However, for parasitoids that lay multiple clutches it can overestimate optimal clutch size because it does not take into account the future reproductive success of the parasitoid.
2. From egg-limitation and time-limitation models, it is theoretically expected that (i) clutch size decreases with age if host encounter rate is constant, and (ii) clutch size should increase with host deprivation and hence with age in host-deprived individuals.
3. Clutch sizes produced by ageing females of the koinobiont gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) that were provided daily with hosts, and of females ageing with different periods of host deprivation were measured.
4. Contrary to expectations, during the first 2 weeks, clutch size did not change with the age of the female parasitoid, neither with nor without increasing host-deprivation time.
5. After the age of 2 weeks, clutch size decreased for parasitoids that parasitised hosts daily. The decrease was accompanied by a strong decrease in available eggs. However, a similar decrease occurred in host-deprived parasitoids that did not experience egg depletion, suggesting that egg limitation was not the only factor causing the decrease in clutch size.
6. For koinobiont parasitoids like M. tristis that have low natural host encounter rates and short oviposition times, the costs of reproduction due to egg limitation, time limitation, or other factors are relatively small, if the natural lifespan is relatively short.
7. Koinobiont parasitoid species that in natural situations experience little variation in host density and host quality might not have strongly evolved the ability to adjust clutch size.     

Evolutionary constraints on polyembryony in parasitic wasps: a simulation model

Polyembryony involves the production of several genetically identical progeny from a single egg through clonal division. Although polyembryonic development allows highly efficient reproduction, especially in some parasitoid wasps, it is far less common than monoembryony (development of one embryo per egg). To understand what might constrain the evolutionary success of polyembryony in parasitoids, we developed Monte Carlo models that simulate the competition between polyembryonic females and their monoembryonic counterparts. We investigated which simulated life‐history traits of the females allow the monoembryonic mode of development to succeed. Published empirical studies were surveyed to explore whether these traits indeed differ between polyembryonic parasitoids and related monoembryonic species. The simulations predict an advantage to monoembryony in parasitoids whose reproduction is limited by host availability rather than by egg supply, and that parasitize small‐bodied hosts. Comparative data on the parasitoid families Encyrtidae and (to a lesser extent) Braconidae, but not the data from Platygastridae, circumstantially support these predictions. The model also predicts monoembryony to outcompete polyembryony when: 1) hosts vary considerably in quality, 2) polyembryonic development carries high physiological costs, and 3) monoembryonic females make optimal clutch size decisions upon attacking hosts. These multiple constraints may account for the rarity of polyembryony among parasitoid species.     

闭弯尾姬蜂与菜蛾盘绒茧蜂寄生菜蛾幼虫时的种间竞争

在室内25℃下,以菜蛾3龄初幼虫作寄主,研究了菜蛾盘绒茧蜂Cotesia plutellae和半闭弯尾姬蜂Diadegma semiclausum的种间竞争。当寄主供2种蜂同时产卵寄生时,2种蜂各自的寄生率与其单独寄生时无显著差异,合计寄生率比一种蜂单独存在时有所提高,但差异不显著。2种蜂均能产卵寄生已被另一种蜂寄生了的寄主幼虫。当寄主被2种蜂寄生的间隔时间很短（少于10 h）时,所育出的蜂绝大部分（80%以上）为绒茧蜂;当寄主先被绒茧蜂寄生,并饲养2天以上再供弯尾姬蜂寄生时,所育出的全为绒茧蜂;当寄主先被弯尾姬蜂寄生,并饲养2天以上再供绒茧蜂寄生时,寄主幼虫绝大部分不能存活,只有少部分能育出寄生蜂,且多为弯尾姬蜂。当2种蜂的幼虫存在于同一寄主体内时,2种蜂的发育均受到另一种蜂的抑制;绒茧蜂1龄幼虫具有物理攻击能力,能将弯尾姬蜂卵或幼虫致死。这些结果表明,菜蛾盘绒茧蜂与半闭弯尾姬蜂在同一寄主中发育时,前者具有明显的竞争优势。     

A comparative study of the relationship between host size and brood size in Apanteles spp. (Hymenoptera: Braconidae)

ABSTRACT. 1. Current models of insect oviposition predict that clutch size in parasitoids should correlate with host size, with a continuum from solitary species at one end to large gregarious broods at the other. This prediction is tested for the genus Apanteles (sensu lato).
2. The distribution of brood sizes in Apanteles is bimodal, with peaks at one (solitary species) and at about twenty (gregarious species).
3. Brood size of gregarious species correlates with host size, but when a measure of the total volume of a parasitoid brood is plotted against host size, solitary species do not lie on the same regression slope as gregarious species.
4. There is a relative shortage of gregarious species on small hosts, and a relative excess of solitary species on large hosts. Solitary species on large hosts do not fully consume the host resource.
5. The possible role of evolutionary constraints to adaptive progeny allocation in Apanteles is discussed.     

Do past experience and competitive ability influence foraging strategies of parasitoids under interspecific competition?

Abstract 1. In solitary parasitoids, several species can exploit the same host patch and competition could potentially be a strong selective agent as only one individual can emerge from a host. In cereal crops, Aphidius rhopalosiphi and A. ervi share the grain aphid Sitobion avenae as host. 2. The present work studied foraging strategies of both species on patches already exploited by the other species. The study analysed larval competition in multi‐parasitised hosts and compared the foraging behaviour of females with and without previous experience. 3. It was found that A. ervi wins larval competition three times more often than A. rhopalosiphi. Both species spent less time on patches exploited by a heterospecific than on unexploited ones. When they foraged on heterospecifically exploited patches, experienced females induced less mortality in aphids than inexperienced ones. 4. Although A. rhopalosiphi is a specialist on cereal aphids and is the most abundant species due to its early appearance in the season, S. avenae is still a profitable host for A. ervi, because: (i) A. rhopalosiphi leaves patches partially exploited, (ii) A. ervi wins larval competition in three out of four multi‐parasitised hosts, and (iii) A. ervi is only slightly deterred by the cornicular secretions of the host and can thus easily parasitise hosts.     

Pseudacteon tricuspis: its behavior and development according to the social form of its host and the role of interference competition among females

We studied how the behavior and performance of Pseudacteon tricuspis Borgmeier varies with the social form of its host Solenopsis invicta Buren, in its native range in Argentina where monogyne colonies are more abundant than polygynes (approximately 75 vs. 25%). Female, P. tricuspis took 44% less time (50 vs. 89 s) to attack monogyne than polygyne ants, but oviposition attempts were similar (23 vs. 18 attacks). The presence of the parasitoid affected the average size of foragers on the trail, with the proportion of minor workers increasing on both social forms. In the laboratory, P. tricuspis selected similar host sizes, although pupal survival was 25% higher on monogynes than on polygynes. Developmental times of both genders were similar (33-35 d), although larger females emerged from bigger hosts. The sex ratio of P. tricuspis was more male biased when exploiting polygyne ants. Intraspecific competition significantly affected parasitoid reproductive success, being significantly higher for a solitary female than when three females were present, although the size of workers selected did not vary. The male:female ratio also changed, being 1:1 without competition but 2:1 with competition. We demonstrated for the first time the consequences of interference competition among P. tricuspis females, a common behavior observed in others parasitoids. We discuss why P. tricuspis sex ratios are always biased toward males in both social forms and suggest that similar studies of interference competition within and between already naturalized Pseudacteon species in the United States could help predict establishment patterns.     

The effect of host size on the sex ratio of Syngaster lepidus, a parasitoid of Eucalyptus longhorned borers (Phoracantha spp.)

The solitary larval ectoparasitoid, Syngaster lepidus Brullé, parasitizes the cryptic larvae of two wood-boring beetles, Phoracantha recurva Newman and Phoracantha semipunctata F. The objective of this study was to determine how the female parasitoids allocated the sex of progeny when presented with larval hosts of uniform size classes. Host size was directly correlated with age of the Phoracantha larval hosts. Groups of Phoracantha larvae of a single age class (2-, 3-, 4-, or 5-week-old) were exposed to parasitoids, and sex ratios of the resulting parasitoid progeny from each host age class were determined. A significant relationship was observed among the sizes of P. recurva and P. semipunctata hosts and the sex ratio of emerging parasitoids. Parasitized 2-week-old beetle larvae of both Phoracantha spp. produced only male S. lepidus progeny, whereas older larval hosts produced increasing proportions of female parasitoids (up to 80% females from 5-week-old hosts). Two-week-old Phoracantha larvae of both species produced fewer parasitoids than host larvae 3–5-week-old. The size of parasitoid progeny consistently increased with host larval age (size), and female parasitoids were larger than males across all host size classes. Male S. lepidus developed in approximately 25 days from 2-week-old hosts, and 19–21 days in 3–5-week-old hosts. Female S. lepidus developed in 22–25 days, with developmental time increasing with host size.     

How two different host species influence the performance of a gregarious parasitoid: host size is not equal to host quality

1. Hyssopus pallidus Askew (Hymenoptera, Eulophidae) is a gregarious ectoparasitoid of the two tortricid moths species Cydia molesta Busck and C. pomonella L. (Lepidoptera, Tortricidae). It paralyses and parasitizes different larval instars of both species inside the apple fruit, which leads to the death of the caterpillar. 2. We assessed the influence of host species characteristics and host food on the performance of the parasitoid female in terms of clutch size decisions and fitness of the F(1) generation. 3. A comparison of clutch size revealed that female parasitoids deposited similar numbers of eggs on the comparatively smaller C. molesta hosts as on the larger C. pomonella hosts. The number of parasitoid offspring produced per weight unit of host larva was significantly higher in C. molesta than in C. pomonella, which is contrary to the general prediction that smaller hosts yield less parasitoid offspring. However, the sex ratio was not influenced by host species that differed considerably in size. 4. Despite the fact that less host resources were available per parasitoid larva feeding on C. molesta caterpillars, the mean weight of emerging female wasps was higher in the parasitoids reared on C. molesta. Furthermore, longevity of these female wasps was neither influenced by host species nor by the food their host had consumed. In addition we did not find a positive relationship between adult female weight and longevity. 5. Parasitoid females proved to be able to assess accurately the nutritional quality of an encountered host and adjust clutch size accordingly. These findings indicate that host size is not equal to host quality. Thus host size is not the only parameter to explain the nutritional quality of a given host and to predict fitness gain in the subsequent generation.     

Effect of host quality of Callosobruchus maculatus (Coleoptera: Bruchidae) on performance of the egg parasitoid Uscana lariophaga (Hymenoptera: Trichogrammatidae)

Development and reproductive success of the solitary egg parasitoid Uscana lariophaga Steffan were examined after development in eggs of the bruchid storage pest Callosobruchus maculatus Fabricius reared at either low or high densities on cowpea seeds and laid at day 1 and 4 of maternal life. Both bruchid larval competition and maternal age negatively affected egg size, but the latter more than the former. Uscana lariophaga reared in small hosts developed slower, were smaller and produced fewer eggs compared to parasitoids reared in large hosts. Fecundity of the parasitoid was heavily influenced by host egg size. This was reflected in the values for the intrinsic rate of increase of U. lariophaga, which differed for wasps that developed in host eggs laid by bruchid females of different age. Wasps allocated marginally more female offspring to larger hosts.     

Effect of host instar on host discrimination of heterospecific‐parasitised hosts by sympatric parasitoids

1. Interspecific competition among hymenopteran parasitoids may shape their behavioural strategies for host resource exploitation. In order to reduce or prevent competition, many parasitoid species have evolved the ability to discriminate between unparasitised hosts and hosts parasitised by another parasitoid species (i.e. heterospecific host discrimination). However, discriminatory ability might be affected by host instar. 2. This study reports the first results on whether host instar can influence the use of heterospecific‐parasitised hosts by sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae). 3. Aphytis melinus and Aphytis chrysomphali discriminated between unparasitised and heterospecific‐parasitised hosts when they found a third‐instar host (high quality), with a tendency to multi‐parasitise. However, this discrimination was not observed in the second instar (lower size). 4. The behavioural strategies adopted towards multi‐parasitise third‐instar hosts varied between both species. Aphytis chrysomphali reduced its clutch size in heterospecific‐parasitised hosts, whereas A. melinus tended to probe them for longer than healthy hosts. 5. Overall, our results highlight the importance of host instar in the study of intrinsic competition between parasitoids.     

Larval competition within seeds: From the behaviour process to the ecological outcome in the seed beetle Callosobruchus maculatus

Abstract Environmental conditions experienced by organisms during development can have profound impacts on adult fitness and behaviour. Internally feeding larvae unable to leave the seed selected by their mother face limitations of resource suitability and competition. The host seed may guide the larval behaviour within the seed leading to differential intensity of competition and determining its process and outcome, which varies in strains of the legume seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae). However, the intensity, process and outcome of larval competition in different hosts have yet to be simultaneously considered, the objective of the present study. Here we assessed the intensity, process and outcome of intrastrain larval competition as related to host type, and how they are interrelated. Larval competition was faced with two distinct strategies – scramble and contest competition depending on the insect strain and host seed species. The intensity of competition did not show any straight link with the process and outcome of competition. Only a single strain showed a contest competition process with likely interference between larvae, while the four other strains studied showed the process of scramble competition. The process of scramble competition, however, led to variable outcomes in mung beans based on larval competition curves. Such differences were not apparent on cowpea seeds and either the plateau or the peak expected on the larval fitness curves were not reached preventing the distinction of the competition outcome, a likely consequence of the egg laying behaviour of these strains limiting the maximum number of eggs laid per seed. Seed host species rather than seed size are the likely cause of the differences observed from the initial expectation. The strain showing the process of contest competition increased larval fitness with density of larvae emerged per seed regardless of the host species, an unexpected outcome based on theoretical models. In this case the egg laying behaviour of the adult female is probably the main fitness determinant of its progeny.     

Fitness costs of intrinsic competition in two egg parasitoids of a true bug

Intrinsic competition in insect parasitoids occurs when supernumerary larvae develop in the same host as consequence of multiple ovipositions by females of the same species (intra-specific competition) or by females of different species (inter-specific competition). Studies on intrinsic competition have mainly focused on understanding the factors that play a role in the outcome of competition, while fitness-related effects for the parasitoid surviving the competition have been poorly investigated, especially in egg parasitoids. Interestingly, even the winning parasitoid can experience fitness costs due to larval development in a host in which multiple factors have been injected by the ovipositing females or released by their larvae. In this paper we studied fitness-related traits associated with intra- and inter-specific competition between Trissolcus basalis (Wollaston) and Ooencyrtus telenomicida (Vassiliev), the main egg parasitoids associated with the southern green stink bug Nezara viridula (L.) in Italy. We investigated the impact of intrinsic competition for the surviving parasitoid in terms of body size, developmental time, number and size of oocytes. Our results indicated that T. basalis adults did not experience fitness-related costs when surviving intra-specific competition; however, adults were smaller, took longer to develop and females produced fewer oocytes after surviving inter-specific competition. A different outcome was found for O. telenomicida where the emerging females were smaller, produced fewer and smaller oocytes when suffering intra-specific competition whereas no fitness costs were found when adults survived inter-specific competition. These results support the hypothesis that the impact of intrinsic competition in egg parasitoids depends on the severity of the competitive interaction, as fitness costs were more pronounced when the surviving parasitoid interacted with the most detrimental competitor.     

Inter- and intra-specific host discrimination in gregarious and solitary endoparasitoid wasps

In nature, most species of Lepidoptera are attacked by parasitoids, and some species may be hosts for several parasitoid species. When hosts are parasitized by more than one female of the same species (=superparasitism) or females of different species (=multiparasitism), then intrinsic competition occurs for control of host resources. To reduce competition, some parasitoids are able to recognize the difference between parasitized and unparasitized hosts. Inter- and intra-specific host discrimination were investigated in the two sympatric species, the gregarious Cotesia kariyai (Watanabe) and solitary Meteorus pulchricornis (Wesmael), endoparasitoids of the Oriental armyworm Mythimna separata (Walker). To measure host discrimination, choice experiments were conducted in which females of both species foraged and chose between healthy host larvae and hosts initially parasitized by either C. kariyai or M. pulchricornis. An olfactory test was also performed to examine the discrimination behavior of the two parasitoids. Our results showed that, in oviposition choice tests, both braconid female wasps were able to discriminate between unparasitized hosts and from four to seven day-old hosts previously attacked by conspecific and heterospecific wasps. On the other hand, superparasitism and multiparasitism occurred even in host larvae that were parasitized two days earlier. However, once the immature parasitoids hosts are at larval stage (1st and 2nd instar), super- and multiparasitism were avoided in the two-choice test, but the latter often occurred in the multiple-choice experiment. Host discrimination abilities may have been based on plant volatile signals incurred from damaged plants and internal mechanisms from four to seven post-parasitized hosts.     

Sperm competition selects for increased testes mass in Australian frogs

Game theory predicts that investment in spermatogenesis will increase with the risk and intensity of sperm competition. Widespread support for this prediction has come from comparative studies of internal fertilizing species reporting positive associations between testes mass and the probability that females mate with more than one male. Data for external fertilizers have generated conflicting results. We investigated how risk of sperm competition affects testes size in two families of Australian frogs: the Myobatrachidae and the Hylidae. We also examined effects of clutch size, egg size and oviposition location as alternative factors that might influence sperm production. Species were ranked according to probability of group spawning, and hence risk of sperm competition. Controlling for body size and phylogenetic relationships, we demonstrated that within the Myobatrachidae, the risk of sperm competition explained a significant amount of variation in testes mass. Oviposition location had a weak influence, with species ovipositing into foam having smaller testes. No significant effects of clutch size or egg size were detected. In hylids, the relationship between testes mass and risk of sperm competition was positive but not significant, again with no predictable effects related to egg size or number. These data provide an important test of sperm competition theory for externally fertilizing taxa.     

Intraspecific competition between healthy and parasitised hosts in a host–parasitoid system: consequences for life-history traits

Abstract  1. In two different treatments, groups of healthy hosts ( Ephestia kuehniella ) or hosts parasitised by Venturia canescens competed for a limited amount of food. The larva to adult survival in each group, as a function of the initial number of hosts and treatment, was fitted to the generalised Beverton and Holt and generalised Ricker survival functions, and a number of life-history traits of the parasitoids was measured.
2. Intraspecific competition was scramble-like, and the parasitised hosts were less susceptible to competition than were their healthy counterparts.
3. For both the healthy and the parasitised hosts, the number of larvae surviving to adulthood gave a good fit to both the generalised Beverton and Holt and generalised Ricker models, but the values of all the parameters differed between the two treatments.
4. Parasitoid size, egg load, and adult survival time decreased significantly with the initial host number.
5. Previous theoretical work suggests that both lower susceptibility to competition by parasitised hosts and scramble competition contribute to the dynamical instability of host–parasitoid systems. Changes registered in life-history traits may also affect host–parasitoid dynamics. These changes have not yet been incorporated into host–parasitoid models.