Size-induced reproductive constraints in an egg parasitoid

Body Part Selection for Size Classification of Two Trichogramma Species

Body size is an important morphological characteristic that covaries with the quality of parasitoids and predators. Data show that the larger the organism is, the better the biological parameters and the host location by natural enemies in the field. The standard way of evaluating the size of parasitoids of the genus Trichogramma (Hymenoptera: Trichogrammatidae) is by measuring the tibia, but using only one body part to estimate the size of organisms can lead to miscalculations. In this paper, commercial Trichogramma pretiosum Riley, 1879 (Hymenoptera: Trichogrammatidae) and Trichogramma galloi Zucchi, 1988 (Hymenoptera: Trichogrammatidae) were mounted on slides for microscopy and photographed, and the photographs were used to measure their antennae, scutellum, ovipositor, tibia, and wing. Principal component analysis (PCA) and linear discriminant analysis (LDA) were performed to select the body part that best represents their size. PCA showed that all body parts represented size in a similar way, and LDA showed that the ovipositor was the most representative. We conclude that the best body parts for representing the size of the Trichogramma species studied are the wing and ovipositor, and at least two body parts are needed to detect two size groups.

Effect of Male Body Size on Female Reproduction in Pyrrhocoris apterus (L.) (Heteroptera, Pyrrhocoridae)

Females and males of the abundant heteropteran species Pyrrhocoris apterus (L.) mate with the larger of the available partners. The male benefits from copulation with a large female, which is more productive than a small female. However, the benefit to females from copulation with a large male has not yet been investigated. Overwintered sexually immature adults were transferred from the field to indoors for a long day at 25 °C and subsequently kept as female-male pairs with different partner size ratios. The female lays eggs in several successive clutches. We investigated the influence of male size on the number and weight of eggs in individual clutches and the time elapsed from the transfer to their deposition. These characteristics of reproduction were first regressed on female size, and residuals of this regression were then regressed on male size. The positive effect of increasing male size on the characteristics of female reproduction manifested as a significant value of the latter regressions. The expected positive effect of increasing male body size on female reproductive characteristics was not detected. Several reasons for this deficiency are discussed.

Ovigeny strategy of the parasitic wasp Cosmocomoidea annulicornis (Hymenoptera: Mymaridae): effect of female age, feeding and host availability on reproductive traits

The reproductive traits of the mymarid wasp Cosmocomoidea annulicornis (Ogloblin) (Hymenoptera: Mymaridae) attacking eggs of the sharpshooter Tapajosa rubromarginata (Signoret) (Hemiptera: Cicadellidae) were evaluated under laboratory conditions. Bioassays were carried out to estimate the realized fecundity and egg load of females. The ovigeny index was calculated and different biological traits, such as body size, oöcyte length, gaster length and wing length, were analysed to assess significant associations between these traits and the species fitness. In addition, the effect of host availability and feeding on longevity and potential fecundity throughout life and the effect of female age on egg maturation dynamics were assessed. The results showed that C. annulicornis is a strongly synovigenic species. A positive correlation was found between fecundity and longevity of the females and between body size and oöcyte length. Contrary to expected, body size was not related to fecundity and longevity. Females lived significantly longer in the presence of hosts and honey than when they were host-deprived and honey-fed or both host and honey deprived. Host availability had a significant effect on the amount of eggs laid by C. annulicornis females. Female age was negatively associated with oöcyte length. Furthermore, females were able to mature additional eggs as they aged, nevertheless, when host-deprived, senescent females presented significantly less mature eggs than younger ones, suggesting a possible egg oosorption. These results might contribute to a better understanding of the reproductive potential of this species as a biocontrol agent.

Common endosymbionts affect host fitness and sex allocation via egg size provisioning

It is hard to overemphasize the importance of endosymbionts in arthropod biology, ecology and evolution. Some endosymbionts can complement host metabolic function or provide defence against pathogens; others, such as ubiquitous Wolbachia and Cardinium, have evolved strategies to manipulate host reproduction. A common reproductive manipulation strategy is cytoplasmic incompatibility (CI) between differently infected individuals which can result in female mortality or male development of fertilized eggs in haplodiploid hosts. Recently, an additional role of endosymbionts has been recognized in the modification of sex allocation in sexually reproducing haplodiploids. This was theoretically expected due to the maternal inheritance of endosymbionts and natural selection for them to increase infected female production, yet the underlying mechanism remained unknown. Here, we tested whether and how Cardinium and Wolbachia causing different CI types interact to increase female production in a haplodiploid thrips species where sex allocation depends on both maternal condition and egg size provisioning. We found that Cardinium augmented female production by increasing maternal fitness and egg size, thereby boosting fertilization rate and offspring fitness. Wolbachia, in contrast, reduced the beneficial effects of Cardinium. Our results demonstrate different invasion strategies and antagonistic effects of endosymbiotic bacteria on host fitness and evolution of sex allocation.

The effect of temperature on host patch exploitation by an egg parasitoid

The effect of temperature during host patch exploitation by parasitoids remains poorly understood, despite its importance on female reproductive success. Under laboratory conditions, we explored the behaviour of Anaphes listronoti, an egg parasitoid of the carrot weevil, Listronotus oregonensis, when foraging on a host patch at five temperatures. Temperature had a strong effect on the female tendency to exploit the patch: A. listronoti females parasitized more eggs at intermediate temperature (20 to 30°C) compared to those foraging at the extreme of the range (15.9°C and 32.8°C). However, there was no difference in offspring sex-ratio and clutch size between temperature treatments. Mechanisms of host acceptance within a patch differed between temperatures, especially at 32.8°C where females used ovipositor insertion rather than antennal contact to assess whether a host was already parasitized or not, suggesting that host handling and chemical cues detection were probably constrained at high temperature. Females spent less time on the host patch with increasing temperatures, but temperature had no effect on patch-leaving rules. Our results show that foraging A. listronoti females behave better than expected at sub-optimal temperatures, but worse than expected at supra-optimal temperatures. This could impair parasitoid performance under ongoing climate change.

Host population density and presence of predators as key factors influencing the number of gregarious parasitoid Anaphes flavipes offspring

The number of parasitoids developed per host is one of the major factors that influences future adult body size and reproductive success. Here, we examined four external factors (host species, heritability, host population density, and presence of predators) that can affect the number of the gregarious parasitoid Anaphes flavipes (Förster, 1841) (Hymenoptera: Mymaridae) wasps developing in one host. The effect of host population density on the number of parasitoid offspring developed per host was confirmed, and for the first time, we also showed that the number of offspring per host is influenced by the presence of predators. Low host density and presence of predators increases the number of wasps developed in one host egg. However, a higher number of A. flavipes in one host reduces A. flavipes body size and hence its future individual fertility and fitness. Our results highlighted the importance of some external factors that distinctly affect the number of wasp offspring. Therefore, in this context, we suggest that in comparison to solitary parasitoids, the gregarious parasitoid A. flavipes can better respond to various external factors and can more flexibly change its population density.

Optimization of a Banker Box System to Rear and Release the Parasitoid Habrobracon hebetor (Hymenoptera: Braconidae) for the Control of Stored-Product Moths

Pyralid moths, such as Ephestia kuehniella (Zeller) or Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae), are among the pests of most concern in mills and food industries worldwide. One option for their control, which presents an alternative to the application of insecticides, is the release of natural enemies. Habrobracon hebetor (Say) (Hymenoptera: Braconidae) is a larval parasitoid of pyralid moths that is commercially available for augmentative release in storehouses. They are delivered as adults that limit their performance. To improve their quality when released at the target location, a banker box has been developed consisting of a rearing box that optimizes the release of the parasitoid. In the present study, the nonpest larvae Galleria mellonella (L.) (Lepidoptera: Pyralidae) have been used as a host, substituting for E. kuehniella larvae which were used in the previous design. The best results were obtained when a mixture of two larval sizes of the host were offered to the female parasitoid, producing five times more adults than with E. kuehniella larvae. Quality of the released parasitoids was optimal because they were delivered in the pupal stage inside the rearing box and adults began to emerge in situ. The banker box released adult parasitoids over a prolonged period of approximately 25 d at the target location. The use of this banker box may significantly help in the biological control of stored-product moths.

Cockroach Oothecal Parasitoid, Evania appendigaster (Hymenoptera: Evaniidae) Exhibits Oviposition Preference Towards Oothecal Age Most Vulnerable to Host Cannibalism

Many female parasitoid wasps optimize host selection to balance the benefits of high-quality hosts and the costs of predator- or hyperparasitoid-induced mortality risks to maximize their fitness. Cannibalism exists in many insect species and affects survival of parasitoid larvae developing in or on parasitized hosts. However, little is known about how parasitoid wasps resolve the fitness consequence of host cannibalism-induced mortality risk during host selection. We examined the effect of oothecal age on cannibalism in the American cockroach Periplaneta americana (L.) (Dictyoptera: Blattidae) and its effect on host age selection and fitness of its oothecal parasitoid Evania appendigaster (L.) (Hymenoptera: Evaniidae). P. americana differentially cannibalized 1-d-old (30‒60%) versus 10- to 40-d-old oothecae (<9%). However, parasitoid females did not avoid but still preferred to parasitize 1-d-old (45%) over 10- to 40-d-old oothecae (1.6‒20%). The parasitism rate was greater and the handling time was shorter on 1-d-old compared to older oothecae. For parasitoid progeny emerging from different-aged oothecae, regression analysis showed that development time increased and body size (measured as hind tibia length) and longevity decreased with oothecal age. These results demonstrate that reduced parasitoid progeny survival due to host cannibalism did not change the parasitoid's oviposition preference for newly laid oothecae, and that E. appendigaster females traded progeny survival for fitness gains for themselves and their progeny.

Number of attacks by Trichogramma dendrolimi (Hymenoptera: Trichogrammatidae) affects the successful parasitism of Ostrinia furnacalis (Lepidoptera: Crambidae) eggs

In laboratories, the parasitism rate of Ostrinia furnacalis (Güenée) (Lepidoptera: Crambidae) eggs by Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae) is low; however, efforts to control O. furnacalis with T. dendrolimi in the field have been successful. In this study, the effects of the number of attacks by T. dendrolimi against O. furnacalis eggs and diet of O. furnacalis larva on wasp development were investigated. The results indicated that more attacks increased significantly not only the successful parasitism rate of O. furnacalis eggs by T. dendrolimi, but also the percentage of host eggs that failed to develop into either O. furnacalis larvae or T. dendrolimi. Both the size and female proportion of T. dendrolimi offspring decreased as the number of attacks increased. The number of T. dendrolimi eggs laid in per host egg increased significantly as the ratio of wasps to host eggs increased from 1:5 to 3:5. Host diet also significantly affected the developmental time of immaturity and the emergence rate of adults of T. dendrolimi. These results illustrate how inundative releases of T. dendrolimi can successfully control O. furnacalis despite the fact that pest parasitism by the subsequent wasp generation decreases sharply in the field. The suitability of O. furnacalis eggs to T. dendrolimi and the superparasitism effects on offspring of T. dendrolimi are discussed.

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.