Remarkable similarity in body mass of a secondary hyperparasitoid Lysibia nana and its primary parasitoid host Cotesia glomerata emerging from cocoons of comparable size

Host trehalose metabolism disruption by validamycin A results in reduced fitness of parasitoid offspring

The general cutworm, Spodoptera litura (Lepidoptera: Noctuidae) is a worldwide destructive omnivorous pest and the endoparasitoid wasp Meteorus pulchricornis (Hymenoptera: Braconidae) is the dominant endoparasitoid of S. litura larvae. Trehalase is a key enzyme in insect trehalose metabolism and plays an important role in the growth and development of insects. However, the specific function of trehalase in parasitoid and host associations has been less reported. In this study, we obtained two trehalase genes (SlTre1 and SlTre2) from our previously constructed S. litura transcriptome database; they were highly expressed in 3rd instar larvae. SlTre1 was mainly expressed in the midgut, and SlTre2 was expressed highest in the head. SlTre1 and SlTre2 were highly expressed 5 days after parasitization by M. pulchricornis. Treatment with the trehalase inhibitor validamycin A significantly inhibited the expression levels of SlTre1 and SlTre2, and the trehalase activity. Besides, the content of trehalose was increased but the content of glucose was decreased 24 h after validamycin A treatment in parasitized S. litura larvae. In addition, the immune-related genes in phenoloxidase (PO) pathway and fatty acid synthesis-related genes in lipid metabolism were upregulated in parasitized host larvae after validamycin A treatment. Importantly, the emergence rate, proportion of normal adults, and body size of parasitoid offspring was decreased in parasitized S. litura larvae after validamycin A treatment, indicating that validamycin A disrupts the trehalose metabolism of parasitized host and thus reduces the fitness of parasitoid offspring. The present study provides a novel perspective for coordinating the application of biocontrol and antibiotics in agroecosystem.

Density-Dependent Growth and Fitness in Dastarcus helophoroides (Coleoptera: Bothrideridae)

The ectoparasitoid Dastarcus helophoroides Fairmaire (Coleoptera: Bothrideridae) is an important natural enemy insect, which is artificially mass-reared and released into woodland to control medium and large longhorn beetle species. This study examined the developmental duration (days) of larvae and adult fitness (including numbers of adults emerging per host and mean body size) by exposing a single substitute host, a pupa of Zophobas morio (Coleoptera: Tenebrionidae), to different densities of D. helophoroides larvae. We showed that there was no significant effect on the rate of successful parasitism and cocoon formation, but emergence success and measures of individual adult body size (length, width, and weight) declined with increasing larval density. Larval period and cocoon period increased with larval density, while total weight of adults emerging per host increased initially before reaching a plateau. Our results suggest that a pupa of Z. morio could be successfully parasitized by a single D. helophoroides larva, but multiple D. helophoroides larvae can share one host. Excessive larval density caused intraspecific competition among D. helophoroides larvae, manifesting in extended developmental duration of immature stage and reduced fitness of adults. Furthermore, the tradeoff between the numbers of adults and body size may stabilize the population dynamics with detectable mutual interference, particularly in competing for limited host resources. These findings suggest six larvae per host would achieve the highest adult fitness and would enhance mass-rearing techniques as part of IPM strategies for longhorn beetles.

Development of a solitary koinobiont hyperparasitoid in different instars of its primary and secondary hosts

Parasitoid wasps are excellent organisms for studying the allocation of host resources to different fitness functions such as adult body mass and development time. Koinobiont parasitoids attack hosts that continue feeding and growing during parasitism, whereas idiobiont parasitoids attack non-growing host stages or paralyzed hosts. Many adult female koinobionts attack a broad range of host stages and are therefore faced with a different set of dynamic challenges compared with idiobionts, where host resources are largely static. Thus far studies on solitary koinobionts have been almost exclusively based on primary parasitoids, yet it is known that many of these are in turn attacked by both koinobiont and idiobiont hyperparasitoids. Here we compare parasitism and development of a primary koinobiont hyperparasitoid, Mesochorus gemellus (Hymenoptera: Ichneumonidae) in larvae of the gregarious primary koinobiont parasitoid, Cotesia glomerata (Hymenoptera: Braconidae) developing in the secondary herbivore host, Pieris brassicae (Lepidoptera: Pieridae). As far as we know this is the first study to examine development of a solitary primary hyperparasitoid in different stages of its secondary herbivore host. Pieris brassicae caterpillars were parasitized as L1 by C. glomerata and then these parasitized caterpillars were presented in separate cohorts to M. gemellus as L3, L4 or L5 instar P. brassicae. Different instars of the secondary hosts were used as proxies for different developmental stages of the primary host, C. glomerata. Larvae of C. glomerata in L5 P. brassicae were significantly longer than those in L3 and L4 caterpillars. Irrespective of secondary host instar, every parasitoid cluster was hyperparasitized by M. gemellus but all only produced male progeny. Male development time decreased with host stage attacked, whereas adult male body mass did not, which shows that M. gemellus is able to optimally exploit older host larvae in terms of adult size despite their decreasing mass during the pupal stage. Across a range of cocoon masses, hyperparasitoid adult male body mass was approximately 84% as large as primary parasitoids, revealing that M. gemellus is almost as efficient at exploiting host resources as secondary (pupal) hyperparasitoids.

Trophic assimilation efficiency markedly increases at higher trophic levels in four-level host-parasitoid food chain

Trophic assimilation efficiency (conversion of resource biomass into consumer biomass) is thought to be a limiting factor for food chain length in natural communities. In host-parasitoid systems, which account for the majority of terrestrial consumer interactions, a high trophic assimilation efficiency may be expected at higher trophic levels because of the close match of resource composition of host tissue and the consumer's resource requirements, which would allow for longer food chains. We measured efficiency of biomass transfer along an aphid-primary-secondary-tertiary parasitoid food chain and used stable isotope analysis to confirm trophic levels. We show high efficiency in biomass transfer along the food chain. From the third to the fourth trophic level, the proportion of host biomass transferred was 45%, 65% and 73%, respectively, for three secondary parasitoid species. For two parasitoid species that can act at the fourth and fifth trophic levels, we show markedly increased trophic assimilation efficiencies at the higher trophic level, which increased from 45 to 63% and 73 to 93%, respectively. In common with other food chains, δ(15)N increased with trophic level, with trophic discrimination factors (Δ(15)N) 1.34 and 1.49‰ from primary parasitoids to endoparasitic and ectoparasitic secondary parasitoids, respectively, and 0.78‰ from secondary to tertiary parasitoids. Owing to the extraordinarily high efficiency of hyperparasitoids, cryptic higher trophic levels may exist in host-parasitoid communities, which could alter our understanding of the dynamics and drivers of community structure of these important systems.

The evolution of body size, antennal size and host use in parasitoid wasps (Hymenoptera: Chalcidoidea): a phylogenetic comparative analysis

Chalcidoid wasps represent one of the most speciose superfamilies of animals known, with ca. 23,000 species described of which many are parasitoids. They are extremely diverse in body size, morphology and, among the parasitoids, insect hosts. Parasitic chalcidoids utilise a range of behavioural adaptations to facilitate exploitation of their diverse insect hosts, but how host use might influence the evolution of body size and morphology is not known in this group. We used a phylogenetic comparative analysis of 126 chalcidoid species to examine whether body size and antennal size showed evolutionary correlations with aspects of host use, including host breadth (specificity), host identity (orders of insects parasitized) and number of plant associates. Both morphological features and identity of exploited host orders show strong phylogenetic signal, but host breadth does not. Larger body size in these wasps was weakly associated with few plant genera, and with more specialised host use, and chalcidoid wasps that parasitize coleopteran hosts tend to be larger. Intriguingly, chalcidoid wasps that parasitize hemipteran hosts are both smaller in size in the case of those parasitizing the suborder Sternorrhyncha and have relatively larger antennae, particularly in those that parasitize other hemipteran suborders. These results suggest there are adaptations in chalcidoid wasps that are specifically associated with host detection and exploitation.

Differing Host Exploitation Efficiencies in Two Hyperparasitoids: When is a 'Match Made in Heaven'?

Host exploitation behavior in two hyperparasitoids, Lysibia nana and Gelis agilis, was compared in single cocoon clusters of their primary parasitoid host, Cotesia glomerata. L. nana reproduces sexually, is fully winged, does not host-feed and matures eggs quite rapidly after eclosion, whereas G. agilis possesses opposite traits. Cohorts of individual hyperparasitoid females of differing age and physiological state were given access to single cocoon clusters of C. glomerata that also varied in age. These results reveal that the reproductive biology of L. nana is well matched to exploit cocoon broods in C. glomerata, suggesting strong a co-evolutionary history with this host. By contrast, G. agilis is much less efficient at exploiting host cocoons and is probably a generalist species that attacks other hosts in nature.