Parasitism plays an essential part in ecology and evolution of host species and understanding the reasons for differential parasitism within and among hosts species is therefore important. Among the very important factors potentially affecting parasitism is the gender of the host. Here, we studied whether either females or males are more likely to harbour parasites among Odonatan insects, by relying on an extensive literature review and new field data. We collected data on numerous dragonfly and damselfly species and their ectoparasites (water mites) and endoparasites (gregarines) to examine the generality of similarities and differences in prevalence, intensity and maximum number of parasites of male and female hosts. We found three main results. Firstly, most of the odonate host species showed no differences between sexes in either gregarine or water mite prevalence and intensity. The only exception was female damselflies’ higher gregarine prevalence and intensity compared to conspecific males. These inequalities in gregarine parasitism may be due to behavioral and physiological differences between conspecific males and females. In comparison, there were no differences in dragonflies between sexes in water mite or gregarine prevalence and intensity. Secondly, damselflies had higher prevalence and intensity levels of both gregarine and water mite parasites compared to dragonflies. Finally, we found a strong species level pattern between female and male parasitism: a certain level of gregarine or water mite parasitism in one sex was matched with a similar parasitism level for the other. This indicates similar exposure and susceptibility to parasites on both sexes. Even though significant differences of parasite levels between the sexes were observed within certain host species, our results strongly suggest that on a general level a more parasitized sex does not exist in the order, Odonata.

1. Parasitism may be an important factor determining the coexistence of closely related species. Although host-parasite interactions can affect the ecology and distribution of the host species, virtually nothing is known about how other interspecific interactions affecting the host, such as competition or predation, relate to the parasite burden of the host.2. We studied parasite-mediated competition between two closely related Calopteryx damselflies, C. virgo L. and C. splendens Harris. We investigated a total of 31 populations, including 18 allopatric and 13 sympatric populations. We measured the occurrence of gut parasites, eugregarines.3. We found that the prevalence of gregarines was higher in C. virgo than in C. splendens. On average, more than half of the C. virgo individuals were infected by eugregarines both in allopatric and sympatric populations. However, hardly any allopatric C. splendens populations had gregarines, but most of sympatric populations had infected individuals.4. According to our results, co-existence of the host species affects the likelihood of the subordinate species showing higher levels of parasitism. Interspecific aggression, lower species genetic heterozygosity, and the difference in host species immunity are proposed as possible explanations for greater parasite burdens in the inferior species at sympatric sites.

Host–parasite interactions are an intriguing part of ecology, and understanding how hosts are able to withstand parasitic attacks, e.g. by allocating resources to immune defence, is important. Damselflies and dragonflies show a variety of parasitism patterns, but large-scale comparative immune defence studies are rare, and it is difficult to say what the interplay is between their immune defence and parasitism. The aim of this study was to find whether there are differences in immune response between different damselfly and dragonfly species and whether these could explain their levels of gregarine and water mite parasitism. Using an artificial pathogen, a piece of nylon filament, we measured the encapsulation response of 22 different damselfly and dragonfly species and found that (i) there are significant encapsulation differences between species, (ii) body mass has a strong association with encapsulation and parasite prevalences, (iii) body mass shows a strong phylogenetic signal, whereas encapsulation response and gregarine and water mite prevalences show weak signals, and (iv) associations between the traits are affected by phylogeny. We do not know what the relationship is between these four traits, but it seems clear that phylogeny plays a role in determining parasitism levels of damselflies and dragonflies.

1. Damselflies and dragonflies are widely parasitised insects and numerous studies have tried to understand this host–parasite relationship. However, most of these studies have concentrated on a single host species, neglecting the larger pattern within the Odonata order.2. The aim of this paper was to examine different damselfly and dragonfly species for common endo‐ and ectoparasites and whether a general infection pattern can be found. Additionally, the goal was to investigate whether the phylogeny of the host species could explain these possible infection patterns. To this end, a dataset from the existing literature was compiled and the prevalence of endoparasitic gregarines and ectoparasitic water mites was analysed for 46 different odonate species.3. Three distinct patterns were found: (i) most of the odonate host species had both gregarines and water mites, rather than only either one or neither; (ii) there appears to be a positive association between gregarine and water mite prevalences across host species; (iii) a weak phylogenetic signal was detected in gregarine prevalence and a strong one in water mite prevalence.4. It is hypothesised that, due to the infection and transmission mechanisms by which water mites and gregarines infect different odonate host species, parasitism is aggregated to common, high‐density species. However, much research is needed in order to fully understand this relationship between odonates and their parasites, especially within the same host populations and host species assemblages.

Cite this article: Suhonen J, Ilvonen JJ, Nyman T, Sorvari J. 2019 Brood parasitism in eusocial insects (Hymenoptera): role of host geographical range size and phylogeny. Phil. Trans. R. Soc. B 374: 20180203. http://dx.One contribution of 18 to a theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.Interspecific brood parasitism is common in many animal systems. Brood parasites enter the nests of other species and divert host resources for producing their own offspring, which can lead to strong antagonistic parasite-host coevolution. Here, we look at commonalities among social insect species that are victims of brood parasites, and use phylogenetic data and information on geographical range size to predict which species are most probably to fall victims to brood parasites in the future. In our analyses, we focus on three eusocial hymenopteran groups and their brood parasites: (i) bumblebees, (ii) Myrmica ants, and (iii) vespine and polistine wasps. In these groups, some, but not all, species are parasitized by obligate workerless inquilines that only produce reproductive-caste descendants. We find phylogenetic signals for geographical range size and the presence of parasites in bumblebees, but not in ants and wasps. Phylogenetic logistic regressions indicate that the probability of being attacked by one or more brood parasite species increases with the size of the geographical range in bumblebees, but the effect is statistically only marginally significant in ants. However, non-phylogenetic logistic regressions suggest that bumblebee species with the largest geographical range sizes may have a lower likelihood of harbouring social parasites than do hosts with medium-sized ranges. Our results provide new insights into the ecology and evolution of host-social parasite systems, and indicate that host phylogeny and geographical range size can be used to predict threats posed by social parasites, as well to design efficient conservation measures for both hosts and their parasites.This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.