Grooming decisions by damselflies, age-specific colonisation by water mites, and the probability of successful parasitism

Cooler and drier conditions increase parasitism in a subtropical damselfly population

Host-parasite interactions are impacted by climate, which may result in variation of parasitism across landscapes and time. Understanding how parasitism varies across these spatio-temporal scales is crucial to predicting how organisms will respond to and cope under a rapidly changing climate. Empirical work on how parasitism varies across climates is limited. Here, we examine the variation of parasitism across seasons and identify the likely climatic factors that explain this variation using Agriocnemis femina damselflies and Arrenurus water mite ectoparasites as a host-parasite study system. We assessed parasitism in a natural population in Sylhet, Bangladesh which is located in subtropical climate between 2021 and 2023. We calculated prevalence (proportion of infected individuals) and intensity (the number of parasites on an infected individual) of parasitism across different seasons. Parasite prevalence and intensity were greater during cooler seasons (autumn and winter) compared to hotter seasons (spring and summer). Mean temperature and precipitation were negatively correlated with parasite prevalence, whereas only mean precipitation was negatively correlated with parasite intensity. Tropical, subtropical and mediterranean regions are predicted to experience extreme climatic events (extreme temperature, less precipitation and frequent drought) as a consequence of anthropogenic climate change, and our finding suggests that this might alter patterns of parasitism in aquatic insects.

Parasitism, immune response, and egg production of the spearhead bluet (Coenagrion hastulatum) damselfly

Theoretical models predict that parasites reduce reproductive success of their hosts, but very few empirical studies have given support to this. Using the spearhead bluet (Coenagrion hastulatum (Charpentier, 1825)) damselfly, we tested how immune response, wing length, and the number of both endo- and ecto-parasites affect egg production of host damselflies. The study was conducted with four different populations in southwest Finland. We found a negative association between endoparasitic gregarines and number of host eggs. Furthermore, immune response increased with the number of water mites, but decreased with the number of eggs. Contrary to previous studies with other damselfly species, the number of ectoparasitic water mites did not affect the number of eggs. Moreover, wing length, used as an indicator of individual size, was not associated with egg numbers. The negative effect of gregarine parasites on egg numbers is likely to affect the composition of host populations, i.e., damselflies that show higher resistance to these endoparasites will have more of their offspring represented in subsequent generations. In future, more experimental research on the varying effects of different parasite species on the number of eggs is needed.

Inter-annual variation in prevalence and intensity of mite parasitism relates to appearance and expression of damselfly resistance

BACKGROUND

Insects can resist parasites using the costly process of melanotic encapsulation. This form of physiological resistance has been studied under laboratory conditions, but the abiotic and biotic factors affecting resistance in natural insect populations are not well understood. Mite parasitism of damselflies was studied in a temperate damselfly population over seven seasons to determine if melanotic encapsulation of mite feeding tubes was related to degree of parasitism, host sex, host size, emergence timing, duration of the emergence period, and average daily air temperature.

RESULTS

Although parasite prevalence in newly emerged damselflies was > 77% each year, hosts did not resist mites in the early years of study. Resistance began the year that there was a dramatic increase in the number of mites on newly emerged damselflies. Resistance continued to be correlated with mite prevalence and intensity throughout the seven-year study. However, the percentage of hosts resisting only ranged from 0-13% among years and resistance was not sex-biased and was not correlated with host size. Resistance also was not correlated with air temperature or with timing or duration of damselfly emergence.

CONCLUSIONS

Resistance in host damselflies was weakly and variably expressed over the study period. Factors such as temperature, which have been identified in laboratory studies as contributing to resistance by similar hosts, can be irrelevant in natural populations. This lack of temperature effect may be due to the narrow range in temperatures observed at host emergence among years. Degree of mite parasitism predicted both the appearance and continued expression of resistance among parasitized damselflies.

Success of ectoparasites: how important is timing of host contact?

Hosts often differ in their degree of parasitism and their expression of resistance. Yet very little is known about how the availability (and allocation) of resources to parasites at pre-infective stages influences their success in initiating parasitism, or in inducing and succumbing to resistance from hosts. We studied a damselfly-mite association to address how experimental variation in the age of first contact with hosts (timing) influenced subsequent parasite fitness. We demonstrate that timing influenced the ability of larval mites to make the transition to parasitism, but was not associated with measures of physiological resistance by hosts. Timing presumably relates to the availability of resources remaining for individuals to exploit their hosts. More research is needed on the importance of such factors, from variation in host resistance and parasite success and, ultimately, to the numbers and distributions of parasites on hosts.

On understanding variation in immune expression of the damselflies Lestes spp

Immune ability and immune expression have been viewed as life-history traits that are influenced by such factors as the likelihood of being parasitized, intensity and costs of parasitism, and trade-offs associated with immune expression. In this paper we show that different patterns of infestation by a generalist ectoparasite, Arrenurus planus Marshall (Arrenuridae: Hydrachnida), do not fully explain the variation in immune expression across four species of sympatric damselflies (Lestidae: Zygoptera). Within species, no gender biases in immune expression were evident. Whereas both males and females of one oft-exploited species did not mount immune responses against attending larval mites, males and females of three other species showed similar immune responses, with variable expression. The immune response was melanotic encapsulation of mite feeding tubes, and was associated with dead mites. Of the three species showing immune expression, the species with the highest prevalence and intensity of infestation had a significantly higher proportion of individuals responding immunologically to mites. In conclusion, current infestation levels only partially predict immune investment; consideration of the timing of emergence of different species suggests that season may be an important predictor of immune investment.

Effects of a mite, Limnochares americana (Hydrachnida: Limnocharidae), on the life-history traits and grooming behaviour of its damselfly host, Enallagma ebrium (Odonata: Coenagrionidae)

We examined whether experimental parasitism by a mite Limnochares americana (Lundblad) affected survivorship and maturation of adult damselflies Enallagma ebrium (Hagen). We then tested whether differences in grooming activity between control and exposed individuals (within different age or sex categories of host) mirrored reductions in fitness that resulted from experimental parasitism. We based our choice of experimental numbers of mites on our finding that adult damselflies had between 0 and 12 mites (71% had 0 mites), and mature adults had a higher prevalence and intensity of parasitism than did prereproductive damselflies in two of three field surveys. Low numbers of mites did not affect survivorship of teneral or mature males and females; however, high numbers of mites significantly depressed survivorship of teneral males and females and mature males, and were associated with a delay in maturation of females. Of teneral individuals, only females groomed more than controls when challenged with low numbers of mites; mature individuals of both sexes groomed more than controls in response to high numbers of mites but not in response to low numbers. Our results suggest that variation in grooming behaviour partially reflects variation in fitness costs, due to mite parasitism, across age and sex categories of hosts.