Interactions between parasitized and unparasitized conspecifics: parasitoids modulate competitive dynamics

Increasing radiation doses in Anastrepha obliqua (Diptera: Tephritidae) larvae improve parasitoid mass-rearing attributes

Doses of 40, 80, 120, and 160 Gy were applied to 5-, 6-, 7-, and 8-day-old Anastrepha obliqua larvae, which were exposed to the Neotropical-native braconids Doryctobracon crawfordi and Utetes anastrephae and the Asian braconid Diachasmimorpha longicaudata. These tests were performed to know the effect of the increase in host radiation on the emergence of the aforementioned parasitoids and the related consequences of oviposition on the host. The study was based on the fact that higher radiation doses may cause a decrease in the host immune activity. There was a direct relationship between the increase in radiation dose and the parasitoid emergence. Both, the weight and the mortality of the host larvae were not affected by radiation. Although the larval weight of the larvae was lower and the mortality was higher in the younger larvae. Both, the number of scars and immature stages per host puparium originated from the younger larvae were lower than those from older larvae. Only U. anastrephae superparasitized more at lower radiation. Superparasitism by D. longicaudata was more frequent at 160 Gy. Qualitative measurements of melanin in the larvae parasitized showed that the levels were lower with increasing radiation. As radiation doses increased, the antagonistic response of the A. obliqua larva was reduced. Host larvae aged 5- and 6-day-old irradiated at 120-160 Gy significantly improve parasitoid emergence. This evidence is relevant for the mass production of the three tested parasitoid species.

Hymenopteran Parasitoids of Leucotaraxis argenticollis (Diptera: Chamaemyiidae) and Leucotaraxis piniperda: Implications for Biological Control of Hemlock Woolly Adelgid (Hemiptera: Adelgidae)

The hemlock woolly adelgid (Adelges tsugae (Annand)) is a serious invasive pest of hemlock trees in eastern North America. Multiple biological control agents have been the focus of research aimed at pest management and conserving hemlock communities. Three promising A. tsugae specialist predators are the beetle Laricobius nigrinus (Fender) (Coleoptera: Derodontidae) and flies in the genus Leucotaraxis (Diptera: Chamaemyiidae), Leucotaraxis argenticollis (Zetterstedt), and Leucotaraxis piniperda (Malloch). However, these flies are vulnerable to parasitism by wasps in the genera Pachyneuron (Walker) (Hymenoptera: Pteromalidae) and Melanips (Walker) (Hymenoptera: Figitidae). This study explores parasitoid wasp interactions with these Leucotaraxis species in their native western North American range and potential impacts on the biological control program in the East. Leucotaraxis, La. nigrinus, and parasitoid emergences were observed from adelgid-infested foliage collected from Washington State and British Columbia in 2018, 2019, and 2020. Undescribed species of Pachyneuron and Melanips emerged from puparia as solitary parasitoids. Parasitoid emergence was positively correlated with Leucotaraxis emergence. Percent parasitism increased between February and July, with the months of June and July experiencing higher parasitoid emergence than Leucotaraxis. Differences in emergence patterns suggest that Pachyneuron may be more closely associated with Le. argenticollis as a host, and that Melanips may be associated with Le. piniperda. High parasitism in Leucotaraxis had no effect on La. nigrinus larval abundance, whereas the combined emergence of parasitoids and Leucotaraxis was positively correlated with La. nigrinus. This suggests that there is limited competition among these predators.

Parasitoid Communities in the Variable Agricultural Environments of Blueberry Production in the Southeastern United States

In blueberry crops, there are multiple pest species, and some of those can be suppressed by natural enemies including parasitoid wasps and predators. Parasitoid wasps occur within the environment often tracking pest species for food resources to complete their lifecycle. These small wasps are also sensitive to agricultural environments including agrichemicals, habitat availability, and climate. We investigated how the structure of parasitoid communities varied between organic and conventional blueberry systems, and how the communities of these parasitoids varied within field spatial scales (forested border vs edge vs interior). With the lower intensity of agricultural interventions occurring in organic systems and forested borders, we predicted more stable parasitoid numbers that would be insulated from predicted climate variability. In our study, parasitoids were observed in low abundance in each cropping system, with community structure dependent on both management practice and field position. Unmanaged blueberry fields and forested field borders contained more parasitoid families, and in conventional systems, we saw fewer families present in the field interior as compared to field borders. In this first study to characterize Southern parasitoid communities in blueberry production systems, we observed over 50 genera of parasitoids, with a few dominant families (Braconidae and Ichneumonidae) that would contribute to biological control in blueberry systems. Overall, we captured few parasitoids, which indicates a potential vulnerability in biological control, and the need for further research using other sampling techniques to better understand these parasitoid communities.

Are sick individuals weak competitors? Competitive ability of snails parasitized by a gigantism-inducing trematode

Parasitized individuals are often expected to be poor competitors because they are weakened by infections. Many trematode species, however, although extensively exploiting their mollusc hosts, also induce gigantism (increased host size) by diverting host resources towards growth instead of reproduction. In such systems, alternatively to reduced competitive ability due to negative effects of parasitism on host performance, larger size could allow more efficient resource acquisition and thus increase the relative competitive ability of host individuals. We addressed this hypothesis by testing the effect of a trematode parasite Diplostomum pseudospathaceum on the competitive ability of its snail host Lymnaea stagnalis. We experimentally examined the growth of snails kept in pairs in relation to their infection status and intensity of resource competition (i.e. food availability). We found that parasitized snails grew faster and their reproduction was reduced compared to unparasitized individuals indicating parasite-induced gigantism. However, growth of the snails was faster when competing with parasitized individuals compared to unparasitized snails indicating reduced competitive ability due to parasitism. The latter effect, however, was relatively weak suggesting that the effects of the parasite on snail physiology may partly override each other in determining competitive ability.

Virus infection suppresses Nicotiana benthamiana adaptive phenotypic plasticity

Competition and parasitism are two important selective forces that shape life-histories, migration rates and population dynamics. Recently, it has been shown in various pathosystems that parasites can modify intraspecific competition, thus generating an indirect cost of parasitism. Here, we investigated if this phenomenon was present in a plant-potyvirus system using two viruses of different virulence (Tobacco etch virus and Turnip mosaic virus). Moreover, we asked if parasitism interacted with the shade avoidance syndrome, the plant-specific phenotypic plasticity in response to intraspecific competition. Our results indicate that the modification of intraspecific competition by parasitism is not present in the Nicotiana benthamiana--potyvirus system and suggests that this phenomenon is not universal but depends on the peculiarities of each pathosystem. However, whereas the healthy N. benthamiana presented a clear shade avoidance syndrome, this phenotypic plasticity totally disappeared when the plants were infected with TEV and TuMV, very likely resulting in a fitness loss and being another form of indirect cost of parasitism. This result suggests that the suppression or the alteration of adaptive phenotypic plasticity might be a component of virulence that is often overlooked.

Differential tolerance to direct and indirect density-dependent costs of viral infection in Arabidopsis thaliana

Population density and costs of parasite infection may condition the capacity of organisms to grow, survive and reproduce, i.e. their competitive ability. In host-parasite systems there are different competitive interactions: among uninfected hosts, among infected hosts, and between uninfected and infected hosts. Consequently, parasite infection results in a direct cost, due to parasitism itself, and in an indirect cost, due to modification of the competitive ability of the infected host. Theory predicts that host fitness reduction will be higher under the combined effects of costs of parasitism and competition than under each factor separately. However, experimental support for this prediction is scarce, and derives mostly from animal-parasite systems. We have analysed the interaction between parasite infection and plant density using the plant-parasite system of Arabidopsis thaliana and the generalist virus Cucumber mosaic virus (CMV). Plants of three wild genotypes grown at different densities were infected by CMV at various prevalences, and the effects of infection on plant growth and reproduction were quantified. Results demonstrate that the combined effects of host density and parasite infection may result either in a reduction or in an increase of the competitive ability of the host. The two genotypes investing a higher proportion of resources to reproduction showed tolerance to the direct cost of infection, while the genotype investing a higher proportion of resources to growth showed tolerance to the indirect cost of infection. Our findings show that the outcome of the interaction between host density and parasitism depends on the host genotype, which determines the plasticity of life-history traits and consequently, the host capacity to develop different tolerance mechanisms to the direct or indirect costs of parasitism. These results indicate the high relevance of host density and parasitism in determining the competitive ability of a plant, and stress the need to simultaneously consider both factors to understand the selective pressures that drive host-parasite co-evolution.

Intraspecific competition: the role of lags between attack and death in host-parasitoid interactions

Many natural enemies do not immediately kill their host, and the lag this creates between attack and host death results in mixed populations of uninfected and infected hosts. Both competition and parasitism are known to be major structuring forces in ecological communities; however, surprisingly little is known about how the competitive nature of infected hosts could affect the survival and dynamics of remaining uninfected host populations. Using a laboratory system comprising the Indian meal moth, Plodia interpunctella, and a solitary koinobiont parasitoid, Venturia canescens, we address this question by conducting replicated competition experiments between the unparasitized and parasitized classes of host larvae. For varying proportions of parasitized host larvae and competitor densities, we consider the effects of competition within (intraclass) and between (interclass) unparasitized and parasitized larvae on the survival, development time, and size of adult moths and parasitoid wasps. The greatest effects were on survival: increased competitor densities reduced survival of both parasitized and unparasitized larvae. However, unparasitized larvae survival, but not parasitized larvae survival, was reduced by increasing interclass competition. To our knowledge, this is the first experimental demonstration of the competitive superiority of parasitized over unparasitized hosts for limiting resources. We discuss possible mechanisms for this phenomenon, why it may have evolved, and its possible influence on the stability of host-parasite dynamics.

Prevalence-dependent costs of parasite virulence

Costs of parasitism are commonly measured by comparing the performance of infected groups of individuals to that of uninfected control groups. This measure potentially underestimates the cost of parasitism because it ignores indirect costs, which may result from the modification of the competitiveness of the hosts by the parasite. In this context, we used the host-parasite system consisting of the yellow fever mosquito Aedes aegypti and the microsporidian parasite Vavraia culicis to address this question: Do infected individuals exert a more or less intense intraspecific competition than uninfected individuals? Our experimental results show that, indeed, infected hosts incur a direct cost of parasitism: It takes them longer to become adults than uninfected individuals. They also incur an indirect cost, however, which is actually larger than the direct cost: When grown in competition with uninfected individuals they develop even slower. The consequence of this modification of competitiveness is that, in our system, the cost of parasitism is underestimated by the traditional measure. Moreover, because the indirect cost depends on the frequency of interactions between infected and uninfected individuals, our results suggest that the real cost of parasitism, i.e., virulence, is negatively correlated with the prevalence of the parasite. This link between prevalence and virulence may have dynamical consequences, such as reducing the invasion threshold of the parasite, and evolutionary consequences, such as creating a selection pressure maintaining the host's constitutive resistance to the parasite.