IAP2-22-378

Investigating the role of sexual conflict in parasitoid-host eco-evolutionary dynamics

Finding ways to grow enough food to sustain the ever-growing human population while minimising detrimental impacts on the natural environment is a pressing global challenge [1,2]. Current food production systems rely heavily on the use of agro-chemicals for pest control, the negative impacts of which are extreme for species, ecosystems and for climate change [3-7]. Integrated pest management (IPM) is one strategy that will be critical if we are to feed the human population without inflicting irreversible environmental damage. IPM prioritizes biodiversity-based pest control and considers the use of chemical pesticides only as a last resort [8].

Parasitoid wasps are a large group of insects, which are commonly used as a chemical-free means of controlling insect pests (biological control). Adults are free-living, females lay their eggs on or in other species and their offspring feed on this host, typically another insect, invariably killing it. In addition to their economic value, parasitoid wasps have long been used by blue skies researchers to understand fundamental questions about evolution, ecology and physiology [9,10]. This research has supported their use in crop protection and assisted biocontrol practitioners in selecting the most appropriate species to control a given pest [9,10].

Despite the abundance of studies on parasitoid behavioural ecology, these species have received little attention in studies of sexual selection and sexual conflict [11]. Sexual conflict occurs when the optimum strategy or trait is different for males and females [12]. For instance, the optimal mating rate is generally higher for males than females. This can result in reduced fitness for females if accepting or resisting superfluous male copulation attempts is costly [12, 13]. If these costs manifest as reduced offspring production, there can be consequences for recruitment and even population viability [13]

For parasitoid wasps in natural and agricultural settings, sexual conflict may have profound consequences for parasitism rates. Understanding the extent to which sexual conflict influences individual and population fitness in these insects provides new and valuable information on how aspects of these species’ mating systems can influence host-parasitoid dynamics and coevolution as well as the efficacy of pest control under different IPM scenarios.

Research Objectives

This project aims to understand how sexual conflict in parasitoid wasps influences parasitism rates and the stability of host-parasitoid systems. The project will use a combination of individual-based modelling, lab-based fitness assays and field sampling using the sexual/asexual aphid parasitoid Lysiphlebus fabarum to address four main objectives:
1. Model the consequences of sexual conflict on parasitism rate and the fitness of parasitoid and host.
2. Assay the fitness of female L. fabarum that experience variation in sexual conflict through exposure to different numbers of males during oviposition in the lab.
3. Estimate variation in sexual conflict L. fabarum populations across the UK based on reproductive mode (sexual or asexual reproduction) and the sex ratio.
4. Test how the sex ratio and reproductive mode (sexual or asexual) influences the stability of host-parasitoid interactions at different spatial scales in L. fabarum [14].

The project will use the resevol R package (developed by supervisor Duthie [15]) as a foundation to model interactions between parasitoids and hosts depending on the sex ratio and its effects on fitness. This extension of the resevol package to include parasitoid-host dynamics and co-evolution will have applications beyond this project for the modelling of the likely success of parasitoid-based biocontrol in a range of agricultural scenarios. The empirical findings generated by this project will provide novel fundamental insights into how sexual conflict modifies host-parasitoid dynamics and the stability of their interactions in natural and agricultural settings.

The student should have a background in evolutionary ecology and experience in running lab-based fitness assays and conducting field work using insect model systems. Ideally the student will have some experience in insect rearing and data analysis and/or programming in R. The student will benefit from supervisor expertise in theory, individual-based modelling, lab-based experiments and field monitoring of parasitoid-host populations.

Methodology

To address the research objectives of this project, the student will interact frequently with supervisors at both the University of Stirling and the University of St Andrews. The student will join the Evolving Organisms research group at the University of Stirling.

The student will build individual-based models using the resevol package, which was developed by project supervisor Duthie to simulate the ecology and evolution of agricultural pest species. The student will work closely with Dr Duthie to modify the existing code to integrate host-parasitoid dynamics and co-evolution into the package and test the effects of different sexual conflict scenarios on parasitism rates and parasitoid-host fitness interactions.

Working alongside the lead supervisor Dr Boulton, the student will design and implement lab and field based assays of host and parasitoid fitness and parasitism rate. They will analyse data to test whether male biased sex ratios impose sexual conflict in the parasitoid wasp, Lysiphlebus fabarum, and establish how this influences its economically important host species, the black bean aphid, Aphis fabae.

Finally, the student will present their research at national and international conferences and the University of Stirling’s BES student symposium and be encouraged to publish their results in international peer-reviewed journals.

Project Timeline

Year 1

Lab-based fitness assays under different levels of sexual conflict (obj 2), literature review, modelling skills development in R (obj 1).

Year 2

Plan 1st field season, collect L. fabarum from populations across the UK and assay reproductive mode (sexual or asexual reproduction) and sex ratio (obj 3), integrate parasitoid-host interactions into resevol package.

Year 3

Plan 2nd field season, collect aphid colonies to measure parasitism rate of L. fabarum across populations with different reproductive modes and sex ratios, integrate sexual conflict scenarios into updated resevol package.

Year 3.5

Writing and completion of thesis.

Training
& Skills

During this project the student will learn how to rear and maintain colonies of insects with complex life histories that rely on host plants and host insects. They will learn to design and conduct lab-based fitness assays. They will learn to carry out advanced statistical analyses of these data in R and will use the results of these tests to plan subsequent field studies. They will also learn to program individual-based models in R.

The combination of lab-based assays, field testing and modelling in parasitoid-host systems will provide the student with networking opportunities across fundamental and applied disciplines.

Finally, the student will present their research at national and international conferences and the University of Stirling’s BES student symposium and will be encouraged to publish their results in international peer-reviewed journals.

References & further reading

[1] European Commission 2020. Evaluation of Regulation (EC) No 1107/2009 on the placing of plant protection products on the market and of Regulation (EC) No 396/2005 on maximum residue levels of pesticides EC Document 52020DC0208.[2] UN 2015. Transforming our World: the 2030 agenda for sustainable development. sustainabledevelopment.un.org, A/RES/70/1.[3] EASAC 2015. Ecosystem services, agriculture and neonicotinoids. European Academies Science Advisory Council policy report 26. ISBN: 978-3-8047-3437-1.[4] Springmann, M, et al. 2018. Options for keeping the food system within environmental limits. Nature 562:519–525.[5] Heimpel GE et al. 2013. Environmental consequences of invasive species: greenhouse gas emissions of insecticide use and the role of biological control in reducing emissions. PLoS ONE 8: e72293.[6] Cech, R, et al. 2022. Pesticide use and associated greenhouse gas emissions in sugar beet, apples, and viticulture in Austria from 2000 to 2019. Agriculture, 12:879.[7] Wyckhuys, KAG, et al. 2022. Carbon benefits of enlisting nature for crop protection. Nature Food, 3:299–301.[8] Hutchison, WD & SE Naranjo. 2014. Sustainable management of insect-resistant crops. In Plant Biotechnology: Experience and Future Prospects, DOI: 10.1007/978-3-319-06892-3-10.[9] Godfray. HCJ. 1994. Parasitoids: behavioral and evolutionary ecology. Princeton University Press.[10] Leung, K, et al. 2020. Next‐generation biological control: the need for integrating genetics and genomics. Biological Reviews, 95: 838-1854.[11] Boulton, RA, et al. 2015. Beyond sex allocation: the role of mating systems in sexual selection in parasitoid wasps. Biological Reviews, 90: 599-627.[12] Chapman, T, et al. 2003. Sexual conflict. Trends in Ecology & Evolution, 18: 41-47.[13] Holman, L & H Kokko. 2013. The consequences of polyandry for population viability, extinction risk and conservation. Philosophical Transactions of the Royal Society B: Biological Sciences, 368:20120053.[14] Pacala SW & MP Hassel. 1991. The persistence of host-parasitoid associations in patchy environments II: Evaluation of field data. The American Naturalist, 183: 584-605.[15] Duthie, AB, et al. 2022. resevol: an R package for spatially explicit models of pesticide resistance given evolving pest genomes. BioRxiv. https://bradduthie.github.io/resevol/

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