The role of interspecific interactions in the mitigation of vector-borne disease

Vector-borne disease is a significant source of mortality and morbidity, and disproportionately affects those who aremost socially and economically vulnerable. ThemosquitoesAedes aegypti andAedes albopictus are vectors of arboviruses such as dengue and Zika, with wide-ranging distributions that cover much of the tropics. There have been calls to manage Aedes populations using methods such as sterile and genetically-modified insect releases, with the efficacy of control contingent on a robust understanding of mosquito ecology.
Ae. aegypti andAe. albopictuscompete aslarvae through resource competition and as adults through reproductive interference, but the relative roles of thesemechanisms had not yet been investigated in anAedes-specific model. Thismotivatedmy second chapter,whereI built a theoreticalmodel that notonlyincluded these competitive mechanisms, but also modelled the process of host choice. I demonstrated that host preference both directly mediated pathogen transmission by changing biting rates while also determining the strength of reproductive interference. This led to complex patterns of coexistence and competitive exclusion that could not be predicted by simpler models of competition.
Experimental and field evidence suggests that the costs of larval competition are context-dependent. Therefore, in Chapter 3, I conducted a set of experiments to measure the consequences of larval competition under a particular per-capita feeding regime. These experiments revealed interesting aspects of larval biology – even when there were sufficient resources for all larvae to pupate, demographic variation in resource uptake led to interspecific competition. While examining particular biotic interactions in isolation is valuable, I did so in a laboratory under tightly controlled conditions. In reality, mosquito distributions arise from a complex multivariate function of both biotic and abiotic drivers.
One such driver is temperature; as mosquitoes are exothermic, they exhibit a number of temperature-dependent ecological and epidemiological traits. No model had yet examined how temperature-dependent rates could interact with competition to cause patterns of coexistence and exclusion. Therefore, in Chapter 4, I developed a multi-species model that included species-specific ecological and epidemiological responses to temperature, along with the competitive mechanisms explored in Chapter 2. The model predicted that Ae. albopictus larvae would out-compete Ae. aegypti in climates where competitive displacements have occurred in the past. I also demonstrated that the hydra-effect – the counter-intuitive increase in population size due to diminished reproductive output – could occur in some climates due to reproductive interference (but only if there were no costs of larval competition). The model also reveals temperature-dependent patterns of disease outbreaks that would be impossible to predict using simpler, temperature-invariant models.
The work in my thesis has elucidated the relative importance of two competitive mechanisms that govern the distributions of these prolific disease vectors. I determined that outcomes of competition are sensitive to temperature and biotic processes (such as host preference). These insights will be vital for those aiming to manage insect populations in the present, and for those seeking to predict distributions in the future.

Advisor: Bonsall, M
Wilson, A