Developing insect population models to support the design of GM control strategies

The stable fly is a major nuisance insect with a global distribution and is capable of mechanically transmitting a range of important livestock and human pathogens. The stress and injury caused by its biting activity are estimated to cost the US cattle industry around $1billion/year. Its impact is projected to increase in regions including Brazil and Australia as a consequence of recent changes to the management of sugar cane and other vegetable waste. Better control methods for stable flies would therefore benefit animal welfare, bioenergy production and food production.

The industrial host of this project, Oxitec, is a world leader in developing methods for manipulating insect populations through the development and release of genetically modified strains. For mosquitoes, this typically involves the release of very large numbers of modified male insects to outcompete wild-type males. This has minimal impact because male mosquitoes do not blood-feed. To use similar approaches to control stable fly populations, there is a greater need for tools to design optimally efficient release strategies because male and female stable flies both blood-feed.

The proposed project will develop process-based models of stable fly populations and use modern statistical approaches to fit them to data. Process-based models explicitly model biological processes such as birth and death rates rather than simply modelling abundance. They are relatively complex and the difficulty of fitting them has historically been a limitation, but they are better for exploring the responses of populations to unusual circumstances such as climate change or control strategies. Recently, better statistical approaches to fitting complex models to data have been developed, such as Approximate Bayesian Computation (ABC).

The Mathematical Biology group at Pirbright has a history of successfully using cutting-edge statistical approaches to fit complex biological models to data. During the recent outbreak of Schmallenberg virus in northern Europe, the group used Approximate Bayesian Computation to fit a complex disease transmission model to the early stages of the outbreak, allowing inferences to be made about key disease transmission parameters which were provided to the European Commission to make outbreak management decisions. The conclusions of the model were later validated experimentally. The group also has a history of facilitating the acceptance of modelling outputs for policy decisions. For example, disease spread simulations provided by the group were recently used to help make the case for licensing a novel bluetongue vaccine product, and during the BTV outbreak in the UK in 2007 the group provided simulation outputs in real-time in response to queries from policymakers.

Process-based insect population modelling is a logical next step for the group's research into the spread of vector-borne diseases, as it will allow the effects of climate change and novel control strategies on insect vector populations to be predicted with greater certainty. The stable fly is an ideal species to begin with for several reasons. Firstly, as outlined above, the species is associated with a substantial and growing direct impact in many areas of the world and represents a large potential market. Secondly, an opportunity exists to develop the model in parallel with a GM control product and then use the project outputs to design optimal release strategies of the new strains, supporting its uptake. The academic partner maintains the only colony of stable flies in the UK and is able to support the project via materials and know-how. Colleagues at EMBRAPA have recently begun collecting a large dataset of population observations that they are willing to share for the project, and an outline population model was already developed during previous research activities.

Not applicable for FLIP applications.

The proposed research will develop process-driven population models of Stomoxys calcitrans, the common stable fly, and Bayesian methods for fitting these models to field data. The general methodology is also likely to be useful for modelling the population dynamics of a range of insect species. The principal advantage of process-based models over statistical models is that they are more capable of exploring responses to change. Because of this, the principal potential beneficiaries of this research include organisations interested in knowing the responses of S. calcitrans populations to novel conditions, specifically environmental change and control strategies.

The first major group of beneficiaries is that of industrial companies developing GM solutions to insect population control and modification. The use of genetic modification to control insect populations is still a novel technology, although small-scale field releases have been used successfully. Reducing the costs associated with the mass-rearing and release of modified insects will reduce the direct costs of these activities and increase their profitability. Oxitec, a UK company, is the global leader in this area.

The second group is that of policy-makers. The legislative and regulatory framework pertaining to the use of GM releases for insect population control in the UK and EU is still developing. This project represents an opportunity to apply lessons learned during the process of integrating mathematical modelling into infectious disease policy decisions to accelerate the integration of mathematical modelling outputs into the policy decision-masking process related to GM insect releases.

The third major group of beneficiaries is farmers, both in the UK and elsewhere. S. calcitrans is a major nuisance insect with a global distribution and is capable of mechanically transmitting a range of important livestock and human pathogens. The stress and injury caused to animals and people by its biting activity are substantial in many parts of the world, and it is a common species on British cattle farms. The model developed during this project will include climatic drivers and so will allow the regional effects of seasonal and longer-term environmental changes, and of control strategies such as the removal of larval haitats, to be better understood. Furthermore, the impact of Stomoxys activity is believed to be increasing in some parts of the world because of changes to the management of agricultural waste, particularly that generated during the production of bioethanol from sugar cane, in an attempt to reduce the environmental impact of these processes. By supporting the development of a product which has the potential to reduce S. calcitrans populations, this research could resolve the conflict between bioethanol production and livestock production in some parts of the world.

Effective control methods for stable flies would benefit animal welfare, bioenergy production and food production. Oxitec is about to begin exploring the potential for using GM control strategies for controlling Stomoxys. The planned project is perfectly timed to exploit this opportunity to integrate mathematical modelling in the development and licensing process for new GM insect products by helping a UK company that leads the world in this field to develop a method of controlling a major and emerging insect problem.