Mathematically modelling the population dynamics of CRISPR gene drive systems in the pine pest Sirex noctilio

Sirex noctilio is an invasive pest of pine that has caused significant economic damage in South Africa and many other Southern Hemisphere countries. Current management tools are not efficient in all cases and consequently there is a need for more efficient and targeted control measures. An emerging tool for pest management is the use of gene editing and associated gene drive systems. In this study, we aim to investigate the use of CRISPR-Cas gene drive systems in the management of S. noctilio in South Africa. As a first step, we developed a model for the population dynamics of S. noctilio , using historical national population monitoring data and incorporating the influence of two main biological control agents of the pest. We then modelled the influence of two different CRISPR-Cas systems on the population dynamics of S. noctilio namely, a baseline CRISPR model and Complementary Sex Determination CRISPR (CSD) model. Each model is used to simulate a male and female only introduction strategy to estimate the effectiveness of different methods of introducing the gene drive system. The model calibration was achieved by optimizing the model fit to existing data using the least squares technique. Results suggest that both CRISPR gene drive systems would be effective at controlling the population growth of S. noctilio at high levels of introduction, but overall population control would be hindered by practical limitations. Although only two CRISPR models were explored, the underlying population model serves as a framework for further studies into the population dynamics of Sirex noctilio , as well as many other CRISPR-Cas gene drive systems.