Bt-crops, engineered to express a variety of toxins derived from Bacillus thuringensis, are an efficient method for controlling agricultural insect pests, particularly moth caterpillars. However, as with conventional insecticides, several insect populations around the globe have found ways to evolve mechanisms of resistance to Bt. Therefore, the sustainable use of Bt-crops is dependent on preventing, or at least greatly slowing, the rate at which resistance evolves, and by developing new Bt-crop varieties that target one or more weak points in the pest defences. Resistance spreads through a population much more slowly when the resistance trait is fully recessive (as opposed to partially recessive or dominant), and when the proportion of susceptible individuals in the population is high (which prevents two copies of the resistance alleles coming together in the same individual). This is why Bt-crops are engineered to deliver a high dose of toxin that is supposed to kill all individuals outright, and they are grown together with non-Bt plants, to sustain a sufficiently large number of susceptible individuals. The problem is that, for various reasons, the assumptions on which this resistance management strategy is based rarely apply to field conditions. This project is about putting these assumptions under the microscope by studying the genetics of Bt resistance in two major moth pests of maize, and incorporating this information into a predictive model to provide a more nuanced basis for designing insect resistance management strategies. The primary focal species is the African maize stalkborer, the major insect pest of maize and sorghum in sub-Saharan Africa. This project aims to discover the genetic changes that have occurred in the African maize stalkborer population in South Africa to confer dominant resistance to Bt maize, which led to the economic failure and replacement of the original Bt-crop variety. Surveys of genetic diversity will also allow us to assess the risk of Bt-resistance evolving in east African countries, where Bt-maize is close to being released. The secondary focal species is the Fall armyworm, a major pest of maize and cotton in the Americas, where it has evolved resistance to some Bt toxins. This species colonised west and central Africa in 2016, and has now spread to eastern and southern Africa, where it has elicited emergency large scale pesticide spraying of maize fields. By establishing the degree of tolerance (and its genetic basis) in the South African Fall armyworm population to the Bt-maize currently in use, and plugging this information into our model, we will provide a timely evaluation of how this non-native pest species is likely to cope with the current Bt-resistance management regime in a new ecological setting. The benefits of this research are that specific knowledge about the genetic identity and diversity of Bt resistance and tolerance mechanisms, when put together with a more realistic model, will aid in making more reliable predictions about the rate of appearance and spread of resistance under alternative refuge design approaches. The results, including comparative analysis of two major pest species, will also contribute to developing new Bt crops that provide longer-term lepidopteran pest control. Finally, the outputs will both provide general scientific insights and be directly relevant to addressing a regional food security problem urgently in need of solutions.