Two examples of structured integro-differential models for populations of crop pathogens will be discussed. The first concerns a nematode species (the potato cyst nematode) whose stage durations are affected by temperature, such that higher temperatures will allow more generations per growing season. A model has therefore been developed to quantify how changes in climate will affect the impact of this pathogen on future potato crops. The second concerns a rice pest (the Brown Plant Hopper) which destroys large swathes of rice ('Hopper burn') in SE Asia. The Hopper is a host for various parasitoid species and also prey for spiders. Thus a model has been developed to investigate whether these parasitiod and predator species (which impact on different stages of the Hopper life cycle) can be used to control Hopper populations and thus avoid the application of pesticides.