Fertilizers are commonly applied to plants in both agricultural and ornamental settings and these fertilizers can affect insect herbivores. Beet armyworms, Spodoptera exigua, are phytophagous insects that are easy to rear in the lab. In this exercise, students design an experiment that manipulates plant quality via fertilizer application and then measure the effect of fertilization on caterpillar performance. In the first class, students design the experiment and apply the treatments. In subsequent weeks, students conduct a feeding trial where they collect and analyze data on caterpillar performance. Each student writes a lab report based on class data.