The study of insect pests of crops is called entomology.
Entomology is the scientific study of insects, including their biology, behavior, evolution, and ecological relationships. Within entomology, there is a subfield that focuses specifically on insects that cause harm to crops, livestock, and other agricultural products. This subfield is called agricultural entomology and it focuses on the study of insect pests and their management.
Pathology is the study of diseases and their causes, while agronomy is the science and technology of producing and using plants for food, fuel, and other products. Synecology is the study of ecological communities and their interactions.
So, the answer to the question is entomology.