Seasonal allergy to peanuts?

My Aunt's boyfriend says that he is allergic to peanuts and sunflower seeds, but only at certain times of year. Apparently he can eat them during the winter. I don't know much about allergies, but that doesn't really make sense to me. I was wondering if he might have an allergy to certain pesticides, because they wouldn't really have to be used during the winter. Does anyone know what could possibly be causing his allergic reactions, or have something similar?