One of the fields (of sweet potatoes) that I regularly walk past has several pheromone traps around it, so I decided to find out what they were trapping. The moths inside the trap were pretty battered and hard to identify, but I searched on the Internet for pictures of pheromone traps and found the same type, together with similar looking moths. It seems that the pest species is commonly known as the “armyworm” (Spodoptera litura) and it is a very serious pests of many kinds of crops, including sweet potatoes. Link to more information here.
Spodoptera litura = hasumon-yotou