The seasonal fluctuations and physiological status of Halyomorpha halys adults captured by traps baited with synthetic aggregation pheromone of Plautia crossota stali were investigated in a coppice in Akita Prefecture in 2001, 2002, 2005 and 2006. Peaks in the numbers of adults captured by pheromone traps were observed in mid-May to early June, mid-July to late July, and early September to mid-September in 2001, in mid-May to early June, and early September in 2002, in early September in 2005, and in late May and early September to mid-September in 2006, reflecting H. halys's activity of searching for food plants. The female ratio of trapped overwintered adults was not significantly different from that of trapped progeny adults. Nutritional levels of trapped adults were significantly lower than those of adults collected from food plants. The degree of ovary development of trapped overwintered females was significantly lower than that of females collected from food plants in May and July. Moreover, all trapped adult females had undeveloped ovaries in September, when they are likely to be in reproductive diapause. These observations suggest that H. halys's attraction to the pheromone of P. c. stali is related to searching for food plants.