Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) is one of the major pests of soybean (Glycine max L. Merr.) in Korea and Japan. The occurrence of two important egg parasitoids of R. pedestris, Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) and Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae), before and after soybean cultivation is poorly known. To record the seasonal occurrences of R. pedestris and parasitism during 2008–2010, aggregation pheromone traps baited with R. pedestris eggs were placed in a field in Andong, Korea where a series of crops, including barley (Hordeum vulgare L.), sesame (Sesamum indicum L.), and soybean, were cultivated during this study. R. pedestris was first recorded in the second week of April on barley and was most abundant in August on soybean. G. japonicum first appeared in May and was the dominant parasitoid until the first week of August on barley, sesame, and early soybean. From late August to September or October when soybean was cultivated, O. nezarae became the dominant parasitoid. The differential pattern of seasonal occurrence between the two parasitoids is discussed. ► Aggregation pheromone trap baited with bean bug eggs was used to monitor parasitism. ► Bean bug was first found in April on barley and most abundant in August on soybean. ► Gryon japonicum first appeared in May and was the dominant parasitoid until August. ► From late August, Ooencyrtus nezarae became the dominant parasitoid on soybean. ► Differential pattern of seasonal occurrence between the two species is new finding.