The introduction of an invasive species into an agroecosystem can alter both the interspecies dynamics and existing management practices. In the area of introduction, seasonality of Halyomorpha halys (Stål) in soybean fields was investigated by comparing monitoring efficiency of sweep net sampling and two sizes of pyramid traps baited with aggregation compound methyl (E,E,Z) 2,4,6-decatrienoate in 2006–2007. The large pyramid trap caught significantly higher densities of H. halys than the small pyramid trap and the sweep net samples each year. Adult males and females were detected in significantly higher densities in the large pyramid trap than other life stages. The pyramid traps caught H. halys adults and nymphs earlier than sweep net samples, during the R3 and R4 phenological stages of soybean growth. Peak abundances in the pyramid traps occurred during the R5–R6 stages, while the sweep samples were highest during the R6 stage. Soybean is sensitive to stink bug feeding damage from the R3–R6 stages. The occurrence of H. halys in soybean coincides with soybean's critical growth stage. H. halys has become the dominant stink bug species in the crop, indicating that damage thresholds need to be determined.