Does the distribution of common buckthorn (Rhamnus cathartica) influence the distribution and abundance of soybean aphid (Aphis glycines) and multi-colored Asian ladybeetle (Harmonia axyridis)?

Widespread occurrence of the invasive shrub common buckthorn, Rhamnus cathartica, has resulted in cascading negative impacts across the north central United States. Common buckthorn invades natural areas where it directly reduces native biodiversity. In addition, it is the primary overwintering host of the invasive soybean aphid, Aphis glycines, allowing it to overwinter and negatively impact soybean production. Moreover, the presence of soybean aphid facilitates an increase in the invasive multi-colored Asian lady beetle, Harmonia axyridis. We are investigating how the level of buckthorn infestation in the landscape influences soybean aphid immigration into soybean fields, secondary dispersal of soybean aphid between soybean fields, and natural enemy presence. To accomplish these goals we placed 36 pan traps within a 15x15 mile grid and 6 within smaller satellite sites in Iowa, Michigan, and Ohio from May through August 2010-11. Sweep sampling and whole plant counts were also conducted within 10 soybean fields in 2010 and 4 soybean fields in 2011 within each grid. Landscape composition and percent buckthorn at these sites were analyzed to determine their effects on the distribution and abundance of the soybean aphid and its natural enemies. We expect to find that landscapes with higher amounts of common buckthorn will also contain higher proportions of aphid migrants and the surrounding soybean fields will be colonized earlier in the season. As a result of earlier colonization of soybean aphid we expect that these fields will provide habitat for successful larval development and thus become a source of Harmonia axyridis adults.