Japanese beetle is becoming a more common field crop pest in Iowa. Literature shows adults need about 1,030 growing degree days (base 50°F) to complete development. Japanese beetles will continue emergence until around 2,150 degree days. Based on accumulating degree day temperatures in 2014, Japanese beetle adults should be active in some areas of southeastern and southwestern Iowa this week (Fig. 1). Expect adults to emerge in central and northern Iowa in about 7-14 days if warm temperatures continue.
Available at: http://works.bepress.com/erin_hodgson/156/