The effects of crop rotation on western corn rootworm adult abundance, root injury, and Bt resistance

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is a major pest of corn.  WCR populations have evolved resistance to several management strategies, including genetically modified corn producing toxins derived from Bacillus thuringiensis (Bt).  We tested whether crop rotation affected adult WCR abundance, root injury, and the development of Bt resistance.  In 2013 and 2014, four types of cornfields were identified: 1) cornfields with a history of crop rotation, 2) cornfields in continuous corn production, 3) past problem fields- cornfields with a history of injury to Bt corn, and 4) current problem fields- cornfields reported by farmers with greater than expected injury to Bt corn.  For each field, cooperating farmers provided a detailed management history.  Root injury was measured by rating roots from each cornfield and adult WCR abundance was measured throughout August using sticky traps.  Furthermore, adult WCR were collected from each field and their progeny were tested for resistance to Cry3Bb1 toxin using single-plant bioassays.  Past problem fields were typified by continuous corn production.  By contrast, current problem fields were a mixture of rotated cornfields and cornfields in continuous corn production.  Adult WCR abundance and root injury were both significantly greater in current problem fields compared to the other field types.  Single-plant bioassays revealed that survival of larvae from all four field types did not differ significantly between Cry3Bb1 expressing corn and non-Bt corn. Understanding how management practices influence rootworm populations and the development of Bt resistance is crucial for informing insect resistance management policies.