M. IVEZIC, E. RASPUDIC
Univ.of J.J. Strossmayer in Osijek, Faculty of Agriculture, 31000 Osijek, Croatia
J.J. TOLLEFSON
Iowa State University, Department of Entomology, Ames , Iowa 50011-3140
B.E. HIBBARD
USDA-ARS, Plant Genetics Research Unit, Univ. of Missouri, Columbia, MO 65211
I. BRKIC
Institute of Agriculture, Department for Corn Breeding, 31000 Osijek, Croatia

The western corn rootworm (Diabrotica virgifera virgifera LeConte (WCR)) is a major pest of corn in the USA. It was first recorded in Europe from Yugoslavia in 1992. The pest arrived in Croatia in 1995, and today over 200,000 ha are infested.  In the USA, insecticides are regularly used to control WCR, but the cost is high, can pose environmental risks, and may become ineffective due to resistance. Growing corn resistant to corn rootworms would be a valuable alternative to insecticides.

Ten Croatian (Institute of Agriculture, Osijek) and two Pioneer Hi-Bred. Int. Inc. (Johnston, Iowa, USA) commercial corn hybrids were evaluated for WCR resistance at four locations using a randomized block design with four replications. Two locations were in east Croatia (Gunja, Osijek) and two were in the USA (Iowa, Missouri). At the Missouri location the plots were artificially infested with WCR eggs; the others contained natural rootworm infestations. At all locations, root injury was rated using the Iowa State University Node-Injury Scale (0-3) and the hybrids’ tolerance to larval feeding was compared by evaluating root size and root regrowth. In addition the Iowa experiment contained side-by-side insecticide treated and untreated rows to measure yield tolerance.

In an analysis of variance combined across locations, root injury, size and regrowth were significantly different among hybrids and there was a significant location by environment interaction for each response variable. Root injury ratings in the USA averaged 1.52 and 1.67 at Iowa and Missouri, respectively, and 0.07 and 0.36 at Osijek and Gunja, respectively. Analysis by location showed the hybrids differed significantly in root injury and root size at Croatia and Iowa, but not Missouri.  Root regrowth was significantly different among hybrids at all four locations.