ROOT RESPONSES OF MAIZE HYBRIDS FOLLOWING CORN-ROOTWORM LARVAL FEEDING DAMAGE

The degree of tolerance of maize (Zea mays L.) to corn rootworm (Diabr otica spp.) larval feeding may be affected by root system growth after insect feeding damage. This study evaluated the change in root pull r esistance (DELTARPR; as determined by the subtracting the root pull re sistance at the time of maximum larval feeding damage from the root pu ll resistance measured 17 days later) caused by western corn rootworm (D. virgifera virgifera LeConte) larval feeding damage in single-cross maize hybrids which were popular with corn producers during three suc cessive 10-yr eras. The 2-yr field study was conducted as a split-spli t plot with plant population as main plot treatments, rootworm infesta tion levelas subplot treatments, and plant hybrid as sub-subplot treat ments. Increased plaNt populations reduced vertical root pull resistan ce and delayed root development (as measured by the number of nodes pr oducing root axes). Under dry soil conditions, there was a significant plant population x infestation level x hybrid interaction because inc reased rootworm larval feeding damage and plant population reduced DEL TARPR to a great extent in 1960-era hybrids, while the 1980 and 1970-e ra hybrids were relatively unaffected. Under wet soil conditions, ther e was no significant effect of infestation level or plant population o n DELTARPR, and no significant 3-way interaction. However, DELTARPR of 1980 and 1970-era hybrids was greater than that of 1960-era hybrids. When the change in root pull resistance after rootworm larval feeding is considered, maize hybrids popular in the 1970's and 1980's were mor e tolerant than the 1960-era hybrids under high plant populations and dry growing conditions.