PRUNING OF NODAL ROOT AXES TO SIMULATE FEEDING BY CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) ON GRAIN CORN

Mechanical root pruning was examined as a technique to simulate damage by northern and western corn rootworm to com plants in a 3-year study. Nodal root axes of com were pruned with steel blades of 4, 6, 8, 10 and 12-cm widths on each inter-row side of plants. Roots were pruned each week for 1 to 6 weeks during the normal period of rootworm infestation to simulate continuous larval feeding. For evaluation of the technique, plants in some treatment plots and in nontreated checks were retrieved immediately after pruning and 1 week later to determine amount of root pruning. In other plots the effect of mechanical root pruning on corn plant growth and yield was determined to compare with descriptions of actual rootworm damage. The effect of soil moisture on plant growth and lateral root development on nodal root axes after root pruning was determined by experiments in controlled environmental chambers. The results indicated that moderate and severe pruning of adventious roots with mechanical devices resulted in a decrease in initiation of nodal root axes and that for each cm width of the tool ca. 3% of the roots in the most recent two nodes were severed. Comparisons between effects of mechanical root pruning and reports in the literature on effects of rootworm feeding on com revealed similar results for compensatory root growth and plant growth and development. Significant increases in lateral root development on nodal root axes were observed after mechanical root pruning and the amount of compensatory root growth was affected by soil moisture conditions. Field experiments were conducted over 2 years employing the technique. In 1987 seasonal rainfall was nearly normal and there were significant differences in plant growth and yield among both the various root pruning treatments and for the different periods of 1 to 6 weeks of consecutive pruning. In 1988 drought conditions resulted in a large soil moisture deficit and there were significant differences in grain yield among the different root pruning treatments but number of weeks of consecutive root pruning did not affect yield, In both years the lowest root pruning treatment of 25% nodal root axes removal resulted in less than a 10% reduction in yield even when the pruning was repeated in each week for 6 weeks. Mechanical root pruning was shown to be an effective technique for causing damage to corn roots in the field similar to that caused by com rootworms in removing a consistent proportion of roots at a designated time.