In a uniform infestation with eggs of the western corn rootworm, Diabrotica virgifera LeConte, the root damage rating was found to be a more sensitive measurement of root worm damage than vertical pulling weight, root lodging and goosenecking, plant stunting, or adult recovery. Also, root damage ratings, unlike vertical pulling weight, were independent of the size of the root system. Adult insects began to emerge from the artificial infestation at the same time as from a nearby natural infestation. The maximum damage the test corn hybrids were able to sustain was reached at ca. 1200 eggs/30.5 cm of row (row-ft); any increase beyond this level resulted in a decrease in the number of adults recovered. The relationship established between yield reduction and a known number of corn rootworms showed that even with almost ideal growing conditions, as few as 100 eggs/30.5 cm of row caused a highly significant reduction.
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Branson et al. (1980) studied this question.