Two kinds of white grubs occur in corn fields; annual white grubs (chafer larvae) and true white grubs (May or June beetle larvae). Both types eat corn roots but the true white grubs are potentially more damaging. These larvae are typical “C” shaped white beetle grubs. Annual white grubs have scattered hairs on the posterior (raster) whereas true white grubs have two parallel rows of hairs on the posterior.
Both types of white grubs eat corn roots, preferring the root hairs and small rootlets. Injured plants seldom die but may be stunted and express signs of nutrient or moisture deficiency (due to the compromised root system). This would only occur when grub populations are high and are primarily true white grub species. White grubs prefer to lay eggs in grasses and the larvae overwinter in the soil. Therefore, high populations are usually limited to corn following pasture or other grassy circumstances. Tillage may also destroy many white grubs prior to planting and high populations are more common in conservation tillage situations, especially strip-tillage or no-tillage.
White grubs seldom justify a structured management approach or the use of a soil insecticide. However, in situations where white grubs are most likely, other soil dwelling insects may also be a threat. In this case the complex of seedling and root feeding insects may justify specific management practices including the use of a seed treatment or a soil insecticide. These situations would be most likely when tillage and rotation are restricted and if grassy circumstances precede the current corn crop.