Tuesday, June 12, 2012

White Grub Damage Evident on Corn.

Doug Johnson, Extension Entomologist

White grubs; photo courtesy of Iowa State University.

Questions concerning corn root pruning by white grubs have surfaced across the state. A number of Ag. & Natural Resources agents have noticed typical damage and found white grubs in place, along with typically pruned roots. Certainly, given the drought status of most of our corn production area, it is within reason to believe that these grubs are placing an unusual stress on an already stressed crop.

Early white grub damage typically appears as stunted, wilted, discolored, or dead seedlings and/or as gaps in rows where plants fail to emerge. White grubs prune roots and can feed on the mesocotyl causing plant death. Obviously we are far past this early damage stage. In larger corn, grub feeding simply prunes the roots resulting in the reduced ability to move water and thus nutrients. The plants simply have an unthrifty look.

While there are some soil insecticides labeled for grub control, their control is somewhat erratic. These products must be applied before or at planting. No insecticides are recommended for rescue treatments. Even if soil applied rescues were available, we would likely need some rainfall to move the insecticide into the soil, especially in no-till. If we got the rain needed for that, we would have the rain we need for vigorous growth in the corn crop; which would likely outgrow the problem.

Consequently, even though one might be looking for a pesticide remedy for this problem, what we really need is rain. In the presence of rain the problem is likely to go away. In the absence of rain…… well the damage just will not matter.

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