In many areas of Central South Dakota, true white grubs (Figure 1) have caused serious damage to pastures and range over the last few years. In the affected areas, patches of damage vary in size from a few square feet to many acres.
As a result, we are beginning to receive calls regarding transitioning these areas to row crops.
Damage to Crops
Transitioning an affected area will initially remove the patches of dead grass and disturb any grubs present in the soil, but there is still a risk of white grubs causing injury to the replacement crop. If true white grubs are not managed they will prune crop roots (Figure 2) and cause patches within fields where crop yields are greatly reduced (Figure 3).
When planting into areas where white grubs have historically been an issue, it is important to consider using an in-furrow insecticide during planting. Although seed treatments may reduce white grub feeding in some instances, larger grubs are often unaffected by this management strategy.
We are likely in the second year of infestations, which indicates that the grubs will be fairly large and tolerant to insecticide seed treatments.