A couple of folks asked me this past week about what to do this year if they observed white grubs in their fields last year. It is probably a good idea to review the life cycle of the true white grubs (Phyllophaga spp.).
True white grubs complete a single generation in 3 years in northern areas, sometimes in 2 years in southern areas. Each female deposits 35 to 60 white eggs in individual cells 1 to 8 inches deep in the soil. Small, C-shaped first instars emerge 2 to 3 weeks after egg deposition and feed on organic matter. Larvae molt once during their first summer. Grubs then move to a depth below the soil frost line. Second instars migrate upward and begin feeding on plant roots in the spring of the second year. Larvae feed heavily on roots the entire second summer; thus damage usually is greatest in the second year of the life cycle. Grubs molt to the third instar by late fall and again descend in the soil. Larvae ascend in the spring of the third year and feed until mid- to late summer. Crop stand reduction as a result of feeding by true white grubs also can occur early in the third summer, although damage usually is not as extensive as during the second year. In late July, grubs form earthen cells in which they pupate. During late August and early September, pupae transform to adults, which emerge the following spring.
Recent studies in the northern Corn Belt have shown that densities of Phyllophaga are larger in fields bordered by cottonwood or willow trees. The adults feed on cottonwood and willow trees in the surrounding shelterbelts (windbreaks) at night. At sunrise, they drop from the foliage to the soil surface. Females fly from the trees and lay eggs in the soil, usually in those areas that are adjacent to the shelterbelts. Consequently, areas of fields near shelterbelts usually suffer the greatest damage caused by the white grub larvae.
This information suggests that if you found full-grown white grub larvae last year, the odds are they will not be present at all this spring or, at most, for only a brief period of time. Also, if you observed white grubs in corn last year and the affected fields will be planted to soybeans, you won't have to worry about injury to the soybeans. Some soil sampling before corn is planted next year would be appropriate.--Kevin Steffey