Elevated numbers of white grub larvae are present in many crop fields, pastures, and lawns. In Missouri we use an economic threshold of 1-2 or more grubs per linear foot of row for most crops. This equates to 1 cubic foot of soil when you sample 6 inches to each side of the row and 12 inches deep. Two general groups of white grubs are typically found causing economic damage in crop and forage fields. They include several species of annual and perennial white grubs.
Annual white grub species have a 1-year life cycle in which they remain in the grub or larval stage for most of their lives. Eggs are typically laid in August and hatch in a few days to a few weeks. Depending on the species, small larvae (grubs) will feed and grow through several instars or stages before overwintering in cells they form in the soil. The next spring they become active, feed on available plant roots, usually grow through 2-3 additional instars and then pupate and emerge as adult beetles in late May, June and July. The adult beetles of some species (Cyclocephala spp) such as the Northern and Southern Masked chafers will congregate on willows, cottonwoods, sycamores to feed, mate, and then move into fields to lay eggs in the soil for the next generation. Other species such as the Japanese beetle (Popillia japonica) will often congregate on host plants such as corn or soybean to feed, mate, and then move to the soil to lay eggs.
Perennial white grub or True white grub (Phyllophaga spp.), includes up to 100 different species commonly referred to as May and June beetles. Most true white grubs have a three year life cycle with grubs slowly growing over a three year period. They pupate the last summer and typically emerge as adults during late April, May and June the following spring.
Grubs from both groups can cause severe damage when they feed on roots of most field and forage crops. They prefer grasses such as corn and wheat, but can also severely damage soybean plants. Damage is often observed in the spring on seedling crops, but may occur throughout the growing season depending on grub species. In surveys this past year, problems in corn and soybean were often caused by both annual and perennial grubs with populations of both grub groups being present in some fields. If only perennial grubs were present, we often observed several overlapping generations present and feeding on plant tissues. Conditions generally favorable to both groups of grubs include soils with high organic matter content, fields where cattle and other livestock have grazed, and fields with borders of certain tree species, especially willow trees. Grub larvae can generally be divided into annual and perennial categories by looking at the raster pattern (hair or spine patterns) found on the lower side of the tail end of the grub. If the hairs of the raster pattern are arranged randomly, then the grub is in the annual group. If the hairs of the raster are found to be in a distinct zipper pattern, then the grubs are from the perennial or Phyllophaga group.
Damage from grubs has increased substantially during the past few years. Several agronomic factors thought to favor grub survival in current agricultural systems include (1) reduced application of pesticides, (2) use of low dose seed treatment which provide partial control, (3) use of transgenic seed which does not target grubs, (4) changes in weed management which allow for more winter annuals and other weeds to remain in the field longer in the spring, (5) adoption of tillage systems which help build organic matter content in the soils, and (6) possibly other factors not yet known.
Management of white grub populations in established crops is difficult at best. In fields with known population levels of white grub, viable management options which will help reduce grub numbers include the use of high dose seed treatments (1250 rate) or 250 rate with additional insecticide, the application and incorporation of pre-plant insecticide applications such as Lorsban 4E, and the use of planting-time liquid Capture LFR1.5 or Regent 4SC, and use of planting-time granular products such as Counter 20CR or Force 3G. See specific labels for rates and placement.
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REVISED: October 2, 2015