During the past few years, white grubs have vied with wireworms for the top spot among secondary insect pests of corn. And as the 2002 season begins, concern about white grub problems continues.|
For years we entomologists focused primarily on "true" white grubs (i.e., white grubs in the genus Phyllophaga with three-year life cycles). Although these white grubs can cause significant damage, evidence suggests that many white grub problems are not caused by Phyllophaga grubs. Hosts for the adults of these grubs are ash, elm, poplar, and willow trees, and more than one study has shown that the risk of infestation of Phyllophaga grubs is greatest in fields near adult food sources. Consequently, infestations of Phyllophaga species probably are not widespread.
So, our attention has focused more on annual white grubs in recent years. All annual white grubs, of which there are many species, have one generation per year. The species that are found most commonly in corn in Illinois are the southern masked chafer, Cyclocephala lurida, and the Japanese beetle, Popillia japonica. According to an article written by Larry Bledsoe, an entomologist at Purdue University, in the Proceedings of the 2002 Crop Protection Technology Conference, southern masked chafer grubs rarely cause economic injury to either corn or soybeans. Therefore, it's likely that much of the grub damage to corn that has occurred in Illinois in recent years has been caused by Japanese beetle grubs. Many observers have verified this statement.
Life cycles and injury. Japanese beetle adults lay eggs in the soil in mid- to late summer. Larvae hatch and feed through the fall, then descend in the soil to escape cold winter temperatures. The grubs move back toward the soil surface in the spring and feed on organic matter. However, they also feed on corn roots, if available, especially if organic matter is limited. When they finish feeding, the grubs pupate, and adults emerge in early summer.
May or June beetles, the adults of "true" white grubs, also lay eggs in mid- to late summer. The larvae hatch and molt once before winter dormancy. They, too, descend in the soil to avoid cold soil temperatures. In the spring, the grubs move toward the soil surface to feed on plant roots, including roots of both corn and soybeans. They continue to feed throughout the summer. After they pass the second winter deep in the soil, they again ascend the following spring to feed on plant roots. This is when most "true" white grub problems are noticed because the larvae are fully grown and consume more root tissue. When they finish feeding, the larvae pupate, and adults emerge in midsummer.
Both types of white grubs chew off the fine hairs on the roots; injured roots do not take up water and phosphorus very well. Consequently, aboveground symptoms of white grub injury includes wilting and purpling of the stem (phosphorous deficiency). Severely infested fields often suffer stand loss when injured plants die. Early-planted corn is more vulnerable to white grub damage because the insects feed early in the spring.
Anticipating white grub problems. It is difficult to anticipate white grub problems. As I stated previously, "true" white grub problems occur most frequently in fields near adult food sources. However, we have not detected a consistent pattern for problems caused by Japanese beetle grubs. Both types of grubs can be found in the soil in late summer, but very few people look for them. Consequently, one of the few ways to detect white grubs is to watch for them during tillage operations. Any type of soil tillage usually brings some grubs to the soil surface. In fact, the presence of lots of birds following a tillage operation usually is a clear indication that grubs are present. The birds feast on the grubs lying on top of the soil.
White grub larva.
Identification. If you find grubs in a field, it is very important to determine the type of grub present. Phyllophaga grubs and Japanese beetle grubs can cause damage to corn; it's unlikely that Cyclocephala grubs will cause economic damage. To identify white grubs, you need to examine the raster pattern--the arrangement of small hairs and spines on the underside of the last abdominal segment (Figure 1). Different species of white grubs have different raster patterns.
The Japanese beetle grub has an arrangement of hairs that form overlapping V-shaped patterns (Figure 2a). These lines of hairs form a V that usually is distinct in the center of the pattern. The Cyclocephala grub has no distinct pattern of hairs on its raster (Figure 2b). The Phyllophaga grub has hairs in the center arranged in nearly parallel rows, resembling an open zipper (Figure 2c).
Insecticides for control of white grubs. If you find either Japanese beetle grubs or Phyllophaga grubs, application of a soil insecticide or seed treatment might be justified. This is especially true if corn will be planted early. Insecticides registered for control of white grubs are presented in Table 1. Please follow all label precautions and restrictions.--Kevin Steffey