As I write this article, the weather in Champaign County seems perfect for fieldwork--bright and sunny with a projected high temperature of 74°F. If this type of weather holds for a while, activity in the fields will increase substantially during the next couple of weeks. And as spring tillage commences and planting begins shortly thereafter, visions of white grubs will dance in your head. (Okay, the "Night Before Christmas" analogy is lame, but you get the point.)
During the past few years, white grubs have captured considerable attention in some areas of Illinois. In May 2002, we received a fair number of reports of seedling corn that had been injured by white grubs, and some parts of some fields had to be replanted as a consequence of the damage. White grub problems in 2002 were most prevalent in east-central, central, southwestern, and northwestern Illinois, but growers elsewhere had their fair share of grubs. Before the pace of corn planting becomes frenzied, now is a good time to review what we know and what we hope to learn about white grubs in 2003.
In the past, we have focused primarily on so-called true white grubs, that is, white grubs in the genus Phyllophaga with three-year life cycles. (From now on, I will refer to these grubs as Phyllophaga white grubs, rather than true white grubs.) Although these white grubs can cause significant damage, evidence is mounting that most white grub problems in Illinois 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 can all of the reports of white grub injury be attributed to annual white grubs? As the common name suggests, annual white grubs 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 Phyllophaga white grubs, also lay eggs in mid- to late summer. The larvae hatch and molt once before winter dormancy. They also 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 Phyllophaga 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 include wilting and purpling of the stem. Severely infested fields often suffer stand loss when injured plants die. Early-planted corn usually 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, Phyllophaga 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.
Entomologists have speculated that the increase in reports of white grub injury in recent years may have been the result of consecutive mild winters, at least in part. In many areas of Illinois, the winter of 2002-2003 was not necessarily mild. Depending on the depth of frozen soil, it's possible that white grub larvae may have frozen. However, keep in mind that white grubs overwinter 15 to 18 inches deep.
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).
Research to be conducted in 2003. Because Japanese beetle grubs and adults have been quite problematic in some areas of Illinois, we will embark on some research in 2003 to learn more about this pest in Illinois cornfields. Nathan Wentworth, an M.S. candidate in the Department of Crop Sciences, will conduct research in as many as six fields in Macon County this spring. He will work with me, Mike Gray, and Ron Estes, coordinator of the Insect Management and Insecticide Efficacy program, to establish trials in fields that have significant densities of Japanese beetle grubs. Nathan sampled for grubs in several fields on March 27, and he had no trouble finding the critters. In fact, one of the fields has a particularly high density of grubs, so we intend to establish two trials in that field. We hope that by establishing as many as six trials we will garner data from at least a couple of them. Our experience with white grubs in the past suggests that injury symptoms do not develop in all fields.
In the trials, we will examine the efficacy of several products registered for control of white grubs--Aztec, Capture, Cruiser, and Regent--and also try to relate different levels of injury with yields. We hope to examine the effects of grub feeding on several plant parameters, including plant population, plant height, and plant injury. In addition, we will monitor the emergence pattern of the adults beginning in June. The fieldwork will be supplemented with a greenhouse experiment to examine the effects of Japanese beetle grubs feeding on corn seedlings treated and not treated with Cruiser. Overall, we hope to gain some important insight regarding Japanese beetle grubs this year, and we intend to share any practical results we obtain. And as always, we will appreciate any observations you obtain from your field experiences this year.
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