Issue No. 2, Article 3/April 1, 2004
Let's Talk Grubs
Grubs are a topic that continues to grow each year. Over the past couple of years, grubs and grub damages have been at the tip of many farmers' tongues. Each year we receive reports of injured seedling corn and replanting of severely injured parts of fields.
In Illinois, primarily two types of white grubs cause damage to seedling corn: true white grubs and annual white grubs. The so-called true white grubs, or Phyllophaga white grubs, have three-year lifecycles. The southern masked chafer, Cyclocephala lurida, and the Japanese beetle, Popillia japonica, are the two most common annual white grubs found in corn in Illinois.
Phyllophaga white grubs may cause considerable damage to cornfields. Hosts for the adults of these grubs include ash, elm, poplar, and willow trees. The risk of infestation of Phyllophaga white grubs is greatest near those adult food sources, and thus their injury is probably not widespread. That fact, in conjunction with research that tells us that the southern masked chafer rarely causes economic injury to corn or soybeans, leads us to believe that most of the grub damage in Illinois has been caused by Japanese beetle grubs.
Japanese beetle grubs lay eggs in the soil in mid- to late summer. Larvae hatch and feed through the fall until temperatures begin to fall. They then descend in the soil (about 15 to 18 inches deep) until temperatures begin to warm again. The grubs feed primarily on organic matter but will feed on corn roots, especially when organic matter is not readily available.
The lifecycle of Phyllophaga grubs is different from that of the annual white grubs. May or June beetles, the adults of Phyllophaga white grubs, also lay eggs in mid- to late summer. The larvae then hatch and molt once before descending in the soil and becoming dormant over the winter. As temperatures rise in the spring, they move toward the soil surface to feed on plant roots, including corn and soybeans. They will feed throughout the summer. They pass a second winter in the soil and feed once again on plant roots in the spring. Problems associated with Phyllophaga white grubs are noticed more this second spring because the grubs are full grown and consume more root tissue. When they finish feeding, the grubs pupate, and adults emerge in midsummer.
Both types of grubs cause similar injury to corn plants. Grubs feed on root hairs of seedling corn plants. This affects the uptake of water and nutrients (phosphorus), causing the plant to wilt and stems and leaves to turn purple. Stand loss may occur in severely infested fields when injured plants die. Early-planted corn may be more susceptible to grub injury because grubs feed early in the spring.
It is very difficult to anticipate the occurrence of injury caused by white grubs. While most Phyllophaga problems occur near adult food sources, there has not been a telltale indicator for where Japanese beetle grub damage may occur. Both types of grubs are found in the soil in late summer, but few people look for them. In the spring, watch for white grubs during tillage operations, which bring the grubs to the soil surface, where they are visible (especially to birds that follow you through the field!).
Identifying what grub species is present in the field is very important; Phyllophaga or Japanese beetle grubs may cause damage to corn seedlings, while Cyclocephala grubs will not likely cause economic damage. To identify white grubs, you need to examine the rastor patternthe pattern of small hairs and spines on the underside of the last abdominal segment (Figure 1). Different species of grubs have different rastor patterns.
Figure 1. Location of raster on a white grub (Illustration modified from the 2000 Corn and Soybean Field Guide, Publication ID-179, Purdue University Cooperative Extension Service, West Lafayette, Indiana
The Japanese beetle has an arrangement of hairs that form a V-shaped pattern (Figure 2a). These lines of hairs usually form a V that is distinct in the center of the pattern. The Cyclocephala grub has no distinct pattern of hairs on its rastor (Figure 2b). The Phyllophaga grub has hairs on its center arranged in nearly parallel rows, resembling an open zipper (Figure 2c).
Figure 2. White grub raster patterns: (a) Japanese beetle, (b) Cyclocephala grub, and (c) Phyllophaga grub. (Grub illustration modified from the 2000 Corn and Soybean Field Guide, Publication ID-179, Purdue University Cooperative Extension Service, West Lafayette, Indiana
If Japanese beetle grubs or Phyllophaga grubs are found, a soil insecticide or seed treatment might be justified, especially if corn is planted early. Table 1 lists insecticides registered for control of white grubs in corn.
As you're in the field this spring, please pass on any observations. We'll continue our research efforts to provide you with more information regarding these insects.--Kelly Cook