This is the time of year when symptoms of injury caused by subterranean insects appear--quite a bit of corn has been planted and much of it has emerged. According to the Illinois Agricultural Statistics Service, 23% of the corn planted in Illinois had emerged by May 4. Several people monitoring emerging corn stands for any types of problems have encountered portions of fields with seedlings injured by white grubs and wireworms. The cool, wet soils in some areas will exacerbate the situation because the growth of corn seedlings will be slowed.
We have received reports of injury caused by both Phyllophaga grubs (so-called true white grubs with 3-year life cycles) and Japanese beetle grubs, Popillia japonica. These white grubs feed on the roots of corn seedlings and usually chew off the fine roots, hampering uptake of water and nutrients. Injured plants wilt and the stems turn purple, evidence of phosphorus deficiency. When infestations are severe, plants may die, resulting in reduced plant populations. Infestations of white grubs generally are patchy within a field.
Corn seedlings injured by white grub. Notice the purple color of the stems.
Close-up of corn seedling injured by white grubs. Notice the purple color of the stem and the wilted leaves.
If you want to know what type of white grub is damaging plants within a field, you have to become skilled at closely examining their rear ends. You can identity white grubs by examining the raster pattern--the unique arrangement of small hairs and spines on the underside of the last abdominal segment. We included an illustration of the raster patterns of Japanese beetles, masked chafer, and Phyllophaga grubs in issue no. 2 (April 4, 2003) of the Bulletin. But if you want to get up close and personal with the butt ends of some white grubs, visit a Web page created by Chris DiFonzo at Michigan State University. The photos on this Web page (http://www.msue.msu.edu/ipm/CAT03_fld/FC5-01-02grubs.htm) are really helpful for distinguishing among different species of white grubs. View raster patterns and anal slits to your heart's content.
Wireworm problems seem to be a bit more numerous than white grub problems this year, at least for now. Like white grubs, wireworm larvae feed on seedling roots, but they also feed on planted seeds and on the mesocotyl of the growing seedling. The latter two types of feeding injury usually cause plant death, resulting in reduced plant populations.
Reduced plant population as a result of wireworm damage.
As you know, there are no rescue treatments for either white grubs or wireworms after the damage has been discovered. Management of both of these pests requires knowledge of their presence before planting so that a soil insecticide or insecticidal seed treatment can be used. We have received a few testimonials this spring that corn seed treated with imidacloprid (Gaucho or Prescribe) or thiamethoxam (Cruiser) has reduced the impact of white grubs or wireworms in some fields. We hope to shed additional light on the efficacy of several products that we applied for Japanese beetle grub control in Macon and Piatt counties this spring. We established five trials this year, and we currently have a greenhouse study under way. We'll share the results at some future time.
If either white grub or wireworm damage is severe enough to reduce plant population significantly, replanting may be necessary. We recommend that you follow the agronomists' guidelines for replanting corn. If you replant because of white grub or wireworm damage, you should consider a soil insecticide or insecticidal seed treatment to protect the replanted corn, especially if you replant during early to mid-May. Refer to issues no. 1 and 2 (March 21 and April 4, respectively) of the Bulletin for products suggested for control of white grubs and wireworms.--Kevin Steffey