Issue No. 4, Article 3/April 16, 2004
Diagnostics Is Key to Making Early-Season Decisions
The corn planters are rolling and will continue to roll across Illinois as long as the weather holds. Some corn has emerged in southern counties. Consequently, reports of insect injury are not far behind. In previous articles in the Bulletin, we have discussed, in some detail, such early-season pests as black cutworms, white grubs, and wireworms. Throw in some armyworms, billbugs, flea beetles, grape colaspis, seedcorn maggots, southern corn leaf beetles, and stink bugs, and you have a full array of so-called secondary insect pests that can threaten corn production. We have stated more than once that early planting has contributed to the increase in reports of problems caused by several of these pests, so we won't be surprised if some of them make their presence known this spring.
Visits to cornfields early in the season are more common than they are later in the season, after corn has attained some height and temperatures are more uncomfortable. And as you know, all sorts of problems can occur early in the season, at a time when everyone in the "neighborhood" can see them. Some problems, of course, can be cured, whereas we often have to live with other (e.g., abiotic) problems. So it is extremely important to diagnose early-season problems (late-season, too, for that matter) so that unnecessary treatments are avoided. Spraying an insecticide to correct a problem that is the result of a fertility issue does not make a lot of economic sense.
In March 2003, several Extension specialists from the University of Illinois conducted a diagnostics workshop in Bureau County. Specialists involved in the workshop were Bob Hoeft (soil fertility), Dean Malvick (plant pathology), Terry Niblack (nematology), Christy Sprague (weed science), and me. Jim Morrison, Extension crops systems educator in Rockford, was the emcee, and he also discussed crop development and abiotic factors that affect crops. The workshop was designed for the specialists to progress chronologically through the season, indicating what types of problems could occur during any given crop-growth stage (e.g., corn emergence [VE] through V4-V5). The specialists interacted during each stage of the workshop so that all problems for a given crop-growth stage were addressed, rather than isolating problems by discipline (e.g., entomology). The workshop was well received because of the interaction among the specialists and those attending the meeting. Plans have been discussed to create a "workbook" of diagnostics, based on the visuals used at the workshop.
I explain the workshop as a setup for a link to some of the slides I used to discuss diagnosis of early-season insect problems in corn. The diagnoses are from planting to emergence (VE) and from emergence (VE) to V4-V5. A handful of slides provide some text for foundation, and the rest of the slides include photographs of injury and some of the insects (or related organisms) involved--armyworm, billbugs, black cutworm, carrot beetle, chinch bug, (other) cutworms, flea beetles, grape colaspis, hop vine borer, seedcorn beetles, seedcorn maggot, slugs, southern corn leaf beetle, stalk borer, webworms, white grubs, and wireworms. I owe thanks to my friend Marlin Rice, extension entomologist at Iowa State University, and to Gary Munkvold (formerly at Iowa State University) for their photos.
The slides include both common and relatively uncommon insects. In fact, you will never encounter some of the insects included (the same would hold true for the insects listed on many insecticide labels). However, I wanted to be as complete as possible. Nonetheless, it's always possible that you could encounter an insect or related organism not included within the slide set. So don't hesitate to send me photos and observations of your encounter.
As you are monitoring fields of corn early in the season, watch for symptoms of insect injury--cut, missing, stunted, or wilted plants; discoloration; distorted growth; gouges in stems; leaf tissue missing; reduced plant populations. Many of these symptoms also can be caused by other factors. So keep your diagnostic skills sharp, and make informed decisions.--Kevin Steffey