Issue No. 6, Article 1/April 29, 2005
Time for Early-Season Corn Insect Pests
We're in that time of year when early-planted corn has emerged or is emerging (64% of the corn has been planted statewide, 16% has emerged, as of April 24 [Illinois Crop Weather, USDA]), corn planting continues, and the vast majority of soybean fields have not been planted, so there is little insect activity to report. Some alfalfa weevils chewing on alfalfa in southern Illinois, some reports of moth captures in pheromone traps, and some reports of flea beetles, subterranean insects, and possible cutworm feeding in corn, but that's about it. We're mostly in a holding pattern, waiting to learn whether insects will become an issue.
Corn in the ground early (some as early as late March) followed by cool to cold temperatures is a recipe for injury by some early-season corn insects if they happen to be present in the field. We have been suggesting for a few years now that the trend toward earlier corn planting has allowed more time for insects including grape colaspis, white grubs, and wireworms to feed on corn in the spring. And when cool temperatures slow down the growth of corn seedlings, there is greater potential for the appearance of aboveground symptoms of injury caused by secondary insect pests. However, it's also possible to mistakenly attribute noninsect injury (e.g., herbicide injury, environmental injury) to insects, so it is essential that you identify the responsible culprit before jumping to conclusions.
Thus far, I have received only one verified report of white grubs causing a little injury to corn in southern Illinois. Lynn Weis, Extension unit leader in Bond County, found some white grubs (species unknown) in a cornfield in her area on April 26, although the injury symptoms she observed were more likely associated with weather than with the spotty infestation of white grubs. In fact, the aboveground symptoms of injury were not characteristic of white grub injury--wilted plants and purpling of the stem as a result of reduced uptake of potassium. However, she found a few white grubs here and there, but she determined that the grubs were causing little to no injury at the time. She noted, however, that the grubs seemed to be smaller than she expected, although not as small as grape colaspis larvae. So this seems like an opportune time for a review of grubs and grape colaspis.
The term white grubs is a generic one that applies broadly to larvae of beetles in the family Scarabaeidae (the scarab beetles), which includes the Japanese beetle and "May" or "June" beetles. However, grub refers to any larva within the order Coleoptera (the beetles). So one could argue that the term white grub also includes grape colaspis larvae. However, for the sake of discussion, I'll use the term white grubs to refer only to Japanese beetle grubs and Phyllophaga grubs (the so-called true white grubs with three-year life cycles) in this article. I'll deal with grape colaspis larvae separately.
In this age of electronic communication, it is unnecessary to repeat some of what has been written before, especially if the biology of a given pest or suite of pests has not changed. Such is the case for white grubs, whose biology remains the same from year to year. So I refer you to an article ("Let's Talk Grubs") written by Kelly Cook in issue no. 2 (April 1) of the Bulletin in 2004. In her article, Kelly reiterates the importance of identifying white grubs to genus, if not to species, by examining the pattern of setae (hairs) on the raster (underside of the last abdominal segment) of the larvae. She also explains the biology of Japanese beetles, Phyllophaga grubs, and the southern masked chafer, Cyclocephala lurida. An important note regarding the April 1, 2004, article: The table of suggested insecticides in 2005 is slightly different from Table 1 in the April 1, 2004, article. So far as we know, neither Counter CR nor ProShield with Force ST is available (although Counter 15G still is available). Also, we have added *Proaxis at 0.66 oz per 1,000 feet of row (in furrow) to our list of recommendations for 2005. Keep in mind that these insecticides are preventive, not curative. If a white grub problem is identified, there are no known, effective "rescue" treatments.
Based on my conversation with Lynn Weis, it is possible that the smaller-than-expected white grubs she found could be Phyllophaga grubs in the second year of their three-year life cycle. If such is the case (we are working on a positive identification), these grubs can cause noticeable injury to corn. We will keep you apprised.
Although grape colaspis larvae have not caused as much stir the past couple of years as they did early in this new century, they always pose a potential threat, especially in southern and central Illinois. Because the injury caused by grape colaspis larvae is similar in appearance to injury caused by Japanese beetle grubs and Phyllophaga grubs, it's important to note the differences between the two types of insects. And once again, I can refer you to an article written in 2004: "Grape Colaspis Injury May Be Causing Some Injury to Corn," in issue no. 10 (May 28) of the Bulletin in 2004. The article includes descriptions and photographs of injury to corn seedlings caused by grape colaspis, as well as a photograph that compares grape colaspis larvae to white grub larvae. The size difference is obvious.
Although it may be a bit early to encounter grape colaspis injury, it's not too early to put these occasional pests on your radar for 2005. And, of course, you may encounter wireworms, seedcorn maggots, or some other insects during your diagnostic trips this spring. And when you add environmental injury, herbicide injury, and injury caused by other animals (e.g., I have received two reports of bird damage this spring), your palette of possible causes and effects in corn is broad. So keep your insect identification and diagnostic skills sharpened.--Kevin Steffey