Issue No. 11, Article 3/June 8, 2007
Insect Dribs and Drabs
Often at this time of year, some leftover early-season insect problems linger while the threat posed by some of our major insect pests begins to build. An overview of the insect situation in Illinois during the last week in May and the first week in June seems to be in order. I'll start with the insects usually of major concern and finish with a few "role players."
Corn rootworms. At least a few consultants have found second-instar corn rootworm larvae by sifting through soil or floating root masses in water. Lisa Coorts with Maxi-Yield Consultant Services found an average of 1.5 larvae per plant in a corn field on the county line between Macoupin and Montgomery counties. Brent Raines with Crop IMS has been finding second-instar rootworms in corn fields in Richland County. Neither reported finding large numbers or significant signs of root injury, primarily because we are still early in the rootworm season. However, these findings suggest that others who are determined to look for rootworm larvae could have some success . . . if the larvae are present. Because we still are not certain about the extent of the distribution of variant western corn rootworms in southern Illinois, now might be a good time to search for larvae in corn planted after soybeans. Let us know what you find, even if you find nothing. We always want to keep tabs on the variant western corn rootworm.
European corn borer. European corn borer moths continue to be captured in a handful of pheromone traps around the state. None of the counts has been noticeably large. However, Brent Raines observed approximately 50% whorl-feeding injury by first, second, and a few third instars in a field of non-Bt corn (about 4 feet tall) in Madison County. Freshly deposited southwestern corn borer eggs also were evident. Brent also found noticeable, although not extensive, whorl-feeding injury caused by European corn borers in some non-Bt corn refuges in fields in southern Illinois. Given some of the large densities of second-generation European corn borers in 2006, especially in western counties, we'll want to scrutinize the development of the first generation this year.
Japanese beetle. In last week's Bulletin (issue no. 10, June 1, 2007), we noted the first emergence of Japanese beetles in deep southern Illinois. At the time this article was written, I had not seen an update of "The Hines Report", but I received one report of a few Japanese beetles in a corn field in the area of Hamilton and Saline counties.
Grape colaspis. According to some observers, grape colaspis injury this year has rivaled the level we observed several years ago, although we have received fewer reports. Although grape colaspis populations ebb and flow over time, based on their survival in relation to environmental conditions, it's safe to assume that grape colaspis larvae are present in lots of fields every year. However, unless development of corn and soybeans is inhibited by less-than-ideal growing conditions, both crops typically grow right past the injury and we never see the effects of the larval feeding. But the dry soil conditions this year have allowed us to observe grape colaspis injury more clearly. Recent rainfall will help alleviate the situation. In addition, the larvae will complete their development soon, so their feeding will be complete.
Burrower bugs. On occasion, we get reports of burrower bugs occurring in large numbers in corn or soybean fields. Burrower bugs are in the family Cydnidae and are related to stink bugs (family Pentatomidae). The nymphs are brightly colored with red and black, and they feed underground, often on plants in the mint family (Lamiaceae), which includes the winter annual weed henbit.
Although some species of burrower bugs are known to cause injury to the underground parts of some crops, most notably peanuts, it is still unclear whether they will cause injury to corn and soybeans. On June 1, I visited a strip-tilled field of corn in Champaign County in which quite a bit of henbit had been killed with herbicides. The corn in several areas of the 80-acre field was noticeably stunted, although there were no other apparent symptoms of any type of injury (for example, by herbicides, grape colaspis, or white grubs). The roots appeared to be normal, and we could find no insects other than lots of burrower bug nymphs, Sehirus cinctus cinctus. (There is a good photograph of these nymphs at North Carolina State University's North Carolina Pest News Web site.) The combination of large numbers of these bugs and large patches of stunted corn certainly seemed suspicious. The recent rainfall in Champaign County likely will help the stunted corn considerably.
Patches of stunted corn infested with burrower bugs, Sehirus cinctus cinctus. Note the dead henbit plants between rows (photo courtesy of Dan Schaefer, Illini FS, Tolono).
Potato leafhopper. Matt Montgomery, University of Illinois Extension educator in Sangamon and Menard counties, has received reports of large numbers of potato leafhoppers in alfalfa fields in his area. After the first-crop injury caused by frost and alfalfa weevils, injury caused by potato leafhoppers, the most important economic insect pest of alfalfa, could really have a negative effect on yields and quality. Alfalfa growers and their advisors should be using sweep nets regularly to determine the densities of potato leafhoppers in alfalfa fields. Depending on the height of the crop, 0.2 to 2 leafhoppers per plant (3-inch to 12-inch alfalfa, respectively) can cause economic damage and would warrant an insecticide application.--Kevin Steffey