INSECTS. Corn Rootworm Larval Injury Evaluated in First-Year Cornfields

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1 INSECTS Corn Rootworm Larval Injury Evaluated in First-Year Cornfields In late July, Susan Ratcliffe, Extension entomologist, coordinated a large root-retrieval project with producers, numerous agribusiness personnel, and Extension Unit leaders and educators. We extend our thanks to all those who participated in this large project. Digging roots from cornfields in late July and early August is not much fun. Ultimately, we hope to refine our economic threshold even more when these data are further evaluated. To begin this task, cooperators dug corn roots from untreated check strips (no soil insecticide used), and we arranged for their journey to Urbana, where they were washed and rated on the Iowa State 1-to-6 injury scale (see Table 1). Root injury from these on-farm plots will be compared with adult Table 1. Corn rootworm larval injury in first-year cornfields. FOR IMMEDIATE RELEASE No. 20 / August 11, 2000 Executive editor: Kevin Steffey, Extension Entomologist Available on the Web at For subscription information, phone , or acesnews@uiuc.edu In This Issue Corn Rootworm Larval Injury Evaluated in First-Year Cornfields, 217 Keep Your Eye on European Corn Borers, 218 Blister Beetles in Corn, As Well As Alfalfa and Soybeans, 218 Cotton/Melon Aphids in Soybeans in Northern Illinois, 219 Diplodia Ear and Stalk Rot, 219 Midseason Soybean Problems, 220 Regional Reports, 220 County Field number Number of roots rated Root rating 1 in check strips DeWitt DeWitt DeWitt DeWitt Douglas Kendall LaSalle Livingston Livingston Livingston Livingston Livingston Livingston Livingston Livingston Livingston Logan Logan Logan McLean McLean McLean McLean McLean McLean McLean McLean McLean McLean McLean McLean McLean McLean Sangamon Vermilion Vermilion Iowa State University 1-to-6 root-rating scale used: 1 = no visible damage or only a few minor feeding scars, 2 = some roots with feeding scars but none eaten off to within 1.5 inches of the plant, 3 = several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed, 4 = one node of roots destroyed or the equivalent, 5 = two nodes of roots destroyed or the equivalent, and 6 = three or more nodes of roots destroyed. See issue no. 15 (July 7, 2000) of the Bulletin. 217

2 capture data collected in soybean fields during 1999 with Pherocon AM traps (yellow sticky traps) and the threshold adjusted as needed. A cursory examination of these preliminary data reveals considerable variation in root injury from field to field. Root ratings in Livingston County (eight fields evaluated) ranged from 1.3 (light scarring) to 4.9 (two nodes of roots destroyed). Similarly, a wide range of injury occurred in McLean County (1.7 to 4.3). By using Pherocon AM traps in soybean fields this season, producers should be able to more accurately pinpoint those firstyear cornfields most at risk to corn rootworm injury in Data such as these, year after year, strongly document that economic larval injury does not occur in every rotated cornfield of east-central Illinois. Remember, even continuous cornfields support economic infestations of corn rootworm larvae only about half the time in any given year. These data show that only 13 of 36 fields exceeded the economic injury index of 3.0 in untreated check strips. The bottom line is that 64% of these first-year cornfields did not exceed the economic injury index. Before an investment is made in a soil insecticide for rootworm control in rotated corn, we encourage you to evaluate the potential need of these pest-management tools. Mike Gray and Kevin Steffey Keep Your Eye on European Corn Borers In last week s issue of the Bulletin (no. 19, August 4, 2000), I suggested that you should continue watching for European and southwestern corn borers, especially in later-planted cornfields. Although the flight of southwestern corn borers laying eggs for the second generation has peaked, I told you that the counts from traps in southern Illinois (as reported by Ron Hines, senior research agronomist at the Dixon Springs Ag Center) were extremely large this year. Alan Mosler with Twin Counties FS indicated on August 4 that second and third instars were present throughout southern counties and that boring into stalks was imminent. For the rest of the state, it s worth noting that European corn borer adults are rather common. I continue to see them every night, either in my yard or flying into my house, and others have reported considerably larger numbers than they had seen earlier this year. I heard one report that a light trap at the Monsanto farm near Monmouth in Warren County had captured more than 600 moths (I don t know the number of nights over which this capture took place). I have also heard of small pockets of significant infestations in several areas of the state. We may be in for a few surprises this fall when we conduct our annual survey. Let us know what you have found in your region the past couple of weeks. We d like to get a better feel for the ranges of percentage of infestations and the locations of infestations. Kevin Steffey Blister Beetles in Corn, As Well As Alfalfa and Soybeans I recently received a report from Dennis Epplin, crop systems educator in Mt. Vernon, about blister beetles stripping some corn plants, including the silks, in a cornfield in Jefferson County. He had been finding intense, localized infestations of blister beetles in alfalfa and soybeans earlier this summer, but the infestation he encountered in corn was the most severe. Although the infestation was not economic (from a whole-field perspective), the injury was devastating in small areas of the field. And to prove his point, Dennis sent three good photographs of clusters of the striped blister beetle feeding on corn leaves and corn silks. The photos appear on the Web version of the Bulletin. Although most of us have probably seen clusters of blister beetles feeding in alfalfa or soybeans, Dennis s observation of these pests in corn is a first for me. Their clipping of fresh silks resembles injury caused by Japanese beetle or corn rootworm adults; the injury they cause by feeding on leaves resembles injury caused by many other defoliators in corn, including armyworms and grasshoppers. The striped blister beetle is about 1/2 inch long, with prominent orange and black stripes. Other characteristics shared by several species of blister beetles are a broad head; narrow neck (prothorax); long, slender legs; and long, pliable wing covers that cover most of the abdomen. As alfalfa producers know, blister beetles produce cantharadin, a toxin that causes painful blisters when it contacts mammalian tissues. Cantharadin can be especially toxic to horses that feed on hay infested with blister beetles. In addition, blister beetles have been implicated as possible vectors of bean pod mottle virus in soybeans. On a good note, blister beetle larvae are predators of grasshopper eggs, destroying as many as 24% of the eggs in an area. Because of this, blister beetles usually are more prevalent during a year when grasshoppers are common and during the following year. In addition, blister beetle adults also are known to feed on pigweed. We consider blister beetles to be more beneficial than pestiferous (although their presence in alfalfa hay is a notable exception). Therefore, it s best not to overreact to their presence, even though injury to the plants can be quite severe in areas of the field where the beetles congregate. Kevin Steffey 218

3 Cotton/Melon Aphids in Soybeans in Northern Illinois Here s one for the record books: cotton aphids (also known as melon aphids) in soybean fields in Illinois. I first became aware of this occurrence when I read a recent issue of the University of Wisconsin s Wisconsin Crop Manager. John Wedberg, Extension entomologist at the University of Wisconsin (and former Extension entomologist at the University of Illinois a looooong time ago; sorry, John), had first observed aphids in soybeans in the southern tier of counties in Wisconsin in late July. Several plants, in a plot in which John and Craig Grau (plant pathologist) are investigating insect/disease/herbicide interactions, were plastered with aphids on leaves, stems, and petioles. The diagnostician at the University of Wisconsin has identified the aphids as cotton aphids, or melon aphids, Aphis gossypii. The folks at Wisconsin have sent samples of the aphids to David Voetglin, an aphid specialist in the Center for Biodiversity at the Illinois Natural History Survey, for his taxonomic opinion. We will either verify, or correct, the species in a future issue of the Bulletin, after David has had an opportunity to see the aphids. Since reading John s article in Wisconsin s newsletter and receiving some information from him via e- mail, we also have received reports of these aphids in fields of soybeans in some counties in northern Illinois, including Kane and Grundy counties. Some FS field men have reported to Ria Barrido with Growmark, Inc., in Bloomington that they can find aphids in soybean fields in the northern tier of counties in Illinois. What do cotton/melon aphids look like? Cotton aphids are very small (about 2 millimeters long) and may be winged or wingless. The aphid usually is pale to dark green but may be yellow to dark brown. Its cornicles ( tailpipes on the rear of the abdomen) are black. If cotton/melon aphids occur in soybeans, what are the noticeable signs of their presence? According to John Wedberg, the first thing you notice in infested fields, in addition to the plastered plants, is the sticky honeydew, a common aphid secretion, on your pants when you exit the field. Leaves on injured plants are cupped or crinkled; they turn yellow if the infestation is severe. John also reported that in their plots sprayed with Warrior T, the healthy plants are taller and the canopy is closing. Injured plants are shorter and less full. Do cotton/melon aphids cause economic injury? Your guess may be as good as ours. Obviously there is no threshold for this insect in soybeans. John Wedberg has suggested spraying a few fields when the fields had signs of severe stress. However, he indicated that most infested fields look fine. We usually expect natural enemies such as lady beetles to keep aphids in check, but John reports that natural enemies have not done their job in Wisconsin; the same probably is true in northern Illinois. Are certain types of soybean fields more likely to be infested with cotton/ melon aphids? John Wedberg indicated that infestations of these aphids are heaviest in late-planted and double-cropped soybeans; however, earlier-planted soybeans also are infested in some counties. If you want to look for aphids in soybeans, focus on later-planted fields first. Are any insecticides labeled for control of aphids in soybeans? The insecticides registered for use on soybeans that also have aphids listed on their labels for use in cotton or melons include *Asana XL, dimethoate, Lorsban 4E, *Penncap-M, *Pounce 32.EC, and *Warrior T. (*Use restricted to certified applicators.) However, we can find no information about control of cotton/melon aphids in soybeans. So, you re on your own if you believe an insecticide application is warranted. Check appropriate labels for footnotes regarding control of aphids in cotton or melons (Does the label indicate suppression?), and don t exceed the highest labeled rate of the insecticide for use in soybeans. Follow all label directions and precautions. We ll provide updates as appropriate. Let us know if you are finding aphids in soybeans and the conditions of fields in which you find them. Kevin Steffey and Mike Gray PLANT DISEASES Diplodia Ear and Stalk Rot The time is here for early ear-rot and stalk-rot development. Eric Suits of Trisler Seed reports diplodia ear rot reaching about 6% to 7% in some fields near Allerton and Sidell, Illinois. Eric indicted that many husks and ears were already fully infected. Environmental conditions this season have set the stage for good development of ear and stalk rots. Leaf blights, insect damage such as corn rootworm feeding, unbalanced fertility, extreme weather conditions, and hail damage are the primary factors that individually or in combination predispose corn plants to infection by stalk rots. Specifically, this year, look in fields that were affected earlier in the season by common rust infection and in fields showing nitrogen deficiency. Fungi cause the ear and stalk rots that render the most damage in Illinois. What these different fungi have in common is that they rob the grain of nutrients during kernel maturation and significantly interfere with stalk integrity. These conditions lead to lodging and lower-quality grain. This season s weather has provided a good environment for the development in particular of one ear and stalk rot called diplodia. Diplodia ear rot is first noticed by the bleached appearance of the husk. When you peel back the husk, you will see a white, fluffy fungus. Diplodia will not produce toxins in the 219

4 grain, but the kernels will be very lightweight and shriveled in appearance. Diplodia stalk rot is easily identified: infection begins at the nodes, and as the disease develops, small, black specks that cannot be scraped off form near the nodes. Scouting for stalk rots is a fairly easy endeavor. Evaluate 20 plants at each of 5 locations in a field. Use the common zigzag scouting pattern to accurately evaluate stalk-rot incidence. Begin scouting when the kernels are at 30% to 40% moisture. You can use either of two methods to evaluate stalk integrity. The first is to lightly grasp the stalk at waist level and push it about 15 from vertical. A second method is to pinch the base of the stalk below the first node; stalks that lodge or collapse when pinched should be marked positive for stalk rot. Fields can endure stalk-rot incidence up to 10%. However, incidences above 10% to 15% call for an early harvest to prevent further damage and lodging. Observe the ears for ear rot at the same time you are scouting for stalk rot. Suzanne Bissonnette Midseason Soybean Problems Frogeye leaf spot. Fungal leaf diseases are beginning to show up in the soybean crop. Omar Koester of the Monroe/Randolph Extension Unit reports very significant frogeye leaf spot in many area fields. He also reports that in their variety trials there is quite a bit of difference in susceptibility of different varieties. Very good observation. Frogeye leaf spot used to be common in the southern part of the state, but use of resistant varieties decreased the prevalence of the disease. However, in the past several years, frogeye leaf spot has been showing up more frequently. Why? Well, it s a good question, but I can only give an indirect answer. First, we need to consider that some varieties are still showing resistance to the disease, so there are obviously still choices of cultivars with that resistance. That s the good news. Second, we need to consider that perhaps a different race of the fungus is increasing, and current resistance isn t effective. At least five races of the fungus have been reported in the United States. Third, we need to consider that cultivars are selected for various agronomic traits and for resistance to what will probably be the most likely diseases to affect them in the area in which they are grown. Selection for frogeye leaf spot may not have been a recent priority. Cercospora sojina is the fungus that causes the disease. Frogeye leaf spot is readily diagnosed in the field. Symptoms are small, round eyespots on the leaves. The center of the spot is light brown to grayish and surrounded by a fairly distinct purple to reddish brown border. Usually just leaves are affected. But stems and pods may be infected late in the season. Heavily infected leaves may die and drop off prematurely. Yield loss is rare; but when susceptible cultivars were grown, loss was reported up to 15% in heavily diseased fields. If pods become infected, the fungus can grow through the pod and infect the seed, resulting in poor seed quality and seed discoloration. Sudden death syndrome. We have had several articles in recent issues of the Bulletin discussing sudden death syndrome (SDS) of soybean. Bill Tarter of Alvey Labs reports the disease is showing up frequently in all of the central-southwestern counties. I did make the mistake of asking which counties, and much to his credit Bill listed all of the counties in that part of the state in rapid succession. Environmental conditions have been very favorable for infection and development of SDS this season. If we enter a weather period that places the crop under some stress, expect symptoms to show up rapidly. Suzanne Bissonnette REGIONAL REPORTS Extension Center educators, Unit educators, and Unit assistants in northern, west-central, east-central, and southern Illinois prepare regional reports to provide more localized insight into pest situations and crop conditions in Illinois. The reports will keep you up to date on situations in field and forage crops as they develop throughout the season. The regions have been defined broadly to include the agricultural statistics districts as designated by the Illinois Agricultural Statistics Service, with slight modifications: North (northwest and northeast districts, plus Stark and Marshall counties) West central (west and west southwest districts, and Peoria, Woodford, Tazewell, Mason, Menard, and Logan counties from the central district) East central (east and east southeast districts [except Marion, Clay, Richland, and Lawrence counties], McLean, DeWitt, and Macon counties from the central district) South (southwest and southeast districts, and Marion, Clay, Richland, and Lawrence counties from the east southeast district) We hope these reports will provide additional benefits for staying current as the season progresses. Northern Illinois Problems with performance of rootworm soil insecticides are still being reported. Beneficial rains have fallen the last few days, putting more yield in the bin for both corn and soybeans. Hot, humid weather has been the rule the past few days, and producers need to monitor for the presence of gray leaf spot in corn. 220

5 U of I Extension Newsletter Service University of Illinois at Urbana-Champaign 528 Bevier Hall, MC S. Goodwin Avenue Urbana, IL West-Central Illinois Rain continues to fall in most areas of the region, allowing the crops to continue to be essentially stress free since the very early stages of growth. Reports of a huge crop seem accurate at this time. Problems, such as SDS and brown spot in soybean and GLS in corn, continue to be observed. Yield reductions should, however, be minor. Effects of soil compaction are evident in some fields. Tillage in wet soils should be avoided. Alfalfa hay harvest continues to be a challenge. Last regular harvest should come off no later than September 10 in central Illinois. University of Illinois Field Day on the Urbana campus is August 24, with tours beginning at 7 a.m. CONTRIBUTING AUTHORS Suzanne Bissonnette (bissonnettes@mail.aces.uiuc.edu), Champaign Extension Center, (217) Mike Gray (m-gray4@uiuc.edu), Extension Entomology, (217) Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217) The Pest Management & Crop Development Bulletin is brought to you by University of Illinois Extension and Information Technology and Communication Services, College of Agricultural, Consumer and Environmental Sciences, University of Illinois at Urbana-Champaign. This newsletter is edited by Erin Cler and formatted by Jerry Barrett, ACES/ITCS. Helping You Put Knowledge to Work University of Illinois U.S. Department of Agriculture Local Extension Councils Cooperating University of Illinois Extension provides equal opportunities in programs and employment. 221