INSECTS. European Corn Borer Moths Reported Statewide

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1 INSECTS FOR IMMEDIATE RELEASE No. 10/ May 29, 1998 Executive editor: Kevin Steffey, Extension Entomologist Web subscriptions available: For subscription information, phone , or In This Issue Update on Corn Rootworm Egg Hatch, 102 Stalk Borers Are Active, 102 More Reports of Southern Corn Leaf Beetles, 103 Don t Forget About Armyworms in Wheat, 103 Certified Crop Advisors & Agrichemical Dealerships, 104 Seedling Blight Problems Continue in Corn, 104 Seed-Treatment Fungicides for Soybean, 105 Another Consideration When Replanting, 106 European Corn Borer Moths Reported Statewide Reports of European corn borer moths throughout the state have been confirmed. As indicated in last week s Bulletin, we can begin to expect the initial spring flight of moths to occur when 374 heat units (base 50 F) have accumulated from January 1. Based upon this old system of tracking heat units from January 1 to predict European corn borer phenology, this first flush of moths should begin to peak when 631 heat units have accumulated. Heat units (base 50 F) from January 1 through May 24 range from 850 in southern Illinois to 550 in northern counties (Figure 1). Have we reached peak flight activity in the southern one-third of the state already? Probably not. To project future biological events such as egg laying, more accurate temperature-based models rely upon an accumulation of heat units beyond the initial capture of spring moths. For instance, peak egg hatch is likely to occur when 100 heat units (base 50 F) have accumulated beyond the first significant moth flight. What is a significant moth flight? This can probably be best defined as the point at which moths are regularly splattering your windshield for the first time during an evening drive this spring. If average daily temperatures of 70 F occur for 5 consecutive days following a significant moth flight, 100 heat units will have accumulated and peak egg hatch will be underway. Bottom line European corn borer larvae will soon begin their leaf-feeding activities. Factors Contributing to the Likelihood of Corn Injury, 106 Figure 1. Actual heat-unit accumulation (base 50 F), January 1 to May 24,

2 What are the survival prospects for first-generation larvae? Late planting and replanting of corn in many areas of Illinois mean that first-generation larvae will not survive on many of the plants having an extended leaf height of less than 18 inches, due to high concentrations of DIMBOA, a plant aglucone (2-4 dihydroxy-7-methoxy- 1, 4-benzoxazin-3-one). The concentrations of DIMBOA begin to decrease as corn plants mature. Survival of corn borer larvae is better on plants in the mid- to late-whorl stage of development (22 to 36 inches extended leaf height). Because of the late planting and replanting experienced by many producers in Illinois this season, the second generation of European corn borers may represent a much more significant threat to corn production than the first generation. We will provide some projected European corn borer phenological developments in future issues of the Bulletin. Entomologist, (217) Update on Corn Rootworm Egg Hatch The timing of the 1998 corn rootworm egg hatch should match the descriptions provided in most entomological textbooks rootworm eggs should hatch in late May and early June. Because the egg hatches in 1996 and 1997 were so late (mid-june), this year s more typical hatch seems early by comparison. Soil heat-unit accumulations (4-inch level, base 52 F) provided by Bob Scott, Illinois State Water Survey, reveal a range of approximately 250 to 600 heat units for the state (Figure 2). Laboratory studies suggest that when 380 to 426 heat units have accumulated, as many as 50 percent of rootworm eggs may have hatched. This implies that for the southern two-thirds of Illinois, egg hatch is underway. Don t be surprised to find signs of larval injury much earlier in June this season than in years past. Because of the late planting and replanting of corn in many fields, Figure 2. Actual 4-inch soil temperature heat-unit accumulation (base 52 F), January 1 to May 24, larvae are likely to put severe pressure on very small root systems. Please pass along your observations when you begin to find this type of injury. In future issues of this Bulletin, we will offer some projections regarding peak larval injury and beetle emergence. Entomologist, (217) Stalk Borers Are Active Howard Brown with Pioneer Hi-Bred International, Inc., visited a field on May 26 in Kankakee County with a severe infestation of small stalk borer larvae. The field was infested with giant ragweed last year, a classic setup for stalk borers. The symptoms of injury in the field were a mixed bag of ragged defoliation of whorl leaves and plants with deadheart injury. We ll describe more about stalk borer management in this article. Because stalk borer adults lay their eggs on weed hosts in late summer and the eggs overwinter, stalk borer larvae begin feeding on young weed hosts early in the spring when they hatch. When the larvae grow too large for weed hosts, they move to larger hosts such as corn. In most fields, injury is confined to the first 4 to 6 rows of corn. However, in fields that were infested with weeds last summer, stalk borers could be more widespread. If corn has emerged in your area, it s time to look for stalk borers. Start looking for symptoms of stalk borer injury and the larvae first in field borders along roadsides and waterways. In northern Illinois where strip planting corn and alfalfa is more common, stalk borers can be particularly bothersome. Ragged leaf feeding, usually accompanied by a lot of frass (insect excrement), is the first sign of the presence of stalk borer larvae. However, the goal of stalk borer larvae is to enter the stalk, as the name implies, and they do so in one of two ways. They may enter from the top after feeding on whorl leaves, or they may drill in from the bottom of the stalk near the ground. In either situation, if larvae severely injure or kill the growing plant, the deadheart symptom appears that is, the center leaves wilt and die. Plants injured in such a way either die or produce many suckers and yield poorly. Stalk borer larvae are 1-1/2 to 1-3/4 inches long, depending upon instar, and dark purple to black with 5 longitudinal white stripes (1 on top, 2 on each side) broken by a dark purple band, or saddle, encircling the body just behind the legs. This band fades as larvae mature. The head capsule is yellow and has a dark stripe on each side. Management of stalk borers, if control is justified, requires timing an insecticide application to kill larvae before they bore into stalks. Application of an insecticide can be timed to coincide with movement of the larvae from weed hosts to corn. Stalk borers first begin to move into corn when about 1,100 heat units have accumulated above a base temperature of 41 F 102

3 Figure 3. Actual heat-unit accumulation (base 41 F), January 1 to May 24, since January 1; 50 percent movement occurs when about 1,400 to 1,700 degree days have accumulated. When about 1,300 to 1,400 degree days have accumulated, scout corn to verify the presence of stalk borers in weeds (dead stems, larvae inside) or border rows of corn. According to Figure 3, stalk borer larvae should have begun their movement into border rows of corn throughout most of the state. By now, 50 percent movement could have occurred anywhere south of St. Louis, and scouting for stalk borers should be underway throughout the southern half of the state. Economic injury levels based upon different leaf stages of corn growth and different prices for corn have been published by Iowa State University. We offered them in table form in issue no. 8 of this Bulletin (see Table 2 on page 79 of the May 15 issue). If treatment is warranted, based upon these thresholds, the following insecticides are suggested for control of stalk borers: Ambush 2E* at 6.4 to 12.8 oz per acre Asana XL* at 5.8 to 9.6 oz per acre Lorsban 4E at 2 to 3 pt per acre Pounce 3.2EC* at 4 to 8 oz per acre Warrior 1EC* at 2.56 to 3.84 oz per acre Some insecticides can be tank-mixed with fast-acting burn-down herbicides to control the borers that are forced from the dying weeds into seedling corn. Check the labels to determine compatibility of insecticides with herbicides. If a slow-acting herbicide is used, an insecticide can be applied 7 to 10 days later. Products marked with an asterisk (*) are restricted for use only by certified applicators. Kevin Steffey (ksteffey@uiuc.edu) and Entomology, (217) More Reports of Southern Corn Leaf Beetles The telephone isn t ringing off the hook, but reports of southern corn leaf beetles feeding on seedling corn continue to trickle in. As has been the case during the past 2 years, most reports of injury by this insect are from western Illinois. Wayne Bailey, Extension entomologist at the University of Missouri, also has encountered this insect and has received reports of damage. In fact, he is following up on one situation in which unidentified larvae are feeding on root systems of seedling corn plants. Although southern corn leaf beetle larvae feed on corn roots, little to no information exists about the impact of their feeding activity. We will keep you posted if we learn anything new about larval injury. We printed a more in-depth discussion of southern corn leaf beetles in issue no. 8 (May 15, 1998) of this Bulletin. Keep looking, and let us know if you find these critters. We are interested in learning as much as we can about this old insect that suddenly has become a new pest. Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217) Don t Forget About Armyworms in Wheat entomologists throughout the Corn Belt have reported finding an appreciable number of armyworm moths in light traps this spring. The moths started showing up in traps about the same time that we began to capture black cutworm adults. Not surprisingly, some folks now are starting to observe considerable larval armyworm activity in wheat fields. Remember, when the adult armyworms arrive in the Midwest, they seek thick stands of grass or grassy crops such as wheat into which they lay eggs. The eggs have hatched, and the larvae are feeding on wheat leaves. Robert Bellm, Crop Systems Educator at the Edwardsville Extension Center, reported a fair amount of defoliation in some wheat fields in southwestern Illinois. However, he has not yet observed either significant feeding on flag leaves or head clipping. Wayne Bailey, Extension entomologist at the University of Missouri, has observed head clipping in some fields in southeastern Missouri, so we should be on the lookout for this yield-threatening activity in Illinois very soon. A detailed discussion of armyworms and their threat to both corn and wheat was printed in issue no. 7 (May 8, 1998) of this Bulletin. However, now that some scouts may come across armyworms as they monitor wheat fields, it s important to repeat some of the information. Young armyworm larvae are pale green and have a looping habit when they crawl. Fully grown larvae are approximately 1-1/2 inches long and have distinct longitudinal white, brown, and orange stripes, most notably the orange stripes just beneath the spiracles (breathing pores) on each side of the body. Black stripes on the prolegs also are noticeable. Obviously, larger larvae cause considerably more injury than smaller larvae. Look first for armyworms in areas of wheat fields where the stand is dense. Part the wheat plants and sift through

4 the litter on the ground. Treatment may be warranted if you find 6 or more nonparasitized armyworms (3/4 to 1-1/4 inches long) per linear foot of row and before extensive head cutting occurs. Remember that armyworms usually don t cause economic losses in wheat until they begin feeding on the flag leaves or they clip off heads. Insecticides suggested for control of armyworms in wheat are Penncap-M* (2 to 3 pt per acre), Sevin XLR Plus (2 to 3 pt per acre), and Warrior 1EC* (2.56 to 3.84 oz per acre). Products marked with asterisk (*) are restricted for use only by certified applicators. Follow all label directions and precautions. On a related note, Robert Bellm also has observed some stem cutting in several wheat fields. He found 4- to 5- inch pieces of cut wheat stems stacked in areas of the field, usually near field edges. Although many growers attribute this to insect feeding, Robert properly diagnosed the problem as rodent activity. In the fields he visited, Robert found evidence of vole activity, and we are aware that mice do the same thing. We do not believe that the injury is economic in a wheat field, so proper diagnosis is important. Another reminder: Watch for armyworm larvae in any corn field that was planted no-till into a grass cover crop or has had an abundance of grassy weeds this spring. Kevin Steffey (ksteffey@uiuc.edu) and Entomology, (217) Certified Crop Advisors & Agrichemical Dealerships: Strengthening IPM Services and IPM Adoption A few weeks ago in this Bulletin, I listed the names of individuals who belong to the Professional Crop Consultants of Illinois (issue no. 8, May 15). The purpose of the article was to encourage an even greater adoption of IPM on the state s corn and soybean acres. One step toward increasing greater adoption of IPM is acknowledging that many producers do not adequately monitor their fields for pest injury nor do they hire professional consultants often enough. agrichemical dealerships and other organizations throughout the state of Illinois hire crop advisors who have passed state and national examinations certifying their expertise as professional agronomists. Increasingly, many dealerships in Illinois have added a scouting and troubleshooting component to their full line of services. Incentives and professional rewards based upon an employee s overall pest management skills, agronomic knowledge, and scouting services offered to clientele and not just based upon volume of chemical sales will strengthen the credibility of everyone involved in agriculture. dealerships in Illinois have taken steps in the right direction by becoming brokers of pest management and agronomic information as a key part of their overall operations. Mike Gray, (m-gray4@uiuc.edu), Extension Entomologist, (217) PLANT DISEASES Seedling Blight Problems Continue in Corn Wet soil conditions are continuing to cause seedling blight problems in corn. The primary pathogen involved appears to be the fungus Pythium, which is favored by wet soils and cool weather. Pythium infections are most common in very wet or flooded areas of fields. Symptoms on plants include a general browning of the seminal, or primary, root tips or of the hypocotyl, as well as poor plant vigor. Infected plants usually do not recover unless there are adequate permanent roots to replace those damaged by the fungus. As far as replant decisions are concerned, corn producers have a few 104 options when dealing with seedling blights. Almost all of the commercial seed is now treated with a fungicide to protect against Pythium. The fungicide may be Allegiance, Apron XL, or Apron all of which should provide equal measures of protection against Pythium. However, this year, some producers have asked if the level of fungicide was adequate to protect seedlings due to the prolonged delays in germination for some fields of corn. In fields where germination did not occur for 10 or more days, the levels of protection offered by the seed treatments may not have been affected by the high soil-moisture level or the delayed germination and seedling emergence. Seed treatments are designed to provide 10 to 14 days of protection under normal conditions (whatever you consider that to be in the Midwest) when germination would be expected to occur within a few days to 1 week after planting. When rains and cool weather continue, the effectiveness of the fungicide can be compromised and seedling blights can be seen. Such was the case this year in many areas of the state. One of two other fungicides will be applied to corn in addition to the Pythium-control material. Depending on the seed company, either Maxim or Captan will be applied. Both products provide a broad-spectrum approach to fungal disease management and should protect seed against soil-borne fungi. Some questions have been asked as to which material is most effective in wet soils. Currently, there are no published data from replicated trials to favor either product. Therefore, producers should not become overly concerned about performance of hybrid seed based upon the selection of a seed treatment by a company. Seedling blight fungi are affected by many factors beyond the seed treatment itself. Walker Kirby (kirbyw@mail.aces.uiuc. edu), Extension Plant Pathology, (217)

5 Seed-Treatment Fungicides for Soybean With weather patterns indicating the continuance of scattered showers for at least the next week or so, soybean producers will want to consider seed treatments, at least in areas of heavy residue from previous crops. Cool or wet soils increase germination time and allow fungi more opportunity to colonize seeds. When selecting seed treatments, you have many options. Our experience has been that seed treatments are the most beneficial when seeds or seedlings are stressed during the first 10 to 14 days after planting. Examples of stress include heavy rains, crusted soils, compaction, deep planting, cool soil, improperly set planters, reduced seed quality, and very dry soils. The choice of a seed treatment also depends upon the target pathogen or pathogens. Typically, these are either water molds (so called because they produce a swimming spore when soil flooding occurs) or other fungi. The water molds include Pythium and Phytophthora species. The other fungi are mainly Rhizoctonia, Fusarium, and Macrophomina species. Water molds produce a soft rotting of the seed, or damping-off, before or after emergence. With damping-off (named for the wet or damp conditions that favor disease development), a dark brownish or blackish soft rot girdles the seedling stems, and plants die. There is no recovery from these infections. Rhizoctonia and Macrophomina produce distinct reddish lesions along one side of the stem and do not commonly girdle stems. Plant growth is reduced in the early season, but death of plants is not common in Illinois. With Fusarium, a generalized dry rotting of the roots may be seen, as well as some reddening of the interior root portions. Fusarium is not a common problem in Illinois and is not seen unless severe stress is placed on the germinating seed and seedling. Seedlings infected with Fusarium die. Suggested seed treatments and diseases controlled are listed in Table 1. This list is not complete and is given for illustrative purposes only. Check with local dealers to determine what products are available in your area. Also, consult the Illinois Agricultural Pest Management Handbook for further information. Walker Kirby (kirbyw@mail.aces.uiuc. edu), Extension Plant Pathology, (217) Table 1. Suggested soybean seed treatments and diseases controlled Product name Diseases controlled Common name of fungicide Comments Phytophthora Metalaxyl Allegiance Pythium Metalaxyl Apron Pythium Mefanoxam Apron XL Rhizoctonia and PCNB PCNB other seedling blights Carboxin Vitavax Captan Thiram Thiram Thiabendazole (TBZ) Controls Phomopsis (pod and stem infections) Maneb + Captan Granox P-F-M Chloroneb + Metalaxyl Nu-Flow AD Controls Pythium/ Phytophthora TBZ + Captan Chloroneb Chloroneb 65W Thiram + Carboxin Bacillus Subtilis Kodiak Use with a chemical seed treatment Other biologicals 105

6 WEEDS Another Consideration When Replanting The last edition of this Bulletin contained information about replanting corn fields and the choice of corn hybrids. The heavy rains last week across much of Illinois have again caused many ponds to appear in planted corn fields. With June rapidly approaching, producers are beginning to consider replanting entire corn fields, or sometimes only the drowned areas, with soybeans. Previously applied corn herbicides can influence the decision to switch from replanting corn to planting soybeans. Almost all corn herbicide labels contain information about replanting options should the initial corn crop be lost. Be aware that many corn herbicide labels restrict planting soybeans during the same season the herbicide was applied. Consult herbicide label(s) to determine if soybeans can be planted in fields previously treated with corn herbicides. We have received a number of questions about planting soybeans in fields previously treated with atrazine. Planting soybeans in corn fields previously treated (this season) with atrazine is not recommended, and most labels of herbicides containing atrazine actually restrict planting soybeans in fields where atrazine has been applied. From a practical standpoint, it s highly likely that some planting of soybeans into atrazine-treated fields will occur. If this happens, the outcome is a gamble! What can you do to increase your odds of success? Anything that can be done to dilute the atrazine in the soil could prove beneficial. The easiest way to achieve this is with tillage, which can help reduce areas of high atrazine concentration in the soil so the soybeans may not be exposed to zones of high concentration all at once. Is this strategy practical given the currently wet conditions? If field conditions are suitable to plant, a tillage pass prior to planting may also be feasible. Certain soybean varieties are more sensitive than others to the herbicide metribuzin (Sencor, Lexone). It may also be beneficial to avoid planting metribuzin-sensitive soybean varieties in fields previously treated with atrazine. Contact your soybean seed representative to determine if the soybean variety you intend to plant is overly sensitive to metribuzin. Along a similar line, producers may want to consider avoiding the application of soybean herbicides containing metribuzin when planting soybeans in fields previously treated with atrazine. Metribuzin belongs to the same chemical family as atrazine and the combined effect of two triazine herbicides may be more than the soybeans can handle. Products containing metribuzin include Sencor, Lexone, Canopy, Turbo, and Axiom. Soybean seed size may also influence tolerance to atrazine. Early research suggested that planting large soybean seeds may be more beneficial than planting smaller seeds in fields treated with atrazine. The larger seed contains more stored food reserves on which the seedling can survive before it must rely upon photosynthesis for its food supply. Finally, producers may want to consider increasing the planting rate slightly to compensate for plants that may be lost due to the atrazine. The later into the growing season the soybean planting occurs, the higher the planting rate adjustment that producers may want to consider making. In short, many factors contribute to the availability of atrazine in the soil for plant uptake. Factors that reduce the availability of atrazine can be beneficial for soybean survival. However, other factors favor enhanced atrazine availability for plant uptake. At this point, it s not possible to predict which factors will predominate. Aaron Hager (hagera@idea.ag.uiuc.edu) and Marshal McGlamery (mmcglame@ uiuc.edu), Department of Crop Sciences, (217) Factors Contributing to the Likelihood of Corn Injury Several factors contribute to the likelihood that a corn crop will exhibit injury symptoms following a herbicide application. In many cases, the cause is relatively clear, but in many other instances, several factors combine to produce the observed injury. If the cause is clear, the explanation can also be clear, but if several factors contribute to corn injury, fingers tend to be pointed in several directions and often little is resolved. Crop genetics can influence the degree of injury response. For example, certain corn hybrids are fairly sensitive to 2,4-D and may exhibit a great deal of injury following a postemergence application. Producers who are concerned about a hybrid being sensitive to a particular herbicide should contact the seed representative for information on the hybrid s response to the herbicide or herbicide family in question. The environment has a large influence on the severity of crop injury symptoms from either soil-applied or postemergence herbicides. High temperatures and relative humidity levels favor enhanced absorption of postemergence herbicides. Adequate soil moisture levels and low relative humidity levels may enhance uptake of soil-applied herbicides. Apart from enhancing herbicide uptake, environment-induced crop stress can often intensify crop injury from herbicides. The excessive soil moisture in many areas of Illinois is a good example of environmental stress. 106

7 Why is a crop under stress more likely to be injured from a selective herbicide? In most cases, herbicide selectivity arises from the crop s ability to metabolize (break down) the herbicide to a nonphytotoxic form before it causes much injury. For example, a grass herbicide used in corn cannot discriminate between giant foxtail and the corn crop it attempts to control the corn just as it does the giant foxtail. When corn is growing under favorable conditions, it will generally metabolize the herbicide well before the corn is injured enough to exhibit injury symptoms. If, however, the corn plant is under stress (which could be caused by a variety of factors), its ability to metabolize the herbicide may be slowed enough for the herbicide to cause sufficient injury for symptoms to be manifested. The herbicide itself can also impact crop response. For example, spray adjuvants applied with a postemergence herbicide can often enhance crop response. Most growth-regulator herbicides should be applied before corn reaches 8 inches in height or exhibits 5 leaves, whichever comes first. Broadcast applications of growth-regulator herbicides to corn larger than 8 inches or having more than 5 leaves greatly increase the probability of corn injury. A large percentage of the corn crop in Illinois is rapidly approaching these growthstage limits, so the window for broadcast applications of these types of herbicides is rapidly closing. Contact postemergence herbicides, often applied with either crop-oil concentrate, a nitrogen fertilizer source (UAN, AMS), or both, can cause leaf speckling or burning. Aaron Hager (hagera@idea.ag.uiuc.edu) and Marshal McGlamery (mmcglame@ uiuc.edu), Department of Crop Sciences, (217)