INSECTS. Are You Ready for the Insect Season?

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1 INSECTS Are You Ready for the Insect Season? During the 1998 Illinois Pest Management Short Course held at the Holiday Inn in Urbana, March 23 to 26, a participant brought in a bean leaf beetle found in an alfalfa field in Jasper County. During a busy, meeting-filled winter season that was coming to an end during the Short Course, I had given little thought about how soon our six-legged friends would be demanding our attention. The active bean leaf beetle I held in my hand reminded me that the insect season is upon us. FOR IMMEDIATE RELEASE No. 2/ April 3, 1998 Executive editor: Kevin Steffey, Extension Entomologist Web subscriptions available: For subscription information, phone , or acesnews@uiuc.edu In This Issue Stewardship of Bt-Corn, 10 Insecticide Guarantees: Who Benefits? 11 Alfalfa Weevils Should Be Active in Southern Illinois, 11 Pherocon AM Traps for Western Corn Rootworm, 12 Rodent Control in Corn, 12 Weed/Crop Competition, 12 Handling Soil Samples for Nematode Analysis, 16 How Early Should We Plant Corn? 16 As the growing season kicks into gear with corn planting and with alfalfa and wheat growth, several insects will become noticeable relatively early. Although alfalfa weevils usually are the first field-crop insect pests we deal with every year, white grubs, wireworms, other soil-borne insect pests, and corn flea beetles will make their presence known as soon as corn is planted. And the presence of bean leaf beetles in alfalfa reminds us that early planted soybeans will be invaded by these pests as soon as the seedlings emerge. In an article in issue no. 1 of this Bulletin, Mike Gray addressed the potential effect of our mild winter weather on several of these pests. Obviously, we need to be prepared for and start scouting diligently for early season insect pests as soon as we know their season has begun. Kevin Steffey (ksteffey@uiuc.edu), Extension Intense Black Cutworm Moth Capture Reported An intense capture of black cutworm moths was reported by Mike Roegge, crop systems educator with the Adams/Brown Extension unit in Quincy, during the weekend of March 28 and 29. Captures of moths have been common in east-central Illinois. Aaron Grote, with United Prairie located in Ivesdale, Champaign County, reported that cooperators have been regularly capturing moths since March 26. Bottom line the moths are here! We will report in later issues of this Bulletin heat-unit accumulations for the different regions of the state. What is an intense capture? An intense capture occurs when nine or more moths are caught during a 1- to 2-day period. Following an intense capture, it is important to keep track of heat units. After 300 heat units (base 50 o F) have accumulated (beyond an intense capture), black cutworm larvae have the potential to begin cutting plants. Black cutworm larvae must reach the fourth-larval instar to begin their cutting activity. Before this stage of development, early instars feed on leaves, creating small holes on the leaf surface. Cornfields most susceptible to cutting and economic losses are those with plants at the 4-leaf stage of development. Most fields in Illinois never require an insecticide treatment for black cutworms. However, certain fields are at greater risk of injury by this insect pest than others. Black cutworm moths are attracted to weedy areas in fields. 9

2 The availability of actively growing (green) weeds creates very attractive ovipositional (egg-laying) sites for moths. Soybean residue is more attractive than corn residue, and bare soil is unattractive for oviposition. Corn grown in rotation with wheat also is at greater risk from cutworms, especially if weeds were present during the egglaying activities of female moths. Certain weeds are more attractive than others when it comes to egg laying by black cutworm moths. Fields that have winter annual and perennial weeds prior to final tillage and planting are most at risk to an infestation of black cutworms. Those fields with common chickweed, mouse-eared chickweed, bitter cress, shepherd s purse, yellow rocket, and pepper grass are likely candidates for the development of a black cutworm infestation. A rescue treatment may be warranted in commercial corn when at least 3% of the plants are cut and larvae are still present. In seed-production fields, rescue treatments may be needed if 3 to 5% of the plants have leaf feeding and two or more cutworms are found per 100 plants. This threshold is based upon the premise that for each 1% of plants with leaf feeding, 3 to 5% of the plants may be subject to cutting at a later date. In general, rescue treatments have performed better, compared with insecticides applied before or at planting. Please take a look at our black cutworm trial (Table 1) established in Many of the compounds performed very well in our experiment despite the fact that we infested the barriers with plenty of third and fourth instars, stages of the cutworm that are just beginning to pose a cutting threat. Also, please note that the transgenic hybrids did not perform in this trial. In fact, cutting was typically more severe than in the control (Pioneer 3489). All five Bt hybrids contained the Cry9C protein. Finally, because this trial was experimental in nature, please consult product labels for information on rates and methods of application for insecticides you may choose to use. Mike Gray (m-gray4@uiuc.edu) and Kevin Steffey (ksteffey@uiuc.edu), Extension Stewardship of Bt-Corn As corn planters begin to roll, we need to re-emphasize the importance of stewardship of Bt-corn. Many more acres of Bt-corn will be planted this year for management of European Table 1. Cumulative mean percentage of corn plants cut per barrier by black cutworm larvae 7, 15, 20 days after manual infestation (DAI), Urbana, Illinois, Mean Percentage of Cut Plants Insecticide Rate2 Application3 7 DAI 15 DAI 20DAI Asana XL 0.66EC 0.04 pre-emerge Pounce 3.2EC 0.1 rescue Warrior 1EC 0.02 band/planting Warrior 1EC 0.02 pre-emerge Pounce 3.2EC 0.1 pre-emerge Warrior 1EC 0.02 rescue Force 3G 0.13 band/planting AsanaXL 0.66EC 0.03 rescue Regent 4SC 0.13 band/planting Pounce 3.2EC pre-plant Lorsban 4E 1.3 rescue Lorsban 30G 1.3 band/planting Lorsban 15G 1.3 band/planting Regent 4SC 0.13 microtubes at planting Transgenic Hybrids EXP83 Bt - at planting VCE Bt - at planting VCB non-bt - at planting EXP85 Bt - at planting VCB Bt - at planting VCE non-bt - at planting EXP83 non-bt - at planting Control (Pioneer 3489) Barriers were infested with 15 black cutworm larvae (third and fourth instars) per barrier on May 29. 2All rates are specified at lb (AI)/acre, based upon a 30-inch row spacing. 3Pre-plant treatments were applied on April 28, planting-time treatments on May 15, pre-emerge treatments on May 21, and rescue treatments on June 1. 10

3 corn borers. The efficacy story about Bt-corn has been told countless times at meetings, on radio programs and spot ads, and in print media articles and ads for months. Although we Extension entomologists have discussed the potential virtues of Btcorn, we also remind growers about their obligation to practice good stewardship of this exciting new technology for insect management. Specifically, we strongly encourage corn growers to plant non-bt-corn refuges in or adjacent to fields of Bt-corn. The objective of planting non-bt-corn refuges is to slow down the development of resistance to the Bt endotoxin within our European corn borer population. Non-Bt-corn refuges encourage the survival of corn borers that have not been exposed to Bt, and these susceptible moths should find and mate with any (hopefully rare) individuals that are homozygous for resistance to Bt. The resulting heterozygous progeny should still be susceptible to the Bt endotoxin. You ve heard many of us say that the arrangement and amount of non-btcorn refuges are based upon theoretical models; and some people have asked, Why should I plant non-btcorn just because these theoretical models indicate it s a good idea? We understand the question, but we cannot support an attitude that non-btcorn refuges are not necessary. They are. If European corn borers become resistant to the Bt endotoxin, we will have repeated the same mistakes we made by overusing insecticides. Entomologists at three different universities have developed laboratory colonies of European corn borers that became moderately resistant to Bt after seven or more generations, proving that the gene(s) for resistance is (are) present in the native population. If we place too much selection pressure on European corn borers by planting too much Bt-corn, resistance to Bt will develop. And we will lose the benefits of the technology and hurt our chances for ever being able to use other transgenic crops. Although the exact amount and placement of the optimal non-bt-corn refuge is not known, and maybe never will be, university entomologists throughout the north-central region have agreed upon the following suggestions. In areas where mostly corn and soybeans are produced, growers should plant at least 20 to 30% non- Bt-corn. If a grower wants to protect the non-bt-corn refuge from an economic infestation of corn borers, the refuge size should be increased to 40%. The non-bt-corn refuge should be planted at a similar time and in close proximity to Bt-corn. These recommendations and other information about Bt-corn can be found in North Central Regional Extension Publication NCR 602, Bt Corn & European Corn Borer: Long-Term Success Through Resistance Management. This publication may be obtained from Information Technology and Communication Services at the University of Illinois. Inquire about this publication by calling (217) or (800) Although the optimal arrangement of Bt-corn and non-bt-corn is not known, ideally the non-bt-corn refuge should be planted as a block within the same field as Bt-corn. This arrangement mostly likely would foster better interaction among corn borer moths that emerge from both Bt- and non-btcorn. However, regardless of how a grower decides to arrange a non-btcorn refuge (within reason), we will be content as long as a refuge is planted. Each grower who plants Bt-corn must accept the responsibility, and, in our opinion, the obligation to protect the technology. Development of corn borers resistant to Bt will be detrimental for everyone involved with agriculture. Kevin Steffey (ksteffey@uiuc.edu) and Mike Gray (m-gray4@uiuc.edu), Extension Insecticide Guarantees: Who Benefits the Most by Marketing Ploys? It s that time of the year for the proliferation of interesting guarantees offered by some companies for certain insecticides in their line of products. As consumers of these products, producers should be wary of so-called guarantees of product performance. In some instances, guarantees are made for insects that in many cases may never infest a majority of fields. If economic infestations do develop, some companies simply offer free rescue treatments of another compound in their arsenal. Pure and simple it s a numbers game, an effective marketing strategy for certain insecticides. Who benefits by this approach most often? I believe the answer to that question is straightforward. Who stands to lose the most? A much broader segment of the agricultural and nonagricultural sectors stands to lose the most. Unless the responsible use of insecticides is recognized as the norm, popular support for agricultural pest management will erode. Mike Gray (m-gray4@uiuc.edu), Extension Alfalfa Weevils Should Be Active in Southern Illinois Although we have not geared up to produce heat-unit maps for predicting development and activity of alfalfa weevils, I am certain that alfalfa weevil larvae are active in some alfalfa fields in southern Illinois. As you probably are aware, we can accumulate heat units above a base temperature of development (48 o F) from January 1 and determine when certain events occur in the life of an alfalfa weevil and when certain activities should commence. Hatching of overwintering eggs usually occurs when 200 heat units accumulate, and we suggest that scouting should begin 11

4 when 300 heat units accumulate. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 heat units; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 heat units. With the mild winter weather we experienced, I am certain that we have exceeded 200, maybe even 300, heat units in some of the counties in southern Illinois. As you begin to monitor alfalfa fields for alfalfa weevil activity, start looking in areas of the field that might warm up first, for example, southfacing slopes or areas of the field with lighter soils. After 300 heat units have accumulated, you should be able to find small, first-instar weevils in the folded terminal leaves. As these small, yellowish larvae with black heads feed on these leaves, you will observe some pinholes. This injury is not economic because the larvae are too small to cause significant defoliation. However, by the time alfalfa weevils grow into third instars, they begin to cause more economic damage by skeletonizing the leaves. At this stage of development, alfalfa weevil larvae are bright green, with a distinct white strip along the center of the back. If you are really interested in learning more about the presence of alfalfa weevils in a field, you can scratch around the crowns of the plants to look for the adults that overwintered. Alfalfa weevil adults are oval, light brown with a darker brown stripe along the middle of the back, and about 3/16-inch long. Like most other weevils, alfalfa weevil adults have an extended snout, at the end of which are their mouthparts. You can also look for alfalfa weevil eggs in stems. The female weevils deposit clusters of about 9 or 10 eggs. The eggs are only 1/32-inch long, oval, and initially light yellow. Just before hatching, the eggs darken and you can see the developing larvae through the shell. We will provide updates about heatunit accumulations beginning with next week s issue of the Bulletin. In the meantime, let us know if you find anything while you re scouting alfalfa fields. Kevin Steffey (ksteffey@uiuc.edu), Extension Pherocon AM Traps for Western Corn Rootworms: How Can I Order Them? We ve had a number of calls from individuals who want to order Pherocon AM traps to use in soybean fields this summer for monitoring western corn rootworm adults. In later editions of this Bulletin, we will offer much more detail regarding how to deploy these traps and will describe what economic thresholds should be used. For now, if you re interested, we offer the following two addresses. Also, there are likely many other sources available for these traps. We encourage you to do some comparison shopping. IPM Great Lakes, Church Road, NE, Vestaburg, Michigan ( glipm@nethawk.com); Gemplers, 100 Countryside Drive, PO Box 270, Belleville, Wisconsin ( Mike Gray (m-gray4@uiuc.edu), Extension Rodent Control in Corn No-till corn growers who have suffered significant damage caused by rodents, particularly voles, have a new tool available for battling these critters. Within the past few months, Zinc Phosphide Pellets, manufactured by Haco, Inc., and marketed by Loveland Industries, Inc., was federally labeled for rodent control in no-till corn. The label indicates that one application of the pellets at 4 to 6 pounds per acre should be made at planting time. This product can be applied only with equipment specifically designed or modified to place the pellets in the seed furrow. Surface broadcast treatments are prohibited. Trials conducted by Ron Hines, senior research specialist at the Dixon Springs Ag Center, have revealed that zinc phosphide pellets applied in the appropriate manner provide good control of rodents in no-till corn. Growers who will be planting corn into CRP acres (Conservtion Reserve Program) this year may wish to assess the potential for rodent damage by scouting the field before planting. An economic threshold of five vole colonies present in a field 30 days before planting has been proposed. We have learned that the numbers of vole colonies in some CRP acres south of I- 74 are considerably larger than 5 per acre. If the mild weather fostered better survival of rodents over the winter months, we should not be surprised to experience some problems with rodents in no-till corn this spring. Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217) WEEDS Weed/Crop Competition: Factors to Consider Weed management strategies attempt to limit the deleterious effects of weeds growing with crop plants. These effects can be quite variable, but the most common is competition for available growth factors (light, water, etc.). Those quantities of growth factors used by weeds are thus unavailable to the crop. If weeds are able to utilize a sufficient amount of some growth factor to the detriment of the crop, the result can be, and often is, an adverse impact on crop yield. The most common method currently employed to manage weeds is the use of herbicides. Producers can choose from many available options, each with its own distinct set of advantages and disadvantages. In addition, there are several different methods for applying herbicides. Regardless of the herbicide or method of application, the 12

5 goal of using a herbicide is to prevent the weeds from contributing to a crop yield loss, by reducing the amount of competition exerted by the weeds. Recently, the concept of weed/crop competition has become a marketing strategy for several herbicide manufacturers. The point of debate tends to focus on when competition (from weeds) should be removed so crop yields (of corn or soybean primarily) are not affected. Some maintain that soil-applied, residual herbicides should be used to eliminate any early season weed competition, while others propose that the crop can compete with weeds for some time without affecting yields. Which view is correct? What type of research activities are needed to determine how and when weed competition reduces crop yield? How should results of studies like these be interpreted? At the 1998 Illinois Agricultural pesticides Conference, Loyd Wax with the USDA/ARS at the University of Illinois presented a paper dealing with many factors associated with weed/ crop competition. The remainder of this article contains Dr. Wax s discussion. Those involved in managing weeds have long recognized how weeds can harm crop growth and productivity by competing for light, moisture, nutrients, and space, as well as by hampering harvesting operations, reducing quality of the harvested crop, and producing weed propagules, thus leading to future problems. Numerous studies over the years have compared weed species and density in various crops and assessed the importance of (1) duration of competition and (2) time of weed removal. From those studies, some general guidelines have evolved as to relative competitiveness of weeds with various crops, the weed-free time needed following crop emergence, and the appropriate time of weed removal with postemergence treatments to preclude loss of crop quantity and quality. However, as tillage, planting, and weed management practices have changed over the years, the former guidelines regarding crop/weed competition perhaps should be revisited, in some instances modified, as new findings are reported. This paper will review crop/weed competition to include earlier as well as more recent results and to raise some concerns and questions about interpreting the data in several aspects of crop/weed competition. Crop and cultural practices. Crops obviously vary greatly in their ability to compete with weeds, ranging from crops that provide essentially no competition to ones that are very aggressive competitors. However, this discussion shall be limited to the major field crops of Illinois, corn and soybeans, for which there is the greatest concern with weed management and perhaps the most questions raised in recent years. Early studies, with a variety of weed species, tended to show nearly equal competitive ability of corn and soybeans, with some differences. Very tall-growing weeds, if left for the entire season, were sometimes less competitive in corn than in soybeans, mainly because the weeds could overtop soybeans and cause greater losses due to shading. For shorter-growing weeds that rarely grew taller than soybeans, yield loss was less in soybeans than in corn, again due to the excellent shading provided by a healthy stand of soybeans. Crop varieties and hybrids can vary substantially in response to weed competition, with those that canopy earlier and provide more shading being the most competitive. For the most part, this aspect has not been exploited to any great degree. A number of studies have shown that increasing crop populations within the row, up to a point, can increase the competitive ability of the crop, with no deleterious effect on crop growth or yield. Crop row spacing and time of planting can greatly influence the crop s competitive ability. Especially for soybeans, reduced row spacings have increased the ability of soybeans to provide additional competition with weeds, so that (under current production practices) soybeans may be more competitive than corn. Conversely, when planted in wide rows, soybeans and corn are probably more nearly equal in their competitiveness. Time of planting for both corn and soybeans presently occurs earlier in the season than it did than several decades ago, but this practice does not always enhance the crops competitive ability. Very early planting, combined with reduced or no tillage, allows for greater weed competition, as well as for different species of weeds to be present, than has historically been common. Clearly, weeds that are established at the time of crop emergence begin to compete with the crop earlier than weeds that emerge only after the crop emerges. With modern production practices and herbicides, do corn and soybeans differ in their ability to compete with weeds? Conclusive evidence is lacking, but many speculate that any difference is minimal in most instances. However, soybeans, especially when vigorous varieties are grown at high populations in narrow (8- to 15-inch) rows, usually have an edge over corn in competitive ability, assuming that complete weed control is achieved with herbicides prior to crop canopy closure and that neither crop will be cultivated. Weed variables. Weeds have been able to reproduce, survive, and compete for centuries, at least partially due to their diversity. Species of weeds, and sometimes biotypes within species, can vary greatly in various growth habits and ultimately in their ability to compete with crops. Germination patterns differ markedly and sometimes erratically, causing differences in potential for competition from year to year. Emergence and growth also vary from slow and even, to rapid and almost unpredictable. Different species and biotypes appear to respond differentially to various 13

6 environmental conditions: Some years are nightshade or smartweed or nutsedge years; whereas, in most areas of the state, every year is a foxtail or velvetleaf year. Most recent years could be described as lambsquarters and pigweed years, and who can dispute the increased prevalence of waterhemp years recently across much of the state? Obviously, as demonstrated in many competition studies, weeds produce markedly differing amounts of growth per individual plant and reach widely varying heights. These studies have allowed the development of relative competitive indices that can be somewhat helpful in determining the severity of problems presented by stands of various weed species. For example, it obviously requires more foxtail plants than cocklebur or giant ragweed plants to produce the same degree of competition with corn or soybeans. The density or population of weeds required to cause a consistent yield reduction in crops has been difficult to establish. Many research studies have addressed this issue and have helped to establish some of the thresholds and guidelines currently available. In general, corn and soybeans can withstand low populations of weeds throughout the season without suffering yield or harvest losses; losses tend to increase linearly with increases in weed population, up to some population level above which further increases in yield reduction tend to subside. Establishing consistent thresholds or numbers of weeds that cause a specific yield reduction has been difficult to derive across many locations, years, and weather patterns. A soon-to-bepublished report summarizing competition studies conducted across several states and over many years concludes that improved techniques may be needed to establish and refine thresholds because variation across locations and years almost always occurs and can be considerable. This finding really should not be surprising and most likely can be attributed to differences in environmental conditions, with special emphasis on weather patterns. General threshold guidelines would be possible, as long as a range of likely responses is given, and could cover a majority of situations. Competition research. Numerous studies over the years have attempted to define the critical duration of weed competition in corn and soybeans and determine the optimal time to implement weed management operations. One type of study is designed to determine the early season weed-free interval that is needed before the crop can shade out later-emerging weeds and subsequently progress independently for the remainder of the season, with no loss of crop quantity or quality and without increased harvesting losses. Studies like these are especially useful in determining how long a soil-applied herbicide needs to be effective after planting the crop. In general, for many weed species encountered in corn- and soybean-production systems of the northern United States, this interval ranges from 3 to 6 weeks, with 4 to 5 weeks being the most frequent requirement. It is important to note in these studies that some initiated the interval at planting, while others began at crop emergence (a potentially significant difference, depending on the season and weather). Most of these studies were conducted with healthy crop stands in 30- to 40-inch rows, with the objective to obtain 4 to 5 weeks without weed competition, after which the weeds were kept under control by crop shading and one or more layby cultivations. In sharp contrast, cultivators today are not used nearly as much as they once were, and weed management after crop emergence is administered in the form of postemergence herbicides if soilapplied treatments do not last long enough. It is also important to note that these guidelines were developed with good crop stands and, for the most part, with the most common row-crop weeds, the majority of which tend to emerge fairly uniformly, not in multiple flushes well into the season. As mentioned here and again later, a review of the available data will indicate that, in most studies, variation from year to year has been considerable, probably due to differing environmental conditions, making it very difficult (if not impossible) to set a specific weed-free interval that will be acceptable with all species and across all locations and years. Another factor to consider is that many of these studies were conducted either by seeding unimbibed weed seeds at various times after crop planting or by removing natural weed populations as needed for a specified period of time. These two methods effect different results, and how these results compare with a herbicide treatment that lasts the same amount of time is not specified. Does a lower dosage of a herbicide still cause some growth inhibition of later-emerging weeds? Such unanswered questions point toward the need for extreme caution in pronouncing exact time periods applicable over a wide variety of conditions. Another type of study is designed to determine how long weeds can remain in the crop and eventually be removed with no resultant deleterious effects on quantity and quality of crop yield. In previous years, this was important so producers would know how early one needed to cultivate between the rows, as many older herbicides were applied only in a band over the row. With the growing prevalence of broadcast, selective postemergence herbicides, studies like these became relatively more important for providing guidelines in timing postemergence herbicide applications. Until fairly recently, these types of competition studies were often conducted by growing various populations of weeds from crop and weed emergence until the weeds were removed, either mechanically or by hand. The weeds were removed at some point after either crop planting or emergence, or until certain weed heights or stages. As a general guideline, many of these stud- 14

7 ies tended to show that a moderate population of weeds could remain growing with the crop for up 3 to 6 weeks after planting and, once removed, cause little or no crop yield loss. Studies like these also appear to have considerable variation in results, so it is again difficult to set specific intervals that will be valid over widely diverse conditions. In assessing these studies, one needs to consider what species were involved and what weed population was present. In general, higher weed populations should be removed earlier, while less-intense populations can be left to compete longer. From an applied standpoint, a problem with many competition studies is that only one weed species is considered, whereas producers fields often contain a number of species at varying populations. Personal bias would be to place the most emphasis on total weed biomass present at crop flowering and fruiting as the best indicator of loss likely to result from competition. However, this stage is generally well past that when control is possible or even feasible, and herbicides undoubtedly should be applied before this stage in most instances. To reiterate, these types of studies are influenced greatly by the environment, which makes establishing concrete intervals arduous. The results of these studies should also be examined closely with respect to how the competition (weeds) was removed. In some cases, weeds were removed by hand, allowing any weeds that emerged afterwards to grow, while in other cases weeds were handweeded throughout the season, to simulate season-long control. Modern day studies tend to focus on controlling either a single species or a mixture growing at whatever population is present in the field, by applying selective postemergence herbicides at various weed sizes or growth stages. In interpreting the results of these studies, it is important to note whether or not the herbicide(s) used possessed any soil bioactivity that may have provided some control of weeds emerging following application. Additionally, the population, the mixture of weeds, or both factors are important to note. And it is of special importance to note whether or not the weeds were actually controlled completely. This point is important because any yield reduction noted and attributed to preapplication-competition stress might actually have been partially due to postapplication stress from uncontrolled weeds or weeds emerging after application. Invariably, these studies lead to a range of intervals for weed removal that work effectively under various conditions. Personal bias would tend to recommend removal of competition at the average or even slightly earlier time because potentially adverse conditions might cause delays in herbicide application, resulting in weeds that would be very difficult to control. This approach may become especially important when dealing with weed species for which later emergence might be a problem with herbicides that do not have soil-residual activity. Under this scenario, the conservative approach might involve adding to the mixture a herbicide that has soilresidual activity. As will be noted in the next section, environmental conditions can cause significant variation in the results of studies like these. Environmental conditions. Factors that probably contribute more to the variation in results than any others are soil and air temperature, along with soil moisture content and rainfall before, during, and after initiation of competition studies. Even the best planned and conducted studies can vary considerably from location to location and year to year, often because environmental conditions vary. These conditions affect weed emergence and growth, herbicide effectiveness, the competitive interaction between crop and weed, and the ability of the crop to recover from early weed competition once the weeds have been removed. Primarily because environmental conditions vary, one should be very cautious in setting precise guidelines for crop/weed competition, including thresholds for density, duration of weed-free intervals, and times of competition removal. It would seem prudent to establish ranges of densities, times, etc. and/or to operate on the conservative side in these matters. Other considerations. The total effect of weeds on crop plants is more correctly termed interference, which is the total of competition plus allelopathy. Allelopathy can and has been demonstrated; but, with most of our soils and cropping situations in the Corn Belt, it is thought to be relatively minor and is very difficult to demonstrate. Thus I have elected to discuss mainly competition, which I think is significantly more important, as it deals with plants competing for light, moisture, nutrients, and perhaps space. I suspect, however, that in dense infestations of weeds (such as grasses in corn) besides competition allelopathy could be a contributing factor to yield loss. Competition for light may be one of the most important factors in reducing yields, especially with weeds that grow taller than the crop. Moisture stress, especially during and after removal of a very dense population of weeds, may be extremely important in crop recovery. One would not consider nutrient stress as greatly important in the rich, fertile soils across much of the Corn Belt; however, in soils with low fertility or coarse texture, it may be important. Some research has demonstrated that weeds can exhibit luxury consumption of some nutrients to the detriment of the crop. Regarding time-of-removal studies with herbicides, several questions remain for which there does not seem to be much information. What are the relative effects of weeds on crop growth and yield when the weeds are removed at some stage by (a) hand- or mechanical weeding, (b) a fast-acting herbicide, (c) a slow-acting herbicide, (d) a herbicide causing little or no injury to the crop, (e) a herbicide causing substantial, temporary injury to the 15

8 With wet weather continuing into early April, it appears that the question of early planting will not be as comcrop, or (f) a herbicide with residual activity, relative to one with no soil residual? It appears likely that when these factors are interacting with environmental conditions, variable results are almost certain to occur. Those involved with developing weed management systems need to remember that the whole subject of crop/ weed competition, while seemingly not simple, is even more complex in the marketplace. That is, weed management decisions are made not only based on a true crop/weed competition situation but also on other factors. Yield and quality loss are not the only factors being considered by those making decisions. Some producers are concerned about harvest difficulties, while others may be concerned about leaving a few weeds that will contribute to the soil weed seedbank. Aesthetic thresholds are real as growers worry about personal pride and the opinions of family and neighbors. Landlord thresholds may be the lowest of all, sometimes necessitating weed control at much higher levels than required based simply on yield losses. Product guarantees and respray programs also have contributed to extraordinarily high levels of weed management expectations on the part of the producers. Summary. Numerous studies have investigated crop/weed competition from a variety of aspects. The results of these studies can be helpful in making decisions about weed management, as guidelines can be prepared indicating, in general, the relative competitive ability of various weeds at various densities in our major crops. These studies also provide guidelines for duration of weed-free conditions needed after crop emergence and for the time when weeds should be removed with postemergence herbicides. Data are somewhat limited, however, on mixtures of several weed species in competition studies. Other types of concerns, such as perception of the producer, neighbors, and landlord, may be as important as yield-loss indications from crop/weed competition studies in determining the types of weed management systems implemented. Aaron Hager (hagera@ideal.ag. uiuc.edu), Loyd Wax (l-wax@uiuc.edu), and Marshal McGlamery (mmcglame@ uiuc.edu), Department of Crop Sciences, (217) PLANT DISEASES Handling Soil Samples for Nematode Analysis Soil sampling for plant-parasitic nematodes can provide timely information for growers in selecting management strategies and tactics. The quality and condition of the samples is extremely important and determines the reliability of the results. Some soil samples continue to arrive at the University of Illinois Plant Clinic or nematology laboratories in poor condition. Some samples are packaged in paper bags and are completely dried out. Plant-parasitic nematodes are living organisms and require moisture. The lack of moisture can kill nematodes and hinder the extraction and identification of the species involved. Soil samples for nematode analysis should be placed in a plastic bag and labeled with an indelible marker. Do not allow soil samples to become overheated because it may increase nematode deterioration and cause inaccurate analyses. Overheating may occur when soil samples are stored in car trunks or inside enclosed vehicles during spring and summer. Nematode diagnostic laboratories usually have special forms to submit with the samples. University of Illinois Extension gives out a form entitled Nematode Soil Sample Form. A copy is included in the University of Illinois Field Crop Scouting Manual. If this form is not readily available, include equivalent information in an attached letter: your name, address, county, and phone number; the name or number of the field and sampling date; any symptoms observed during previous plantings and their patterns in the field (scattered, clustered in spots, uniform, etc.); the approximate size of the area sampled; the cropping history for at least the last 3 years; the crop or crops to be grown in the current season; and pesticides used during the previous growing season. Please indicate on sample forms or letters whether you want a soybean cyst nematode (SCN) analysis, a complete nematode analysis (types other than SCN), or both. Check with your local Extension office to see if it can process soil samples for SCN or if a nearby private laboratory is capable of performing the same service. For more information on collecting, handling, and shipping soil samples for nematode analysis, refer to Report on Plant Diseases no and 1107 (published by the Department of Crop Sciences). Dale I. Edwards (diedward@uiuc.edu), Extension Plant Pathology, (217) CROP DEVELOPMENT How Early Should We Plant Corn? 16

9 mon as it was in Here are some points to consider about planting corn in April: Based on research in Illinois, yields of corn are generally highest when the crop is planted up to a week earlier or later than the last week of April, then yield losses begin to accelerate. Our work showed yields an average of 3 to 5 bushels per acre lower for corn planted in mid-april. Even with slightly lower yields with early planting, most farmers will proceed to plant in the first 2 weeks of April to finish on time. Delays in late April due to weather push planting later than the optimum and also delay soybean planting. Risk management, in other words, favors early planting. Even though we worry about placing corn seed in soil when temperatures are in the 40s, experience particularly that in 1997, when it stayed quite dry after planting has shown that the crop usually emerges well under such conditions. We thus no longer use soil temperature as a guide to planting after April 1, but we strongly recommend that soils be dry enough to allow placement of seeds into good soil conditions. Planting into mud is never a good idea, but it can be a disaster if done when soils are cold as well. In addition, excessive soil compaction from working and planting wet soils may decrease yields some years. Very early application of some insecticides and herbicides may mean loss of activity and less protection of the crop. Although we never want to replant corn, very early planting may allow replanting, if needed, to be done early enough so that little yield is lost. Keep in mind that very good yields are still possible even if planting is delayed until mid-may. But once soil conditions are good for planting in April, there probably should be little hesitation in getting started. Emerson Nafziger (ednaf@uiuc.edu), Crop Sciences, (217)