Field Crop Insects is a publication that is a cooperative effort between the Iowa Soybean Association and the College of Agriculture and Life Sciences and Iowa State University Extension and Outreach. The publication contains descriptions and images of many pest insects as well as information on insect life cycle, damage, scouting and management options. Correct recognition and identification of insect pests is an important first step to making a proper management decision regarding any insect species found in soybean and corn.
Resistance is showing up in weeds, insects and disease pathogens in Iowa and has the potential to impact yields, increase the cost of production, and limit farmers’ future
pest management options.
Examples of Resistant Weeds
Common cocklebur, common lambsquarters, common sunflower, giant foxtail, giant ragweed, horseweed, Kochia, Pennsylvania smartweed, waterhemp, and recently, Palmer amaranth.
Examples of Resistant Insects
Western corn rootworm, also soybean aphid populations near, but not yet inside, Iowa.
Examples of Resistant Diseases
Soybean cyst nematode and frogeye leaf spot.
Pilot projects will be selected from these pest options and will be used to inform management option strategies as well as collaborative efforts within communities to resolve resistant pest issues.
What is the Iowa Pest Resistance Management Program?
The Iowa State University Integrated Pest Management (IPM) Program is pleased to present the eighth annual Crop Scouting Competition for Iowa Youth. High school students (those completing grades 9-12) from Iowa are invited to compete and showcase crop scouting abilities in corn and soybean. The competition will be a one day event focusing on outdoor learning.
The Iowa Pest Resistance Management Program (IPRMP) is an Iowa-specific effort to address pests--including weeds, insects and diseases--that can adapt and become resistant to chemical, genetic, and agronomic control practices. The IPRMP outlines approaches for effective, integrated management solutions that will sustainably control pests. By fostering methods to detect resistance, resistance can be delayed or even prevented, limiting the spread of pest resistance.
Upon reviewing the data collected in the study, researchers concluded that corn growers struggle with “balancing the conflicting roles of environmental stewardship and successful businessperson. In reality, short-term profit-making trumps the environmental stewardship role.”
A recently published study in the American Phytopathological Society’s (APS) Pathology Journal, “Perceptions of Midwestern Crop Advisors and Growers on Foliar Fungicide Adoption and Use in Maize” (Sept. 18), explains the various ideologies behind the exponential increase of foliar fungicide on corn crops from the year 2000-2010. The study surveys certified crop advisors (CCAs) and corn growers across Iowa Wisconsin, Ohio and Illinois.
Fungicide use in the United States increased rapidly during the first decade of the new millennium, despite lack of intense pressure of fungal foliar diseases in corn. According to the study, fungicide use in corn production before the year 2000 in the U.S. was a rare practice; however, by 2007, nearly 10 percent of the country’s corn was being treated with fungicide. In 2010, a report by the Agricultural Resource Management Survey (ARMS), conducted by the U.S. Department of Agriculture (USDA) found that 22.5 percent of planted corn in Illinois had been sprayed with a fungicide.
For background information on the economical and agricultural technology landscape of the time periods being investigated, the report states that fungicides were commercially available to corn farmers since the 1980s. The Farm Crisis was occurring at this point in American history, and low commodity prices and low farm income continued until around 2000, when corn prices began to increase. In 2008, corn price topped out at approximately $6 per bushel. Foliar fungicide use remained a rare practice through 1998, at which point it was mainly used in the production of high value hybrid corn seed. According to the report, “at that time, extension and crop advisors predicted no appreciable change in fungicide use over the next five years, because crop rotation and hybrid resistance would continue providing economically acceptable levels of disease control.”
However, this prediction did not come true. At the turn of the millennium, the mass adoption of shorter crop rotations and reduced tillage increased residue-born disease presence. Gray leaf spot (incited by Cercospora zeae-maydis) in corn was propelled — as predicted by researchers, according to the study — from the “sideline to international status.” An uncertainty in being capable of predicting foliar disease outbreaks was also on the rise, as corn prices began to trend upward. Around this time also came the registration of quinone outside inhibitor (QoI) fungicides for corn. The mass marketing of these products to farmers was integral at this time, due to the mechanism of which QoI’s worked. These types of fungicides were able to last longer in direct sunlight, and had activity against a broad spectrum of fungi Perhaps more intriguing than the aforementioned benefits were claims that this class of fungicides could boast plant health benefits, such as green leaf longevity and improved stalk strength.
The study sought to understand CCAs and corn growers’ perceptions on fungicide use during this time period, as well as measure the average perceived benefit and/or consequences of this increase in fungicide use. Increased fungicide use with no increase in disease pressure went against researchers’ observed behavior of corn farmers, whose decision making has been very much dependent on profitability. These findings also go against the practices of integrated pest management (IPM), which urges responsible pesticide use through information-based decision making and multiple other management tools for pest control. The report states that multiple university trials across the U.S. Corn Belt had shown inconsistent profitability under low foliar disease severity.
The survey was sent to 188 CCAs and 188 corn growers in each states, totaling 1,504 individuals. The survey response rate was 47.4 percent. From 2005 through 2009, 73 percent of all CCAs recommended the use of a foliar fungicide on corn. Just 35 percent of corn growers applied foliar fungicide. Of this total, 84 percent were applying a fungicide for the first time. 68 percent of CCAs who recommended spraying had done so for the first time. When it came to ranking threats to corn production, weeds were the number one concern (90 percent by CCAs and corn growers), followed by insects and then diseases. The perception on behalf of corn growers when it came to disease impacts on yield loss varied among the states evaluated, however the study did not examine the disease pressures and other circumstances in each state.
When asked to rank corn production factors from most to least important, approximately 23 percent of respondents ranked the use of foliar fungicide as being “very-extremely important”. The top four factors were maximizing profit, yield, commodity prices and plant population, followed by “other factors” (which includes trait resistance and Roundup Ready seed). Those who held disease as more important to impacting yield loss were more likely to perceive foliar fungicides as more important.
The breakdown of CCA recommendation of foliar fungicide use varied by the use of the end product: 98 percent recommended applications to grain, 35.7 percent to seed crops and 24.3 to silage. Of corn growers who applied fungicide, 94 percent applied fungicide to grain, 24.8 percent applied to seed crops and 27.7 percent applied to silage.
The impacts on crop yield, conditional on having sprayed a foliar fungicide were as follows: 94.4 percent of CCAs and 65.1 percent of corn growers saw a 5 to 9 bushel per acre increase (a 4.08 percent increase per year average); 47.4 percent of CCAs and 25.6 percent of corn growers saw a negative yield response of 1 to 4 bushels per acre.
The study also examined how much CCAs and corn growers were willing to spend on foliar fungicide, as a way to examine the economic decision making during this period. Approximately 20 percent of CCAs believed that their corn growers were willing to spend $25 or more per acre, while corn growers reported a willingness to spend between $16 and $38 per acre. The use of fungicide in previous crop production years had an influence on the amount that corn growers and CCAs were willing to spend. For those who had sprayed fungicides in the last 5 years, the odds of being willing to spend $25 or more per acre on fungicides compared with less were 5.2 times the odds for those who did not spray fungicides being willing to spend $25 or more per acre compared with less.
Upon reviewing the data collected in the study, researchers concluded that corn growers struggle with “balancing the conflicting roles of environmental stewardship and successful businessperson. In reality, short-term profit-making trumps the environmental stewardship role.” Researchers further concluded that the use of fungicide in the absence of disease pressure, while disagreeing with IPM protocols, was seen as a form of insurance to protect yields with commodity prices spiking in the period examined. Since fungicides at this time covered a broad spectrum of fungi while also boosting plant health, the cost might have been worth the investment. The survey indicated that, on average, farmers observed yield responses above the break-even point. Though CCAs are influencers, and were more likely to recommend fungicide if they perceived disease to be important, corn growers were more skeptical, and viewed disease resistance as a seed trait/treatment to be more beneficial. Given the current state of commodity prices, it can be inferred that prices per bushel will influence growers’ attitudes toward using a fungicide.
A research study published in the American Phytopathological Society (APS) Plant Disease journal examined the effects of cropping system diversification on management of soybean sudden death syndrome (SDS), caused by Fusarium virguliforme, that provides some insight into preventing the disease for growers. SDS is a major disease that impacts North and South America. Estimates of annual yield losses in the United States due to SDS have ranged between 0.6 and 1.9 million metric tons during the years from 2006 to 2014, accounting for $200 to 750 million in monetary losses over the same time.
The study included a 2-year, 3-year, and 4-year crop rotation as well as a control. The rotations were as follows:
· 2-year: corn-soybean
· 3-year: corn-soybean-oat (with a red clover cover crop)
· 4-year: corn-soybean-oat-alfalfa
The study also examined differing crop practices, with manure application on the 3 and 4-year rotations and reduced rates of fertilizers, as well as weed management regimens. The study took place between 2010 and 2015.
Analysis shows cropping system diversification can be an effective strategy to manage SDS, especially when incorporating oat, clover and/or alfalfa into an annual crop rotation. Even in years when SDS disease pressure was low, the strategies’ effect was consistent over six years. SDS severity was 17 times higher in two-year cropping system plots, compared with extended and diversified systems, specifically four-year systems. F. virguliforme was isolated at higher frequency from roots from the 2-year system compared with the more diversified systems in 2012 and 2013.
Low levels of SDS in 2011 and 2012 may be explained by the amount of precipitation in those years. Development of SDS is highly dependent on soil moisture, and higher-than-average precipitation, especially during soybean reproductive stages, has been associated with SDS epidemic years. All 4 years of the study when severe SDS symptoms were observed had one or more months during the growing season where rainfall greatly exceeded the monthly average.
When it came to yield results, three and four-year systems showed 40 percent greater yields than in two-year systems. Between 50-87 percent of yield variance was explained by SDS incidence; SDS severity caused 30-70 percent variation in yields.
Extended and diversified cropping systems were correlated with lower F. virguliforme population densities. In both soybean and corn plots sampled in 2012 and 2013, the pathogen’s density in the soil was approximately fivefold greater in the two-year system, as compared to the four-year system. F. virguliforme can survive in soil for years as thick-walled chlamydospores and rotation with corn has been shown to be ineffective for reducing F. virguliforme soil populations or SDS symptoms.
The explanation for higher SDS severity and pathogen presence in the two-year system may be the result of the greater frequency of soybean planting, which creates inoculum build-up in the soil. Corn residue can support survival of the SDS pathogen, which could help explain increased SDS in two year rotations. It is possible alternating corn and soybean is more conducive to SDS than is interruption of that cycle with other crops.
Alfalfa and clover are susceptible to F. virguliforme infection, thus pathogen reduction in the extended rotation may be more closely attributed to the inclusion of oats. In addition, oat used in rotation or as a cover crop has been shown to suppress root rot diseases on certain crops because it produces avenacins, which are active saponin compounds that have antimicrobial activity against several fungal pathogens.
Diversified cropping systems, especially those integrated with livestock production, offer numerous potential environmental and agronomic benefits, including improved soil quality, greater nutrient cycling and retention, greater water-holding capacity, lower rates of soil erosion, and improved control of weeds, diseases, and insect pests. The loss of crop diversity in the Midwestern United States, due especially to reductions in small grain, hay, and pasture production, is linked with significant reductions in livestock production in the region, thus the reduction of manure application to fields. The study asserts that adoption of diversified systems will require growers to learn new practices and adapt to new farming equipment, which could serve to benefit growers financially in the long-run, by limiting yield loss.
Since being brought to market in 1974, glyphosate has become the most widely used herbicide in the U.S. From less than 44 million pounds used annually in agriculture in 1995 in the U.S., to over 253 million pounds in 2015, the herbicide has become a heavily relied on weed management tool. This can be explained through the advent of glyphosate-resistant row crops, which allowed growers to commit to using the herbicide on weeds without causing any damage to plants. The conventional usage of glyphosate has resulted in numerous incidences of weed resistance to the herbicide over the last decade.
In order to understand the evolutionary outcomes of weeds subjected to strong, continued selection as a result of prolonged glyphosate use in row crops, researchers from both Iowa and Ohio examined the variations of resistance in self-pollinating Conyza canadensis (horseweed). Their research was published this past summer in the Nature Scientific Reports journal. Researchers chose to focus on horseweed due to the fact that this weed has become prevalent among low-til to no-till crops. Horseweed has become a problem in fields due to the fact that in tillage cropping systems, seeds could not germinate in the low tillage depths. With the availability of glyphosate-tolerant crop breeds, low-till to no-till cropping systems could be implemented, which increased horseweed germination rates. Glyphosate was a widely adopted herbicide used to kill horseweed, which has resulted in high resistance rates. Glyphosate resistant horseweed was first reported in Ohio and Iowa in 2002 and 2010, respectively.
Researchers collected seeds from a single maternal plant from 74 biotypes from six north-central Ohio counties, and 74 biotypes from 32 southern Iowa counties, from agricultural and non-agricultural land (parks and ditches). Researchers targeted counties with large areas of soybean production and known horseweed populations, due to the the weed’s problematic rise in no-till soybean fields.
This procedure was used because maternal seed families are expected to be full-siblings due the fact that horseweed self-pollinates, virtually making copies of itself, to very high representing evolutionary units on which selection for herbicide resistance can act. These seeds were then germinated and grown, and then sprayed with various rates of glyphosate, and the rates of tolerance to the herbicide were measured.
Results from the study show that nearly all Ohio agricultural biotypes of horseweed (approximately 76 percent) were classified as R4 or extremely resistant (the study ranked tolerant varieties on a scale of 1-4, with R1 being least resistant to glyphosate exposure and R4 being extremely resistant to glyphosate exposure), as were 62 percent of biotypes from the non-agricultural sites. In Iowa, glyphosate resistance levels were more diverse among the seeds collected. R4 biotypes were clustered in south central/southeast parts of the state, where no-till agriculture is more common (R4 biotypes made up only 26 percent of plants). Approximately 45 percent of non-agricultural biotypes were between R1–R4 resistance, with the remainder being susceptible to the herbicide. These results illustrate that horseweed resistance levels to glyphosate can be very high in both states, and that even non-agricultural sites likely serve as a refuge for glyphosate-resistant biotypes.
The study concludes that there are multiple mechanisms through which weeds can be resistant to glyphosate, with those in the R1 biotype having less mechanisms of resistance than those classified as R4, which allows for horseweed to be more tolerant of the herbicide. These mechanisms for horseweed include reduced translocation and vacuolar sequestration, which would be the common injuries that lead to plant death in glyphosate. There have been other studies that show that some horseweed varieties can actually metabolize glyphosate, which is another mechanism of resistance, as well as target site resistance.