Pest management and yield losses are estimated to cost U.S. corn producers over $1 billion annually. Recently, the corn rootworm has proven to be especially challenging in some regions of Illinois, Indiana, Michigan and Ohio. Growers in these areas are faced with managing a rootworm “variant” that has developed the ability to circumvent traditionally effective biological control provided by a corn-soybean rotation. Most growers have increased their reliance on soil insecticides.

How widespread is the rootworm variant problem? How have farmers responded to the emergence of this new type of pest? Further, are there rootworm management alternatives available or potentially available that are acceptable to corn growers? If so, what information avenues can producers use to learn more about these alternative techniques? Determining the answer to these questions is important to corn producers for successful rootworm management; to researchers as they develop extension education efforts; and to farm input suppliers as they design more effective product marketing programs.

Brief Background on Rootworm Management and the Variant Emergence

 

Corn rootworms adversely impact growers in two ways. First, farm profitability may be reduced when a producer has to incur the cost of applying insecticides to manage rootworm infestations. Soil insecticides are usually applied at planting time to control rootworm larvae, while aerial sprays can be applied later in the growing season in order to suppress adult rootworm beetle populations in cornfields. Second, rootworm infestations can impact yields. Rootworm larvae feed on the corn root system, which can hinder plant growth and contribute to plant lodging. Larvae that reach the adult stage feed on the silk and pollen, which can interfere with pollination, thereby reducing yields.

Corn growers in Indiana typically control rootworms through the routine application of soil insecticides, the use of a crop rotation, or both. Historically, adult rootworm beetles would feed, mate, and lay eggs only in cornfields during summer months. Rootworm eggs deposited in a cornfield during late summer remained dormant during the winter and hatched into a rotated soybean crop the following spring. In the past, soybean roots were considered an inadequate food supply for rootworms, and the larvae starved. Until the 1990’s, adult rootworm beetles had not been known to lay eggs in soybean fields. So a field rotated back to corn after a soybean crop was not threatened by rootworm larvae infestations.

The primary benefit of rotating annually between corn and soybeans is the disruption of the rootworm life cycle. Hence, the need for soil insecticides is eliminated when corn is grown in alternate years with soybeans. However, when corn is grown continuously in the same field, there is no break in the rootworm life cycle, and soil insecticides are generally applied to control rootworms.

The aggressive adoption of a corn-soybean rotation over the past quarter-century appears to have contributed to the emergence of a western corn rootworm strain capable of laying eggs in soybean fields (Sammons et. al., 1997). The evolution of such a variant beetle has reduced or eliminated the effectiveness of a corn-soybean rotation in much of the Eastern Corn Belt, including a significant portion of Indiana.

The Indiana Corn Rootworm Management Survey

To assess the severity of and management response to the emergence of the rootworm variant, a rootworm management survey was mailed to a random sample of Indiana corn and soybean producers. Questionnaires were mailed to 6,000 farmers during February and March 2001. Respondents were asked to: 1) assess the effectiveness of crop rotation for controlling rootworms on their farm, 2) indicate alternative management tactics they used when they perceived a failure of crop rotation to control rootworms, 3) report any acceptable management practices that they would be willing to use in the future, and 4) specify the information avenues they use when seeking information on new and emerging rootworm management strategies. Information on farm and farmer characteristics, producer attitudes, and management information sources also was collected. A total of 1,135 usable surveys were returned (19% response rate). Every county in the state was represented with the exception of Crawford, Floyd, and Ohio, which are not major corn producing counties and have not reported problems with the corn rootworm variant.

Rootworm Problem Areas in Indiana

Entomological data suggest that the variant problem is not uniform throughout Indiana. Variant pressure is greatest in the northern portion of the state. It is generally accepted that the variant phenome-non originated in east-central Illinois in the late 1980’s, and then spread in a north-eastwardly direction into Indiana (Levine and Oloumi-Sadeghi, 1996). Consequently, counties located on the Illinois/Indiana border north of Interstate 70 are being impacted the most by the variant, while corn producers in southern Indiana appear to be largely unaffected.

Accordingly, Indiana was stratified into four geographical regions based on variant infestation levels. These levels were determined by county-level, multi-year (1998-2001) Western Corn Rootworm Sweep Net Surveys in soybeans reported by Purdue University entomologists. The Severe Rootworm Problem Area experiences the greatest levels of rootworm variant pressure and is comprised of ten counties located near the Indiana/Illinois border (Figure 1). These ten counties have experienced extremely high average rootworm beetle numbers in soybeans. The Emerging Problem Area in central Indiana does not have average beetle counts as high as those in the Severe Rootworm Problem Area, but beetle counts in soybean fields have been persistent in recent years. The Potential Problem Area has beetle counts that are moderate to low and typically includes counties located in the northeastern section of the state. Finally, the Unaffected Area comprises the vast majority of the state located south of Interstate 70 and represents those counties that have virtually no adult rootworm beetles present in soybean fields.

One hundred sixty-two usable surveys were obtained from farmers in the Severe Rootworm Problem Area. Emerging Problem Area data included responses from 317 surveys. The largest number of questionnaires (386) was received from the Potential Problem Area. A total of 286 questionnaires were received from the Unaffected Area.

Effectiveness of Crop Rotation What is the extent of the rootworm variant problem in Indiana? Survey responses reveal that crop rotation is less effective in those areas of the state with greater corn rootworm variant pressure. As variant pressure increases, so does the likelihood of reporting that crop rotation was less effective for controlling rootworms. Over one-third of the producers operating in the Severe Rootworm Problem Area (36%) indicated that crop rotation was less effective in 2000 when compared to the early 1990’s. Nearly a fourth of respondents from the Emerging Problem Area also so indicated. In contrast, only eighteen percent and 12% of respondents from the Potential Problem Area and the Unaffected Area, respectively, indicated that crop rotation was less effective.

The likelihood of having problems with rootworm larvae in first-year corn is highest in areas with greater rootworm variant pressure as reported in the entomologist’s sweep net surveys. Based on the farmer survey responses, the region of the state that is most affected by the rootworm variant is the Severe Rootworm Problem Area. Corn producers operating in this area have a 14% probability of having a major problem with rootworms in first-year corn. The Emerging Rootworm Problem Area has the second highest probability, with 8% of producers indicating that they have a major problem. The Potential Corn Root-worm Problem Area has the third highest probability, while in the Unaffected Area barely 1% of producers had a major problem with rootworms in first-year corn in 2000.

About half (51%) the respondents from the Severe Problem Area reported that rootworms were a minor problem in rotated corn acres. The percentages decrease across areas as the rootworm pressure becomes less severe. Forty-one percent of the farmers in the Emerging Problem Area had a problem with rootworms in first-year corn in 2000. The remaining two areas had 30% of the producers who reported a minor problem with rootworms.

In all, nearly two-thirds of the producers operating in the Severe Rootworm Problem Area reported a problem with rootworms in corn rotated after soybeans. Forty-nine percent of respondents from the Emerging Problem Area indicated that they had a problem in 2000 with rootworms. Thirty-five percent from the Potential Problem Area had a problem, while the lowest percentage of farmers who had a problem was located south of Interstate 70 (31%).

Management Response to the Corn Rootworm Variant

Because crop rotation is no longer an effective biological control measure in many areas of Indiana, affected farmers appear to have increased their reliance on soil insecticides. This may pose a problem because the U.S. Environmental Protection Agency (EPA) may limit the use of some soil insecticides. The Food Quality Protection Act (FQPA) of 1996 requires the EPA to review the tolerances for pesticide residue in food. Organophosphates, the primary compound in several commonly applied soil insecticides, are currently under review.

A comparison of production characteristics among the four areas indicates that farm managers in Indiana now rely heavily on soil insecticides. In 1999, the Severe Problem Area had the highest proportion of rotated corn acres to total corn acres (94%) as well as the highest percentage of treated first-year corn acres (73%). The percentage of treated rotated corn acres in the Severe Problem Area increased to 77% in 2000. In 1999 and 2000, the Emerging and Potential Problem Areas had similar proportions of first-year corn acres to total corn acres, 87% and 88%, respectively. However, the Emerging Problem Area appears to have applied soil insecticides to a greater proportion of rotated corn acres than the Potential Problem Area in both years. In 1999, the difference is about 9-percentage points, with growers from the Emerging Problem Area treating 42% of first-year corn, and farmers from the Potential Problem Area treating 33%. In 2000, the gap narrowed to 5-percentage points.

The group of counties classified as the Unaffected Area had the smallest percent of first-year corn acres (82%) in both years. As expected, this area also had the lowest percentage of first-year corn acres treated with insecticides in 1999 and 2000 (28%).

Survey results further reveal that in certain areas there has been a decrease in the number of farmers who use crop rotation as a management practice to control rootworms. The number of Severe and Emerging Problem Area producers who chose to alternate between corn and soybeans as a management practice to control rootworms has fallen since the early 1990’s (Graph 1). There has been no change in the number of growers who alternate crops for rootworm management in the Potential Problem Area. However, in the Unaffected Area crop rotation has actually increased by 8% since the early 1990’s.

Alternatives to Soil Insecticides Given the recent failure of crop rotation, and the potential future restrictions on chemical controls used to manage corn rootworms, alternative control measures must be identified. The recent approval of transgenic corn resistant to rootworms is expected to offer producers affected by the corn rootworm variant a viable alternative management option.

In early 2003, Monsanto Company announced that it received registration approval from the EPA for its YieldGard rootworm resistant technology. Monsanto initiated the commercialization of the first transgenic corn designed to control the corn rootworm pest during the 2003 season (Monsanto 2003). Bio-engineered corn can produce substantially higher yields in situations where there is heavy rootworm pressure (Caspers-Simmet, 2004). As long as costs to use transgenic corn are comparable to that of applying soil insecticides, the use of rootworm resistant corn may rival that of insecticides. Transgenic corn is also advantageous for farmers because of a decreased exposure to chemicals (Caspers-Simmet, 2004). In addition, the use of rootworm resistant corn reduces the amount of pesticides released into the environment

(Caspers-Simmet, 2004).

In addition to transgenic corn, experimental programs such as Areawide Pest Management (AWPM) have been evaluated as an approach to rootworm control. AWPM pro-grams involve integrating control tactics over many adjacent fields with similar crops and target pests with the goal of suppressing rootworm populations over time. A sixteen square mile site was established in 1996 located in Benton and Newton Counties in Indiana, and Iroquois County in Illinois. The site was a joint USDA-ARS/Land Grant University Research Project. The AWPM approach involves scouting and selective aerial spraying of neighboring fields with a semiochemical bait to suppress adult rootworm beetles and reduce egg laying.

Responses from the Indiana farmer survey indicate that collabo-rating with neighbors appears to be an acceptable option for about a third of the corn producers. About 37% and 31% of Severe and Emerging Problem Area producers expressed a willingness to collaborate with neighbors in an AWPM program (Graph 2 ). Slightly less than a third of Potential Problem and Unaffected Area producers (29%) reported that working with fellow producers is a feasible alternative to applying soil insecticides. A higher percentage was expected in the Severe Problem Area. This may reflect greater producer awareness of the USDA-ARS research to determine the feasibility and effectiveness of AWPM that was conducted in this region. These findings are similar to those obtained from a recent survey (2003) of AWPM program participants (Howell, 2004). Survey responses from Howell’s survey suggest that at most, about 18% of farmers in the Indiana/Illinois area would be willing to participate in an AWPM program in the future.

More interest in transgenics exists among corn farmers in those counties impacted by the variant. Nearly half of the corn growers who operated in the three affected areas indicated that they would consider growing bio-engineered corn as a feasible option to soil insecticides (Graph 2). In Howell’s 2003 survey of the Illinois/Indiana AWPM participants, more than 90% expressed an interest in growing rootworm-resistant corn. Although there is no apparent variant problem in the Unaffected Area of the state, farmers there also are interested in growing bio-engineered corn. Thirty-six percent of growers located south of Interstate 70 expressed a willing-ness to use transgenic corn as an alternative to soil insecticides.

Many corn growers reported that they would be willing to rotate with another crop besides soybeans in an attempt to control rootworms. Fifty-three percent of Severe Problem Area growers favor rotation with a non-soybean crop as a feasible alternative (Graph 2). Results further indicate that as the variant problem becomes more severe across Indiana, interest in rotating with another crop increases. Fifty-four percent, 61%, and 65% of the Emerging, Potential, and Unaffected Area growers, respectively, expressed an interest in alternative crops to rotate with corn. In Southern Indiana, double cropping soybeans with wheat is common, and thus a higher interest in alternative crops is expected. However, in Northern Indiana, no other economically viable crop has been identified which can be rotated with corn on a large-scale basis. Hence, the challenge is to identify a profitable crop that can be rotated with corn and can biologically disrupt the corn rootworm life cycle.

Rootworm Management Information Sources

 

Producers were asked to identify their primary source of information concerning corn rootworm management. Information sources included farm suppliers or chemical dealers, crop consultants or scouting services, news media and trade publications, the Internet, and extension educators and specialists. About three-fourths indicated farm suppliers or chemical dealers were the primary source of information on rootworm management. Other major information sources are producer associations and publications, Purdue extension specialists, and county extension educators (Table 1). There is relatively little difference in information sources across the four rootworm pressure areas.

Farmers located in the northern region of the state more often use Purdue University Extension Specialists than county educators as information sources on rootworm management. In contrast, Unaffected Area respondents are more likely to rely upon management suggestions from county educators. The probability of a Severe Problem Area respondent utilizing University extension resources is 37%, and the likelihood for an Emerging Problem Area producer is 31% (Table 1).

Twenty-nine percent of Potential Problem Area producers get information from Purdue University Extension educators, while only twenty-four percent of Unaffected Area respondents do the same.

Concluding Remarks

 

Corn growers operating in the northern portion of Indiana appear to be the most affected by the Western corn rootworm variant. More specifically, growers located along the Indiana/Illinois border appear to have experienced the greatest problems with adult root-worm variant beetles. To date, an increase in the application of soil insecticides has been the primary response. Indiana growers are interested in transgenic corn as well as to a very limited extent in an areawide approach to manage corn rootworms.

The results of this rootworm management survey have implications for extension specialists and educators as well as for input suppliers. Under EPA guidelines for the approved use of transgenic corn to control rootworms, farmers must plant a 20 percent refuge of non-YieldGard Rootworm corn adjacent to or within the YieldGard rootworm cornfield (Monsanto, 2004). Educational programs and materials from extension educators and seed dealers are critical to ensure that farmers comply with EPA regulatory requirements.

Very few Indiana farmers have expressed an interest in adopting the areawide management approach to rootworm control (Harbor, 2002; Howell, 2004). Input suppliers and/or extension educators will need to provide considerable organizational leadership if the areawide approach is to be adopted by Indiana corn growers.

References

Caspers-Simmet, Jean. “YieldGard May Dominate Rootworm Battle” http://webstar.postbulletin.com/agrinews/ 280574622836464.bsp (2004)

Harbor, Anetra L. M.S., Purdue University, December 2002. Managing the Corn Rootworm Variant: Results of an Indiana Farmer Survey. Major Professor: Marshall A. Martin.

Howell, Aaron W. M.S., Purdue University, May 2004. Areawide Pest Management of Corn Rootworms: An Analysis of Potential Technology Transfer. Major Professor: Marshall A. Martin.

Levine, E. and H. Oloumi-Sadeghi. “Western Corn Rootworm (Coleoptera: Chrysomelidae) Larval Injury to Corn Grown for Seed Production Following Soybeans Grown for Seed Production.” Journal of Economic Entomology. 89 (1996): 1010-1016.

“Monsanto’s Rootworm-Protected Biotech Corn Receives Final Regulatory Clearance.” http://www.monsanto.com/monsanto/layout/media/ 03/02-25-03.asp (2003).

Sammons, Amy E., C. Richard Edwards, Larry W. Bledsoe, Philip J. Boeve, and Jeffrey J. Stuart. “Behavioral and Feeding Assays Reveal a Western Corn Rootworm Variant that is Attracted to Soybean.” Environmental Entomology. 26(6): 1336-1342.