Western Corn Rootworm Injury in First-Year Corn: A Diminished Threat?

The economic impact of the variant western corn rootworm that evolved in the eastern Corn Belt continues to reverberate nearly twenty years later. The overall impact includes yield losses in first-year corn and the additional input costs of Bt seed and/or soil insecticides to rotated corn. In 1995, severe root injury and punishing yield losses occurred in first-year cornfields across east-central Illinois and northern Indiana. These were fields in which the annual rotation of corn and soybeans had rigidly taken place for decades. Producers had unwittingly selected for a rotation-resistant western corn rootworm in which females were laying at least a portion of their eggs in the soil of soybean fields. This behavior enhanced the survival of their progeny the following spring in first-year cornfields. In 2012, some University of Illinois researchers determined that the variant western corn rootworm could tolerate soybean foliage to a greater extent than the non-rotation resistant population. Based on their investigations, they discovered that the variant western corn rootworm had 3 to 4 times more of a key digestive enzyme (cathepsin L-like protease) enabling them to feed on soybean foliage for longer periods of time. Consequently, these western corn rootworms spent more time in soybean fields laying eggs. The reference for this very nice contribution to the entomological literature is provided.

Curzi, M.J., J.A. Zavala, J.L. Spencer, and M.J. Seufferheld. 2012. Abnormally high digestive enzyme activity and gene expression explain the contemporary evolution of a Diabrotica biotype able to feed on soybeans. Ecology and Evolution: DOI 10.1002/ece3.331.

As mentioned previously, the economic impact of this unique adaptation continues to be sobering in the eastern Corn Belt. Prior to the evolution of the variant western corn rootworm, crop rotation was considered to be a sound pest management strategy to limit root injury and yield loss caused by this insect pest. Since the mid-1990s, producers who felt they were at risk to first-year corn rootworm damage began to use planting-time soil insecticides on rotated corn ground. Western corn rootworms began to move across the Corn Belt in the 1950s and 1960s and continuous corn was the primary sink for soil insecticides delivered during planting. After 1995, concern over potential injury to rotated corn was most common in the northern two-thirds of Illinois and northern Indiana. In 2003, Bt hybrids that offered corn rootworm protection were commercialized. Producers quickly transitioned away from soil insecticides as their primary corn rootworm control tactic. However, many of these producers who were using soil insecticides on rotated and non-rotated corn ground continued to use Bt corn rootworm hybrids across all their corn acres regardless of crop history. Recently, it has become increasingly common to use both a planting-time soil insecticide and a Bt corn rootworm hybrid in some areas of the eastern Corn Belt even though crop rotation has been practiced.

A journal article was published recently (February 2013) by Mike Dunbar and Aaron Gassmann, entomologists at Iowa State University, that clearly pointed out the viability of crop rotation as a corn rootworm management strategy for northern and western corn rootworms in eastern Iowa. They indicated rotation resistant western corn rootworms were present in eastern Iowa at very low levels. By comparing their data with previous surveys, they reported no measurable expansion of this behavior in the last 5 years. They concluded their paper by recommending that growers scout, familiarize themselves with economic thresholds, and consider crop rotation as a viable corn rootworm management strategy in eastern Iowa. Based upon their findings, it would seem reasonable to suggest that producers in eastern Iowa could potentially save on their input costs for corn rootworms in rotated corn. The citation for this research is provided.

Dunbar, M.W., and A.J. Gassmann. 2013. Abundance and distribution of western and northern corn rootworm (Diabrotica spp.) and prevalence of rotation resistance in eastern Iowa. Journal of Economic Entomology 106(1): 168-180. DOI: http://dx.doi.org/10.1603/EC11291

What do the densities of western corn rootworm adults look like in Illinois soybean fields? In 2011 (late July and early August), we conducted a statewide (47 counties) survey of both corn and soybean fields for various insect pests. In each county, five soybean and five cornfields were selected randomly and sampled. In each soybean field, 100 sweeps were taken with a sweep net. In corn, 20 consecutive plants were examined for western corn rootworm adults. In both corn and soybean fields, western corn rootworm densities were exceedingly low. With one exception (Ford County, 25 western corn rootworm adults per 100 sweeps), on average, fewer than 10 western corn rootworm adults were recovered after 100 sweeps per field in all 47 counties. Historically, 10 adults captured per 100 sweeps was considered the benchmark that indicated the presence of variant western corn rootworm adults in a soybean field. For the vast majority of Illinois soybean fields sampled in 2011, this threshold was not even approached. In fact, it was exceeded only once (Ford County) and nearly reached in Vermilion County (9.4). In 28 counties, no western corn rootworm adults were found in soybean fields that we sampled. It’s worth noting, that Ford County still has the greatest density of variant western corn rootworms. Many will recall that Ford County was the epicenter of the outbreak that erupted in 1995.

Our data collected in 2011, look far different from survey results of western corn rootworm densities in soybeans collected approximately 1 decade ago (2002 and 2003). State surveys (2002 and 2003) conducted by Dr. Joe Spencer, an entomologist with the Illinois Natural History Survey, revealed densities of western corn rootworm adults that commonly went well beyond the 10 per 100 sweep threshold mentioned previously. Western corn rootworm densities such as those observed 10 years ago in soybean fields, would likely justify the use of a planting-time soil insecticide or a corn rootworm Bt hybrid on first-year corn. Can the same argument be made today even at the higher commodity prices? Unfortunately, most producers are no longer scouting for western corn rootworms in soybean fields (or cornfields) and assume the input costs will be worth the investment. In many instances, this may not be a wise decision with these very low western corn rootworm densities observed in Illinois soybean fields in recent years.

Why are the densities of variant western corn rootworm adults so low in Illinois and Iowa soybean fields in recent years? I have stated previously that I believe three key factors are involved: 1) excessively wet springs and saturated soils (2012 being an exception) over many successive years leading to significant larval mortality during the spring hatch, 2) continued significant use of Bt corn rootworm hybrids in first-year corn and continuous corn production systems, and 3) popular broadcast applications of tank-mix treatments (fungicide/insecticide) to corn and soybean fields across a large swath of the Corn Belt. With increasing concern over emerging western corn rootworm resistance to the Cry3Bb1 protein expressed in some Bt hybrids, the use of crop rotation, along with a non-Bt hybrid in first-year corn would seem to make increasing sense. This would reduce the selection pressure on various Cry proteins targeted at corn rootworms. Ultimately, in order for management decisions to be optimized, they should be made on a field by field basis and rely upon scouting inputs the previous season and knowledge of rootworm densities and economic thresholds.

Is the variant western corn rootworm threat diminished as compared with the threat it posed 10 to 15 years ago? Recent data would suggest this to be the case. We should not expect this reduced threat to be permanent. Instead, producers are encouraged to scout soybeans for western corn rootworm adults and to make more informed management decisions regarding this insect pest in first-year cornfields. By more carefully targeting inputs (Bt hybrids, soil insecticides), their longevity can be enhanced.

Mike Gray