By Nick Seiter and Joe Spencer
Producers across east-central Illinois have enjoyed low western corn rootworm pressure for several years, due to a combination of saturating rains during rootworm egg hatch and widespread use of Bt corn hybrids. Following a low point in the rootworm population in 2015, statewide monitoring of corn and soybean fields has documented a slow western corn rootworm population rebound in some areas.
Recent low corn pest abundance (combined with lower commodity prices) provides an opportunity to become reacquainted with rootworm monitoring and non-Bt options for their management. While relying on soil insecticide or a seed treatment to protect corn roots may not fit into every growers’ operation every year, rotating among different rootworm management tactics should be considered a part of the best management practices for corn rootworms in the transgenic era.
Rotating between different rootworm management tactics and Bt modes-of-action is necessary because western corn rootworm populations are evolving resistance to the Bt proteins expressed in Bt corn hybrids. In addition, monitoring adult populations in fields that will be planted to corn the following year will help to assess the need for control (whether a Bt trait or an insecticide).
In 2018, we conducted a series of field trials to evaluate control options for corn rootworm. These trials were planted following a 2017 “trap crop” of late planted corn and pumpkins to artificially increase rootworm populations in the field. Root masses (5 per plot) were removed during the early reproductive stages (R1-R3), cleaned using pressure washers, and rated for corn rootworm damage using the 0-3 Node-Injury Scale developed by researchers at Iowa State (Oleson et al. 2005).
The rootworm population at this location consisted almost entirely of western corn rootworm, and previous bioassay data indicated a high level of resistance to the “Cry3” Bt traits within the population. Note that additional information and data for these trials (as well as additional insect and disease management trials) are available in the recently published “Applied Research Results on Field Crop Pest and Disease Control,” available at the following link: http://cropdisease.cropsciences.illinois.edu/wp-content/uploads/2018/12/Pestpathogenappliedresearchbook2018.pdf. In addition, readers are encouraged to consult “on Target” for summaries of applied research trials conducted by University of Illinois personnel from 2004-2014: http://ipm.illinois.edu/ontarget/.
Seed treatments are nearly ubiquitous on seed corn planted across the Corn Belt. In our trials, the seed treatments Poncho Votivo and Poncho Votivo 2.0 offered significant root protection from corn rootworm larvae compared to an untreated control (Table 1). For many years, some corn hybrids have been marketed with seed treatments at what has been described as the ‘rootworm rate’. These data indicate that at modest larval pressure (ca. 1.9 on the 0-3 Node Injury Score scale), these seed treatments provide some root protection; however, based on previous studies these treatments should not be relied upon alone for control under heavy rootworm pressure. Note that all hybrids used in this trial expressed Cry3Bb1 for root protection. The relatively high root pruning observed in the untreated plots illustrates that resistance to the “Cry3” proteins is an issue at this site.
We tested soil-applied insecticides with a non-Bt hybrid for rootworm control, and all insecticide materials tested in 2018 reduced injury from corn rootworm larval feeding compared with the untreated control. This trial was conducted under relatively low larval pressure (1.07 on the 0-3 node-injury scale in the untreated plots), and no distinctions among the different insecticides could be made.
Before commercialization of Bt corn hybrids, a soil-applied insecticide was one of the only options available to growers anticipating economic rootworm injury in continuous or rotated corn. Over the years, soil-applied insecticides were regularly evaluated in University of Illinois Insect Management Trials (see previously linked “on Target” reports). They typically provided significant reductions in corn rootworm larval damage to corn roots compared to untreated controls.
Oftentimes, soil-applied insecticides provided root protection equivalent to, or approaching that provided by single trait Bt corn hybrids with similar yield results (see 2013 “on Target” report). Ultimately, Bt corn’s season-long root protection that was as good as or better than a soil-applied insecticide, reduced pesticide exposure, and simplified planting operations were powerful motivations that drove adoption of Bt corn. However, use of a granular or liquid soil-applied insecticide on a non-rootworm Bt corn hybrid remains a viable tactic to protect corn roots without the use of a Bt corn hybrid.
If you are interested in using one of these products and have not done so in a while, now is a good time to verify that your application equipment is in good shape. Rotating corn hybrids that incorporate Bt traits with non-Bt corn treated with a soil-applied insecticide should be considered as a strategy to slow resistance evolution, especially in areas that are currently experiencing only moderate corn rootworm pressure.
Oleson, J. D., Y. Park, T. M. Nowatzki, and J. J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 98: 1-8.
Table 1. Mean (± standard error) node-injury ratings of corn rootworm larval feeding injury on corn hybrids expressing the Bt trait Cry3Bb1 treated with Poncho Votivo, Poncho Votivo 2.0, or Untreated at Urbana, IL in 2018.
a Means followed by the same letter within a column are not different based on the Fisher method of least significant difference (α = 0.05)
Table 2. Mean (± standard error) node-injury ratings of corn rootworm larval feeding injury on non-Bt corn treated with granular and liquid insecticides at planting at Urbana, IL in 2018.
a Means followed by the same letter within a column are not different based on the Fisher method of least significant difference (α = 0.05) b Note that Ampex EZ is not labeled for use in corn at the time of this publication
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