By Ashley N. Dean and Erin W. Hodgson – Department of Entomology, Iowa State University
Western corn rootworm (WCR) and northern corn rootworm (NCR) larvae cause considerable yield loss in the corn-producing regions of North America, especially where continuous corn is prevalent. Lodging, severe root injury and high numbers of beetles in fields are usually the first indication of an economic issue. Adult beetles can further reduce yield by feeding on reproductive parts of corn and interfering with pollination.
Scouting and proper identification of these pests can help farmers make effective management decisions. There are several scouting techniques available for farmers and agronomists to use for estimating corn rootworm larval populations. However, many are labor intensive and time consuming. Using yellow sticky cards is an effective way to passively scout for corn rootworm adults, and trap counts can assist in making management decisions the following year.1
Corn Rootworm Trapping Network Goals
During the spring of 2021, university, industry and government personnel from 12 U.S. states and 5 Canadian provinces met to discuss goals for a regional corn rootworm trapping network and develop a shared protocol. The goals were to:
Increase scouting efforts in corn
Understand changes in populations between years
Raise awareness about changes in WCR and NCR activity
Use appropriate management strategies based on scouting information
We provided free yellow sticky traps to volunteer cooperators (typically farmers or their advisors) in exchange for information about the fields where they placed the traps (e.g. cropping history, performance issues with rootworms, etc.) and weekly trap capture data.
The standard protocol was to place 4 traps along a single row of corn and check them for 4 weeks, exchanging traps for new ones each week, beginning in mid-July (near silking or after the first beetles were spotted in the field). Most cooperators chose to report data on the go with an online reporting system called Survey1231; the data entered in this system is summarized in this article.
Cornfield and Soybean Locations
In 2021, we had 619 unique locations entered in Survey123, which represented 14 U.S. states and 5 Canadian provinces (Figure 1). IA had the most total sites, largely because Syngenta had an immense network of cooperators. Most sites were located in cornfields (560 sites), but 59 sites were in soybean fields. Most soybean sites were in IL and IN, where concern for the WCR variant (females laying eggs in soybeans) is higher. Very few beetles were captured from the soybean sites. The maximum beetles per trap per day was 1.63 compared to 339.57 for corn sites (Table 1).
The primary crop rotation reported was continuous corn (at least 2 years; 391 sites), followed by corn-soybean rotation (205 sites), and “other” (23 sites).
Figure 1. The number of corn rootworm trapping sites in each state that were entered in Survey123 in 2021
Table 1. Average and maximum beetles (each species and total) captured per trap per day at corn and soybean sites
*NCR = northern corn rootworm; WCR = western corn rootworm; total = northern and western corn rootwormCorn Rootworm Issues and Management Tactics
We asked cooperators to report any past or current corn rootworm issues in the field. Only 122 sites reported issues with corn rootworm (Graph 1). The most reported issue was high beetle populations (61 sites), followed by a combination of goosenecking/lodging and high beetle populations (28 sites), and goosenecking/lodging only (19 sites). 7 sites reported 3 or more issues, and very few sites reported expected resistance to Bt traits or crop rotation.
Graph 1. Reported corn rootworm issues in the fields where traps were placedWe also asked cooperators to report any corn rootworm management tactics that were used in the field during the current growing season. Out of the 115 sites that reported corn rootworm management tactics, 16 sites reported using no corn rootworm management, 50 sites reported using a single tactic to manage rootworms and 49 sites reported using more than 1 corn rootworm management tactic (Graph 2).
Graph 2. Reported corn rootworm management tactics used in 2021 in the fields where traps were placedBeetle Counts
A threshold exists for yellow sticky traps, regardless of species: 2 or more corn rootworm beetles/trap/day suggests that alternative management is needed the following year.1 Approximately 29% of sites in 2021 met or exceeded the trapping threshold in their peak week (the week when the most total beetles were captured at each site; Figure 2). Of the sites that exceeded the threshold:
Figure 2. Relative trap captures during the peak week at each site. Blue dots indicate sites that did not reach the trapping threshold,
while orange dots indicate sites that exceeded the threshold of 2 beetles/trap/daySites that exceeded the threshold would indicate regions where corn rootworm populations were particularly high in 2021. Most of these sites (80%) were in NE, IA, WI, and Ontario, Canada, which accounted for 67% of the total sites in the trapping network, and 90% were in continuous corn. Continuous corn is the most important factor for high corn rootworm populations. Of the continuous corn sites, 41.4% exceeded the trapping threshold, while only 8.8% of sites with a corn-soybean rotation exceeded the threshold.
For choosing whether to manage corn rootworms, it is not important to distinguish between the 2 species of rootworm beetles. However, knowing which species is most prevalent in the field could help determine which management decisions are best. Figures 3 and 4 show where each species was the dominant species (species that comprised >50% of the total beetles) during the peak week at each site.
Figure 3. Each trap site where NCR was the dominant species during the peak week
Figure 4. Each trap site where WCR was the dominant species during the peak weekKey Management Strategies for Northern and Western Corn Rootworm
Since corn rootworm populations are highly dependent on in-field management practices, it is difficult to extrapolate this data and make assumptions for fields not scouted. However, we plan to continue this network in 2022 (and maybe beyond) to build a dataset that could uncover trends or changes in rootworm activity that may be useful to farmers, industry agronomists and Extension personnel. Some key management considerations:
Every cornfield should be scouted every year to assess fresh root injury from mid to late July.3 Monitor for adult activity later in the season.4
Continuous corn production is the biggest driver for high corn rootworm populations and development of resistance to Bt hybrids.5
Crop rotation is the most effective way to confuse corn rootworm. Rotating every 3-5 years will break up the life cycle and reduce resistance development in the field.
Weedy fields and borders are attractive food sources after corn pollination is complete.
Alternate management strategies are needed to prolong the efficacy of these tactics.
Rescue treatments (foliar sprays for larvae or adults) have not shown consistent results.
References
1Dunbar, M., and A. Gassmann. 2013. Abundance and distribution of western and northern corn rootworm (Diabrotica spp.) and prevalence of rotation resistance in eastern Iowa. J. Econ. Entomol. 106: 168-180.
2ArcGIS Survey123: https://survey123.arcgis.com/
3Interactive Node-Injury Scale: https://www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html
4Hodgson, E. and A. Dean. 2019. Assessing corn rootworm activity using sticky traps. Integrated Crop Management News. Iowa State University Extension. July 23, 2019.
5Gassmann, A. 2021. Resistance to Bt maize by western corn rootworm: effects of pest biology, the pest-crop interaction and the agricultural landscape on resistance. Insects, 12: 136. DOI: https://doi.org/10.3390/insects12020136
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