Western Corn Rootworm (“Soybean”) Variant Survey 2005
Eileen Cullen, Extension Entomologist, UW Entomology Department
Risk management pertaining to corn rootworm is becoming more complicated regionally as variant strains of both northern and western corn rootworms have adapted behaviors to succeed in rotated corn. In Wisconsin, the variant rootworm concern is that of the Western corn rootworm, sometimes referred to as the “soybean variant”. It gets the nickname because unlike normal western corn rootworm beetles, the Variant western corn rootworm can lay heavy populations of eggs in soybean fields, resulting in risk of economic injury to corn planted in the same field the next year. As a new pest in Wisconsin, there is a need for information on the growing range of Variant western corn rootworm and uncertainty of crop rotation efficacy to manage rootworms in southeastern and southern Wisconsin.
The Southeast Wisconsin Variant Western Corn Rootworm Trapping Network (Network), formed in 2003, is a working group of UW Entomology, UW Extension County Agents, UW Nutrient and Pest Management, WI DATCP Pest Survey, and cooperating corn/soybean farmers. In 2005, the Network monitored 71 soybean fields in southeastern/southern Wisconsin to help producers determine the distribution and risk of Variant western corn rootworm in and around the affected area.
2005 Variant Western Corn Rootworm Survey:
Integrated pest management (IPM) research supports the recommendation of a trap-based field scouting protocol for Variant western corn rootworm in soybeans to estimate egg-laying activity and provide information to guide treatment decisions for corn planted the next spring. The sampling protocol and threshold of 5 B/T/D over the sampling period were developed by entomologists at the University of Illinois Champaign-Urbana where the Variant western corn rootworm was first discovered.
The Network uses 12 Pherocon AM yellow sticky traps evenly spaced throughout a soybean field to be rotated to corn. In Wisconsin, trapping begins the last week in July and lasts for four weeks. The August sampling period represents the time after which corn rootworms have completed development on corn, emerged from the soil, mated and started laying eggs of the overwintering population. Traps are secured to a metal or wood post, and positioned above the soybean canopy as the beans grow. Every 7 to 10 days total western corn rootworms are recorded from each trap and traps replaced. At the end of the sampling period, the average number of western corn rootworm adults caught per trap is calculated:
(Total WCR ÷ 12 traps) ÷ Total # of trapping days = Average Beetles/Trap/Day
An average between 5-10 Beetles/Trap/Day over the entire four week sampling period has been documented to result in economic root injury to corn planted in the field the next season. A University of Illinois IPM fact sheet, explaining the Variant western corn rootworm sampling protocol, sampling period and threshold calculation in soybeans, is posted at
http://ipm.uiuc.edu/fieldcrops/insects/western_corn_rootworm/wcr.pdf
The research study on which the trapping protocol is based was published in 2001 in the Journal of Economic Entomology and is cited at the end of this article.
2005 Survey Results:
The 2005 Network map (Figure 1) displays results of the Variant western corn rootworm survey in southeast and southern Wisconsin and indicates areas at greater risk. The map legend explains symbols for soybean fields that trapped at or above the 5 beetles/trap/day threshold, close to threshold, and below threshold. Table 1 presents the 2005 beetle counts in soybeans by field.
In 2005, the Network found 13 of the 71 soybean fields had western corn rootworm averages exceeding the 5 B/T/D threshold: two fields in Kenosha County, three in Racine County, three in Rock County, four in Walworth County, and for the first time, one Dodge County field. The detection of threshold level western corn rootworm beetles in soybeans in southern Dodge Co. indicates a northward expansion of the Variant western corn rootworm.
The 0 to 3 Node Injury Scale http://ipcm.wisc.edu/wcm/pdfs/2005/05-18insect2.htm was used to evaluate corn rootworm larval feeding damage to first year corn roots following soybean in 39 Network corn fields in 2005. A rating of 0 represents a root with no rootworm pruning; a rating of 3 is given to roots with three full nodes of roots pruned, the maximum damage rating on this scale. 10 randomly selected roots were dug from throughout each field (untreated fields or untreated strips within fields) and rated. A root damage rating between 0.25 to 0.50 (several roots pruned, 25% or 50% of one node, but never the equivalent of one entire node) has potential to cause yield loss.
Of the 39 first year corn fields contributing root samples to the Network in 2005, Walworth (10 fields) and Rock (10 fields) Counties averaged a 0.5 root rating score, while Columbia (4 fields), Dane (5 fields) and Jefferson (10 fields) averaged root rating scores of 0.0, 0.1 and 0.0, respectively. In Walworth and RockCounties, 30% (3 of 10 fields) and 60% (6 of 10 fields), respectively, sustained root injury scores higher than 0.5 in first year corn following soybean.
Twenty-four of the 39 first year corn fields in the 2005 Network were trapped as soybean fields during the 2004 growing season. When comparing average beetle/trap/day counts in soybean to first year corn root damage we examined how many of the fields trapping at or above the 5 beetles/trap/day threshold in soybeans met or exceeded a root rating of 0.5 in corn the next year. Based on this small sample size (n=24), if a grower had based corn rootworm treatment decision upon the threshold of 5 beetles/trap/day, 75 percent (18 out of 24 fields) would have made the correct decision (below 5 B/T/D a No Treat decision; fields above 5 B/T/D a Treat decision). Of the remaining 25 percent (6 of 24 fields), the threshold was conservative in two fields (Walworth 6 and Walworth 10), and in four fields damage was higher than predicted (Rock 4, Rock 5, Rock 9 and Kenosha 4) (Table 2).
Figure 1. Variant Western Corn Rootworm Survey Map 2005. 71 soybean fields monitored by the Network.* Average western corn rootworm beetles per trap per day. (Legend below).
* In addition to 69 soybean fields scouted in 10 contiguous southern WI counties, 2 fields were trapped in southwest WI in Grant county. Average beetles/trap/day in the two grant county fields (not pictured on map) were well below threshold at 0.24 and 0.56 B/T/D.
Table 1. 2005 Network Trapping Results by field.
Table 1. continued …
Table 2. Network fields comparing 2004 soybean sticky trap counts relative to a threshold of 5 beetles/trap/day against 2005 first year corn root ratings.
Summary and Network plans for 2006:
Field surveys by entomologists and Extension educators in areas with the variant such as east central IL and southeast WI have found that not every first year corn field warrants rootworm protection, even in the most severely affected pockets. Trap-based scouting and use of the IPM threshold for adult beetles in soybean, is currently the most accurate method available to determine treatment needs for first-year corn.
The Network will continue the Variant western corn rootworm soybean survey in 2006 with the support of the Wisconsin Soybean Marketing Board, and collaboration with the WI DATCP Pest Survey. For more information on implementing a scouting program in your area, a list of Network cooperators is included following this article. Beginning in 2006, the UW Entomology and UW Agricultural Economics departments will begin a more intensive joint research project in the affected area of Walworth/Rock Counties to evaluate the costs and returns of trap-based IPM decision support versus insurance treatment of first year corn in the absence of soybean trapping data. We will also monitor adult western corn rootworm activity in other crop rotations (corn following winter wheat and corn following alfalfa), as well as reevaluate lower intensity trapping methods (fewer traps or fewer weeks) as part of the new study.
References
O’Neal, M.E., M.E. Gray, S. Ratcliffe and K.L. Steffey. 2001. Predicting western corn rootworm (Coleoptera: Chrysomelidae) larval injury to rotated corn with Pherocon AM traps in soybeans. Journal of Economic Entomology 94:98-105.
SOUTHEAST WISCONSIN VARIANT WESTERN CORN ROOTWORM TRAPPING NETWORK
EILEEN CULLEN, UW Madison, Entomology Department
(608) 261-1507 / cullen@entomology.wisc.edu
BRYAN JENSEN, UW Madison, Entomology Department, UW IPM
(608) 263-4073 / bmjense1@facstaff.wisc.edu
TED BAY, UW Extension Grant County
(608) 723-2125 / ted.bay@ces.uwex.edu
TIM BENDER, UW Extension Jefferson County
(920) 674-7295 / tim.bender@ces.uwex.edu
DAVID FISCHER, UW Extension Dane County
(608) 224-3716 / david.fischer@ces.uwex.edu
MATT HANSON, UW Extension Dodge County
(920) 386-3790 / matt.hanson@ces.uwex.edu
LAURA PAINE, UW Extension Columbia County
(608) 742-9682 / laura.paine@ces.uwex.edu
PEG REEDY, UW Extension Walworth County
(262) 741-3181 / peg.reedy@ces.uwex.edu
KEVIN SHELLEY, UW Nutrient & Pest Management Program
(608) 262-7846 / kevin.shelley@ces.uwex.edu
JIM STUTE, UW Extension Rock County
(608) 757-5696 / stute@co.rock.wi.us
KAREN TALARCZYK, UW Nutrient & Pest Management Program
(608) 723-2240 / karen.talarczyk@ces.uwex.edu
RACHEL KLEIN-KOTH
WI Department of Agriculture Trade and Consumer Protection (WI DATCP)
(608) 224-4544 / Rachel.klein-koth@datcp.state.wi.us
KRISTA LAMBRECHT
WI Department of Agriculture Trade and Consumer Protection (WI DATCP)
(608) 224-4594 / Krista.lambrecht@datcp.state.wi.us