Variant Western Corn Rootworm Survey Results 2006

Eileen Cullen, Extension Entomologist

Variant western corn rootworm beetle abundance and egg-laying in soybean fields, with potential root damage to first year corn in the same field the following year, present an economic risk to corn and soybean producers in the affected region(s). Previous issues of the Wisconsin Crop Manager, and neighboring state newsletters, have introduced the variant western corn rootworm in detail. This is a behavioral variant of the western corn rootworm population that has adapted its egg-laying behavior to include non-corn crops, notably soybeans. As the variant western corn rootworm expanded its range into Wisconsin, UW-Extension responded by forming the variant western corn rootworm trapping network (Network).

Since 2004, the Network has been conducting an extensive survey to confirm incidence of the variant by comparing Pherocon AM yellow sticky trap beetle counts in soybeans in one year with corn root node-injury scores from untreated areas of the same field the following year. The extensive survey method allows us to track changes in variant western corn rootworm population densities from year to year, assess the boundaries around initial hot spots in southeastern Wisconsin, and detect new occurrences.

Network participants (see 2006 Acknowledgements) include 1 UW Madison Extension Entomologist, 11 UW-Extension County Agents, 3 UW Madison Outreach Specialists, and 5 individuals from the WI DATCP Pest Survey. The Network has expanded to include 10 contiguous southern WI counties (Rock, Walworth, Racine, Kenosha, Jefferson, Waukesha, Columbia, Dane, Dodge and Green). Grant County, southwestern WI, participated with one field.

In 2006, 106 Wisconsin soybean fields were monitored for variant western corn rootworm beetle abundance, using the 12-trap grid Pherocon AM yellow sticky trap method developed by colleagues at University of Illinois (Gray 2004) and published in the Journal of Economic Entomology (O'Neal et al. 2001). Most of the fields monitored in 2006 were located in the 11 southern and southeastern counties named above. The remaining fields were located in the Western region, with two soybean fields each in Monroe, LaCrosse, Trempealeau, Buffalo and Vernon Counties.

The scouting protocol deploys 12 Pherocon AM yellow sticky traps evenly spaced throughout the soybean field to be rotated to corn. Trapping begins the last week of July and lasts for four weeks, spanning the month of August when adult rootworm beetles are laying eggs. Every 7 to 10 days, total western corn rootworm beetle adults are counted from each trap and traps replaced. At the end of the 4-week sampling period, the average number of adults caught per trap per day is calculated. A count of 5 beetles/trap/day (B/T/D) over the sampling period is likely to result in economic root injury for corn planted in the field the next season. We quantify this damage at a node-injury score of 0.5 or greater on the Iowa State University 0 to 3 node-injury scale (equivalent of one-half of a node of roots pruned, or more).

Photo Credit: Sarah Schramm, UW Madison Entomology Department

Figure 1 displays the map for ten contiguous southeastern counties monitored in 2006. All field points are soybeans, and points are shape and color coded to display the range of adult western corn rootworm beetle densities detected per trap per day over the 4-week sampling period. We use 5 B/T/D as a threshold to indicate liklihood of corn root injury (0.5 or greater node-injury score) the following year. Figure 2 displays the map for soybean fields monitored in 2005.

Comparing UW-Extension Network monitoring results between years, we found that yellow sticky trap captures in soybeans were lower in 2006 than 2005. In 2006, two fields exceeded the 5 B/T/D threshold; four fields ranged between 4 and 5 B/T/D; and four fields between 3 and 4 B/T/D. These fields were clustered for the most part in eastern Rock County in 2006, while the majority of Network soybean fields trapped well below threshold at 0.1 to 3 B/T/D as shown by the black circles on the 2006 map (Figure 1). 2006 results for the one field sampled in Grant County (southwestern WI) and the 10 fields scouted across five western counties all trapped well below threshold, and were less than 1 B/T/D.

By contrast, our 2005 map showed a higher proportion of soybean fields at or above the 5 B/T/D for the 4-week sampling period, and these threshold fields were more widely dispersed with an apparant northward movement into southern Dodge County.

Tables 1 and 2 present the results of our Network on-farm trials examining first year corn root injury as related to yellow sticky trap data for the same field in soybeans the previous year.

Based on 24 fields in the 2004-2005 soybean-corn cycle, we observed the 5 B/T/D threshold and scouting protocol to be 75% accurate (18 of 24 fields), 8% conservative (2 of 24 fields would have received unnecessary rootworm treatment), and 17% inaccurate (4 of 24 fields sustained more root damage than predicted by the 2004 B/T/D number). For the 2005-2006 soybean-corn cycle, the scouting protocol was 90% accurate (27 of 30 fields), with three fields sustaining more root damage in 2006 than predicted by 2005 B/T/D.

Table 1. Yellow sticky trap (B/T/D) over 4-week sampling interval in soybeans August 2004, and corresponding corn root node-injury rating from untreated fields or strips in the same field in 2005 (10 roots per field). A check-mark is placed in the fourth column each time a correct treatment decision would have been made using the 5 B/T/D threshold in soybeans.

County	2004 B/T/D Soybean	2005 Corn Root Node-Injury Rating (0 to 3)	Correct Treatment Decision (Ö),  Root Injury (higher) or (lower) than predicted by 2004 B/T/D
Rock	2.22	0.1	Ö
Rock	1.34	0.3	Ö
Rock	1.57	0.6	Higher root injury
Rock	0.97	0.7	Higher root injury
Rock	1.17	0.2	Ö
Rock	1.52	0.6	Higher root injury
Rock	7.33	0.7	Ö
Rock	8.30	0.7	Ö
Rock	5.10	0.4*	Ö
Walworth	8.83	0.5	Ö
Walworth	5.36	0.1	Lower root injury
Walworth	5.59	1.9	Ö
Walworth	4.16	0.3	Ö
Walworth	5.66	0.2	Lower root injury
Kenosha	2.39	0.8	Higher root injury
Kenosha	2.34	0.2	Ö
Kenosha	2.68	0.2	Ö
Racine	1.96	0.2	Ö
Columbia	0.21	0.0	Ö
Columbia	0.78	0.0	Ö
Columbia	0.23	0.0	Ö
Columbia	0.71	0.0	Ö
Dane	0.67	0.3	Ö
Dane	0.42	0.0	Ö

 *Counted as a correct decision, as 0.45 node-injury rating is very close to 0.5.

Table 2. Yellow sticky trap (B/T/D) over 4-week sampling interval in soybeans August 2005, and corresponding corn root node-injury rating from untreated fields or strips in the same field in 2006 (10 roots per field). A check-mark is placed in the fourth column each time a correct treatment decision would have been made using the 5 B/T/D threshold in soybeans.
 

County	2005 B/T/D Soybean	2006 Corn Root Node-Injury Rating (0 to 3)	Correct Treatment Decision (Ö), Root Injury (higher) or (lower) than predicted by 2005 B/T/D
Dodge	4.82	0.03	Ö
Dodge	3.48	0.52	Higher root injury
Columbia	0.66	0.01	Ö
Columbia	0.29	0.02	Ö
Columbia	0.75	0.00	Ö
Columbia	0.10	0.00	Ö
Columbia	0.06	0.00	Ö
Dane	2.99	0.12	Ö
Dane	0.93	0.01	Ö
Dane	0.78	0.00	Ö
Dane	0.63	0.10	Ö
Jefferson	1.99	0.18	Ö
Jefferson	3.76	0.07	Ö
Jefferson	2.25	0.02	Ö
Jefferson	1.73	0.02	Ö
Jefferson	4.38	0.02	Ö
Jefferson	2.84	1.57	Higher root injury
Waukesha	3.60	0.06	Ö
Green	1.04	0.03	Ö
Green	1.21	0.00	Ö
Green	0.46	0.00	Ö
Green	0.77	0.01	Ö
Walworth	2.68	0.23	Ö
Walworth	5.00	2.45	Ö
Rock	2.01	0.10	Ö
Rock	6.71	0.60	Ö
Rock	5.43	0.66	Ö
Rock	2.37	0.00	Ö
Rock