1. Rotation-Resistant Corn Rootworms 2007
Jon Tollefson
Patti Prasifka
and
Nick Kiley
Tollefson

2. northern corn rootworm
Corn Rootworm Species
Two major species of corn rootworm in Iowa, western crw (typically yellow with black strips) and northern crw (typically pale yellow to bright green in color with no black markings).
western corn rootworm
northern corn rootworm
Tollefson

3. Corn Rootworm Life Cycle
May
June
July
This is the life cycle of CRWs. Eggs overwinter in the soil and hatch in early June. Larvae (the most damaging stage) feed on corn roots throughout June and into July. There are 3 larval instars. Larvae pupate in late June to early July and adults start emerging around the first week in July. As adults mate and lay eggs (typically in corn) they also feed on corn silks (silk clipping) which in extreme cases can cause damage by decreasing seed set.
The main objective of CRW management is to prevent larval injury to maize roots
In the US, current management options are crop rotation (where it is still effective), insecticides applied at planting time, and now genetically modified, Bt, maize
Because all of these management practices must be applied at the time of planting, the decision to us them must be made based on the previous egg or adult numbers
Planting
Tollefson

4. Corn Rootworm Larval Injury
The left picture is an example of goose necking – seen when roots are injured by CRW and rain, hail or wind causes the plants to lodge early in the season. The plants have enough time to try and grow back up and then we see this “goose necking” effect. The pictures on the right are examples of the actual root injury. The top 3 roots have severe injury (2-3 nodes eaten) and the bottom roots have very little injury (little to no injury, definitely less than 0.25 nodes eaten which is the economic threshold).
Tollefson

5. Extended Diapause Corn Corn Year 1 Year 2 Year 3 Soybean X X X
Some populations of the northern corn rootworm have successfully adapted to the corn-soybean crop rotation. This adaptation is called extended diapause. With extended diapause, a small proportion of the population contains individuals that remain dormant as eggs in the soil for two or more years and successfully avoid death by crop rotation. As this diagram shows, some of the eggs that are laid in year one will not hatch until year three and this can result in damage to first year corn. This is generally an occasional problem in Iowa and in most cases, economic losses are rare.
CRW eggs
Tollefson

6. Extended Diapause Northern Corn Rootworm Distribution Pre-2005
This map shows the estimated distribution of extended diapause in Iowa before our monitoring efforts started in One of the questions we asked in 2005 was, has the distribution of extended diapause changed (expanded)?
Tollefson

7. Rotation Resistance Year 1 Year 2 Female leaves corn,
lays eggs in soybean
Eggs hatch,
damage 1st year corn
Corn
Corn
Field A
Field B
Field A
Field B
Soybean
Soybean
Researchers in Illinois and Indiana are dubbing this new problem as “Rotation Resistance” in western corn rootworm populations. So what is happening here? Although the answer is still not clear, it appears that long-term use of a corn-soybean crop rotation has selected for a behavioral change in some western corn rootworm populations. After the beetles emerge in the cornfield, they move to a nearby soybean field to lay their eggs. So in the following year, when corn is planted back into the field, previously soybeans, the eggs hatch and damage the first year corn, successfully circumventing the crop rotation as a management option. X X
CRW eggs
Year 2
Tollefson

8. Rotation-Resistant Western Corn Rootworm
Iowa
2005
Before our monitoring project started in 2005, this was the estimated distribution of the variant western CRW (there were some reports of it being present in eastern Iowa but nothing was 100% confirmed).
Tollefson

9. Corn Rootworm Resistance Update
Where is the rotation-resistant western corn rootworm in Iowa?
Are we seeing an increase of extended-diapause northern corn rootworm?
Is the northern corn rootworm laying eggs in soybeans?
These were the 3 main question that we had for our 2005–2007 research.
Prasifka

10. Variant of the Western Corn Rootworm Field Locations, 2005
Variant detected, not economic
This map shows the counties where we found the variant of the western CRW present in So in the yellow counties, we captured western CRWs emerging from first year corn fields but the sticky traps did not detect economic levels (all sticky traps captures in nearby soybean fields were less than 3–5 beetles per trap per day).
Prasifka

11. Variant of the Western Corn Rootworm Field Locations, 2006
Variant detected, not economic
Variant not detected
Variant detected, economic
This map shows the counties monitored for the variant of the western CRW in No variant beetles were detected in the green country (no emergence trap captures). In the yellow counties, we captured western CRWs emerging from first year corn fields but the sticky traps did not detect economic levels (all sticky traps captures in nearby soybean fields were less than 3–5 beetles per trap per day). In the two red counties (Scott and Clinton counties), we found high numbers of beetles emerging from first year cornfields and economic levels of beetles captured in soybean fields with sticky traps (greater than 3–5 beetles per trap per day).
Prasifka

12. Variant of the Western Corn Rootworm Field Locations, 2007
Howard
Chickasaw
Floyd
Butler
Bremer
Buchanan
Tama
Benton
Iowa
Keokuk
Mahaska
Jefferson
Monroe
Davis
So, variant western CRWs are at economic levels in some of the eastern most counties (Scott and Clinton) and present in most of the eastern 2-3 counties in Iowa. Our next question was, how far west has the variant spread? To address this question, we selected locations that were on the western edge of previous monitoring efforts and fields one county further west than any previous monitoring efforts (see black dots above in the blue counties).
Prasifka

13. Variant of the Western Corn Rootworm Sampling
Emergence traps in first-year corn
Pherocon AM sticky traps in soybean fields
At each of these locations, we placed 12 emergence traps in a first year cornfield and 12 Pherocon AM sticky traps in a nearby soybean field. All emergence traps were checked and sticky traps were changed weekly. Emergence traps were out from the end of June through mid August and sticky traps were out from the last week in July through the 3rd week in August.
Prasifka

14. Emergence Trap Data 2007 Prasifka
These are the emergence trap data for Note that the emergence for western CRWs (variant western CRWs) from these areas were very low (4 of the fields had no emergence and the other 14 had very low numbers). Expect for perhaps the Butler and South Buchanan sites, these low levels of emergence should probably not be considered evidence of the presence of the variant (catching 1-2 beetles over the course of the summer could just be incidental or accidental captures – e.g., if the ground got dry and a crack formed, a beetle could have crawled under the cage and gotten into the capture unit). The captures of Northern CRWs (extended diapause CRWs) from these areas were fairly high (exceptions include Davis and Monroe counties were little or no northerns were captured). We don’t have economic thresholds for northern CRW beetle emergence cage captures but it is probably reasonable to assume that emergence greater than 15 or 20 beetles per cage could cause economic injury.
Prasifka

15. Sticky Trap Data 2007 Prasifka
These are the sticky trap data for Note that the captures of western CRWs (variant western CRWs) from these soybean fields were very low (beetles were only captured in 2 of the 18 fields and in those fields just 0.2 and 0.3 beetles per trap per day!). Consequently, we would conclude that even in the two counties were emergence was slightly higher (Butler and South Buchanan) the variant is not present at economic levels. In other words, in all the counties listed above, at this time, it would probably not be economical to treat first year corn to protect it from the variant of the western CRW.
We do see two fields with slightly higher northern CRW activity (Iowa Country, east and west fields) but these levels are still below the western CRW threshold of 3–5 beetles per trap per day. It is also important to note that we don’t have experimental evidence that the same threshold holds for northerns so these data should be interpreted with caution.
Prasifka

16. Conclusions from Variant Western CRW Monitoring 2007
Very low levels detected in 14 of the 18 locations (emergence trap data)
No location anywhere near economic levels (sticky trap data)
This could change – growers should monitor their soybean fields with sticky traps, 6/field (especially the eastern edge of this region)
These are the major conclusions from the 2007 variant western CRW study.
Prasifka

17. General Recommendation
Soybean fields in eastern Iowa should be monitored with yellow sticky traps for western CRWs
Use Pherocon AM traps
If the number of western beetles caught exceeds 3–5 / trap / day,
protect the corn from
rootworm larvae the following
year
Prasifka

18. Are we seeing an increase of extended-diapause northern corn rootworm?
Distribution – yes, found in all counties
Abundance – possibly, the proportion of the population that is extended diapause has historically be ≈ 30%
Data from 2006 shows some areas with higher percentages
Prasifka

19. Extended Diapause Northern Corn Rootworm Distribution
Pre-2005
Post -2005
Prasifka

20. Estimate of Extended-Diapause Northern Corn Rootworm, 2006
Prasifka

21. What can we do about extended diapause
What can we do about extended diapause? Can extended diapause be predicted?
Tollefson

22. Relationship of Root Injury to Number of Northern Beetles 2 Years Earlier
Tollefson

23. Pay-Back on Insecticides
1988 – 1993
59 fields
in 12 fields yield protection paid for insecticide
20 % chance of breaking even or making money (1 in 5)
Tollefson

24. Gain when Treating for Extended Diapause
Year n
Untreated
Yield (bu)
Average
Gain (bu)
Corn
Price
Pay-
Back #
Econ
1988 9
103.4
0.4
2.45
4.9 1
1990 12
152.2
0.3
2.21
5.4 3
1991 23
147.8
2.3
2.30
5.2 7
1992
165.6
1.5
2.00
6.0
1993 6
105.2
2.8
2.50
4.8
Using $12.00 / acre insecticide cost.
Tollefson

25. Can we predict what will happen in 2009?
Not with a high degree of reliability
So, what are your options?
Option 1 – play the odds (5 to 1), don’t protect your first-year corn
Option 2 – use a soil or seed applied insecticide, or plant RW corn (recommended if: history of extended-diapause problems, extensive lodging, high beetle counts (?))
Option 3 – rotate out of corn for 2 years
Tollefson

26. Are northern CRW laying eggs in soybean?
2006
Emergence traps in extended non-corn rotation (not planted to corn for at least 3 yrs)
Pherocon AM sticky traps in soybean field (corn in 2005)
Soil sampling in soybean field (corn in 2005)
June sample = estimate of extended diapause
 in Sept sample = evidence of egg laying
Prasifka

27. Field Locations, 2006          Prasifka Chickasaw Wright
Butler   
Buchanan
Webster  
Boone
Story 
Cass
Prasifka

28. Northerns Laying Eggs in Soybean: 2006
Emergence and sticky traps caught almost no beetles (very low numbers)
Soil sampling showed there may be some evidence of egg laying in eastern counties, but the level of egg laying is very low
Prasifka

29. Comparison of June and Sept Soil Samples
East Buchanan
West Buchanan
Prasifka

30. Are northern CRWs laying eggs in soybean?
Egg laying OR extended diapause???
2007
Egg trap placed in soybean fields
Eliminate complication of extended diapause
5 locations: Story, Chickasaw (2), & Buchanan (2) counties
Data being processed
New egg sampling method for 2007.
Prasifka

31. Evaluation of CRW Hybrids, 2007
Planted on continuous corn (trap crop)
Ames, Crawfordsville, Nashua, Sutherland
Node injury 0–3 scale
Product consistency = % of times nodal injury was ≤ 0.25 (¼ node eaten)
Lodging = % of plants leaning at least 30° from vertical
Stand Count = # plants per 17.5 row-ft
These are some of the methods for Jim Oleson’s strip test study.
Tollefson

32. Ames, IA 2007 Treatment Node-injury Consistency % Lodging
Stand ct. (17.5 row-ft)
YieldGard Plus
0.01a
100a
29.17
Herculex XTRA
27.17
Agrisure CB/LL/RW
0.11a
89a
Aztec 2.1 G (T-band)
29.33
CHECK
1.21b
17b
Strip test results from Ames.
Tollefson

33. Crawfordsville, IA 2007 Treatment Node-injury Consistency % Lodging
Stand ct. (17.5 row-ft)
YieldGard Plus
0.01a
100a
49a
30.00ab
Herculex XTRA
0.02a
54a
29.50ab
Agrisure CB/LL/RW
0.04a
44a
29.75b
Aztec 2.1 G (T-band)
0.07a
34a
29.75ab
CHECK
2.53b 0b
80b
30.75a
Strip test results from Crawfordsville.
Tollefson

34. Nashua, IA 2007 Treatment Node-injury Consistency % Lodging
Stand ct. (17.5 row-ft)
YieldGard Plus
0.04a
100a 0a
34.33
Herculex XTRA
0.02a
28b
Agrisure CB/LL/RW
0.82b
17b
27b
34.42
Aztec 2.1 G (T-band)
0.87b
21b 3a
34.08
CHECK
2.21c 0b
100c
34.50
Strip test results from Nashua.
Tollefson

35. Evaluation of CRW Hybrids, 2007
Nashua, IA
Strip Test Check Rows
Tollefson

36. Sutherland, IA 2007 Treatment Node-injury Consistency % Lodging
Stand ct. (17.5 row-ft)
YieldGard Plus
0.02a
100a 0a
32.00
Herculex XTRA
14b
Agrisure CB/LL/RW
0.12a
92a
4ab
34.13
Aztec 2.1 G (T-band)
0.10a
32.75
CHECK
1.99b 8b
74c
32.88
Strip test results from Sutherland.
Tollefson

37. Summary
Injury was variable, YieldGard Plus & Herculex XTRA provided most consistent root protection at all locations
Agrisure CB/LL/RW & Aztec 2.1G provided high levels of root protection at three of the four locations
Lodging ratings were inconsistent possibly not strongly related to CRW injury this year
Node Injury
The amount of injury from corn rootworm larvae was variable across the spectrum of products evaluated. YieldGard Plus and Herculex XTRA consistently provided the greatest amount of root protection at all four locations. Agrisure CB/LL/RW and Aztec 2.1G also provided high levels of root protection at three of the four locations, but at the Nashua site, there was significantly more corn rootworm injury and than occurred in the YieldGard Plus and Herculex EXTRA. The reasons for this failure to adequately protect corn roots in the Agrisure CB/LL/RW and Aztec 2.1G treatments are unknown.
Product Consistency
Product consistency represents the percentage of times in which the roots had ¼ node or less eaten back to within 1½ inches of the stalk. This distinction allows a small amount of injury to occur without penalizing the performance of a product. If more than ¼ node was eaten on an evaluated corn plant, then the product was determined not to provide adequate root protection, and it was marked as being an inconsistent product in that particular test. Consistency works on the same principle as a batting average in baseball—the larger the number, the better the performance.
Only YieldGard Plus and Herculex XTRA were 100 percent consistent in providing total root protection across all four locations during 2007.
Lodging and Stand Counts
Lodging ratings were inconsistent and did not appear to be strongly related to corn rootworm injury this year. For example, at Nashua where the YieldGard Plus and Herculex XTRA were 100 percent in consistent root protection, the Herculex hybrid suffered 28 percent lodging, which was nearly identical to Agrisure CB/LL/RW but that had a poor consistency rating at this location.
There were was only one difference in the stand counts; at Crawfordsville Agrisure CB/LL/RW has approximately 3,000 fewer plants per acre than the untreated check.
Summary
These data provide a brief insight into several performance characteristics of genetically-engineered corn rootworm hybrids under continuous corn rotations in Iowa. It is important to understand that variations in performance could be expected for any corn rootworm hybrid (see also Bt rootworm corn failures: Understanding the issues and that cost per unit of seed, yield production, and individual grower expectations are additional factors worthy of consideration when making a seed purchase. At the end of the season, products may not perform as advertised because of a variety of agronomic, genetic, environmental, and pest related issues. Therefore it is imperative that corn growers have an appreciation of these factors and the potential influence on net profit and future pest management decisions.
Tollefson

38. Thank you

39. Iowa State University, 2003-05
Insecticide
Placement
Node-
Injury
Percent
Consistency
Lodging
Yield
(bu/a)
YieldGard RW
----
0.03 a
99 a
1 a
183 a
Aztec 2.1G
Furrow
0.24 ab
82 b
0 a
159 b
Aztec 4.67G
Furrow SB
0.28 bc
78 b
157 b
Force 3G
T-band
0.29 bc
76 bc
162 b
0.30 bc
75 bc
151 bc
0.35 bcd
72 bc
Fortress 2.5G
0.49 cd
68 bc
10 a
153 bc
Fortress 5G
0.57 de
61 c
4 a
155 b
Lorsban 15G
0.80 ef
44 d
6 a
150 bc
Capture 2EC
42 d
7 a
Poncho 1250 ST
0.98 f
21 e
158 b
Cruiser Ext 1250
1.53 g
8 ef
31 b
152 bc
CHECK
2.00 h
2 f
40 c
130 c
Tollefson

40. Relationship of Yield to Number of Northern Beetles 2 Years Earlier
Tollefson