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The U.S. Environmental Protection Agency judged that the use of transgenic
hybrids for corn rootworm larval control would serve the public interest by
replacing or reducing the use of soil insecticides (particularly
organophosphates) deemed to be of greater risk (environmental, human health,
and safety). In addition, the use of transgenic hybrids for corn rootworm
control was judged to have "comparable or greater efficacy than current
chemical alternatives." As the 2004 growing season revealed, this
recombinant technology is still relatively new with respect to corn rootworm
control, and we have much to learn.
As we stated last year in an article in issue no. 1 of the Bulletin (March
20, 2003), producers should anticipate finding numerous corn rootworm adults
in their fields of YieldGard Rootworm corn. These adults are immigrants from
other cornfields as well as survivors from YieldGard Rootworm corn.
Different from the poor survival of European corn borers on Bt hybrids used
to control them (hybrids resulting from high-dose transgenic events), some
rootworms survive after feeding on the roots of current transgenic hybrids
for control of corn rootworm larvae. Why? The MON 863 event is a
non-high-dose product. In August 2002, the EPA's FIFRA Scientific Advisory
Panel believed it was not necessary to distinguish between low- and
moderate-dose events. Regardless of the precise characterization of the
YieldGard Rootworm event (low dose vs. moderate dose), it remains certain
that corn rootworm survivorship is evident. In 2001, we placed adult
emergence cages over transgenic plants (MON 863) that were planted on May
10. The cages were checked for adult emergence every 2 days. A total of 96
emergence cages were used in this replicated split-plot experiment. Based on
the number of adults emerging into these cages, we projected that 2,450 male
western corn rootworm adults would emerge per acre from the MON 863-event
corn. The number of females emerging per acre from the transgenic corn was
25,320. Overall, we projected that 27,770 adults would have emerged from 1
acre of transgenic corn (MON 863, Cry3Bb1), and 91% of these were females.
Emergence of males and females also from the transgenic corn rootworm hybrid
was delayed when compared with emergence from the non-transgenic isoline.
Delayed emergence and survivorship data such as these have been reported by
other entomologists from land-grant institutions. Although MON 863 is a
non-high-dose event, root protection afforded by this technology in
university research plots had been exceptional.
In 2003, we established three corn rootworm product efficacy trials in
trap-crop settings located near DeKalb, Monmouth, and Urbana. Average
root-injury ratings (1-to-6 scale) in our untreated check plots (DeKalb
60-15 hybrid) were 4.06 (one node destroyed), 5.75 (2-3/4 nodes destroyed),
and 5.45 (2-1/2 nodes destroyed), respectively. Planting dates in 2003 for
DeKalb, Monmouth, and Urbana were May 28, May 16, and May 13, respectively.
In 2003, corn rootworm larvae began hatching by May 29 across much of
central Illinois. Despite the intensity of larval feeding, the DeKalb
YieldGard rootworm hybrid offered excellent root protection at each
experimental site: at DeKalb, the average root rating was 1.45; at Monmouth,
2.05; and at Urbana, 1.35.
In 2004, we again established corn rootworm product efficacy plots in our
three standard experiments in trap-crop settings. Our trap crop consists of
late-planted, mixed-hybrid corn interplanted with pumpkins. Planting dates
for DeKalb, Monmouth, and Urbana were April 28, April 27, and April 19,
respectively. These planting dates were considerably earlier in 2004 than
the corresponding planting dates in 2003: 1 month earlier in DeKalb, 19 days
earlier in Monmouth, and 24 days earlier in Urbana. Corn rootworm larval
hatch in central Illinois was estimated to be about 1 week to 10 days
earlier in 2004 than in 2003. In 2004, we planted a Golden Harvest (H-8588
RW) YieldGard Rootworm hybrid and its isoline (Golden Harvest H-8799). Root
injury (1-to-6 scale) in the untreated checks was severe at each location
and averaged 5.0 (2 nodes destroyed), 5.75 (2 3/4 nodes destroyed), and 5.8
(nearly 3 nodes destroyed) at DeKalb, Monmouth, and Urbana, respectively.
Root evaluations were conducted for each of these locations on July 21, July
15, and July 10, respectively. Average root ratings for the Golden Harvest
YieldGard Rootworm hybrid at DeKalb, Monmouth, and Urbana were 2.35, 1.80,
and 3.15 (pruning occurred, considerably less than 1 node), respectively.
The level of root injury to the YieldGard Rootworm hybrid in the Urbana
experiment was greater than anticipated. Plants from our trials near DeKalb,
Monmouth, and Urbana were checked (by Monsanto personnel) for expression of
the Cry3Bb1 protein, and the results were positive (i.e., the roots were
expressing the protein).
On July 13, a severe storm swept across central, north-central, and
east-central counties of Illinois. High winds and torrential rains
characterized this impressive storm. Within a few days, we began to receive
telephone and e-mail inquiries concerning severely lodged corn, including
concerns from some producers who had planted YieldGard Rootworm hybrids. We
visited some of these producers' fields. In some cases, root injury was
greater than expected. In others, the level of root pruning was minimal in
theYieldGard Rootworm fields, yet the fields were severely lodged and
harvestability was seriously compromised. Not long after these observations
from commercial cornfields became more common, Ron Estes (insect management
and insecticide evaluation program manager, Department of Crop Sciences,
University of Illinois) noted that the YieldGard Rootworm hybrid (Golden
Harvest H-8588RW) in our Urbana experiment was severely lodged. At that
point, we elected to take more root-injury and lodging evaluations from this
treatment at all three of our experiments.
During the first week of August, we extracted 40 roots (10 per replication)
from the YieldGard Rootworm plots in the Urbana experiment and rated them
for larval damage (1-to-6 root-rating scale). Slightly more than 3 weeks had
elapsed since our original ratings on July 10. Average root ratings per
replication were 3.6, 3.4, 3.6, and 3.8 (overall average = 3.6). The range
in root injury was 3 to 4 (60% of the roots were rated 4.0). None of the
roots was assigned a root rating of 1 or 2 during the root evaluations in
August. To quantify the level of pruning on these roots more precisely, we
also used the 0-to-3 rating scale for node injury developed at Iowa State
University. Root-injury ratings per replication were 1.43 (nearly 1-1/2
nodes destroyed), 1.08 (1 node destroyed), 1.64 (slightly more than 1-1/2
nodes destroyed), and 1.24 (slightly more than 1 node destroyed). The
node-injury rating for YieldGard Rootworm corn in the Urbana experiment was
1.35 (1-1/3 nodes destroyed). The level of injury we observed in August was
much greater than what we observed in July.
In addition to the root evaluations, we measured the level of lodging that
occurred in the YieldGard Rootworm corn. The percentage of plants lodged in
each of the replications (four-row plots) was 40%, 79%, 61%, and 39%.
Because of our experience with YieldGard Rootworm corn in our Urbana
experiment, we took additional root ratings (August 12) and lodging counts
in our DeKalb and Monmouth experiments. Average root ratings (1-to-6 scale)
in the DeKalb experiment (by replication) were 2.6, 2.3, 2.3, and 2.2
(overall average = 2.35). The range in root ratings in our DeKalb experiment
was 2 to 5. None of the YieldGard Rootworm corn plants was lodged in the
DeKalb study. Average root ratings (1-to-6 scale) in the Monmouth experiment
(by replication) were 2.5, 2, 2.7, and 2.2 (overall average = 2.35). The
range in root ratings was 2 to 4. The percentage of plants lodged in each
replication (four-row plots) was 29%, 3%, 9%, and 0%.
Many questions have arisen from producers regarding performance issues
relative to some YieldGard Rootworm hybrids and severe lodging that has
occurred in certain commercial cornfields. The observations from our
experiments will hopefully begin to help us find answers to many of these
perplexing questions. Most notably, why was the level of injury in our
Urbana experiment much greater than we had anticipated? The following set of
questions and answers is designed to help improve our understanding of this
Has resistance to YieldGard Rootworm hybrids already occurred?
We don't believe so. Keep in mind that YieldGard Rootworm hybrids were
released commercially for the first time in 2003. It does not seem probable
that rootworm resistance to the Bt protein has occurred this quickly. The
FIFRA Scientific Advisory Panel (August 2002) suggested that the likelihood
of resistance development within the first 3 years of commercialization is
unlikely, regardless of refuge size. In April 2003, the U.S. EPA issued a
document titled "Event Mon863 Bt Cry3Bb1 Corn Biopesticide Registration
Action Document". The authors clearly point out many of the assumptions
underlying model predictions for potential resistance development. If these
models have underestimated the actual initial resistance allele frequency,
then resistance development could proceed more rapidly. We simply don't have
the required data to answer this question.
What steps should be taken if unexpected damage or suspected resistance does
The FIFRA Scientific Advisory Panel (August 2002) specifically established
four steps a registrant should follow to confirm whether resistance may have
1. "Request the grower check planting records."
2. "Rule out damage from nontarget insects, weather, or other environmental
3. "Conduct tests to verify MON 863 was planted and that the correct
percentage of plants are expressing."
4. "If plants are MON 863 and damage approaching a 0.5 (node-injury scale)
is found on any expressing plant, evaluate roots from the corresponding
The following paragraph from the April 2003 EPA document (page IID 11) is of
particular interest with respect to resistance confirmation.
"Resistance should be confirmed by a standard diet bioassay or evaluation of
root node injury. An insect diet bioassay with the Cry3Bb1 protein that
results in a LC50 that exceeds the upper limit of the 95% confidence
interval of the LC50 established from baseline measurements of susceptible
populations could be used to confirm resistance. Alternatively, resistance
may be confirmed when one or more root nodes of at least 50% of Cry3Bb1
plants grown in the laboratory are destroyed. [Recall that 60% of the roots
evaluated from the YieldGard rootworm treatment in Urbana rated 4.0 (1 node
destroyed)]. A discriminating concentration bioassay may also be used to
confirm resistance; however, this method may take a long time to develop.
The August 2002 SAP also recommended investigating the potential of using
samples of populations surviving on Bt corn or an evaluation of larval root
tunneling to confirm resistance."
Resistance confirmation must be reported to the U.S. EPA within 30 days.
What other explanations may account for the loss of performance with the
YieldGard Rootworm hybrid in the Urbana experiment?
In 2003 (root rating in the untreated check = 5.45) and 2004 (root rating in
the untreated check = 5.8), we had severe corn rootworm larval injury in our
Urbana trials. So the explanation for the reduced efficacy of the YieldGard
Rootworm hybrid in 2004 cannot be blamed solely on severe corn rootworm
larval pressure. However, considerable differences in planting dates
occurred between the two growing seasons. In 2003, the Urbana experiment was
planted on May 13 (late by today's standards). In 2004, the Urbana study was
planted on April 19, more than 3 weeks earlier than the 2003 planting date.
We, as well as other researchers, have determined that development and
emergence of rootworms are delayed by the MON 863 event. Is root protection
compromised significantly when certain factors converge, such as (1) early
planting (early to mid-April), (2) delayed corn rootworm development
resulting from larvae feeding on MON 863 hybrids, and (3) intense larval
pressure (root ratings in the untreated check of two or more nodes of roots
destroyed)? Might these factors explain the compromised efficacy in our
Urbana experiment as well as in some producers' fields?
Other questions to ponder:
Does the expression of the Cry3Bb1 protein diminish as the season
progresses, compromising root protection in late July and early August?
Are there critical differences in expression of the Cry3Bb1 protein across
Are some hybrids with less than satisfactory root characteristics poor
candidates for the YieldGard Rootworm technology?
As indicated at the outset of this article, we still have much to learn
about transgenic hybrids and their role in an overall corn rootworm
management program. One thing seems certainthere continues to be no silver
bullet that can be directed at this impressive and resilient insect pest of
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