EDITOR'S NOTE — The following article was written by R.R. Youngman and C.A. Laub who are with the Department of Entomology at Virginia Tech University.
Many eastern Virginia corn farmers saw their fields infested with unusually high numbers of European corn borers in spring 2004, and are understandably concerned about what to expect in 2005.
This article speculates on why European corn borer numbers were so high in 2004, and addresses some issues regarding the use of Bt corn to manage this pest.
Much of this article reflects years of farmer comments and observations from field trials we have conducted, as well as from those of other in-state specialists and agents, and from my counterparts in other corn growing states. We have restricted our comments in this article to grain corn production in eastern Virginia and to Bt corn hybrids labeled for European corn borer.
Although this may be viewed as somewhat unorthodox, we have decided to give you our take home message now: When deciding on Bt corn hybrids for managing European corn borer in 2005, your decision should first be based on selecting hybrids that exhibit those traits which will give you the best agronomic performance for your farm, and secondly on the Bt trait.
Just as marked differences in performance exist among conventional corn hybrids, the same is true for Bt hybrids. It is important to realize that the vast majority of corn fields that experience little or no European corn borer infestation in 2005 will not benefit from the Bt trait in regard to increased yields.
And apart from rare exceptions (such as a long-term history of economic infestation by European corn borer in a particular field), it is not possible to predict what the infestation severity of this pest will be before planting.
Make no mistake: The transgenic Bt technology that protects corn from European corn borer damage arguably represents one of the most significant contributions to corn insect pest management in over half a century. Under moderate to heavy corn borer infestations, Bt hybrids more than pay for themselves from superior yield performance and, in many cases, lower levels of mycotoxins in the grain when compared to non-Bt hybrids.
However, when European corn borer is not part of the equation, the Bt technology benefits all but disappear when comparisons are made to un-infested non-Bt corn hybrids. Why?
Because the Bt protein toxin itself is not capable of expressing yield enhancing or fungicidal properties. Should you read or hear claims of increased grain yields in Bt corn hybrids when compared to un-infested non-Bt hybrids, please realize that the reason for the yield benefit in these instances is due to factors other than the expression of the Bt toxin in the plant.
From 1997-99, our lab conducted replicated, large-plot field trials in two locations in eastern Virginia in which we compared machine harvested grain yields among several commercially-available Bt hybrids and the non-Bt standard Pioneer 3394.
European corn borer pressure was very low in both locations during the three years of this study. In five of the six trials, the non-Bt standard produced yields as good as or better than most of the Bt hybrids evaluated. In the one location (Holland, Va; 1997 trial) where the non-Bt standard produced the lowest yield, no significant differences were detected due to a severe drought that caused high variability and low yields across all treatments.
For complete details on these trials please contact your local extension office for a copy of the Virginia Corn Hybrid and Management Trials, or visit the following Internet addresses: http://www.ext.vt.edu/pubs/grains/424-031/424-031.html#sect2, http://www.ext.vt.edu/pubs/grains/424-031-98/424-031-98.html#sect4, or http://www.ext.vt.edu/pubs/grains/424-031-99/424-031-99.html#sect4
In addition, some of the conclusions from the 1997 North Central Regional Extension Publication, NCR 602, Bt Corn & European Corn Borer: Long-Term Success Through Resistance Management, bear repeating:
“Bt corn provides “insurance” against European corn borer infestation; it only protects the yield potential of the hybrid. Select hybrids carefully.”
“No predictive tools exist to pin-point where or when Bt corn will be advantageous; Bt corn is essentially insurance.”
“Refuge areas of non-Bt corn on each farm form an essential component of resistance management. Associated logistical issues and unprotected losses represent a cost of growing Bt corn.”
“Bt corn provides nearly season-long protection from European corn borer.”
“Late-season effectiveness of Bt corn against European and southwestern corn borer, and control of other caterpillars (cutworms, stalk borer, armyworms, corn earworm) varies among events. The spectrum of activity is limited.”
What about other (non-Bt) methods for managing European corn borer?
According to the 1999 Handbook of Corn Insects (published by the Entomological Society of America), cultural control tactics such as crop rotation, crop isolation, and destruction of crop residue do not provide effective management of European corn borer. This is due to the high mobility of European corn borer moths over wide areas, and the large number of plant species (over 200) that support development of this insect throughout the spring and summer months.
The time of planting, however, can play a role in population development. It is generally accepted that the earliest planted corn and the latest planted corn within a region are at greatest risk to economic corn borer infestation. However, in eastern Virginia, recent field studies along with years of farmers' observations concur that only late-planted corn is routinely at high risk to European corn borer. Consequently, farmers are strongly encouraged to plant appropriate Bt hybrids in all late-planted fields.
For early-planted (late March — early April) cornfields in eastern Virginia the corn borer management strategy is not as clear. This conclusion is based on years of pre-2004 observations by agents and farmers who have reported few problems from first-generation corn borers.
There are two plausible explanations for the historical lack of corn borer pressure in whorl-stage corn in eastern Virginia. The first is that the typical population size of over-wintering corn borers in most years is too low to economically impact whorl-stage corn.
The second is that there probably is a lack of synchrony between the stage of early corn plant development that is most attractive to egg-laying moths and the time when egg-laying activity is at its peak.
So what happened in 2004?
We suspect that the problems in 2004 resulted from a combination of much higher than normal numbers of over-wintering European corn borers coinciding with corn plants at a stage most susceptible to the egg-laying moths.
Recall that the 2003 growing season was one of the wettest on record, and corn planting dates were considerably delayed throughout much of eastern Virginia. Much of this late-planted acreage was in non-Bt corn hybrids, which means it is likely that much higher than usual populations of second generation corn borers developed and subsequently over-wintered from 2003-04. This in turn led to unusually high populations of emerged moths in spring 2004.
In addition, the unseasonably warm May of 2004 (credit Rex Cotten, City of Suffolk Extension agent for this observation) caused corn to develop much earlier than in previous years, which may have helped synchronize susceptible plant growth with peak egg laying in May 2004.
Unfortunately, what this all means for the 2005 growing season in eastern Virginia is anyone's guess.
What we do know is that for those of you who decide to plant Bt corn this year, you have the ideal opportunity to make your own on-farm comparisons between your Bt corn and non-Bt corn (refuge) hybrids. And be sure to let your local Extension agent and seed dealer know what you find.