Assistant Professor of Maize Breeding and Genetics
Department of Crop Sciences
(217) 244-2536; firstname.lastname@example.org
|Figure 1. European corn borer larvae in a corn stalk
(left photo) and a larval entry
whole with secondary fungal disease (right photo). Source: T. Magg.
The European corn borer is a devastating pest of corn. Larvae of this insect species cause damage to the corn plant mainly by feeding in the stalk and ear shank (Figure 1). Yield losses are largely attributable to a reduction in kernel number and weight, owing mainly to physiological disruption of plant growth and only to a minor extent to broken stalks, dropped ears, and larval feeding on the grain or lodging. In field experiments, total grain yield reduction due to European corn borer infestation ranged from 5 bushels to more than 50 bushels per acre. The average annual yield loss in production fields is estimated to vary between 5.0 to 7.5 percent. In addition, cavities of the European corn borer larvae increase the occurrence of secondary infections from stalk- and ear-rotting pathogens such as Fusarium species. These reduce yield and deteriorate the quality of grain and forage maize through contamination by mycotoxins. European corn borer damage especially favors Fusarium species that produce Fumonisin, a toxin known to induce serious health disorders in animals and humans (Figure 1).
The use of corn hybrids carrying the Bt gene is the standard procedure to control the European corn borer in the U.S. The Bt gene enables the corn plant to produce a toxin with adverse effects on the development of European corn borer larvae. Intensive studies showed that the Bt toxin is highly specific for the European corn borer and has negligible effects on other lepidopteran species, like the monarch butterfly, or other insect species. Insecticides are used as an alternative tool to manage European corn borer. However, the time window for a successful application is small. Larvae seek protection by boring into the plant whorl or stem and then are inaccessible for insecticides. With the introduction of Bt hybrids, the use of insecticides effective against the European corn borer declined by 2.6 million pounds per year, resulting in less pollution of the environment with pesticides. This is surely a positive side effect of the use of transgenic Bt corn hybrid. However, effective resistance management is essential to prevent the rapid development of ECB individuals resistant to the Bt toxin. In addition, ECB populations are partly controlled by natural antagonists and diseases. Therefore, a direct control using Bt hybrids is not economically sensible in every year.
Furthermore, conventional maize breeding can reduce yield losses by improving natural host plant resistance against ECB. The natural host plant resistance of corn against the European corn borer depends on three mechanisms: nonpreference, tolerance, and antibiosis. Nonpreference is a lack of attractiveness of the host plant as an oviposition site or shelter for the insect. Tolerance is the ability of a maize plant to withstand a certain population density of the insect without economic loss of yield or quality. Antibiosis increases the mortality and hampers the growth and feeding of larvae on the host plant. Antibiosis to first generation European corn borer can be traced back to high concentrations of DIMBOA, a plant compound toxic to corn borer species. However, as the plant grows older, the DIMBOA concentration drops drastically to a level with practically no effect on European corn borer larvae. Antibiosis to European corn borer in corn plants at an advanced growth stage seems to depend on higher levels of tissue toughness caused by improved cell wall fortification as a result of increased amounts of detergent fiber, cellulose, lignin, and biogenic silica. Plants with highly fortified cell walls prevent larval penetration and are of less nutritional value to the insect.
The success of a conventional breeding program for improving ECB resistance depends on the availability of resistant germplasm sources and detailed knowledge about the inheritance of the resistance traits. Hybrids with improved resistance can be produced only if resistant corn lines are available. Therefore, the genetic diversity of the breeding material is of key importance. If the genetic diversity in maize germplasm adapted to the U.S. Corn Belt is limited, it is necessary to screen exotic germplasm (e.g., from the tropics and subtropics) for new sources of resistance. The breeding strategy used depends on the number of genes that are involved in the inheritance of European corn borer resistance and their genetic effects.
In the last 10 years, several genetic studies were conducted to identify regions on maize chromosomes containing putative genes involved in the resistance against the European corn borer (Figure 2). These chromosomal regions carry genes involved in cell wall fortification. An association study will be performed to validate the hypothesis that these “cell wall” genes are related to European corn borer resistance. In human genetics, where the use of controlled crosses and populations is not possible, association mapping approaches were developed to correlate diversity at candidate gene loci across populations with phenotypic variation. This approach was efficiently used to identify molecular markers associated with diseases like Alzheimer. The advantages of the association tests are their speed and their high resolution. However, although it is attractive to use this technique in corn, caution is warranted. New statistical methods are in development to account for the complex breeding history of corn that may otherwise increase the probability of identifying false associations between genes and the trait expression.
Why make these efforts toward improving host plant resistance in maize, if Bt-maize hybrids with an extremely high level of ECB resistance are already available? Monogenic resistances are likely to be overcome quickly by the target pest. In the future, it may make sense to place the Bt gene in a genetic background that provides an improved level of quantitative resistance against ECB larvae feeding. This combination might help to conserve the effectiveness of the Bt genes. In addition, not all farmers have access to Bt hybrids. National regulations might prevent the use of genetically modified organisms, or farmers might produce maize products for markets that do not accept genetically modified crops. Therefore, non-transgenic host plant resistances will continue to play an important role in securing maize production in the future, and the design of new and improved breeding strategies are of great importance.
|Figure 2. Putative chromosomal cluster containing quantitative
trait loci (blue ovals) involved in corn borer resistance of
temperate and tropical maize with genes involved in cell wall fortification.