Corn Borers and Transgenic Bt Corn Technology

Fangneng Huang, Leonard, Billy R., Baldwin, Jack L.  |  11/1/2006 4:14:26 AM

Fangneng Huang, B. Rogers Leonard and Jack Baldwin

Field corn represents substantial acreage and contributes significant crop value to Louisiana agriculture. In 2005, more than 330,000 acres of corn were harvested in Louisiana for a total farm value of $100 million. A growing threat to corn production in the state is a complex of corn stalk boring insect pests.

Three species of corn stalk borers – European born borer (ECB), southwestern corn borer (SWCB) and sugarcane borer (SCB) – infest Louisiana field corn. ECB is an uncommon pest that usually occurs at low numbers in the northeastern parishes. The SWCB and SCB are more important and have the potential to be major pest problems every year. Heavy infestations of these pests can cause significant yield loss.

These corn borers can have three to four generations each on corn in Louisiana. The late generations also infest grain sorghum and rice. Several management tactics are recommended for controlling corn borers:
  • Post-harvest crop destruction to reduce overwintering populations.
  • Early planting to escape late-season damage.
  • Foliar insecticides to control young larvae.
  • Soil insecticides to suppress whorl stage infestations.
  • Use of transgenic Bt corn hybrids.
Bt Corn  
Bt corn gets its name from the bacterium, Bacillus thuringiensis (Bt) – a naturally-occurring microorganism commonly found in soils. The bacterium produces crystalline (Cry) insecticidal proteins. Upon ingestion by insects, Cry proteins are activated in insect midguts and become toxic only to target pests. Bt is an environmentally friendly control measure and is practically non-toxic to vertebrate organisms including birds, fish, and humans.

Various commercial formulations of Bt insecticides have been used as foliar insecticides on a wide range of crops for many years. Advances in biotechnology have allowed scientists to transfer Bt genes into crop plants. Transgenic Bt crops produce insecticidal Bt proteins within plant tissues. Bt crops usually provide full season protection against target pests. Commercial Bt corn cultivars have been available in the United States since 1996 and used as a primary tool for managing corn borer problems in Louisiana since 1999. Most Bt corn hybrids produce only a single Cry protein, but newer transgenic lines express two or more Cry proteins.

Recently, Herculex corn, which expresses the Bt protein Cry1Fa2, has become available in Louisiana. Herculex corn varieties are effective against all corn borers and offer good control of other lepidopterous pests such as the fall armyworm, corn earworm and black cutworm (Table 1). Effectiveness and low environmental risk have encouraged corn growers to rapidly adopt the Bt corn technology in the United States. Bt corn currently accounts for about 38 percent of the total corn acreage nationwide and more than 40 percent in Louisiana. The widespread acceptance of Bt corn, however, has the potential to accelerate development of resistance, thus threatening its longterm sustainability as an effective corn borer management tool.

Bt Resistance
In corn borer populations, most individuals are susceptible to the Bt Cry proteins and do not carry resistant genes. Some insect possess one copy of the resistant genes (heterozygote), and a few have two copies of resistance genes (resistant homozygote). On Bt corn plants, corn borers with Bt resistant genes may survive and produce offspring, thus giving them a selective advantage over the Bt-susceptible borers. As more Bt corn is planted, the Bt-resistant borer population will increase with each succeeding generation and could eventually reach infestation levels on Bt corn. Therefore, management of insect resistance has been an important issue in the proper use of Bt corn technology.

In 2004, a major Bt resistance gene was documented in a northeast Louisiana sugarcane borer population. The sugarcane borer, a key pest of sugarcane, has been expanding its geographic range and has become a dominant corn borer species in central Louisiana and in northeast Louisiana where sugarcane is not grown. This sugarcane borer strain can complete larval development on commercial Bt corn hybrids. The resistance detected in the sugarcane borer is the first major resistance to commercial Bt corn in any corn borer species. In addition, the sugarcane borer is less susceptible to Bt Cry proteins compared to other corn borer species.

High Dose/Refuge Resistance Management Strategy
To delay Bt resistance development, a “high dose/refuge” resistance management strategy is currently implemented in the United States. This strategy involves planting a portion of the corn acreage in an area to high dose Bt corn that can kill all susceptible and heterozygous resistant insects. The remaining area is planted to non-Bt hybrids that serve as a refuge for Bt-susceptible borers. The relatively large populations of Bt-susceptible moths from refuge areas should mate with rare surviving resistant homozygotes. Thus, most of their offspring will be heterozygous for the resistance. Since heterozygotes should be killed by high dose Bt corn, resistance frequencies in the corn borer populations should be maintained at low levels for several decades or longer.

As required for the high dose/refuge strategy, corn producers must plant a minimum refuge of at least 20 percent non-Bt corn outside cotton-production regions but are required to plant at least 50 percent non-Bt corn across the cotton production areas. Cotton is a major Louisiana crop, and Bt corn growers in the state must plant at least 50 percent of their acreage to non-Bt corn cultivars. In addition, all Bt corn plants must express a regulatory-defined high dose of Bt.

LSU AgCenter researchers are validating the high dose/ refuge strategy for managing Bt resistance in the SCB. Greenhouse evaluation suggests that commercial Bt corn hybrids in Louisiana appear to meet the high dose requirement against SCB. Although, the Bt resistance frequency in SCB is greater than in other corn borer species, it appears to be low enough to meet the requirements of the current resistance management strategy. These data have provided initial evidence that the high dose/refuge strategy may be effective for SCB. In addition, an effective technique for monitoring changes in Bt resistance frequency for SWCB and SCB has been developed at the AgCenter. With such a Bt susceptibility monitoring program, LSU AgCenter scientists will be able to recommend appropriate management actions before field corn borer populations become resistant.

Acknowledgment: Louisiana Soybean and Feed Grain Promotion Board and the National Science Foundation Center for IPM

Fangneng Huang, Assistant Professor, Department of Entomology; B. Rogers Leonard, Professor, Northeast Research Station, Winnsboro, La.; and Jack Baldwin, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.

(This article was published in the fall 2006 issue of Louisiana Agriculture.)
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