Jarrod T. Hardke, B. Rogers Leonard and Karla D. Emfinger

The fall armyworm is a common insect pest of field corn in Louisiana and other states across the South. Conventional chemical control strategies used to manage the fall armyworm in corn generally provide inconsistent results because the larvae feed within the whorls of vegetative-stage plants or on immature grain in corn ears. Therefore, proper timing of insecticide applications is difficult and must target eggs or larvae as they hatch, but before they move into the plant and become protected from the sprays.

Recent developments in insect biotechnology have become available for managing the fall armyworm and other caterpillar pests attacking corn. Insecticidal proteins from Bacillus thuringiensis (Bt) are being expressed in transgenic corn plants. Both crystal (Cry) and vegetative (VIP) proteins produced from Bt are toxic to selected caterpillar pests when ingested by larvae feeding on corn plant tissue. The first commercial Bt corn (YieldGard) expressed a Cry1Ab protein with corn stalk borers as the primary targets. Since the introduction of the Yield- Gard technology, corn plants have been transformed to express other Cry proteins with a wider spectrum of control, including the fall armyworm.

Growers now have the option of planting corn with YieldGard technology or the new Herculex technology that expresses a Cry1F protein. Cry1F has been shown in laboratory tests to be inherently more toxic than Cry1Ab to fall armyworm. In the near future, the agrichemical and seed industries will commercialize transgenic Bt corn technologies with multiple combinations of novel insecti cidal proteins, which will provide nearly complete caterpillar pest control.

In laboratory and field studies during 2007-2008, LSU AgCenter entomologists evaluated YieldGard and Herculex technologies for their effects on fall armyworm survival and corn damage. All studies were conducted at the Macon Ridge Research Station near Winnsboro, La. The performances of YieldGard and Herculex technologies were compared with a conventional non-Bt corn line. In no-choice laboratory tests, fall armyworm caterpillars were offered freshly harvested leaves from field plots of vegetative- stage corn. Fall armyworm survival was monitored daily. Survivors were offered fresh tissue as needed (every two to three days) until all larvae were classified as either dead or having successfully pupated. Larval consumption of leaf tissue was evaluated in separate tests by offering fresh leaf tissue of a known size to the insects. After a period of four days, leaf consumption was measured. In field trials, corn plants were artificially infested with fall armyworm caterpillars during appropriate growth stages. After 14 days, larval damage to leaf tissue was evaluated using a visual rating scale (1-9). A rating of 1 was defined as no visible leaf injury, and a rating of 9 was defined as most leaves exhibiting long (greater than 1 inch) lesions.

In the laboratory tests, no differences in fall armyworm larval survival were observed between those placed on Yield- Gard (71 percent) or conventional non-Bt tissue (78 percent) (Figure 1). However, leaf consumption was significantly lower for YieldGard plants compared with the non-Bt line. In field trials, leaf tissue injury ratings were similar between YieldGard (4.28) and conventional non- Bt (5.37) corn plants. Significantly lower survival of fall armyworm larvae was observed on Herculex corn tissue (34 percent) compared with that on conventional non-Bt tissue (78 percent) (Figure 2). Larvae also consumed significantly less Herculex corn tissue compared with the conventional non-Bt tissue. In another series of field trials, significantly lower leaf injury ratings were observed on Herculex corn plants (1.59) than conventional non-Bt (5.37) corn plants.

Results from these studies indicate that YieldGard demonstrates limited efficacy against fall armyworm. The Herculex technology demonstrated significant insecticidal activity against fall armyworm, which resulted in less foliage injury. These studies support previous laboratory trials with purified proteins that show a Cry1F protein is more toxic to fall armyworm than Cry1Ab in corn. Results from these studies should aid Louisiana corn producers in choosing the best corn technologies to utilize given their particular set of pest problems.

Jarrod T. Hardke, Graduate Assistant, Department of Entomology, LSU AgCenter; B. Rogers Leonard, Jack Hamilton Regents Chair in Cotton Production, Macon Ridge Research Station, Winnsboro, La., and Department of Entomology, LSU AgCenter, Baton Rouge, La.; Karla D. Emfinger, Research Associate, Macon Ridge Research Station, Winnsboro, La.

(This article was published in the summer 2009 issue of Louisiana Agriculture.)