David S. Wangila, B. Rogers Leonard and Fangneng Huang

Field corn expressing single or multiple Bacillus thuringiensis (Bt) traits has been planted in the Midsouth to manage a complex of corn caterpillar pests – including armyworms, earworms and stalk borers – since 1999. Bt corn, a transgenic crop, gets its name because it contains genes from a bacterium, Bacillus thuringiensis (Bt), which is a naturally occurring insecticide.

In Louisiana, the most important corn stalk borer causing economic loss to non-Bt corn is the sugarcane borer. Larvae of the first generation corn borers usually attack leaf whorls, causing dead hearts in vegetative stage of plants. In the second generation, they often bore into the corn stalk and develop tunnels within it. Larvae also feed on corn kernels or tunnel into ear cobs.

The development of Bt resistance in target pests is a major threat to the sustainability of Bt corn technologies. Before 2010, corn hybrids (i.e. YieldGard, Herculex1) planted in the Midsouth produced only a single Bt protein targeting caterpillar pests. A regional Bt-resistance-monitoring program coordinated by AgCenter scientists showed that the frequency of resistance to the Bt protein in YieldGard corn increased significantly in Louisiana populations of sugarcane borer in 2009.

To delay Bt resistance, a gene-pyramiding strategy has been employed using transgenic plants that express multiple Bt proteins. The first two commercialized pyramided Bt corn products in the United States are Genuity VT Triple Pro and SmartStax. They were first commercially planted during the 2010 crop season.

LSU AgCenter researchers recently evaluated the efficacy of pyramided Bt corn against a YieldGard-resistant sugarcane borer population. Two greenhouse trials were conducted during 2010-11 to determine larval survivorship and plant injury of YieldGard-susceptible, YieldGard-resistant and heterozygous populations of sugarcane borers on two non-Bt corn and three Bt corn hybrids.

Heterozygous populations have inherited different forms of a particular gene from each parent while homozygous populations have inherited identical forms of a particular gene from each parent. The heterozygous sugarcane borers were developed by crossing Bt-susceptible and Bt-resistant insects.

Three Bt corn hybrids representing different transgenic Bt corn technologies (YieldGard Corn Borer, Genuity VT Triple Pro and Genuity SmartStax) and a non-Bt control corn were included in these trials. YieldGard expresses a single Bt protein; Genuity VT Triple PRO produces two Bt proteins; and Genuity SmartStax corn expresses three Bt proteins for controlling caterpillar pests. Newly hatched larvae of the three corn borer populations were manually placed on plants during reproductive plant stages in the greenhouse. Larval survivorship and tunneling by borers inside each stalk were recorded after 21 days.

Performance of the corn hybrids against sugarcane borer infestation was consistent between the two trials.

On non-Bt corn plants, 45.8-57.5 percent of larvae survived after 21 days (Figure 1). Larval survivorship rates on YieldGard plants were 5.2 percent for the susceptible population, 32 percent for the heterozygous population and 41.1 percent for the resistant population. The Bt-resistant population was highly resistant to YieldGard. Both pyramided Bt corn hybrids were very effective against the sugarcane borer regardless of Bt resistance levels, with a 21-day mortality of greater than 98 percent for the susceptible and heterozygous populations and greater than 96 percent for the YieldGard-resistant population.

Stalk tunnel length by the three populations on the four corn hybrids was highly correlated to larval survivorship. Heterozygous and resistant populations produced significant tunneling inside the plant stalks of non-Bt and YieldGard corn plants, but little injury was observed on the two pyramided Bt corn hybrids (Figure 2).

Pyramided Bt corn lines expressing Genuity VT Triple Pro or SmartStax traits were effective against the YieldGardresistant sugarcane borers and should offer one tool for Bt resistance management of sugarcane borers. Since 2010, areas planted with single Bt corn such as YieldGard or Herculex in the Midsouth have reduced considerably because of the availability of pyramided Bt corn products. The timely replacement of single Bt technologies with pyramided Bt corn products should delay further increase in Bt resistance frequency in sugarcane borer and ensure the continued success of Bt corn in the state.

David S. Wangila is a former graduate research assistant; B. Rogers Leonard is a professor; and Fangneng Huang is an associate professor in the Department of Entomology. Leonard is also associate vice chancellor in the LSU AgCenter.

(This article was published in the winter 2013 issue of Louisiana Agriculture magazine.)