First documented case of pest resistance to biotech cotton

A pest insect known as bollworm is the first to evolve resistance in the field to plants modified to produce an insecticide called Bt, according to a new research report. Bt-resistant populations of bollworm, Helicoverpa zea, were found in more than a dozen crop fields in Mississippi and Arkansas between 2003 and 2006.

"What we're seeing is evolution in action," said lead researcher Bruce Tabashnik. "This is the first documented case of field-evolved resistance to a Bt crop.”

Bt crops are so named because they have been genetically altered to produce Bt toxins, which kill some insects. The toxins are produced in nature by the widespread bacterium Bacillus thuringiensis, hence the abbreviation Bt.

The bollworm resistance to Bt cotton was discovered when a team of University of Arizona entomologists analyzed published data from monitoring studies of six major caterpillar pests of Bt crops in Australia, China, Spain and the U.S. The data documenting bollworm resistance were first collected seven years after Bt cotton was introduced in 1996.

"Resistance is a decrease in pest susceptibility that can be measured over human experience," said Tabashnik, professor and head of UA's entomology department and an expert in insect resistance to insecticides. "When you use an insecticide to control a pest, some populations eventually evolves resistance."

The researchers write in their report that Bt cotton and Bt corn have been grown on more than 162 million hectares (400 million acres) worldwide since 1996, “generating one of the largest selections for insect resistance ever known."

Even so, the researchers found that most caterpillar pests of cotton and corn remained susceptible to Bt crops.

"The resistance occurred in one particular pest in one part of the U.S.," Tabashnik said. "The other major pests attacking Bt crops have not evolved resistance. And even most bollworm populations have not evolved resistance."

The field outcomes refute some experts' worst-case scenarios that predicted pests would become resistant to Bt crops in as few as three years, he said.

“The only other case of field-evolved resistance to Bt toxins involves resistance to Bt sprays," Tabashnik said. He added that such sprays have been used for decades, but now represent a small proportion of the Bt used against crop pests.

The bollworm is a major cotton pest in the southeastern U.S. and Texas, but not in Arizona. The major caterpillar pest of cotton in Arizona is a different species known as pink bollworm, Pectinophora gossypiella, which has remained susceptible to the Bt toxin in biotech cotton.

Tabashnik and his colleagues' article, "Insect resistance to Bt crops: evidence versus theory," will be published in the February issue of Nature Biotechnology. His co-authors are Aaron J. Gassmann, a former UA postdoctoral fellow now an assistant professor at Iowa State University; David W. Crowder, a UA doctoral student; and Yves Carrière, a UA professor of entomology. Tabashnik and Carrière are members of UA's BIO5 Institute.

"Our research shows that in Arizona, Bt cotton reduces use of broad-spectrum insecticides and increases yield," said Carrière. Such insecticides kill both pest insects and beneficial insects.

To delay resistance, non-Bt crops are planted near Bt crops to provide "refuges" for susceptible pests. Because resistant insects are rare, the only mates they are likely to encounter would be susceptible insects from the refuges. The hybrid offspring of such a mating generally would be susceptible to the toxin. In most pests, offspring are resistant to Bt toxins only if both parents are resistant.

In bollworm, however, hybrid offspring produced by matings between susceptible and resistant moths are resistant. Such a dominant inheritance of resistance was predicted to make resistance evolve faster.

The UA researchers found that bollworm resistance evolved fastest in the states with the lowest abundance of refuges.

The field outcomes documented by the global monitoring data fit the predictions of the theory underlying the refuge strategy, Tabashnik said.

Although first-generation biotech cotton contained only one Bt toxin called Cry1Ac, a new variety contains both Cry1Ac and a second Bt toxin, Cry2Ab. The combination overcomes pests that are resistant to just one toxin.

The next steps, Tabashnik said, include conducting research to understand inheritance of resistance to Cry2Ab and developing designer toxins to kill pests resistant to Cry1Ac.

Source: University of Arizona

Related stories:

Mars teams up with IBM to map cocoa genome
US chocolate and sweets giant Mars is teaming up with IBM to study and map the cocoa genome in a move which could enhance the production of stronger plants, the companies said Thursday.
Ancient Mexican maize varieties
Maize was first domesticated in the highlands of Mexico about 10,000 years ago and is now one of the most important crop plants in the world. It is a member of the grass family, which also hosts the world's other major crops including rice, wheat, barley, sorghum, and sugar cane. As early agriculturalists selected plants with desirable traits, they were also selecting genes important for transforming a wild grass into a food plant. Since that time, Mexican farmers have created thousands of varieties suitable for cultivation in the numerous environments in the Mexican landscape—from dry, temperate highlands to moist, tropical lowlands. Because of its importance as food, the need to improve yield, and the challenges presented by changing climate, the maize genome of the B73 cultivar is being sequenced. However, because maize has a complex genome and many varieties, the genome sequence from just one variety will not be adequate to represent the diversity of maize worldwide. Mexican scientists are also sequencing and analyzing the genomes of the ancient landraces to recapture the full genetic diversity of this complex and adaptable crop.
Research required urgently to control planthopper pests
June 23-25 conference to address major threat to Asian rice production
A small insect that has devastated millions of hectares of rice in southern China and Vietnam over the past few years—causing the loss of thousands of tons of the grain at a crucial time for global production—is the focus of a critical and timely conference this week in the Philippines.
New study shows that transgenic plants don't hurt beneficial bugs
Genetically modified (GM) plants that use Bt (Bacillus thuringiensis), a common soil bacterium, to kill pests won't harm the pests' natural enemies, according to new research by Cornell entomologists.
Oregano oil works as well as synthetic insecticides to tackle common beetle pest
New research in the Society of Chemical Industry’s Journal of the Science of Food and Agriculture shows that oregano oil works as well as synthetic insecticides to combat infestation by a common beetle, Rhizoppertha dominica, found in stored cereals.
Fungus fight: Researchers battle against dangerous corn toxin
The spiraling use of corn for food and fuel is creating heightened concerns about contamination of this staple crop with deadly aflatoxin. Produced by certain fungi that grow on corn, this contaminant is a known human carcinogen that especially threatens food safety in the developing world and can potentially cause the loss of hundreds of millions of dollars in the United States each year.
Insecticide combo delivers knockout punch
A cocktail of insecticides containing a plant protein and a common insecticide may be more lethal to crop pests than either ingredient used alone, according to biologists. The one-two punch also inhibits the insects' growth rate and reduces their chance of developing resistance.
Purging the plantain pests in Africa
A tiny pest threatening the staple diet of millions in Africa could soon be eradicated in a project announced today, bringing together plant experts from Leeds and Uganda.

News discussion:

General Science news