Cotton growers know the pest as the bollworm. Corn growers call it corn earworm. Tomato growers don it tomato fruitworm. By any name, the pest is Helicoverpa zea, and it’s the first pest to develop resistance to Bacillus thuringiensis.
“Bt” is a natural insecticide produced by the bacterium Bacillus thuringiensis that works by creating pores within an insect’s gut (see this link for more detail on how Bt kills insects).
In its biological form, Bt is sprayed on crops and can be used in organic agriculture. Bt interacts with special proteins carried only by certain insects. Because people do not have the same proteins, Bt is harmless to people. In fact, different strains of Bt are needed for each insect species; what kills one species will not kill another.
Bt was discovered in 1901 by Japanese biologist Shigetane Ishiwatari, who was searching for the cause of dead of large populations of silkworms. Later, in 1911, German scientist Ernst Berliner isolated the same bacteria, which was killing Mediterranean flour moths, in Thuringia, Germany, giving it the name, Bacillus thuringiensis. German growers began using Bt as a pesticide in 1920, in a product named Sporine. Only a few growers used Bt at the time, since synthetic pesticides were still very effective and cheaper.
US growers began using Bt as a pesticide in 1958, and by 1961, EPA registered Bt as a pesticide. Bt was still used primarily by organic growers, but scientists discovered several strains of Bt, each with a toxin unique to a specific species. In the 1980s, as pests began developing resistance to synthetic pesticides, and environmentalists found that the chemicals had negative environmental impacts, more growers began to use Bt. By the mid-1990s, scientists had found a way to insert the Bt gene into corn.
In the South, cotton growers waged a constant battle against the boll weevil and bollworm. Some growers began spraying with Bt as part of an integrated pest management system to rotate away from chemistries that were no longer working. When Bt cotton hit the market in 1996, cotton growers thought they had their secret weapon.
EPA set guidelines around planting Bt crops, in order to prevent future Bt resistance. Growers were required to plant non-Bt crops in “refuge” fields near the Bt fields, ensuring that insects beginning to develop resistant genes to Bt would mate with Bt-susceptible insects in the refuge fields. The concept worked as long as the resistant genes in an insect species were recessive.
For at least a decade, growers reaped several benefits from planting Bt crops. Yield increased because the toxin killed the pests that typically fed on the crops. Because Bt was so effective at killing major pests, growers sprayed fewer of the synthetic pesticides, benefiting both the environment and their profits. Biotechnology organizations that produced Bt crops advertised the crops as “environmentally-friendly.”
However, during this time, environmental groups began to voice concerns about the widespread use of Bt and its ultimate safety. These groups argued that the products were not as environmentally friendly as biotech companies promoted. A 1999 Cornell laboratory study showed that milkweed leaves dusted with pollen from Bt-corn increased mortality of the monarch butterfly. Because the study started a public outcry against the use of Bt crops, other scientists questioned the validity of the data, collected from laboratory specimens of monarch butterfly who were force-fed large amounts of Bt in closed containers. In 2004, USDA Agricultural Research Study scientists discovered that in the field, levels of the Bt toxin from corn pollen on milkweed leaves was too low to be fatal to monarchs – thereby officially debunking the Cornell study. One exception was the pollen of Bt 176-corn, which ARS scientists discovered expressed high enough amounts of the toxic protein to kill butterflies. As a result of the ARS study, the EPA did not re-register Bt 176-corn.
Controversy over Bt-engineered crops, as with other genetically-modified crops, has continued to rage. Advocates of Bt crops cite advantages of reduced synthetic pesticides and reductions in crop losses. A University of Arizona entomologist, Bruce Tabashnik, claimed in 2005 that resistance to Bt-cotton seemed to be decreasing, according to his studies. In 2007, Tabashnik joined a research team in Mexico working on a designer toxin that could kill Bt-resistant insects.
As the number of acres of both Bt-corn and Bt-cotton increased, and some growers began cutting corners on refuge acreage, several Extension entomologists predicted the inevitable: growers would soon see the day when resistance would creep into the Lepidoptera pest pool. Some growers relied exclusively on Bt-crops to manage pests, while others added insecticides to try to increase yields even more.
They were right. Bt-resistant populations of Helicoverpa zea, the cotton bollworm, were detected in over a dozen fields in Mississippi and Arkansas between 2003 and 2006. In a peer-reviewed article published in the Journal of Economic Entomology in 2009, Tabashnik stated that bollworm resistance to Bt was present in southern states and seemed to be increasing.
Unfortunately for growers of Bt crops, H. zea defied the characteristic trait needed to keep resistance from occurring: the resistance gene for H. zea is dominant, not recessive.
So far, H. zea seems to be the only insect species to have developed resistance to Bt-cotton. Cotton growers continue to rely on Bt crops, and critics continue posing questions about the wisdom of relying on one form of pest management. Somewhere in the middle is an integrated pest management answer to this debate.