Bt cotton gene: Differences prompt questions

Since the introduction of Bollgard and Bollgard II cotton varieties, growers have reported differences in control of bollworm. These differences may indicate growing resistance, but may only be the natural course of variation in nature that prevents plants from uniform expression of the insect killing protein toxin in these varieties.

North Carolina State University Entomologist J.R. Bradley says, “Variation is one thing that leads to uncertainty in agriculture. And, uncertainty leads to risk and farmers are constantly trying to avoid risk.”

Even before bacillus thurengiensis, or Bt-containing products came on the market in 1996 and 1997, Bradley’s research showed some significant differences in yield on Bt cotton that was over-sprayed with insecticides to control bollworm.

“In 1993, we increased yields by 15 percent at one location by intensively treating Bt cotton to insure that no escapes could damage the crop. A few miles (65 miles) away the exact same tests showed no significant difference in treated versus non-treated Bt cotton. Just by changing location, we saw a significant variation,” he explains.

The following year, Bradley says they repeated the treatments at the same locations and recorded a 19 percent improvement by over-spraying at one location and a 26 percent yield increase at the same site at which the previous year showed no difference.

At the same time, researchers in other parts of the country were showing different results — results that indicated Bt-containing cotton varieties did not need any additional insecticide to control budworm or bollworms. “If they were dealing with tobacco budworms, these early Bt-containing lines were absolutely good as a stand alone material, but in our part of the world, we were dealing with a much heavier concentration of bollworms and under heavy pressure these lines did not always provide adequate control,” Bradley notes.

This is a perfect example of how geographic differences can create variation that affects overall performance of Bt cotton varieties. “These differences emphasize how important it is to consider location when designing systems to manage insects,” the veteran North Carolina entomologist stresses.

In the late 1990s Bradley and other researchers at North Carolina State looked at a number of Delta & Pine Land varieties and one Stoneville variety, which constituted the major breeding lines containing the Bt gene, to determine whether variation exists among different varieties. “We saw little difference among varieties, except for the Stoneville 4740 variety. For some reason that variety did not express the Bt protein,” Bradley says.

“That was the first indication the Bt gene does not behave the same in all varieties. We didn’t see much increase in yield by over-spraying in most locations, except for the one Stoneville variety,” he adds.

Bollgard II varieties upped the ante by adding two new protein toxins. “Still in some locations, we saw no differences between conventional cotton and Bollguard and Bollgard II. At those locations, there was no advantage to having the Bt protein in the plant”, Bradley says.

Using Bt-containing varieties is not the same as mixing an insecticide from a can and spraying it on cotton. If you have a known insecticide at a known dose, and if applied appropriately and in a timely manner, a grower can reasonably expect a certain level of insect control. With Bt cotton you have living, breathing organisms that must produce a protein toxin to protect the plant. If that plant is under stress, it may or may not produce the necessary amount of protein toxin to kill insects.

A gene expresses through the synthesis of a protein or an enzyme (most proteins are enzymes), which is the means of functioning of a gene. Gene expression varies with the nucleotide sequence of the gene, its promotor, and the point of insertion of the gene in the DNA of the transgenic variety, the internal cell environment, as well as several external factors in the environment. All of which make predicting exactly how a Bt gene will work in a cotton plant difficult to predict.

To make the understanding of exactly how much protection from bollworms and budworms a grower can expect from Bt-containing varieties a little more difficult, Bollgard and Bollgard II work slightly different.

The original Bollgard varieties produced a higher amount of the insect killing toxin protein in cotton leaves than in cotton squares. The new Bollgard II varieties show a higher level of toxins in the squares than in the leaves.

In 2002, the North Carolina researcher says he and fellow North Carolina State Entomologist John Van Duyn got a new revelation about Bollgard cotton varieties. “In that year there was an extremely high number of bollworms at all our research locations. In the non-treated (neither with commercially available insecticides nor Bt-containing varieties) the total weight loss of some of these plots exceeded 85 percent. By far, the Bollgard II varieties fared the best, but there was still significant variation from one site to another,” Bradley says.

“In 2002, we learned it is very difficult to manage Bollgard cotton in the presence of unusually high populations of bollworm,” he says.

In 2006 Bradley says they saw some varietal differences in Bollgard II varieties that may or may not be trends. For example, he says Stoneville 6611, under heavy bollworm pressure had 10 percent boll damage, compared to 2.5 percent for other varieties. “By comparison, when we started testing Bollgard II varieties, in particular DPL 50, we did not see the varietal differences we are seeing now,” Bradley contends.

“From my perspective, I don’t think you can stick these genes in different varieties and expect the same level of control. The Bollgard II I saw for several years may not be the same Bollgard II growers are seeing in different varieties,” Bradley says.

The veteran North Carolina State researcher says it is difficult to get a real grip on the differences in how the Bt protein gene expresses itself in different varieties or even under different geographic conditions or timelines. It can probably be done easier in a lab, but unfortunately lab insects are not as vigorous as those out in a farmer’s fields, so even that data may not be absolute.

There is no way to make an accurate guess as to when protein levels crash in Bt-containing plants. Thus, there is no way to know precisely the ability of a plant to express the amount of protein toxins needed to control bollworms, according to Bradley. If the plant’s ability to express the Bt protein is low when bollworm counts are peaking, there may not be enough of the toxin to control such high levels of insects, he explains.

Variation is often confused with resistance, but Bradley contends true resistance is much more likely to come from tobacco budworms than from bollworms. Until the introduction of synthetic insecticides in the late 1940s, tobacco budworm was not a recognized pest of cotton. Only when synthetic insecticides enabled it by eliminating natural predators did budworm become a treatable pest of cotton.

By comparison, bollworm survivors captured from Bollgard-treated corn and cotton fields and raised in the lab could not survive in the natural population in a field.

“We don’t think resistant bollworm can sustain populations, but that is definitely the case with tobacco budworm,” Bradley stresses.

“I don’t think anyone outside the companies that are developing these multi-trait or multiple toxins knows. We don’t know which varieties will have which traits, much less how these traits will react under different conditions and locations,” he concludes.

“It will take the science that we can contribute, plus the art that farmers contribute to effectively manage the boll worm/budworm complex on cotton,” Bradley says.

e-mail: [email protected]

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