Modern insect-protected transgenic corn combines several toxins, all of which derive from the bacterium Bacillus thuringiensis (Bt). This is why we use the term Bt corn. However, newer technology that does not rely on Bt is on the horizon.
Genes from the Bacillus thuringiensis bacterium have been placed into the corn. The genes code for the production of proteins from the Bt bacteria that are toxic to insects. When a susceptible insect eats these proteins, they break down into smaller subunits, some of which bind on the wall of the insect’s gut. This binding eventually causes a small hole to form in the gut wall, and the bacteria in the gut then move in to the insect’s body. Death is caused not by the Bt protein crystals, but by bacterial septicemia from the natural bacteria in the insect gut.
Susceptibility to Bt toxins ranges widely among broad insect groups (such as beetles, caterpillars, and flies) and among even closely related insects within a small group (caterpillars). The acidity of the insect gut is key—the wrong acidity prevents the crystal subunits from forming. The insects must also have the right type of receptors on the gut wall for the protein subunits to bind, and many insect species lack the appropriate receptors for the Bt toxins.
Table 2 lists the Bt toxins that are registered against corn insects.
This varying activity by Bt proteins on different, even closely related, insect species explains why some Bt toxins work better on some pests than on others. For example, the toxin Cry1Ab affects fall armyworm relatively less than does the toxin Cry1F. However, some stalk borers—the southwestern and European corn borers—are extremely susceptible to both toxins, so much so that farmers have reduced their populations just by planting Bt corn.
|Lepidoptera (caterpillars)||Cry1Ab, Cry1F, Vip3a, Cry1A .105, Cry2Ab2|
|Corn rootworm||mCry3a, eCry3 .1Ab, Cry3Bb1, Cry34/35Ab1|
Stacked traits are combinations of toxins that act against very different groups of insects. An example is a corn hybrid that has one toxin (Cry1F) to kill caterpillar pests and a different toxin (Cry34/35) to kill corn rootworm larvae. Cry1F has no effect on corn rootworm, and Cry34/35 has no effect on caterpillars, but the combination protects against both types of pests. Similarly, herbicide-tolerant genes are also added to the mix of traits, which makes this a threeway stack of traits.
Pyramid traits are combinations of two or more types of Bt toxins that act on the same group of pests. A simple example is the combination of Cry1F and Cry1Ab, both of which target caterpillars. This type of corn has no toxins for corn rootworm but is pyramided for toxins to protect against caterpillars.
Stacked pyramids are currently the highest evolution of Bt technology: They are stacked for toxins that target very different types of insects; and they are pyramided for two or more toxins that target a particular type of insect or pest group.
An example is a hybrid containing three toxins targeted at caterpillars (Cry1F, Cry1A.105 and Cry2Ab2). It is a pyramid toxin plant for caterpillars; it also has two toxins directed specifically at corn rootworm larvae (Cry3Bb1 and Cry34/35), making it pyramided against rootworms as well. (Technically speaking, Cry34/35 is a binary toxin, but it acts as a single toxin because both components are always present.) The hybrid also has genes that make it tolerant to two types of herbicides.
The seed/technology companies have begun to cross-license Bt toxins from each other to build multitoxin pyramids for caterpillar pests and corn rootworm. These multi-toxin pyramids control the target pests better than would a single toxin; they also help prevent insect resistance.
Pyramids are critical for delaying the development of resistance. Some insects have become able to survive some individual toxins in hybrids if the pests encounter them one at a time. For example, fall armyworm is resistant to Cry1F in Puerto Rico and parts of the U.S. South, where continuing to use Cry1F (only) corn will make a larger and larger percentage of the population resistant each year.
One answer to the problem is to add a second or even third toxin so that the insects with genes to live through Cry1F will most probably not have the genes to live through the second and third toxin. This effectively removes the Cry1F-resistant insects from the population and prevents the continued development of Cry1F resistance.
Pyramid Bt corn enables farmers to avoid most significant economic losses to direct feeding by caterpillars. Don’t expect the pyramid Bt corn to be damage-free, but know that the damage is less—often far less—than would be the case with non-Bt corn.
Growers whose fields have heavy caterpillar infestations year after year should consider pyramid Bt corn for caterpillars. Corn earworm and fall armyworm usually infest late-planted corn more heavily than corn planted in the normal window. In this situation, the value of Bt toxins for caterpillars often increases.
Because of the complexity involved in choosing Bt corn, consult with seed dealers before making a decision.
Note: Bt corn offers no protection against spider mites, which can threaten any type of Bt or non-Bt corn in Texas.
Insect resistance management and transgenic corn
The EPA requires all companies that register hybrid corn with plant-incorporated protectants to have a resistance management plan. These plans are based on several factors, including scientific studies on the pests, their survival on a particular toxin, and their movements before and after mating. The purpose of the plans is to prevent resistance from developing for a certain number of years—not forever. That goal is not practical.
Because the EPA will not grant permission for a company to sell Bt corn without a resistance management plan in place, the seed companies require growers to sign stewardship agreements each year. Each agreement is based on the EPA resistance management plan for the particular set of Bt toxins in the hybrids. Following the stewardship agreements is vital because it delays the development of resistance to the corn toxins.
All current resistance management plans in corn rely on a “refuge,” a certain percentage of the corn seed that does not contain the Bt toxins. The idea is to let some insects develop on non-Bt corn, where they are not selected for resistance; these will then mate with selected insects and dilute the resistance alleles, or alternate forms of genes, in the next generation.
Seed companies must report compliance data to the EPA each year. Compliance is the number of growers who are following the stewardship guidelines and the number who are not. In recent years, the EPA has noted a slip in compliance, especially in the South (cotton zone), and it has turned up the heat on companies to increase compliance. The seed companies also know that a lack of compliance means that resistance to their Bt toxins is more likely to develop. For these reasons, corn growers can expect visits from seed company representatives to ensure that they comply with the stewardship agreement(s).
Resistance development can cause two serious problems:
- The EPA could remove certain Bt technologies from the market. This has already happened in Puerto Rico.
- The Bt hybrids could stop working, and growers would once again lose money to the pests that have developed
Growers in the Midwest with resistant corn rootworms are now paying the technology fee for rootworm Bt corn (with the highest levels of seed treatment available), paying for soil-applied insecticides to put on top of it, and paying again to spray adult rootworm beetles in the summer, all the while losing yield to corn rootworm. Resistance means vastly increased costs and lower farm profits.
Across the United States, there are two zones with different refuge requirements in Bt corn. The Corn Zone includes the northern Texas Panhandle and all of the traditional Corn Belt. The Cotton Zone comprises the areas where cotton is traditionally a significant crop. The refuges required for Bt corn are larger in the Cotton Zone than in the Corn Zone.
The different types of Bt corn have varying refuge requirements, depending on where the corn is grown. Texas is split in to two zones: Counties north of the line in Figure 1 follow the refuge requirements for the Corn Belt. Counties south of the line must have larger refuges because of the dominance of Bt cotton that contains some toxins similar to those found in Bt corn.
All corn grown in Texas south of the line in Figure 1 have a mandated 20 percent or 50 percent caterpillar refuge, depending on whether the corn has pyramid toxins (20 percent) or a single toxin (50 percent) and whether it is active against corn rootworms, caterpillar pests, or both.
All refuges in the Cotton Zone must be “structured,” meaning that the non-Bt refuge corn must be either in a 4-row or wider strip or in a block planting in part of the field or in a field next to it (with restrictions depending on the Bt traits).
Seed blend refuges, sometimes referred to as “refuge in the bag,” are mixes by the seed companies of non-Bt seeds with Bt seeds at the right ratio to give the minimum refuge stated in the stewardship agreement. (It is illegal for growers to blend their own seeds.)
Seed blend refuges may be sold in both the Corn Zone and Cotton Zone, but if they are planted in the Cotton Zone then an appropriate structured refuge must be planted as well. In the Cotton Zone, none of the refuge corn in the seed blends counts toward the block or strip refuge that must be planted.
It can be very difficult to keep up with the proper stewardship practices required for the many types of Bt corn on the market today. As a response to this problem, the seed companies have partnered with the National Corn Growers Association to publish the stewardship practices for any type of Bt corn grown anywhere in the country. A web-based tool is posted at http://www.ncga.com/for-farmers/issue-briefs-ipm/ irm-refuge-calculator, which also links to downloadable apps for iPhone and Android devices.
Also, each year, Michigan State University produces The Handy Bt Trait Table, an excellent, concise, two-page summary of the toxins in Bt corn, the pests controlled, and the refuge requirements.
Two versions of the publication are published, one for the Corn Zone and one for the Cotton Zone. They are posted at http://www.texasinsects.org/corn.html.