Genetically Engineered Bt Corn

FIELD CROP NEWSLETTER, Department of Agronomy, Penn State University

December 9, 1996

By Steve Spangler and Dennis Calvin
This was the first commercial year of Bt hybrid availablility for corn growers in the U.S. Bt corn is a "transgenic" crop, that is, the plants have a gene from the bacterium Bacillus thuringiensis, inserted into their own genetic material. This new gene produces a protein which kill insects after the protein is ingested. The results so far indicate that dramatic new levels of corn borer control can be acheived: well over 90%, compared to an average effectiveness of about 60% for a typical insecticide. In addition, with these transgenic crops, there is no cost of scouting or application of a pesticide to a crop, so the only additional cost will be the premium placed by the company on a bag of seed. Several companies have developed and patented the technology for these transgenic lines, and are working with several seedcorn companies. Ciba and Mycogen marketed Bt corn in Pennsylvania in 1996, and NK had full registration in August 1996. In 1997, several other seed companies will be using the Bt gene from Monsanto in some of their hybrid lines. Economic analysis indicates that the economic benefit in Pennsylvania from using these trangenic corn hybrids will range between $6 to $26 per acre, depending on the conditions. There will be about a $7/acre premium on transgenic corn hybrids. Therefore the economic returns per acre will vary between $0 and $21 per acre. On average most corn fields will benefit economically from a trangenic corn hybrid.

But, there is an important assumption to this analysis which states that Bt corn will pay off in 50-80% of the corn fields in the U.S., and this is that the Bt hybrids have equal yield potential of the non-Bt hybrids. Put simply, it is quite possible that a superior non-Bt hybrid will still out-perform an inferior Bt hybrid. In the end, yield of the corn hybrid will be some combination of having a good yielding hybrid to begin with, and having the addition of the Bt gene, or for that matter future gene insertions which may give protection for other insects (we are told that corn rootworm transgenics are just around the corner!). Thus, hybrid trials are still as important as ever.

A major concern with these transgenic crops is the potential for development of resistance of corn borer to the Bt toxin if they are over used. Most insect populations, if they are exposed to a toxin excessively, can develop resistance, making pest control products less effective. Any potato grower knows of this problem with Colorado potato beetle. Resistance is also common in many other crops, including cotton and apples. Also, there have been two documented cases of resistance of insects in the field to Bt products: diamondback moth in cole crops, and indian meal moth in stored grain. Fortunately, however, corn borer has not shown resistance to any chemicals to date. It is everyone's interest - biotechnology companies, growers, and corn agriculture in general - to maintain the usefuleness of this valuable technology, so resistance management needs to become part of everyone's thinking. You may be hearing a new word in our vocabulary from now on: "refugia". Specifically, seed companies will probably be specifically recommending that you not plant all your acres to transgenics. Planting too many acres to transgenic corn may lead to resistance. However, leaving a small portion (probably 4-20%) of your acres as non-Bt corn will maintain insect susceptibility to the Bt corn, in others words, you will more likely retain the effectiveness of this exciting new technology.

8/19/97