Several kinds of transgenic plants have been genetically engineered. They possess beneficial traits formerly lacking in the original plant that are now internally present as part of the plant's own genetic material. Added traits can include a broad range of desirable characteristics, such as ability to tolerate environmental extremes, increased growth or yielding potential, improved shelf life of the marketable product, and factors that provide pesticidal qualities or substantially improve plant resistance to the pest. Pesticidal transgenic corn varieties will increase in commercial availability in the near future and could significantly modify strategies for insect control in crops.

Figure 36. Whorl-stage transgenic corn plant containing the Bacillus thuringiensis,, subspeices kurstaki, gene undamaged by European corn borer (B. D. Barry).

Figure 37. Whorl-stage susceptible corn plant of the same heritage as that in Figure 36, except without the Bacillus thuringiensis,, subspeices kurstaki, gene showing damage by European corn borer (B. D. Barry).

Some companies are developing transgenic corn lines that incorporate Bacillus thuringiensis, sub-species kurstaki, genes into corn plants. The resulting plant can produce toxic protein crystals. One method of incorporation involves bombarding small units of corn tissue with microprojectiles carrying Bt genes. The Bt genes carrying the insecticidal traits merge with the genetic material from the corn tissue. New plants cultured from the treated corn tissue contain the Bt characteristics. A plant line containing the Bt genes exhibits no damage (Figure 36) compared to the same line lacking the genes (Figure 37).

The release of transgenic Bt corn varieties is expected to offer corn producers a new tool that will expand pest management opportunities. Some of the transgenic corn lines and hybrids have proven to be very effective in controlling European corn borer and show great potential against southwestern corn borer. The insecticidal quality is maintained throughout the growth of the plant as development progresses. Some lines have been engineered to have Bt in the leaf and stalk tissues, but do not contain the toxin in kernels, pollen, or other plant parts. The concentration of Bt toxin in leaves, sheath, and collar sites, where young European corn borer larvae initially feed, is very effective in controlling both first and second generations of the insect. The larvae usually survive only a brief time after emerging from the egg. They feed only enough to make a tiny scar (not even a hole) on the corn leaf or sheath. Most European corn borer larvae on Bt corn die within their first day after attempting to feed or shortly thereafter.

As with any management tool, transgenic pesticidal plants should be considered as a component of an overall, comprehensive management plan. With the potential wide usage of Bt corn, a significant concern of researchers is the development of tolerance to Bt within European corn borer populations. Over time, some members of the European corn borer populations that can tolerate high doses of Bt are likely to increase, making Bt corn less effective. Researchers are investigating strategies to prevent or delay development of tolerance to Bt in European corn borer. These strategies include maintaining European corn borer populations that are genetically susceptible to Bt by: (1) having areas of corn where the Bt genes are not used (refugia), (2) introducing more than one kind of Bt gene into corn lines, (3) incorporating another effective insecticidal component into transgenic corn other than Bt, and (4) combining Bt insecticidal qualities with other insecticidal traits in transgenic corn.