Host Plant Resistance
Host plant resistance pertains to a plant’s ability to resist damaging insect invasions. Some plants use their physical appearance as a deterrent such as plants that have hair covering their leaves or plants with a thick leaf cuticle. With biotechnology, came a new form of host plant resistance. The ability to insert genes into plants, creating transgenic hybrids, gave a whole new meaning to plant resistance. Plants that previously had no resistance mechanisms now are able to resist damaging insects. This is the case with the development of the YieldGard® corn traits. The first YieldGard on the market was developed for plant resistance against European corn borer. It used the Cry 1Ab protein from Bacillus thuringiensis to control the European corn borer. This protein is specific to the insect order Lepidoptera and can only be activated in the gut of the larva (caterpillar). The same concept for creating this type of host-plant resistance was used for YieldGard® Rootworm. YieldGard Rootworm was developed to allow corn plants to have resistance to damaging corn rootworm larvae.
YieldGard Rootworm contains Bt (Bacillus thuringiensis) that is specifically targeted to affect the corn rootworm. The Bt protein inserted into YieldGard Rootworm corn is Cry 3Bb and can only be activated inside the corn rootworm gut to cause death. Additional research has been conducted to determine effects on other non-target organisms. YieldGard Rootworm has been found to pose minimal risk to mammals, birds, fish, and other non-target insects including lady beetles and monarch butterflies (Pilcher, 2003).
Mode of Action
The use of Bt as an insecticide is an effective, plant delivered approach to controlling certain insects. There are three criteria that need to be met for an insect to be affected by Bt. First, the correct Bt protein must be used against the specific insect that is susceptible. For example, YieldGard® Corn borer contains the Bt protein that will control corn borer but will not control corn rootworm. The second criterion that must be met is for the insect to have the correct pH within the gut system to activate the Bt protein. The third criterion requires that the insect targeted by the Bt protein, have the appropriate midgut receptor. If an insect does not have the correct receptors, there is no binding site for the protein and therefore, no toxic activity will take place inside the insect (Pilcher, 2003).
There are many advantages to using transgenic hybrids including reduced exposure to insecticides, ease of use, and proven product performance. The handling practices required when using transgenic hybrids, such as YieldGard, minimize exposure to pesticides. Since the insect resistance is contained within the plant and is not applied on the outside, there is never any contact with insecticides for the producer.
Transgenic hybrids are very easy to use, as well. These hybrids can be planted using the same equipment and plant populations as non-transgenic hybrids. The insect protection technology is in the seed, therefore eliminating any need to calibrate insecticide application equipment.
Proven product performance can be seen through examples such as improved crop harvestability and the ability of the corn hybrid to reach its maximum yield potential. The hybrid is protected season long from CRW damage due to the internal insect protection offered by YieldGard, therefore reducing the possibility of lodging. In addition, the plants’ chances of producing the maximum amount of grain have increased since the adverse affects of CRW feeding have been drastically reduced (Pilcher, 2003).
There are disadvantages to using transgenic hybrids including possible development of insecticide resistance, the limited spectrum of the transgenic hybrid, harvested grain channeling, and unnecessary use of transgenic technology. Insecticide resistance occurs when the insect becomes resistant to the management option, in this case, the transgenic hybrid. This can occur in a number of different ways and should be closely monitored. More on insecticide resistance management is discussed in the next section.
The limited spectrum disadvantage pertains to the transgenic hybrids ability to only control one insect, CRW. YieldGard Rootworm is only able to protect itself from corn rootworm larvae. Therefore, if another root damaging insect is present, such as the white grub, YieldGard will not provide plant protection. In these cases another method of management will have to be implemented.
The third disadvantage of using transgenic hybrids is the difficulty in which the producer may have in marketing the grain after harvest. Some grain elevators and export markets will not accept YieldGard Rootworm grain. The producer must then determine where the grain can be stored and ultimately into which market it can be sold.
Finally, the disadvantage of using transgenic hybrids when not needed can directly affect producers’ profit. The insecticide technology is always in the plant whether or not the target insect is present. Producers must pay a fee to use this technology when purchasing the seed. Therefore, in the event of low insect pressure the producer who planted transgenic seed has unnecessarily applied insecticide to the field.