INSECT CONTROL: TRANSGENIC CONTROL of the COLORADO POTATO BEETLE
(modified from a talk by Jennifer Feldman, 1992)
Recent advances in the biotechnology of crop plants present new options for agricultural pest control and horticultural improvements, and offer a number of advantages over traditional plant breeding techniques. Because transgenic modifications involve single gene traits, commercially acceptable cultivars can be improved without compromising growth characteristics. As a result, the development of new clones is considerably faster than that of conventionally bred cultivars. In addition, genes maybe selected from a wide variety of sources, including unrelated plant species, bacteria and animals, which vastly expands the germplasm available for crop improvement.
Insertion Using Agrobacterium tumefaciens
The insertion of foreign genetic material is accomplished through the use of a bacterium known as Agrobacterium tumefaciens, a plant pathogen which causes "crown gall" disease in many broadleaf plant species. A. tumefaciens infects its host by inserting genetic material into the plant genome. In this way, it directs the plant to produce a tumor-like gall.
In the laboratory, scientists have modified A. tumefaciens so that it no longer causes the crown gall disease, but still infects the plant in the same fashion. When the gene for a beneficial trait is spliced to A. tumefaciens, the bacterium acts as a vector and carries the new gene into the plant. The beneficial gene is then inserted into the plant DNA as a stable part of the genome. Genetically "transformed" cells are grown from callus and propagated through tissue culture so that the new gene is present throughout the plant tissue. From this point on, the plant carries and expresses the new gene.
Bt Gene in Potato
Colorado potato beetle (CPB) resistance has been achieved through the incorporation of a gene for the Bacillus thuringiensis (Bt) protein into potatoes. Bt is a very desirable pest control agent because it is non-toxic to humans, animals or beneficial insects (predators) but is highly effective against specific groups of pest insects. When ingested, Bt acts as a gut poison, causing susceptible insects to stop feeding and eventually die.
Although Bt has been available to potato growers as a foliar insecticide for CPB control for some years, use is limited because foliar Bt applications must be precisely timed to the CPB life cycle for adequate results, and short field persistence necessitates frequent applications. Alternatively, transgenic Bt-expressing potatoes manufacture the Bt protein at a sufficient level throughout the foliage. The protein is present when the insects are most vulnerable, and individuals do not escape control due to poor spray coverage or residue degradation. In field trials, transgenic Bt-expressing potatoes exhibited season-long control of all stages of CPB, and provided greater protection from defoliation than did conventional foliar insecticides.
Transgenic Bt expression provides "built in" control of the CPB. As it requires no application, it is safe to use and creates no harmful pesticide residue. Unlike broad spectrum insecticides, Bt is non-toxic, environmentally sound and will not negatively impact natural enemy populations. Data suggest that the incorporation of transgenic Bt into pest management programs will result in increases of beneficial predators and parasites such as damsel bugs (Nabid sp.), spiders and big-eyed bugs (Geocoris sp.), which in turn can contribute to control of other potato pests. Aphids, which are responsible for a large number of insecticide applications in many potato growing regions, lend themselves readily to natural enemy control. However, as growers have previously been reliant on insecticides for CPB control, natural enemies have contributed little to pest management programs.
Managing Insect Resistance
Insecticide resistance management is an important element of CPB management programs. Transgenic Bt represents a "high dose" strategy of resistance management, as it is hypothesized that those beetles able to survive on transgenic plants will be very rare or nonexistent. However, in order to ensure that potential resistant genes do not become established in the population, it is necessary to provide a "refuge" for susceptible individuals, where beetles do not undergo selection pressure. In this way, resistance genes that occur infrequently through random mutation will be "diluted out" of the population. This type of approach, in combination with cultural practices such as crop rotation to reduce overwintering adult populations, will be important in extending the longevity of Bt as a pest control tool.
Progress is being made toward the commercial development of potatoes resistant to insects (such as Colorado Potato Beetle), diseases (such as Potato Leaf Roll Virus and Late Blight). In addition, it has been demonstrated that gene modifications can be used to produce potatoes with higher dry matter, lower sugars and a better ability to store.
The potential benefits of transgenic technology are enormous. Eventually, traits may be combined in a single plant so that crop plants defend themselves from pests without the need for supplementary pesticides. In addition, quality improvements may make it possible to process food with less fat, store food longer and improve flavor and texture. One day, genetic engineering will work cooperatively with plant breeding to develop new varieties which are tailored to the wishes of consumers and needs of producers.