16.6.1. Genetic engineering of host resistance and the potential problems
Molecular biologists have used genetic engineering techniques to produce insect-resistant varieties of a number of crop plants, including corn, cotton, tobacco, tomato, and potato, that can manufacture foreign antifeedant or insecticidal proteins under field conditions. The genes encoding these proteins are obtained from bacteria or other plants and are inserted into the recipient plant mostly via two common methods: (i) using an electric pulse or a metal fiber or particle to pierce the cell wall and transport the gene into the nucleus, or (ii) via a plasmid of the crown-gall bacterium, Agrobacterium tumefaciens. This bacterium can move part of its own DNA into a plant cell during infection because it possesses a tumor-inducing (Ti) plasmid containing a piece of DNA that can integrate into the chromosomes of the infected plant. Ti plasmids can be modified by removal of their tumor-forming capacity, and useful foreign genes, such as insecticidal toxins, can be inserted. These plasmid vectors are introduced into plant cell cultures, from which the transformed cells are selected and regenerated as whole plants.
Insect control via resistant genetically modified (transgenic) plants has several advantages over insecticide-based control methods, including continuous protection (even of plant parts inaccessible to insecticide sprays), elimination of the financial and environmental costs of unwise insecticide use, and cheaper modification of a new crop variety compared to development of a new chemical insecticide. Whether such genetically modified (GM) plants lead to increased or reduced environmental and human safety is currently a highly controversial issue. Problems with GM plants that produce foreign toxins include complications concerning registration and patent applications for these new biological entities, and the potential for the development of resistance in the target insect populations. For example, insect resistance to the tox- ins of Bacillus thuringiensis (Bt) (section 16.5.2) is to be expected after continuous exposure to these proteins in transgenic plant tissue. This problem might be overcome by restricting expression of the toxins to certain plant parts (e.g. the bolls of cotton rather than the whole cotton plant) or to tissues damaged by insects. A specific limitation of plants modified to produce Bt toxins is that the spore, and not just the toxin, must be present for maximum Bt activity with some pest insects.
It is possible that plant resistance based on toxins (allelochemicals) from genes transferred to plants might result in exacerbation rather than alleviation of pest problems. At low concentrations, many toxins are more active against natural enemies of phytophagous insects than against their pest hosts, adversely affecting biological control. Alkaloids and other allelochemicals ingested by phytophagous insects affect development of or are toxic to parasitoids that develop within hosts containing them, and can kill or sterilize predators. In some insects, allelochemicals sequestered whilst feeding pass into the eggs with deleterious consequences for egg parasitoids. Furthermore, allelochemicals can increase the tolerance of pests to insecticides by selecting for detoxifying enzymes that lead to cross-reactions to other chemicals. Most other plant resistance mechanisms decrease pest tolerance to insecticides and thus improve the possibilities of using pesticides selectively to facilitate biological control.
In addition to the hazards of inadvertent selection of insecticide resistance, there are several other environmental risks resulting from the use of transgenic plants.
First, there is the concern that genes from the modified plants may transfer to other plant varieties or species leading to increased weediness in the recipient of the transgene, or the extinction of native species by hybridization with transgenic plants. Second, the transgenic plant itself may become weedy if genetic modification improves its fitness in certain environments. Third, non-target organisms, such as beneficial insects (pollinators and natural enemies) and other non-pest insects, may be affected by accidental ingestion of genetically modified plants, including their pollen. A potential hazard to monarch butterfly populations from larvae eating milkweed foliage dusted with pollen from Bt corn attained some notoriety. Milkweeds, the host plants of the monarch larvae, and commercial cornfields commonly grow in close proximity in the USA. Following detailed assessment of the distance and Bt content of pollen drift, the exposure of caterpillars to corn pollen was quantified. A comprehensive risk assessment concluded that the threat to the butterfly populations was low.
Crop plants engineered genetically for resistance to herbicides may impact deleteriously on non-target insects. For example, the widespread use of weed control chemicals in fields of herbicide-resistant corn in the mid-western USA is leading to the loss of milkweeds used by the larvae and flowering annuals used as nectar sources by the adults of the monarch butterfly. The monarch has received much attention because it is a charismatic, flagship species (section 1.7), and similar effects on populations of numerous other insects are unlikely to be noticed so readily.
