2.9.1 Pest Resistance to Control Tactics

The history of agricultural pest management has demonstrated that insects, weeds, and microbial pathogens have the evolutionary potential to overcome or circumvent most control tactics imposed by farmers (Barrett 1983; Green et al. 1990; Gould et al. 1991). That over 400 insect species have become resistant to at least one insecticide (Georghiou 1986) is often cited as evidence of the genetic ability of arthropods to evolve resistant strains. In addition, weeds and pathogens also have an impressive record of successful adaptation to control measures (Green et al. 1990). Although the number of cases of resistance by weeds to herbicides is smaller than that of insects, the percentage of weed species that has developed resistance is greater than that of insects (Gould 1995). It is well documented that microbial pathogens can successfully adapt to crop cultivars that are bred to resist specific diseases (Lamberti et al. 1981). In examining a random selection of 63 cases of viruses that live in association with specific plant hosts, Fraser (1990) found that in 28 of the cases there were good data indicating that adapted isolates of the virus existed (in only five cases was there good evidence that there had been no adaptation by the virus). Fungal and bacterial adaptation to pest-protected cultivars has caused serious crop losses. In many cases, pathogen resistance has occurred less than 5 years after a classically bred resistant cultivar was released for commercial use (see section 3.1.1). Experience with both insect and pathogen adaptation to genetically modified pest-protected (GMPP)6plants indicates that the more intensively a control tactic is used, the more rapidly pests will adapt to it (Gould et al. 1991). History also indicates that pests adapt more rapidly to some types of GMPP plants than to others (Lamberti et al. 1981).

If a GMPP cultivar is lost because the target pest adapts to the cultivar, replacing the cultivar with a new GMPP cultivar can have a number of associated costs. Even if new GMPP genes are available, moving those genes into a modern cultivar is expensive. Although the health and environmental safety of the plant protectant in the new cultivar can be tested in laboratory experiments, the new cultivar will need to be monitored for impacts that could not have been detected in the laboratory experiments. If new pest-protection genes are not available, farmers might need to move back to reliance on broad-spectrum pesticides. Decreasing the rate at which target pests adapt to GMPP cultivars can therefore produce societal benefits.


As a reminder to the reader, GMPP plants include both transgenic and conventional pest-protected plants. See section ES.3.

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