Eugenics & Birth control
Some control tactics are designed to suppress a pest population by altering its genetic makeup and/or reducing its reproductive potential. As a group, these tactics are frequently known as genetic controls because they affect the accuracy or efficiency with which a pest species passes its genetic material (DNA) from one generation to the next. Genetic control usually works in one of two ways: either by causing (inducing) reproductive sterility, or by incorporating new and potentially deleterious genes (or alleles) into the genetic makeup of a pest population. In effect, some members of a pest species are transformed into biological time bombs that eventually destroy other members of their own species. Because of the self-destructive nature of these tactics, they are sometimes called autocidal control.
Insects can be sterilized by exposing them to certain chemical agents (chemosterilants) or to non-lethal levels of ionizing radiation (X-rays or gamma rays). Chemosterilants are really a form of chemical control. They usually work by blocking the onset of sexual maturity, by inhibiting the production of eggs and/or sperm, or by damaging the chromosomes. These compounds will be covered more fully in the section on semiochemical insecticides.
Exposure to radiation also damages chromosomes (usually by breakage or mutation). Since cells with damaged chromosomes cannot divide correctly, they do not form normal gametes or produce viable offspring. Although the susceptibility of each insect species is different, a proper dose of radiation administered at an appropriate stage of development (usually to pupae) can often induce sterility without causing other deleterious side effects. Sterile individuals, reared in large numbers and released into the environment, can mate with "normal" individuals but they produce no viable offspring. The more sterile individuals released, the fewer "normal" matings are likely to occur.
The effects of radiation on insect development was first studied in 1916, but this knowledge was not put to practical use until the 1930's when E. F. Knipling suggested that it might be possible to suppress pest populations by flooding the environment with large numbers of males that had been rendered infertile by irradiation. By maintaining a constant population of sterile males that was large in comparison to the number of virgin females, Knipling calculated that the number of "normal" matings would decrease each generation until the population was forced into extinction. (see Table 1)
Sterile Male Mathematics
The first large-scale test of Knipling's sterile male technique was conducted in 1954 on the island of Curaçao against the screwworm, Cochliomyia hominovorax, a Dipteran pest of livestock. After just 13 weeks (four to five generations of the fly) and release of nearly a million sterile males, the screwworm population was completely eradicated from this tiny Caribbean island.
Nailing the Screwworm
The dramatic success of Knipling's first project led to larger screwworm eradication programs in the southeastern and southwestern United States and to intense interest in the sterile male technique for eradication of other pest species, including Mediterranean fruit flies (Ceratitis capitata), boll weevils (Anthonomus grandis), horn flies (Haematobia irritans), tsetse flies (Glossina spp.), pink bollworms (Pectinophora gossypiella), and codling moths (Cydia pomonella).
Overall, the sterile male technique has had a spotty record of achievement. Despite isolated successes, no other eradication program has ever measured up to the outstanding performance of the screwworm project. From a theoretical standpoint, the use of sterile males is highly attractive because it is non-polluting, species specific, and becomes more effective as the target population decreases. But in practice, the technique simply doesn't work unless a pest population meets the following criteria:
- Easy to mass-produce. A sterile insect release program cannot even be considered unless large numbers of insects can be produced at relatively low cost.
- Females mate only once. This characteristic is uncommon among insects, but it is important for success of the sterile male technique because multiple matings only increase the probability that a female will receive sperm from a fertile male.
- Males can be sterilized without loss of competitive vigor. Sterile males must be able to compete successfully for mates with normal (fertile) males. Sterile males must be physiclly and behaviorally identical to normal males.
- Low initial population. The pest population must be small enough so the initial release of sterile males will outnumber the population of normal males. In some cases, alternative control tactics may be used to reduce initial populations to practical levels.
- Restricted geographic range. Sterile males must disperse to all parts of the range. Small, localized populations are easier to control than large dispersed populations.
Within the last few years, geneticists have begun to devise new genetic control strategies that work by altering DNA or by adding new genes or alleles into the genetic makeup of a pest population. Most of these new tactics are still untested in field applications, but they do offer intriguing possibilities for the future. One such approach involves breeding members of a pest species that have been genetically altered to make them more susceptible to cold winter (or hot summer) temperatures. This type of genetic trait is known as a conditional lethal mutation; it causes mortality only when triggered by extreme environmental conditions. Genetically altered individuals are mass-reared, released into the environment, and allowed to breed with other members of their species. Hopefully, the mutation will spread throughout much of the pest population before the "carriers" are killed by cold (or hot) temperatures.
Another promising approach involves cytoplasmic incompatibility. Although insects lack an immune system, some species do have different "strains" that are not reproductively compatible: the egg cytoplasm of one strain may contain substances that block or inhibit sperm from another strain. By mass-rearing and releasing large numbers of these "incompatible" insects, it may someday be possible to eradicate a pest population and replace it with a more "benign" strain.
Next page: Chemical Control - Semiochemicals