STERILE MALE TECHNIQUE
(Dr. Girish Chandra)
Sterile Insect Technique (SIT)
Sterile Insect Release Method (SIRM)
G. A. Runner, in 1916, found that low doses of X-rays decreased the reproductive capacity of the cigarette beetle, while higher doses killed them. By 1926 H.J. Muller had demonstrated that X-ray radiation will cause heritable changes in fruit flies (Drosophila melanogaster). Raymond Bushland and Edward Knipling first developed the technique to eliminate screw-worms preying on cattle herds in the late 1930s, when both scientists were working at the United States Department of Agriculture Laboratory in Menard, Texas.
E. F. Knipling (1937) had conceived an approach to insect control in which the natural reproductive processes of the screwworm fly (Cochliomyia hominivorax Coquerel) are disrupted by chemicals, rendering the insects sterile. These sterile insects are then released into the environment in very large numbers (10 to 100 times the number of native insects) so that they compete with the native insects that are present in the environment. Females that mated with sterile males will produce unfertilized eggs that will not hatch. Since there are 100 times more sterile insects in the population than native insects, most of the eggs will be unfertilized. As time goes on, the number of native insects will decrease, thus driving the native population to extinction.
The classical example is eradication of screwworm blowfly (Cochliomyia hominivorax Coquerel), which is flesh-parasite on cattle, goats and sheep in the island of Curacao, north of Venezuela in 1940. Male flies were sterilized in large numbers by gamma radiation from Cobalt-60 and then dropped from airplanes @400 million/square mile/week. The whole pest population was eradicated in 12 months. Later complete eradication was also achieved in Florida in 1958-59. In Texas 99.9% control was achieved in only 3 years. The method was also effectively used against the Mediterranean fruit fly, Ceratis capitata in Hawaii in 1959-60, but immigration from the untreated areas prevented the control from being long-lived.
The methods that are presently employed for the successful use of the sterility principle (Bartlett 1990) have not changed significantly since E. F. Knipling’s original formulation:
In USA the screwworm eradication program started in Florida in 1957 and by 1966 all screw-worm colonies were eliminated. But re-infestations from flies migrating from Mexico failed the program. In 1972, the joint US-Mexico program was initiated which enabled Mexico to be officially declared free of screwworms in 1991.
Pink bollworm control
One of the more successful and long-term sterile insect release programs involves the pink bollworm in the San Joaquin Valley of California (Staten et al. 1993). Over a 24 year period (since 1968) there has been a continuous release of sterile pink bollworm adults in the cotton growing season. From 1970 to 1991 the pink bollworm rearing facility in Phoenix produced from 99 million to 826 million dye-marked moths per year. Most of the moths were radiation sterilized and released from airplanes over cotton fields in the San Joaquin.
Tsetse fly in Africa
Female tsetse flies copulate only once in their lifetime. If this is with a sterile male, they cannot reproduce. The mass release of sterile male tsetse flies to control the insect population raises ethical problems. The sterile males are, indeed, not refractory to trypanosomes and their release thus temporarily increases the number of potential vectors. This technique was nevertheless used, for example in Zanzibar from 1994 to 1998.
Sweet potato weevil in Japan
A small scale experiment was carried out to eradicate the sweet potato weevil, Cylas formicarius (Fabricius) by the sterile weevil release method from 1994 to 1996 on the islands of Kiyamajima, Amami Islands and Kagoshima Prefecture in Japan. Weevils were mass-reared on fresh sweet potato roots at 27°C and roots filled with the weevils were irradiated with gamma ray, 80 Gy on the 27th to 28th days after oviposition. Sterile weevils were stained with fluorescent dyes and released by hand on host plant foliages. Monitoring was done by both pheromone traps. The wild population in the release zone was reduced to nearly zero in 1995.
Ceratitis capitata, the Mediterranean fruit fly is a species of fruit fly capable of wreaking extensive damage to a wide range of fruit crops. It is native to the Mediterranean area, but has spread to many parts of the world, including Australasia and North and South America. In the United States, C. capitata has invaded four states (Hawaii, California, Texas and Florida), but has been eradicated from all but Hawaii. It has also been eradicated from New Zealand and Chile.
· Repeated treatment is required to exterminate the population.
· Sex separation is difficult.
· Radiation treatment affects health of males, so the sterilized insects are at a disadvantage when competing for females
· Species specific. There are 22 species of Tsetse fly in Africa, for instance, and the technique must be implemented separately for each.
· Many fertile pest insects must be grown before sterilisation which must be housed securely to prevent their escape or release: in Feburary 2003, the irradiation machinery at a plant in Mexico failed and 4 million fertile screw worm were released before the problem was spotted.
A large number of chemicals have been known to cause sterility among insects. Such chemosterilzation could be produced in the field in both sexes by luring insects to bait containing chemosterilants. This concept was advanced by Knipling in 1955. The chemosterilants can be grouped under the following 3 categories:
1. Alkylating agents: They are effective on males and include azaridines (tepa, metepa, apholate, thiotepa), which act on fast dividing germ cells. They are used in 0.025-0.5% concentrations as sugar baits. Esters of sulphuric acid and nitrogen mustards are also good male chemosterilants.
2. Antimetabolites: They are analogues of purines and pyrimidines, folic acids or aminoacids and affect the germ cells of females.
3. Miscellaneous: They are chemicals like hempa, colchicines, S-triazines, tin-derivatives like triphenyltin acetate, coumarin.
All chemosterilants are carcinogenic and hazardous to human beings and non-target animals. They have been tried against housefly and Mexican fruit fly.