Alternative Control Strategies-Sterile-Insect Technique, Biology tutorial

Definition of Sterile-Insect Technique:

The utilization of sterilized insects and manipulations of pest genetics have developed slowly since around 1916. Such tactics are pest-selective, aimed mainly at decreasing insect populations by lowering reproductive potentials and they comprise a few of the most innovative and unusual procedures in insect pest technology. Pest insects are employed against members of their own species to decrease population levels which is a kind of autocidal control termed as SIT, that is, the sterile-insect technique.  

SIT received main input and support from the work of E.F. Knipling, a USDA entomologist in the late 1930s. In the year 1950, R. Bushland starts it as a strategy of replacing normal mating in a population having infertile ones in effect inducing sterility. Basically, the sterility principle aims at flooding a population having sterile mates, which seek out and mate by normal females. Such mating outcome in inviable (or unfertilized) eggs, and by means of continued sterile male releases, the population declines. As decline takes place, the ratio of sterile to normal male rises till virtually no normal males remain. At this point, the population becomes vanished for lack of progeny. As a result, in most of the instances, the major aim of SIT has been annihilation, not just suppression as in insect pest management.

Definition of Sterility method:

Control of pest population accomplished by discharging large number of sterilized male insects that will compete by the normal males and decrease the population of insect in the later generation.  

It is generally termed to as SIT (that is, Sterile insect technique) or SIRM (Sterile insect release method). Sterile insect release process is a genetic control procedure. This is as well termed as Autocidal control since insects are employed against the members of their own species. 

The sterile-insect method builds up of studies of the screw-worm fly (Cochliomyia hominivorax), a parasite of cattle mainly in tropical and subterranean areas of the New World. Knipling and coworkers noted that the adult females mated just once (were monogamous) to fertilize their eggs. By employing this knowledge, these workers postulated that when males could be sterilized devoid of impairing their mating behavior, discharges into the wild population would outcome in infertile mating and isolated populations could be removed.

As a result, there have as well been many successes in controlling the species of fruit flies, most specifically the Medfly (Ceratitis capitata), and the Mexican fruit fly (Anastrepha ludens). As well, Tsetse fly removed from Zanzibar, Melon fly (Bactrocera cucurbitae, Coquillett) eliminated from Okinawa, Japan.

Circumstances for Application:

Some of the pests conditions are envisaged for the use the sterile-insect method are as follows:

1) Against well established pests if they reach low points in their density cycles (that is, either in a season or between seasons).

2) Against lately introduced pests or established pests spreading to latest regions.

3) By other tactics such as insecticides and cultural methods that precede sterile-insect discharges.

4) Against isolated populations such as those on islands and other such conditions. In all the conditions, an area wide program should be imposed for the complete success.

Sterilizing Insects in a Natural Population:  

The main idea of sterilizing insects in a natural population was advanced some years ago if it was found that some chemicals termed as chemosterilants, had potential in this regard. The succeeding discovery which these chemicals presented unacceptable human health and ecological hazards discouraged further improvement of the approach. Even although not practical presently, future growths could modify the acceptability of the method; thus, a short description of this idea is suitable. Chemosterilants employed are TEPA, HEMPA, BUSULFAN and so on.

Sterilizing insects in a natural population must not be confused by the sterile-insect release procedure. However sterility is comprised in both the instances, the methods of suppression of each is rather different. While mating is needed for the suppression in sterile-insect discharge programs, population reproductive rates are decreased directly through the sterilization of individuals in the natural population. In this, both females and males of the population are sterilized that in regard to reproductive capacity is similar as killing them; they will not add individuals to the subsequent generation. Though, the sterilized individuals, still being active, can mate by individuals of the population which were not sterilized, further decreasing the reproductive potential. This later phenomenon behaves likewise to the release method and by this approach has been termed as the bonus effect.

Methods of Sterilization

The theory of autocidal control is sound in principle; however its application relies mainly on the acceptable procedures to sterilize the insects. Devoid of these, as seen in the theory of sterilizing natural populations, progress toward application can be agonizingly slow or halted entirely. Throughout the growth of the approach, both ionizing radiation and chemicals have been found to cause insect sterility however the former has been employed more successfully.

1) Ionizing Radiation:

The sterilizing effects of X-rays on insects were noticed as early as 1916 with adult cigarette beetles, Lasioderma serricorne and were the primary form of radiation investigated.

These and other investigations by means of X-ray caused mutations in pomace flies, Drosophila melanogaster, finally led to the discovery by R.C. Bushland which pupae of the screwworm, if irradiated close to adult emergence could outcome in competitive, sterile adults.

By the growth of the atom bomb after World War II, it became much more proficient to work by the manufactured isotopes, mainly those generating gamma rays. Further studies showed little difference among certain X-rays and gamma rays in treating the insects and most ensuing programs have used gamma radiation, by cobalt or cesium as the source.  

High-energy radiation exacts its genetic effects through chromosome breakage and point mutations of the DNA. After irradiation, gametes are generated; however they carry dominant lethal mutations in such a way that the zygotes made die early in growth.

2) Chemosterilization:

Chemosterilants are the chemicals which interfere by the reproductive capacity of the insect. The capability of several chemicals to sterilize insects and also some of the drawbacks of this process has already been mentioned. If applied prior to onset of meiosis, chemosterilants prevent gamete production. Knowing this, we are not surprised to learn that such chemicals prevent all kinds of cell reproduction and have received much development in the cancer therapy (chemotherapy). In fact chemotherapy research probably gave impetus to the proposals of chemical use for insect sterilization.

Chemosterilants can be categorized into four fundamental groups:

  • Alkylating agents
  • Antimetabolites
  • Phosphorus amides
  • Triazines

Sterile-Release Programs:

A lot effort has been directed toward building up the sterile-insect release method for practical use. The USDA has been specifically responsible for numerous studies and pilot programs which have advanced the theory and the agency has been the main force behind the large-scale implementations.

A development from such area wide programs has been the so-called theory of total population management (TPM). The TPM approach tries to employ all available means to eliminate a pest over a broad region. This objective is not new, however until the growth of the sterile-male method, it had been considered almost impossible.

Pest annihilation has been recognized in just a few instances where sterile-insect releases were made, however successful degrees of suppression have been accomplished in some other instances.

A few of the most significant illustrations of the method follow.

1) Screwworm eradication and suppression:

The most dramatic success by the sterile-insect method was accomplished with the eradication of the screwworm fly from the Mexico, United States and most of Central America. As a parasite of livestock, this fly lays 250 to 300 eggs in wounds and developing larvae feed on tissues and expand the open wound. Such feeding fascinates still other flies to oviposit, and death of the parasitized animal might take place in as few as 10 days.

2) Sterile fly for African Trypanosomiasis:

Sleeping sickness or the African trypanosomiasis is the parasitic disease in humans. It is caused due to protozoa of genus Trypanosoma and transmitted through the Tsetse fly; the disease is endemic in some areas of Sub-Saharan Africa, covering around 36 countries and 60 million people.

Needs of Sterile-Insect Programs:

1) A process inducing sterility devoid of impairing sexual behavior of insects. 

2) Mass rearing of the insects economically: Even with low numbers in the natural population, most programs call for millions of insects to be generated and discharged weekly over a period of some weeks. This capability at times necessitates growth of a synthetic diet on which healthy and environmentally competitive insects can be reared. 

3) Information on the population density and its rate of increase (that is, reproductive rate)  

4) The released insects should not cause damage to the crops, live-stock or human beings: For example, the inundation of insects capable of transmitting arboviruses or plant pathogens in an area probably would be intolerable

5) Good intermingling of natural and released population. 

6) Releasing sterilized insects if the wild population is plentiful.

7) This procedure is efficient against newly introduced pest or isolated insect population as in the island. 

8) There must be high sterile to fertile (S: F) ratio for faster control. 

Limitations or demerits of Sterile-Insect Programs:

1) Not efficient against the insects that are the prolific breeders.

2) Sterilizing and mutagenic consequence of chemosterilants and irradiation cause problems in the higher animals and man (that is, Carcinogenic and mutagenic) 

3) As by means of insecticide treatment, repeated treatment is at times needed to suppress the population before the utilization of sterile insects.

4) Sex separation could be hard for several species, although this can be simply carry out on Medfly and screwworm. 

5) Radiation treatment in several cases influences the health of males, so sterilized insects in these cases are at a drawback if competing for females.

6) The method is species specific. For example, there are 22 species of Tsetse fly in Africa, and the method should be implemented individually for each.

7) Standard operating methods of mass rearing and irradiation don't leave room for mistakes. Since the fifties, if SIT was first employed as a means for pest control, some failures have occurred in various places all around the world where non-sterilized artificial generated insects were released prior to the problem was spotted.

8) Application to big regions must be long lasting, or else migration of wild insects from outside the control region could repopulate. 

9) The main demerit to this method is that the cost of producing such a big number of sterile insects is frequently prohibitive in the poorer countries.

SIT programs will gain tremendously when genetic processes can be build up that enable just male insects to be reared as has already been completed for the Medfly. Furthermore, more suitable artificial diets for larvae and hormonal, nutritional, microbiological and semiochemical treatments for adults, could make main contributions via enhanced economy and insect quality.

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