Sterile Insect Technique: Pest Control

• The sterile insect technique (SIT) is an environmentally-friendly pest control method.
• It involves mass-rearing, radiation sterilization of a target pest, and systematic area-wide release of sterile males by air.
• Sterile males mate with wild females, resulting in no offspring and a declining pest population.
• SIT is an environmentally friendly insect pest control method.
• Irradiation, such as with gamma rays and X-rays, sterilizes mass-reared insects, which remain sexually competitive but cannot produce offspring.
• SIT does not involve transgenic (genetic engineering) processes.
• The International Plant Protection Convention categorizes sterile insects as beneficial organisms.
• SIT differs from classical biological control due to several factors.
• Sterile insects are not self-replicating and cannot become established in the environment.
• Breaking the pest's reproductive cycle, also called autocidal control, is species-specific.
• SIT does not introduce non-native species into an ecosystem.
• The IAEA, jointly with the FAO, helps Member States develop and adopt nuclear-based technologies.
• These technologies optimize agricultural insect pest management practices, supporting crop production and natural resource preservation.

Benefits of the Technique

• SIT was first developed in the USA and has been used successfully for over 60 years.
• It is currently applied on six continents.
• The four strategic options for deploying sterile insects in area-wide integrated pest management are: suppression, eradication, containment, and prevention.
• The Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture has studied SIT for over five decades.
• SIT involves research to improve the technique and develop it for new pest insects.
• The SIT package is transferred to Member States through field projects, benefiting plant, animal, and human health; environments; crop and animal production; and economic development.
• Integrated with other control methods, SIT has controlled high-profile insect pests, including fruit flies, tsetse flies, screwworms, moths, and mosquitoes.
• Retrospective economic assessments show a high return on investment in countries using the technology.
• Benefits of using the technology include a significant reduction in crop and livestock production losses.
• It also includes protection of the horticultural and livestock industries through prevention of pest introductions.
• SIT provides conditions for commodity exports to high-value markets without quarantine restrictions.
• It protects and creates jobs and significantly reduces production and human health costs.
• Environmental protection is achieved using a reduced use of insecticides.

The Sterile Insect Release Method (SIRM)

• SIRM of pest population suppression was first conceived by E. F. Knipling (1955).
• It is also known as "sterile male technique" and "autocidal control".
• The method involves releasing large numbers of sterilized insects into the environment.
• This reduces the probability that members of a natural population of the same species will successfully reproduce.
• The method has been almost exclusively associated with efforts to eradicate particular species from well-defined and limited geographic areas.
• Mathematical models by Knipling (1979) demonstrate that the method can bring about the total eradication of a defined population in a small number of generations.
• Its practical feasibility has been demonstrated by the eradication of the screwworm, Cochliomyia hominivorax, from Curaçao and peninsular Florida.
• It has also been demonstrated elimination of isolated infestations of the Mediterranean fruit fly, Ceratitis capitata, other fruit flies, and the gypsy moth, Lymantria dispar.
• It has also been applied in regional suppression programs against the cotton boll weevil, Antonomus grandis, mosquitoes and the codling moth, Cydia pomonella.
• Large numbers of insects are produced in a rearing facility and sterilized by radiation or chemical means.
• The sterilized insects are released into the natural population in sufficient numbers to ensure that most wild individuals will mate with a sterile insect, resulting in failure to produce viable offspring.
• Additional releases of the same number of sterile insects are made in subsequent generations, so that the ratio of sterile to fertile insects increases dramatically over time.
• Within a few generations, no fertile matings are likely to occur, and the wild population is eliminated.
• The method operates in an inverse density-dependent, or destabilizing, way.
• As the size of the target population becomes smaller and smaller, the effectiveness of a given number of released, sterile insects increases.
• This feature is what makes eradication theoretically possible.
• The method works best in conjunction with other approaches that first reduce the size of the target population so that fewer sterile insects need be reared and released.
• Success of the method depends on several assumptions that must hold true.
• Insects can be reared and sterilized in large quantities.
• Methods exist for distributing the sterile insects throughout the target area so they thoroughly mix with the wild population.
• The release is timed to coincide with the reproductive period of the target insect.
• The released, sterile insects compete successfully for mates in the natural environment.
• The release ratio (sterile insects to native, fertile insects) is large enough to overcome the natural rate of increase of the population, so that the trend in population size is downward after the first release.
• The target population is closed; i.e., there is no immigration of fertile insects from outside the release zone.
• Calculating the required release ratio for a particular situation is difficult.
• This requires an accurate estimate of the size of the natural population and its spatial distribution.
• It also requires knowledge of the natural rate of increase of the population.
• For a number of reasons, it may be desirable to release only sterile males.
• This requires a way of separating the sexes in the rearing facility and knowledge of the sex ratio of the wild population.

The Model

• The simulation is a version of the original model by A. J. Sawyer (1987), which has now been adapted to run under Microsoft Windows.
• It was written to illustrate the effects of changes in the assumptions of the simple mathematical models of Knipling (1979).
• It is named "Curaçao" after the Caribbean island on which Knipling first demonstrated the feasibility of sterile insect release by eradicating the screwworm fly.
• The basic model (using the default parameters in the setup file) uses the original assumptions of Knipling (1979).
• It is assumed that the target population exists in a well-defined zone.
• Its initial size is 1,000,000 individuals, half male and half female.
• The population will increase 5X each generation (up to a point) (200 eggs/female x 0.05 survival rate to the adult stage x 0.5 female = 5).
• There is no dispersal.
• Sterile males are released in a ratio of 9:1 (sterile to fertile males).
• The population is distributed uniformly throughout the zone, and the released insects are well mixed with the indigenous population.
• Sterile males are completely competitive with wild males in mating with females.
• The model is deterministic, in that the mean sex ratio, survival of immatures, proportion of females mating with fertile males, and fecundity all apply exactly, even in extremely small populations (those near extinction).
• Thus, random, or stochastic, events do not alter the outcome of a release program.
• Countless variations of the basic simulation can be made by changing parameter values or even the structure and underlying assumptions of the model.
• One of the more interesting variations is to introduce space into the problem.
• Three different levels of spatial resolution are possible: low, medium and high.
• In the low resolution case, the central target zone is surrounded by a second zone that is divided into 6 cells.
• This second zone may or may not have an initial population, as you wish.
• When a zone consists of more than one cell, the population may or may not be heterogeneous in that zone all cells need not have the same population density).
• Rates of survival and reproduction can be different in the second zone, to represent habitat of different suitability.
• Most importantly, now that there is space, you can have movement.
• You can specify emigration probabilities for each zone.
• This is the fraction of the population that will leave each cell during the dispersal phase of each generation.
• Order of events: Release (of sterile males), Mating, Dispersal (if any).
• The model can be changed to a stochastic one in which random events play a role, especially in small populations.
• The competitiveness of released sterile males can be set to some value less than 1.0.
• Estimation of native population size in the model is usually set to an estimate.
• Number of sterile insects released can be specified in each zone