S TERILE INSECT TECHNIQUE

The sterile insect technique (SIT) is an environmental-friendly control method also referred to as a kind of birth control (2.2). In principle, a large number of reproductively sterile male insects are released into a wild population of the same species so that they mate with wild females and thereby block their reproductive capacity (Fig. 1) (Knipling, 1955).

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A successful use of SIT for pest control requires a sufficiently high overflooding ratio of mass reared, sterilized, and released males, which are able to compete with wild males and to mate with wild females. There are several different means to implement SIT in area-wide IPM (AW-IPM) programs, known as eradication, suppression, containment, and prevention (Knipling, 1979).

First of all, the eradication strategy is defined as an application of phytosanitary measures to eliminate a pest from an area (FAO, 2005). It was first shown to be successfully eliminating the New World Screwworm Cochliomyia hominivorax from the island of Curacao (Baumhover et al., 1955). This eradication technique was then used for more than 40 years to eliminate the screwworm from the USA, Mexico, and Central America up to Panama (Vargas-Teran et al., 2005). Eradication programs have the ability to eliminate complete pest populations species-specifically and lead to a reduction in the use of insecticides implying a long-term benefit for the environment. In addition, eradication allows the establishment and declaration of “pest free areas”, which can permit access to otherwise closed export markets (Malavasi et al., 1994). In the context of AW-IPM programs the eradication strategy is often used during the last phase of the program and mainly for eliminating an established pest population (e.g. the tsetse fly Glossina austeni in Zanzibar) or to eliminate outbreaks of invasive species before their full establishment (e.g. New World Screwworm from Libya (Krafsur and Lindquist, 1996), painted apple moth from New Zealand (Suckling, 2003) or the Mediterranean fruit fly from the Los Angeles Basin in 1980-1996).

Second, the suppression strategy is an alternative to the eradication. By definition suppression is the application of phytosanitary measures in an infested area to reduce pest populations (FAO, 2005).

Suppression strategies are used to maintain the pest population below defined levels to ensure the economic health. SIT strategies and especially long-duration suppression strategies as a part of AW-IPM programs have become more popular since several premises and views have been changed. Thus, increased restrictions on the use of insecticides in combination with the improved excess to mass-reared key pest species have significantly improved the cost-efficiency of SIT as part of AW-IPM programs (Caceres et al., 2004; Matteson, 1995). In addition, an increasing demand of organic products leads to an expansion in using environmental-friendly pest control strategies like the SIT (Economist, 2001), which can replace the use of several chemicals. SIT suppression programs for pest species are less complex and management intensive than eradication programs and can be achieved more quickly and cost-effective in the initial years (Mumford, 2005). However, suppression strategies require continuing releases of sterile insects to maintain a low population level. Suppression programs were successfully run for the oriental fruit fly in Thailand (Enkerlin, 2003), the codling moth in British Columbia, Canada (Calkins et al., 2000), and the Mediterranean fruit fly in Israel and Jordan (Rössler et al., 2000), in Madeira (Pereira et al., 2000) and in South Africa (Barnes et al., 2004).

Third, the containment or prevention strategy is defined as an application of phytosanitary measures to prevent spread of pest in and around an infested area or to avoid the introduction of a pest into pest free areas, respectively (FAO, 2005). Examples for containment strategies are the Queensland and

Mediterranean fruit fly programs in Australia (Jessup et al., 2007), the New World screwworm program in Panama or the Mediterranean fruit fly program in Guatemala-Mexico and Peru-Chile. All these programs avoid the establishment of invading exotic pests or consolidate the progress made in an ongoing eradication program. Containment programs are able to protect neighboring pest free areas, which can be expanded gradually. Achieving an intensive cooperation as well as the disruption of trade and free movement of commodities between infested and non-infested areas is absolutely necessary, but at the same time displays two major problems to solve in containment programs.

In contrast, preventional programs are carried out in pest free areas to maintain a pest free status. This preventional strategy was described by Edward F. Knipling as the probably most cost-effective variant of using SIT (Knipling, 1979). A preventional strategy is useful if an area is under constant threat of pest invasions, which can be prevented from their development and establishment by releasing sterilized males of the same species. E.g. preventional releases of sterile melon flies are applied in Okinawa (Japan) to avoid reestablishment of melon fly coming from Taiwan (Kuba et al., 1996). After technically, politically and environmentally successful eradications in the year 1996, the probably most visible preventional medfly SIT pest programs were started to prevent the areas of the Los Angeles Basin (California, USA) and Tampa-Miami (Florida, USA) from new Mediterranean fruit fly infestations (Hendrichs et al., 2002). For all parties of a pest management program there seems to be no more biologically and economically efficacious, environment-friendly and cheaper method to prevent and exclude medfly from these areas (CDFA, 2000).

Several examples described above showed that it is possible to run a successful AW-IPM program including a SIT strategy, but these programs have to be well planned and various important program phases have to be managed. The anticipated phases for a SIT program are as follows:

Pre-intervention phase. Data on the distribution and the population dynamics of the target species have to be collected (Ito and Yamamura, 2005; Vreysen, 2005). In addition, an infrastructure for mass-rearing, sterilization, packing, releasing, and quarantine has to be established and public relations work has to be started (Dyck et al., 2005b).

Population suppression phase. Due to the required overflooding ratios in SIT programs, SIT is most effective on relatively low-density populations. Prior to any releasing action from within SIT programs, the population has to be decreased by other pest control methods (2.2), if not already at a low level as a result of climatic conditions (changes summer-winter) or natural decline (changes due to the life-cycle of the target species) (Mangan, 2005).

Release phase. Repeated releases of sterile insects over the infested area are carried out to reduce target populations to an acceptable level (suppression strategy), to eradicate target populations (eradication strategy), or to avoid new pest infestations (containment and preventional strategy) (Dowell et al., 2005).

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Maintenance and verification phase. If the aim of a SIT program was the eradication of a pest species, a pest free status is confirmed and preserved by permanent implementation of monitoring and quarantine activities. Once a low prevalent status of a target area has been achieved, suppression or containment and preventional releases are further carried out (Barclay et al., 2005).

Beside the fact that SIT is a successfully practiced component of area-wide pest management programs, limitations exist in various steps of the described SIT program, which can be optimized to widen the possibilities for this environment-friendly pest control technique.