Through genetic and molecular manipulations, strains can be developed that are more suitable for the sterile insect technique (SIT). In this chapter the development of genetic sexing strains (GSSs) is given as an example. GSSs increase the effectiveness of area-wide integrated pest management (AW-IPM) programmes that use the SIT by enabling the large-scale release of only sterile males. For species that transmit disease, the removal of females is mandatory. For the Mediterranean fruit fly Ceratitis capitata (Wiedemann), genetic sexing systems have been developed; they are stable enough to be used in operational programmes for extended periods of time. Until recently, the only way to generate such 427

BackgroundThe sterile insect technique (SIT) is an environment-friendly method used in area-wide pest management of the Mediterranean fruit fly Ceratitis capitata (Wiedemann; Diptera: Tephritidae). Ionizing radiation used to generate reproductive sterility in the mass-reared populations before release leads to reduction of competitiveness.ResultsHere, we present a first alternative reproductive sterility system for medfly based on transgenic embryonic lethality. This system is dependent on newly isolated medfly promoter/enhancer elements of cellularization-specifically-expressed genes. These elements act differently in expression strength and their ability to drive lethal effector gene activation. Moreover, position effects strongly influence the efficiency of the system. Out of 60 combinations of driver and effector construct integrations, several lines resulted in larval and pupal lethality with one line showing complete embryonic lethality. This line was highly competitive to wildtype medfly in laboratory and field cage tests.ConclusionThe high competitiveness of the transgenic lines and the achieved 100% embryonic lethality causing reproductive sterility without the need of irradiation can improve the efficacy of operational medfly SIT programs.

One of the main obstacles for a wider use of the Sterile Insect Technique (SIT) against the Mediterranean fruit fly (medfly) is the damage commercial fruit suffers due to sterile female stings. To overcome this obstacle, the Joint FAOlIAEA Division of Nuclear Techniques in Food and Agriculture has played a leading role in sponsoring and carrying out research to develop medfly genetic sexing strains that allow male-only SIT releases.Recently, as a result of this continued FAOjlAE.4 effort, genetic sexing strains based on a temperature sensitive lethal (/sl) mutation have been developed at the IAEA Laboratories at Seibersdorf. Unlike previous pupal color sexing strains, these 'second generation' sexing strains allow female killing at an early (embryonal) stage. In addition, they are essentially stable under mass rearing conditions. This represents an important breakthrough because both of these attributes were considered indispensable for genetic sexing strains with any potential to replace conventional strains with both sexes in large scale sterile medfly production facilities. Besides the considerable savings in the costs of release and field monitoring, genetic sexing strains in field tests have shown severalfold increases in the effectiveness of the SIT as compared with the standard strains involving males and females. When releasing both males and females, sterile males are apparently not used effectively, because they use their limited sperm mostly to mate with sterile females and because they do not disperse widely in the presence of these females. When males only are released, however, they disperse much further in search of wild females and compete more intensely with wild males for wild females.As a result of the availability of usable male-only strains, and the demonstration of their increased effectiveness, the applicability of the SIT against medfly has increased in two different ways. Highly developed commercial fruit growing regions, that previously had excluded application of SIT because of the fruit damage due to sterile female stings, are now reconsidering such free area/exclusion programs. More-importantly, sterile male releases can now also be used for routine control purposes, rather than only for eradication programs, partially or fully replacing chemical bait-sprays during the fruiting seasons.

When considering a mosquito release programme, one of the first issues to be addressed is how to eliminate/separate the females. The greatest number of options might eventually be available for those who can use transgenic mosquitoes, but the inherent characteristics of the target species may also provide possibilities for interim measures until more efficient methods can be developed. Differences in intrinsic size, in behaviour and in development rate between females and males are often available and useful for sexing. Efficient species-specific systems for eliminating females at the embryo stage have been developed, but most have since been discarded due to lack of use. Ideal systems specifically kill female embryos using some treatment that can be manipulated during production. Such killing systems are far more efficient than using intrinsic sexual differences, but they systems require selectable genetic markers and sex-linkage created by rare random chromosomal rearrangements. While intrinsic sexual differences should not be considered as long-term candidates for the development of robust and efficient sexing approaches, in the absence of these, the accessibility and integration of less efficient systems can provide a stop-gap measure that allows rapid start up with a minimum of investment. The International Atomic Energy Agency is funding over a 5 year period (2013-2018) a new Coordinated Research Project on "Exploring Genetic, Molecular, Mechanical and Behavioural Methods of Sex Separation in Mosquitoes" to network researchers and to address the critical need of genetic sexing strains for the implementation of the sterile insect technique (using radiation-sterilised or transgenic male mosquitoes) and for insect incompatibility technique programmes against disease-transmitting mosquitoes.

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