In a recent review, Lutrat et al. (2019) provide a comprehensive summary of current (since 2003) sexing methods available for various dipteran pest species. The review distinguishes between early and late acting sexing methods and elaborates on the techniques used to create the various sexing mechanisms. It highlights the advantages and limitations of each method and also compares the various aspects of the sexing techniques in terms of efficiency, costs and the released strain characteristics in the views of public acceptance.
Genetic control is all about interfering with the genetics of a pest species – typically by interfering with it’s reproduction. The best known example of genetic control is the use of sterile males which are released en mass to compete with wild males for wild females. To have a significant impact on the fertility of the target pest, sterile males need to outnumber the wild male population – a concept known as population overflooding.
Usually, genetic control acts through males, hence mass rearing of both sexes is waste of logistics and resources. Moreover, the release of both sexes can negatively affect the released males dispersal and deplete their mating resources. Sexing becomes even more important in the case of disease vectors, in which females are blood feeders.
Area wide genetic control programs typically require weekly release of millions of individuals. Therefore, even a small (<1%) proportion of female contamination is problematic as it may lead to increased biting (public nuisance) and spikes in disease transmission. Furthermore, in programs based on the release of Wolbachia infected incompatible males, the accidental release of fertile females has the potential to undermine the long-term sustainability of the program by replacing the target population with one that is Wolbachia infected.
Separating males from females in the laboratory – aka “sexing” can be approached in many ways, from the simplest mechanical separation based on naturally occurring sexual dimorphisms to the more sophisticated genetic engineered strains harboring for example mutations in key sex determination genes. Between different pest species and technologies there are dazzling number of varieties of sex separation methods.
Lutrat et al. (2019) provide the first review on the topic to report the numbers behind each parameter, allowing a side by side comparison for each method. The authors outline the main characteristics required for sexing. Accordingly, an ideal sexing method should achieve female elimination early in development while maintaining a high male recovery rate. The method should be adjustable for large scale production with a low initial investment and operational costs. In terms of public acceptance, the authors discuss issues relating to public concerns on the release of transgenic strains.
Despite all the latest developments in the field, well summarized in this review, the gold standard for any sexing strain system which actually meet all these requirements, remains the two decades old Mediterranean fruit fly ‘Vienna 8’ strain. Despite of its semisterility, and the necessity for two different colonies for its maintenance this strain is still, to date, widely considered as the state of art for any sexing system, and it is still the most fitted for large scale production. For other pest species, many genetic control programs (e.g. SIT, Gene drive, IIT) are still waiting for the large scale implementation of these sexing methods.
Although important, it should be pointed out that sexing is only the first hurdle for a successful, efficient and cost effective genetic biocontrol program. Labor safety and environmental effects should also be considered – especially when using antibiotics or insecticides as part of the sexing system. But ultimately, it is a matter of the production of a vigorous males that is capable to compete for mating with the natural population.
Célia Lutrat, David Giesbrecht, Eric Marois, Steve Whyard, Thierry Baldet, Jérémy Bouyer (2019).
Sex Sorting for Pest Control: It’s Raining Men!, Trends in Parasitology, 35:649-662, https://doi.org/10.1016/j.pt.2019.06.001.