The biotech approach for the insect pest control

Novel sustainable methods for insect pest control, alternative to insecticides, are required today because of the negative effects of these substances on human health, the environmental impact of insecticides and the development of insecticide resistance. In the last decades, several approaches have been investigated (trapping via pheromone attraction, biological control through the introduction of natural enemies, mating disruption strategies, etc.) but these methods cannot address severe infestations on wide geographical areas at a cost-effective manner [1].

The Sterile Insect Technique (SIT) [2] is the first method involving insect genetics for population control of pest species. SIT is a species-specific and environmentally friendly method based on release of sterilized (by X or gamma ray) insects, reared in bio-factories, to decrease the progeny of an infesting population.  By competing for the mating with their wild counterparts, sterile insects can reduce the reproductive potential of the infesting population until the eradication (Fig. 1). SIT has been already successfully used against several pest insects (Glossina austeni – Zanzibar; Cochliomyia hominivorax - Libya and North and Central America; Bactrocera cucurbitae and B. tryoni – respectively, Japan and Australia; Ceratitis capitata, California, Florida, Mexico and Chile) [1].

Due to the preferential mating between sterile released insects, the release of insects of both sexes is less effective than male-only release [3]. Furthermore, the sterile females can still lay non-vital embryos into the fruit causing physical damages and consequent fungal and viral infections. In the same way, blood-feeding females of disease vector species are responsible for transmitting infections. For this reasons, females have to be removed from the mass production and release procedures in order to achieve economic advantages and effectiveness.

Despite the effectiveness and the advantages of the SIT, its application to date has been limited to only a few target species for two main reasons: 1) the damage to somatic tissues from the sterilizing dose of radiation results in reduced fitness and/or lifespan of released insects and 2) the lack of large-scale sex-separation methods for most pest species [1]. Effective genetic sexing strains have been produced for some species through classical genetics. These are based on translocation of a dominant selectable marker to the Y chromosome, present only in males [1]. However, these translocations are unstable resulting in labor intensive and costly mass-rearing systems [1-4]. Furthermore, a new sexing strain must be developed ex novo for each new species [1-5].

The recombinant DNA and the transgenesis technologies, combined with the knowledge of insect sex determination/differentiation systems, could be usefully applied to molecularly engineered insects for pest control. This has opened the road to a wide array of techniques to improve the SIT method [1-6]. As an example, genetically modified strains for mass-rearing have been designed to kill female offspring using sex-specific alternative splicing [7], to render the males “genetically sterile” while maintaining male sexual activity and competitiveness [8], or to transform XX female individuals in fertile and viable XX males [9].

These new technologies and the increasing knowledge in sex determination systems could help in future to improve the SIT and to apply this technique to a wide range of new species of interest.