Genetic sex conversion could improve sterile pest control

Converting female insects into males using genetic techniques could make one of agriculture’s most successful pest-control methods significantly more efficient, say scientists.

Researchers from North Carolina State University examined recent advances in “conditional sex conversion” systems, which aim to transform genetically female insects into functional males only under controlled laboratory conditions.

The technology is being developed to improve the Sterile Insect Technique (SIT), a pest-control approach that has been used for decades against damaging agricultural insects. SIT works by mass-rearing insects, sterilising them and releasing the males into the environment, where they mate with wild females that then produce no offspring.

While the technique has proved highly effective, many programmes still release both sexes. The researchers say that releasing females alongside males reduces efficiency because sterile males may mate with sterile females instead of wild ones. In some species, released females can also damage crops or spread plant, animal or human diseases.

The review suggests that converting females into males, rather than simply eliminating them during production, could overcome both problems.

“From the perspective of an SIT program, sex conversion is appealing because it not only solves the problem of sex-sorting (by making it unnecessary) but also doubles the male yield, which translates into more efficient population suppression,” the authors wrote in the journal Current Opinion in Insect Science.

Controlled conversion

Unlike permanent genetic modifications, the systems under development are designed to be conditional. This means sex conversion is only triggered under specific conditions controlled within insect-rearing facilities, such as changes in temperature or the removal of tetracycline, an antibiotic commonly added to insect diets in laboratory and mass-rearing settings to control gene expression systems, from the insects’ diet. That allows breeding colonies to be maintained normally before producing male-only release populations.

Researchers have tested several different approaches in species including Mediterranean fruit fly, spotted wing drosophila, mosquitoes and the Australian sheep blowfly.

One promising strategy uses temperature-sensitive mutations in genes involved in insect sex determination. In spotted wing drosophila (Drosophila suzukii), females carrying the modified gene were reliably converted into males when reared at elevated temperatures.

However, the system also highlighted some of the remaining challenges. Survival rates were initially only 5-10% under the temperatures needed for complete sex conversion, although changing the timing of temperature exposure improved survival to around 80-90%. Even then, the converted insects were sterile rather than fully functional males.

Unexpected complications

Other systems use a genetic switch known as Tet-Off, where gene activity is controlled through the presence or absence of tetracycline. These have successfully altered sexual development in several pest species, but have also revealed unexpected biological complications.

One of the biggest obstacles is a process known as dosage compensation, which balances gene expression between the sexes. In some insects, disrupting genes that control sex determination also interferes with this balancing mechanism, causing insects to die before adulthood instead of developing as healthy males.

The review also discusses newer CRISPR-based approaches, including systems that use heat-activated gene editing to disrupt female development or male fertility genes. Although these methods demonstrate the potential of precise genetic control, none has yet reached the reliability needed for commercial pest-control programmes.

Instead, the authors argue that the recent work has substantially improved scientists’ understanding of insect biology while identifying where further research is needed.

They point to advances in genome sequencing, CRISPR gene editing, transgenic technologies and the discovery of sex-determining genes across multiple pest species as providing an expanding toolbox for future development.

Flexible genetic switches

The review also notes that researchers are beginning to investigate alternative gene-control systems beyond temperature and tetracycline. Plant hormones such as auxin and other inducible genetic switches may eventually provide more flexible ways to activate sex conversion without some of the drawbacks associated with current methods.

Although no conditional sex-conversion system is currently ready for field deployment, the authors conclude that progress has accelerated in recent years.

“Given recent advances in the field, effective conditional sex conversion systems appear to be within reach and could greatly aid in the fight against insect pests,” they wrote.

If successful, the technology could allow sterile insect programmes to produce only males without the need for physical sex sorting, improving efficiency while reducing the costs of controlling damaging agricultural pests.