Pink bollworm larvae (source: U.S. Department of Agriculture)

The pink bollworm is a small invasive insect that plagued U.S. cotton harvests for a century. Just by eating seeds and fibers, the pink bollworm larvae led to tens of millions of dollars in lost crops and pest control.

But, thanks to genetic biocontrol and other invasive species management strategies, these pests are now fully eradicated from U.S. cotton- producing areas, as of 2018.

Keep reading to learn more about genetic biocontrol and how it helped stamp out the pink bollworm and how it can help address other invasive species.

So, what is genetic biocontrol?

Genetic biocontrol focuses on how individual organisms and their genes can be used to control populations of invasive species that pose a high risk to their environment. When invasive species specialists release individual organisms with certain genetic qualities, like sterility, the species will stop reproducing. This keeps harmful invasives from multiplying.

How does it work?

The general strategy is that individual organisms of a species will either already have a gene or be scientifically altered to have a gene that will disrupt the reproductive process. Then, these individuals will be introduced into the general population. Once they mate, there are two possibilities: 1) the gene keeps the individual from reproducing or 2) the individual can reproduce but passes on the gene so its offspring will not be able to reproduce.

The scientific techniques used to modify individuals varies. Some can involve actually editing genes (similar to genetically engineered organisms) and some use different processes, like radiation.

One of the oldest is the sterile insect technique. Using this method, scientists create males that are sterile, and therefore unable to reproduce, by exposing them to gamma radiation. If a wild female mates with a sterile male, no offspring will be born, eventually leading to the population declining.

This technique has been used for a variety of species. The most successful application was with the screwworm, a parasite known to feed on live animals. The species was fully eradicated from the U.S. in 1966. This method was also used for the pink bollworm.

Another, newer technique is gene drives. This strategy is a form of genetic modification where a specific gene can be engineered so it is always passed on to the offspring.

If there was a gene drive for infertility, the first individual with the gene drive could still reproduce because the gene is not yet activated. But, its offspring would be unable to reproduce because the gene for infertility would now be activated. It takes several generations for the gene drive to fully spread through a population.

So far, this method has only been used in experiments but many in the conservation technology community are excited about its potential.

What species are genetic biocontrol used for?

The typical targets of genetic biocontrol are insects. It has also been used on fish, amphibians, and rodents.

Because insects can have a lot of offspring at once, their populations can grow extremely fast. This makes it especially challenging to curb the spread of invasive insects. 

Additionally, these bugs cause major problems for public health and  agriculture.

The main threat insects pose to public health is spreading disease. Vector-borne diseases, or human illnesses carried by other living organisms (like insects), result in over 700,000 deaths globally each year.

Asian Tiger mosquito (source: Center for Disease Control)

For example, the Asian tiger mosquito, an invasive species in the U.S., is known to carry West Nile virus and eastern equine encephalitis (which some might remember an outbreak of in 2019).

Using genetic biocontrol, invasive species specialists can keep these insects from reproducing, which would decrease risk of disease transmission.

Invasive insects also have significant economic impacts, particularly on agriculture. Approximately $13 billion in crops are lost to invasive insects in the U.S each year. 

In order to address these losses, the U.S. Department of Agriculture has spent $670 million on projects to protect crops and other natural resources from “plant pests and diseases.” It is unclear why this number is much lower than $13 billion but it could be that the projects do not require an equivalent amount of money to be effective.

Through using genetic biocontrol, the number of pests attacking agricultural crops and the amount of effort put towards stopping them could be decreased dramatically.

What are the risks?

For the sterile insect technique and sterile releases for other types of animals, the main danger is that they require adding invasive species into the environment. The sterile individuals are still invasive so, theoretically, they can cause the same amount of damage as the wild individuals. 

The number of sterile individuals required would vary depending on the species targeted. A study on West Indian fruit flies found a 10:1 ratio of sterile to wild males would be necessary. So, in a population with 1,000 wild males, 10,000 sterile males would need to be added to cause sterility in 80% of the population.

This could be particularly bad if sterilization didn’t work. Not only would the invasive population increase because of natural reproduction, it would also increase because of the introduced individuals themselves.

So far, there have not been complete failures, but this danger has stopped the use of sterile release for some species. For example, bullfrogs are a particular destructive invasive species in Europe. Knowing this, researchers have not used this method in order to avoid environmental damage from adding more bullfrogs.

For gene drives, a serious and ironic risk is that the gene drive could spread. There is a chance that individuals in the target population of the invasive species could move to a different, non-target population of the species and spread the gene drive. Once the gene drive is in a new population, it would act as it is designed and wipe out that particular, unintended population. Because of this, gene drives have not yet been tested in the wild. 

Although these risks exist, genetic biocontrol is an important solution to consider. Not only are there multiple strategies available, it has also been successful across different kinds of animals (i.e. mammal, reptile, amphibian, etc.). Additionally, there is already research being done on how to contain gene drives to target populations. With this research and the continued use of established methods, genetic biocontrol can continue to be a low effort approach to addressing invasive species and the harms they cause.