Using bacteria to control mosquitoes

Living inside the cells of insects is a type of bacteria that is looking increasingly like the key to controlling the spread of dengue fever, the Zika virus and other mosquito-borne diseases.

The Wolbachia bacteria, which occurs naturally in about half of insect species, interferes dramatically with insect reproduction. In mosquitoes, a male infected by certain types of Wolbachia can sterilise the eggs of a female without Wolbachia after mating with her – resulting in a complete loss of reproduction when the female mates only once, as is typically the case.

On the other hand, female mosquitoes that carry the same type of Wolbachia are resistant to this sterilisation.

One way this phenomenon can be used to control mosquitoes, and the diseases they carry, is to release millions of purpose-bred male mosquitoes that carry Wolbachia into ‘wild’ populations so that they can mate with and sterilise ‘wild’ female mosquitoes that lack that same type of Wolbachia.

The world’s most invasive mosquito, Aedes albopictus, has recently migrated from tropical into more temperate climes, and presents a suitable target for this technology.

But, efforts to suppress wild populations by introducing mosquitoes carrying Wolbachia are complicated because any Wolbachia-infected females accidentally released in the process will be resistant to sterilisation.

But in new research published in Nature, an international team of researchers, including our Pest & Environmental Adaptation Research Group (PEARG) at the University of Melbourne’s Bio21 Institute, have successfully used Wolbachia to suppress Aedes populations at trial release sites in Guangdong province near Hong Kong.

It’s a region which has suffered several dengue outbreaks caused by Aedes mosquitoes, particularly in 2014.

By combining Wolbachia infection with low-level radiation treatment to first sterilise any Wolbachia-infected females that may be released, the team have shown that releases involving millions of male mosquitoes could decrease the size of the Aedes mosquito population by more than 95 per cent.

Moreover, this suppression was maintained for many months and persisted for a time even after releases were terminated.

And because male mosquitoes don’t bite, there was a reduction in bites received from Aedes mosquitoes in the community. This helped generate widespread public support for the release program.

As a consequence of these trials, there’s now an alternative to pesticide for suppressing Aedes mosquitoes.

But, whether permanent elimination of mosquitoes can be achieved through this approach remains to be seen.

One important point to consider is that suppressed regions may become reinvaded from outside. Invasive pests by definition are very capable of moving or being moved into new areas.

Our previous PEARG research has indicated that Aedes mosquitoes are frequently transported into Australia by aeroplane from as far away as South America.

So it’s vitally important to understand mosquito dispersal in order to develop effective release strategies.

Our genetic analyses of Aedes albopictus in Guangdong have indicated that transportation by humans along roads and rivers is common, and, as the region has many options for travel like this, it’s likely that releases may need to be repeated frequently to maintain strong suppression.

Transportation by humans may be more important for reinvasion than mosquito flight, as we recently found that Aedes mosquitoes in high-rise apartment buildings tend to remain within their building of origin, with only occasional movement between nearby buildings.

Closer to home, PEARG is helping to evaluate whether these strategies might work in the Torres Strait Islands, where recently established Aedes populations threaten to invade the Australian mainland.

Circumstances are quite different to Guangdong, as movement between islands is likely to be wholly reliant on human transport, though further research is needed to quantify mosquito migration between the islands.

Low rates of mosquito migration between islands may make it possible to achieve lasting suppression of individual islands without a high frequency of releases, which would make Wolbachia-based suppression a suitable option for the Torres Strait.

But high migration, including influxes of Aedes albopictus from Indonesia, Papua New Guinea, or the Pacific Islands, could make it harder to achieve effective suppression through these releases.

As the advantages of Wolbachia-based mosquito control become more widely acknowledged, it’s likely that the methodology will start to be adapted for other insect pests.

Agricultural pests like aphids cause enormous economic damage worldwide, so these represent an obvious next step for the technology.

There are also many Wolbachia-like organisms that can affect their insect host’s biology, and we may expect to see further attention paid to these following the demonstrated successes of Wolbachia.

Banner: Public Health Image Libary/Center for Disease Control/James Gathany