Figure 1: Fall armyworm life cycle.

Source: Coretext/iStock

Detailed genomic analysis is a powerful new way to track and manage internationally invasive insect pest species, such as fall armyworm

Despite international and national biosecurity precautions, the invasive fall armyworm (FAW) moth (Spodoptera frugiperda) is on the move.

At CSIRO, researchers led by Dr Wee Tek Tay have spent the past five years determining how it spreads to better understand which global biosecurity pathways to prioritise as surveillance hotspots. A key aim was to better prepare for future biosecurity incursions and improve post-invasion control measures.

What his team and collaborators found has shattered pre-existing narratives about the moth’s global movement eastward from its native range in the Americas via the occurence of a single introduction.

“What we actually found was a westward migration that did not rely on single introductions,” Dr Tay says. “Instead, we discovered multiple, independent introductions occurring across many sites in Asia and South-East Asia, with both  trade route introductions and natural movement implicated in the spread of  the fall armyworm from Western Africa.”

Ancestry DNA tests for moths

With investment from GRDC, Dr Tay’s Pest Genomics Team used next generation sequencing techniques to analyse FAW genetic diversity. This strategy identified unique signatures across the whole genomes of diverse FAW populations. These signatures allowed more accurate detection of the multiple independent introductions of diverse populations. For example, they enabled Eastern African FAW populations to be linked to specific South-East Asian FAW host populations.

In essence, the method works like ancestral DNA testing for humans, in which a sample taken from an individual can reveal that person’s most likely ancestors, where those ancestors originated and their migration routes.

For the large-scale population analysis, the CSIRO team used CSIRO’s Applied Genomic Initiative (AGI) to collaborate with scientists across Africa, Asia, South-East Asia, the Pacific and the European Union.

The study found that rather than jumping from the Americas to Western Africa, as previously assumed, the moth first made it to Asia before reaching Africa, highlighting Asia and South-East Asia as the biosecurity hotspots for FAW arrivals.

The new study also discovered genetically diverse FAW populations, revealing a far richer (and more complex) series of invasions.

For example, FAW populations in Myanmar and China were genetically distinct at the early reporting stage. This suggests they had different source populations.

“Separate and distinct populations of the pest have actually been introduced into Asia multiple times,” Dr Tay says. “This was likely due to international trade.

Identifying where human activity introduced FAW into new locations helps us to better target biosecurity measures and protect against further FAW introductions. It also helps to identify how the next exotic species may be able to utilise the same pathways.

This is important because the study also found subpopulations that carry genetic resistance to certain pesticide types. Avoiding additional incursions with pesticide-resistance subtypes is now a biosecurity priority.

The nightmare scenario is the coming together of multiple resistances within a population that leaves growers with no viable chemical control options.

Australian borders breached

The unexpected nature of the findings held true when Australia’s FAW situation was analysed.

FAW populations in Australia proved genetically diverse, with distinct genetic groupings in Queensland, the Northern Territory and Western Australia. This is indicative of multiple distinct introductions.

The incursions began in January 2020. The route for first spread was from South-East Asia and Papua New Guinea (PNG) through the Torres Strait and into North Queensland.

“Queensland populations from Walkamin, near Cairns, and further south at Mackay, are genetically closely related to the PNG populations,” Dr Tay says. “The PNG population shows links back to Yunnan
in China.”

In contrast, the Western Australian population is genetically more closely related to FAW in Malaysia and India, which suggests a different entry point in north-western Australia.

Importantly, different pesticide resistance profiles were detected within the Western Australian and Queensland populations.

“Interestingly, a follow-up study using samples from the second and third-year populations since the initial 2020 detection showed, we still have unique populations that have not yet assimilated, potentially indicating additional continuous arrivals,” Dr Tay says.

“That raises an urgent need to better understand how populations move between and within Australia – with an eye to tracking any pesticide resistance genes they carry – in order to frame more effective national biosecurity responses.

“Similarly, we suspect there may be populations with different responses to currently used pheromone lures. We urgently need a deeper understanding of these from a genomic perspective so that growers, policymakers and industry can understand, predict and manage this rapidly changing problem.”

A stronger biosecurity system

Overall, Dr Tay says that the global genetic profiling of FAW revealed that international biosecurity defences are more porous or  leaky than previously assumed. FAW populations were identified in close  neighbour countries that had been  unaware an incursion had occurred.

The moth is exploiting two modes to make its way around the world. The smaller larval and caterpillar stages favour human-assisted modes along long-distance trade routes, such as via air transportation of agricultural and horticultural commodities.

However, the adult moths are able to hitch a ride on trade winds. This later mode has been implicated as the pathway into the northern regions of Australia.

With all of Australia’s neighbouring countries already affected, the situation points to the importance of working with countries like PNG and Indonesia to improve biosecurity.

In that spirit, CSIRO hosts the Secretariat of the Association of South-East Asian (ASEAN) Action Plan on Fall Armyworm, which is funded by the Australian Department of Foreign Affairs and Trade.

Through the South Korean Rural Development Administration’s ASEAN Food and Agriculture Cooperation Initiative (AFACI), CSIRO is also providing ongoing support for FAW-related projects in Sri Lanka, Bangladesh, Bhutan, Cambodia and other Asian nations.

“Improved biosecurity in our neighbouring nations helps to both protect their agricultural production systems and create a biosecurity buffer zone at borders,” Dr Tay says.

Additionally, CSIRO is undertaking research on non-chemical methods to control FAW using genetic methods such as RNA interference (RNAi) technology. The genomic data will be immensely valuable to future R&D activities, and as a resource to develop monitoring tools to track resistance changes in the current populations.

The spread of FAW also highlights the need to get ahead of high-risk pests that are not yet in Australia. About 10 economically important pests have been identified and targeted for genomic profiling. The aim is to pre-emptively boost the ability to monitor, track and detect these pests. This will help countries in the region to delay and manage their spread.

“We were not able to prevent FAW reaching Australia or eradicate the moths when they arrived, but we can learn the lessons from that experience and be better prepared for the next pest threat,” Dr Tay says.

This project was co-invested by CSIRO, GRDC, the Australian Centre for International Agricultural Research, the Cotton Research and Development Corporation, FMC Australasia and Corteva Agriscience. It involved research partners from Australia, Africa (Uganda), South-East Asia (Cambodia, Laos, Vietnam, Myanmar, Malaysia, Indonesia, Philippines, Singapore), East Asia (South Korea), the Pacific (PNG) and the EU (France).

More information: Dr Wee Tek Tay,  weetek.tay@csiro.au