Fall armyworm (Spodoptera frugiperda) has swept sorghum and maize crops throughout Queensland, with the state experiencing severe pest pressure. The southern grains region is advised to stay informed and be prepared for their arrival.
Here’s what you need to know about the latest threshold recommendations and a forecasting tool for the prediction of fall armyworm arrival in crops.
What’s gone down?
The invasive pest, first documented in Australia in January 2020, spread from the Torres Strait to Queensland by February of the same year, and further expanded into New South Wales and northern Victoria by February 2021. It has established a firm presence in the agricultural landscapes of northern Australia, but is not established in southern Australia.
As fall armyworm is a sub-tropical/tropical species, it faces a climatic challenge in Victorian winters, which are too cold for these moths to establish year-round populations. However, there are yearly incursions of fall armyworm into southern regions when it migrates south via wind currents, and during warmer seasons the pest can reproduce locally and pose a threat to crops in the south-eastern regions of Australia.
The ongoing spread of the fall armyworm increases the likelihood of more frequent southerly migrations, particularly in areas where the pest finds local conditions suitable for reproduction and establishment. The recent cyclone weather patterns may have facilitated the migration of fall armyworm populations into central Queensland.
Fall armyworm hits Queensland crops
We’ve already seen a high level of fall armyworm pressure on maize and sorghum crops in Queensland this year, indicating the challenges that may be on the horizon for more southern growers.
Unlike the last two years, the fall armyworm season in 2024 kicked-off early with a mild winter and warm spring, allowing for additional generations over the summer season. Each generation likely contributes to an overall increase in the population, resulting in elevated fall armyworm numbers, as seen in other fall armyworm outbreaks overseas when conditions are ideal for reproduction and development of the pest.
In 2023 the majority of sorghum and maize crops had already entered the reproductive phase by January-February, a less attractive growth stage to fall armyworms. This year, the challenging field conditions and increased rain activity experienced in some regions led to late planted crops that are now in the vulnerable vegetative growth stage.
The early onset of the fall armyworm season this year, coupled with late-planted crops, has created a perfect storm for heightened fall armyworm activity during a critical phase of crop development.
Local activity
There have been early reports of fall armyworm sightings surfacing in southern New South Wales. Whilst no reports have been received yet for Victoria, understanding the insect’s local activity is crucial for implementing effective management practices, as early infestations can cause major damage during the vulnerable emergence stage of young crops.
Cesar has conducted predictive modelling that suggests that the period from December to March offers ideal conditions for their population to thrive in the southeast regions of Australia potentially leading to more local migrations in the southern regions and a rise in population numbers. As we progress through the last stretch of summer and roll into autumn, there is the continued potential for fall armyworm population growth in Victoria.
We can anticipate a decline in April when conditions become less favourable for the invasive species in much of south-eastern Australia. Cooler temperatures, below 12 °C hinder larval development into adults. However, it’s important to monitor the seasonal forecast. Warmer-than-usual autumn conditions could extend the period of suitability for fall armyworm in south-eastern Australia.
Crop hosts and damage
Fall armyworm has a large potential host range, but it particularly favours grass species with whorls, like maize and sorghum.
Early instar larvae cause ‘windowing’ damage on leaf surfaces. In plants like maize and sorghum the larger larvae will hide inside the whorl, which can offer protection against insecticide exposure. They cause internal damage that shows up as ‘shot holes’ when the leaves unfurl and in severe cases, this damage can lead to the complete loss of upper leaves.
Continuous and heavy infestations can impact crops from emergence to the crucial grain-filling period. Worryingly, this year Queensland saw the persistence of fall armyworm infestations post tassel and post head emergence, something not observed in 2021-2023. Acting fast in high-risk situations and closely monitoring your crops from emergence is vital to mitigating the impact.
Counting costs: economic thresholds
While definitive thresholds are not yet established, ongoing research aims to refine our understanding. Current recommendations suggest that Helicoverpa armigera thresholds can be used for now.
Trials being undertaken at the Gatton research station, funded by the Queensland Government and GRDC in collaboration with QAAFI and Cesar Australia, has indicated yield losses in sorghum when fall armyworm densities exceed more than 1 med-large larva per plant in the crop for more than a week.
But rather than using this information as a definitive, it should instead be used as a flexible rule of thumb and considered with in-crop factors. Defoliation alone doesn’t uniformly lead to yield loss; key factors include larval density, infestation duration, and larvae development stage.
Younger plants are more vulnerable; larger larvae can eliminate a plant with up to 5 leaves by chewing through the base. Additionally, prolonged infestations lasting two weeks or more during the vegetative stage can be a cause for concern, especially if you’ve got one or more large larvae down in the whorl and preventing leaves from growing fully.
Despite defoliation, crops can compensate during early vegetative growth. Additionally, there is a considerable rate of natural mortality among small larvae, so if you’re not seeing large larvae in your crop, it might be worthwhile to delay action and revaluate after a few days. At this point, natural enemy populations are also on the rise and should be actively promoted and encouraged to help manage fall armyworm populations effectively.
Monitoring the march of Fall armyworm
Accurate control of fall armyworm depends on counting larval number and observing the current size range of the larvae. Relying on visible damage only offers a retrospective view, limiting effective control and potentially underestimating the actual threat.
Destructive sampling, involving opening up the whorl and counting the larvae, is the most reliable way to collect data on larval presence and size. Pheromone-baited traps, suspended at canopy level at multiple points along the crop, can also be used to detect early moth arrival.
Fall armyworm countermeasures
Before spray decisions are made, it’s important to note that fall armyworm is resistant to synthetic pyrethroids, organophosphates and carbamates, and these chemicals will be ineffective in providing control. Despite these challenges, effective insecticides like chlorantraniliprole, indoxacarb, spinosyns, and emamectin benzoate remain viable options for pest management. To manage resistance, always rotate chemical modes of action.
Trapping efforts in 2023 by The NSW fall armyworm trapping network (run by NSW DPI and Local Land services) revealed local persistence of fall armyworm for several generations, raising concerns about potential insecticide resistance, drawing parallels with Helicoverpa armigera which has exhibited similar patterns of resistance evolution.
However, endemic parasitoids, like the trichogramma family, and generalist predators such as ladybirds, spiders, native earwigs, predatory shield bugs, assassin bugs, and tachinid flies play key roles in natural control. This diverse array of biological control options underscores the importance of adopting sustainable approaches in pest management, such as using the beneficials chemical toxicity table to protect beneficial insects.
Resources
Plant Health Australia – FAW Quick Guide.
Wind Dispersal Forecasting Tool – this tool, developed by Cesar Australia, can be used to help predict when fall armyworm might arrive in crops based on wind forecasts.
Acknowledgements
Thank you to Paul Umina (Cesar Australia) and Lisa Bird (DPIRD) for their technical advice, as well as Lizzy Lowe (Cesar Australia) and Alexander Slavenko (Cesar Australia) for their review of this article.
Cover image: Photo by Frank Peairs, Colorado State University, Bugwood.org, CC BY 3.0 US
