Kill-all no cure-all for fall armyworm
Australia’s voracious new pest, the fall armyworm, is rapidly spreading south and a leading front-line industry advisor says while drastic extermination measures often fail, a more integrated approach is preserving crop yields.
Toowoomba technical services manager Maree Crawford is spearheading Elders’ response to the fall armyworm, after it was detected in February this year in a maize trial site at Georgetown, Queensland.
“This pest is a serious threat to some crops and there is a tendency for growers and advisors to overuse synthetic pyrethroids up front and this approach is not proving successful,” Ms Crawford said.
“Elders agronomists address fall armyworm outbreaks in a very structured way using an integrated pest management approach based on the individual circumstances.
“You have to take a lot of factors into account, like the weather, the lifecycle stage, the crop species and its risk profile, and the extent of the infestation.”
Ms Crawford said the greatest challenge was to get adequate coverage of the larvae with pesticides because they burrowed deep into plant, increasing the risk of chemical resistance.
For that reason, as well as their devastating impact on beneficial insect populations, Ms Crawford advised against blanket applications of synthetic pyrethroids.
“If you nuke everything, you’re not going to have the beneficials to clean up new egg lays and hatchings underneath the leaf where your sprays have failed to reach all the fall armyworms,” she said.
“Where people have tried a total knockdown approach, we’re seeing them back there in big numbers in as little as 14 to 21 days, depending on temperature and day length.”
Rotating chemicals that preserved beneficial insects like the predatory Cotesia and Trichogramma wasps in conjunction with biotech products like Magnet, which targeted moths and reduced the amount of eggs laid, worked very successfully when infestations were caught early, Ms Crawford said.
Once fall armyworms got past the 10 millimetre-long third instar stage, Ms Crawford said, it was almost impossible to prevent large-scale crop damage.
“When they are transitioning from the third to the fourth instar, we find them inside the whorl of the plant,” she said.
“They cover themselves down in the whorl with frass, which is their waste, and the chemical can’t penetrate that, it just sits on top and doesn’t get to them.”
It was critical to spray in the evening, when the largely nocturnal larvae were out feeding, Ms Crawford said.
Water rates needed to be exceptionally high, too, and she said Elders was working with a spray company to develop a new nozzle application specifically targeting fall armyworms in maize crops.
The pests seemed to favour all types of maize crops, as well as sweet forage sorghum, she said, and could move up to 500 kilometres in as little as two days in the right conditions.
“We watch weather fronts, moth flights, and work out where the most likely incursions are going to be,” Ms Crawford said.
“With this particular pest, you can’t afford to take anything for granted. You need to be scouting constantly to get onto moths and hatchings at the early stages. If you get a change in the weather, get out with a head torch and look for them every night for about a week.”
Elders advisors were also using satellite technology and drones to scout for crop damage, deploying trap networks and teaming with government entomologists to track new infestations.
The fall armyworm may make it as far south as Adelaide and Ms Crawford said it had already been detected in the southern wheatbelt of Western Australia and into southern New South Wales.
In the northern regions, fall armyworm would be a year-round challenge but she said outbreaks would be limited to summertime further south.
Part of the challenge was to correctly distinguish the fall armyworm from the also voracious but still less devastating northern armyworm.
In the first instars, the two looked extremely similar but four raised spots on the hind quarters of the fall armyworm grubs and pale patches near the moths’ wingtips were the tell-tale signs.
And, while experts were still divided on the right threshold for spraying, Ms Crawford said as few as two larvae per plant in maize crops could cause “a fair bit of grief”.
“We have seen up to 80 per cent leaf surface area damage and barren plants due to the damage to reproductive tissue,” she said.
The good news was that, unlike sorghum, maize crops with high levels of leaf damage were still able to yield well if reproductive tissue comprising silks and tassels were intact, Ms Crawford said.
“It is essential to use an integrated pest management approach to fall armyworm infestations from the outset, using a wide range of strategies such as soft chemistry, biotechnology, Magnet and beneficials to protect the reproductive tissue so there is less likelihood of major damage to cobs,” she said.
“It’s been a concern, but I think it’s manageable, I really do.
“With proactive, vigilant practices and the array of chemistry that we’ve got access to, we can manage the pest in most crops and limit damage.”
Find out how Elders can help you