The pest putting sorghum to the test - understanding the fall armyworm

A surge in the fall armyworm population throughout Queensland earlier this year has been a steep learning curve for growers and agronomists, who are striving to understand the pest's impact on Australian farming systems.

Since its arrival to Australia in early 2020, the highly destructive fall armyworm, a species known for its significant migratory capacity, had caused relatively limited damage.

However, this changed over the past 12 months due to a “perfect storm” of conditions.

Nutrien Ag Solutions Clermont Branch Manager and Senior Agronomist, Anthony Lee, believes that a milder winter in 2023, followed by early pre-Christmas rain in Central Queensland, created an abundance of weed and pasture biomass favored by the pest, leading to a population surge.

“The early crop had a severe infestation of fall armyworm, right up to flowering, then it tapered off over a couple of weeks and as the season turned dry, they basically became non-existent,” Mr Lee said.

Known to attack more than 350 commercial and non-commercial plant hosts worldwide, the fall armyworm's behavior in Australian conditions and effective management strategies have been the major research focuses for scientists.

“Since the pest’s arrival in Australia, the advice has been to not grow corn in this climate,” Mr Lee said.

“There’s now a lot of information coming together on how to manage corn to limit the damage of the pest, but part of the challenge for us over summer was there’s not a huge amount of information on sorghum.

“We’ve never had a huge amount of fall armyworm in sorghum, and we’ve often been told sorghum isn’t one of their favoured crops to attack, so no one really knows the economic thresholds and the pressure points.”

Learning the pest

The uncertainties of the fall armyworm made ongoing crop monitoring crucial for understanding and managing the pest.

“What this season did show us is sorghum can withstand quite a bit of pressure. Some paddocks were completely chewed out, but they did regrow. Whether they had an economic loss or would have been worth anything, is harder to say,” Mr Lee said.

“The sorghum plant is very persistent - it will continue to try to grow fruit, because that’s its job, and it’s got a very aggressive root system, so it will always try to continue reshooting.”

This persistence and ability to yield even after significant pest pressure was evident in the sorghum crop Dave Daniels grew on his property, Tarvellon, in the Kilcummin district.

Mr Daniels planted 1000 hectares in January and said fall armyworm affected a large area of the crop, with some parts “decimated”.

“In the past, we’ve had a little patch here and there, and we’ve not worried too much about them, but this year they came in and did a lot of leaf damage, which frightened the hell out of us,” Mr Daniels said.

“We were told you can have a lot of leaf damage in sorghum and not get any yield reduction, because the plant has a lot of extra leaf area than it needs.

“After looking at it, we decided not to treat the crop.”

In addition to “one little shower of rain, which bucked the sorghum up and gave it more leaf”,

Mr Daniels believes he was lucky to only be affected by one generation of the pest population.

“They came through and laid once. Whereas I think if we’d gotten another laying, it would have been disastrous,” he said.

Management options and limiting resistance development

This measured approach to applying insecticide is one which Mr Lee believes will be widespread in coming years, due in part to the learnings of the recent crop, the pest’s behaviour and life cycle, and its ability to tolerate many chemical groups.

“These guys will put an egg raft right down low on the underside of a leaf, and the minute they hatch, the larvae will very quickly move to the leaf extremities and try to get caught on the wind to move away from the initial hatch, or they will start crawling immediately to the whorl of the sorghum plant, because they don't like sunlight and are trying to avoid predation,” he said.

Additionally, controlling the fall armyworm will be influenced by factors such as temperature, soil moisture levels, soil nutrition, and commodity prices.

“More focus on the season leading up to the sorghum plant and our timing of planting will be important for managing the pest,” Mr Lee said.

“We understand now that if there’s a wet build-up and we’re planting early, we’re going to have increased pressure.

“We also know we can tolerate a bit more defoliation from the fall armyworm than we first thought, as long as we’ve got moisture in the system.”

If the decision to apply insecticide is made, Mr Lee said the key will be giving the chemistry every opportunity to work.

“Correct application is better than more chemistry, and we know one really well-timed spray can get us through in most cases, but the gross margin and return to the grower will be the dictating factor,” Mr Lee said.