Fall armyworm is causing 2t/ha yield losses, but it is manageable - at a cost

That's one of the issues experts are grappling with and one they discussed at the Warra GRDC grains research update on Tuesday.

Queensland Alliance for Agriculture and Food Innovation agronomist Joseph Eyre has been working on a FAW project with other researchers and presented an update on FAW impacts on grain yield.

"The key message I want to get across is that fall armyworm is manageable, but losses can occur and they can be substantial," Mr Eyre said.

"In three different seasons now, we've measured yield losses in sorghum - some of them 16 per cent - but it's important to note this is large larvae-infested crops."

Mr Eyre said sorghum yield reductions of up to 1.2t/ha occurred when the crop with 10 to 12 fully emerged leaves was infested with high densities of large larvae (5th or 6th instar or development stage).

He also said sorghum defoliation due to FAW infestation didn't always cause yield losses and ongoing research was investigating contributing factors.

While researchers are beginning to understand how much of an impact FAW has on yields, there is still ongoing discussion about if it's more costly to treat than accept a reduction in yield.

Interim results suggest that FAW control should be considered when medium to large FAW (4th to 6th) are present on seedlings i.e. crops with less than six fully emerged leaves on the main stem.

FAW control should also be considered when FAW infestations begin to damage crops at V6 to V8 development stage and establish greater than one large larva (5th to 6th instar) as the 10th leaf fully expands.

Queensland Department of Agriculture and Fisheries principal entomologist Melina Miles has also been studying the pest in great detail.

"Fall armyworm is manageable, but at what cost?" Ms Miles said.

"Until we have a better idea of what that numerical threshold might be, the best we can do is to say that the risk of incurring significant defoliation and potential yield loss is there for the late summer plant period rather than the spring plant period.

"You would be budgeting for sprays if you were planting in December-January or February even - depending on the season."

Ms Miles reiterated Australia had only experienced three seasons of FAW and knowledge was evolving.

"[At Gatton, from] January to May, there is a lot of activity, then it drops off and doesn't pick back up till December," she said.

"If that's the pattern we see year in, year out, then you'd be quite comfortable saying there's a much higher risk for the late summer plant as opposed to the earlier summer or spring plant."

The researchers have also made a new discovery on the way the pest affects crop development.

"This year, for the first time we observed that the fall armyworm was not only eating the foliage - reducing the amount of canopy and reducing the amount of light converted into energy - but we also saw yellowing, [where] the efficiency of that conversion of light into energy was reduced," Mr Eyre said.