Foundation for Arable Research (FAR) biosecurity officer Ash Mills says by late January, some Northland crops had well exceeded the 20 per cent threshold necessitating chemical intervention.
In maize and sweetcorn in New Zealand, the insecticide Sparta (active ingredient spinetoram) by Corteva is on label for use against fall armyworm (FAW). The product is also effective on other pest species.
By late summer, FAW activity had also been reported in other milder regions of the North Island including Auckland, Waikato, Taranaki, Bay of Plenty and Gisborne. However, with only small populations detected and harvesting of maize silage crops underway, these had not reached economic thresholds.
Populations were also detected in the South Island in Tasman, Marlborough, Westland and Canterbury, but at levels below economic thresholds.
Mills says that actively scouting crops and monitoring for any signs of FAW presence allows growers time to assess infestations and consider numbers in terms of economic damage and treatment thresholds.
Economic thresholds vary depending on crop, end use and growth stage. For example, damage to maize silage crops is unlikely to cause a severe economic loss, whereas any pest damage on sweetcorn cobs intended for human consumption will.
It is also important not to confuse FAW with other maize pests, particularly Cosmopolitan armyworm and corn earworm which can cause similar damage.
Overuse of chemicals can disrupt beneficial insects such as the parasitoid wasp Cotesiaruficrus and generalist predators such as spiders, which help manage egg and early larval stages of FAW.
Mills encourages maize and sweetcorn growers to share experiences with their neighbours, as well as talking to their advisor; communication at a local, regional and national level is important to learn how to manage this pest.
Up to three generations of FAW have been recorded in Northland in previous years, but indications suggest that up to four generations are likely this season.
Living up to its fall armyworm name, this means bigger numbers and potentially more damage as populations peak in autumn or ‘fall’.
“In more tropical climates, more than five or six generations of FAW can easily develop in a season; this is when severe crop damage occurs.
“Modelling shows that New Zealand is not expected to get five generations, but this year we will likely get four. However, it is also anticipated that by the time that happens, most crops will have been harvested,” Mills says.
An MPI Sustainable Food and Fibre Futures (SFFF) project, led by FAR and supported by Vegetables NZ and Process Vegetables NZ, is covering a range of topics including identifying economic thresholds for chemical control for maize silage, maize grain and sweetcorn as well as FAW lifecycle predictions and Integrated Pest Management strategies.
Use ta table available from FAR to guide FAW decision making. Only apply chemical treatments when the percentage of infested plants exceeds the threshold for your crop stage. This approach optimises control, protects chemistry and prevents unnecessary cost.