Infestation of invasive armyworm pest has more than doubled since January

The number of farms where a tropical agriculture pest that destroys maize and sweetcorn crops has more than doubled since January, Foundation for Arable Research says.

Foundation general business and operations manager Ivan Lawrie said the fall armyworm moth larvae were found on 119 farms this week, up from 45 farms in January.

Biosecurity New Zealand deputy director-general Stuart Anderson said eggs from the pest were found in Tauranga for the first time in March last year. In December the larvae was found on 18 farms in the North Island, and by January it had spread to the South Island.

The moth was probably windblown from Australia, Anderson said.

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Lawrie said the udate did not include Hawke’s Bay and Gisborne because the of the Cyclone Gabrielle recovery efforts.

Maize and sweet corn crops in areas not affected by the cyclone should be inspected, Lawrie said.

Armyworm had not yet spread to Canterbury, a large grain growing area, Lawrie said.

Damage to maize crops would be limited at this time of year because the crops were past the growth stage where the larvae did the most damage, he said.

Damage seemed to be under control, he said.

As a tropical insect, the moth could not hybernate to survive New Zealand’s cold winter and was likely to die out, according to Biosecurity New Zealand.

But last year’s mild winter meant the moth had survived, Lawrie said.

A Niwa report from January said winter last year was the wettest and warmest winter on record because of the La Nina weather pattern, which was forecast to continue this year.

The foundation’s modellers were working to determine how the moth would spread given different winter conditions, Lawrie said.

If the next winter was not cold enough to kill the infestation it would need to be dealt with again in spring, he said.

The damage the crop caused in South Africa was compared to bilbical plagues and in Brazil the damage cost more than US$400m (NZ$650m) a year. It was estimated that Brazil spent about US$600m annually controlling outbreaks.

A predatory wasp introduced to control other pests, also attacked fall armyworm, he said.

The hope was that the wasp would make a dent in fall armyworm populations, he said.

Farmers and growers were obligated to report any finds of the larvae, which to help map of its spread, he said.

Earlier Lawrie said he was concerned that some growers were not monitoring crops and were not reporting infestations, because many farmers previously had bad experiences when reporting M bovis and were put under notice, he said.

There were no consequences for growers for reporting, because the pest could not be controlled by removing crops. An insecticide had been approved for use.

Director of readiness and response at Biosecurity New Zealand, John Walsh, said the likelihood of a fall army worm overwintering in many parts of New Zealand was low, as it needed an average winter temperature above 10C to survive.

However, climate modelling suggested it would be able to overwinter at the top of the North Island, and in other areas if there was a warm winter, Walsh said.

It was unlikely that fall armyworm could be eradicated because it was windblown from Australia, and more were likely to arrive again, he said.

Fall armyworm is unlikely to become the very serious problem it is in other parts of the world. It thrives in tropical climates. Because of New Zealand’s more temperate climate, it is not likely to have the high number of breeding cycles in a season to boost the population to really problematic numbers,” Walsh said.