North Island farmers report bumper summer for native plantain moth

Farmers in the North Island are reporting to AgResearch plantain crop damage from a normally benign New Zealand native.

The Epyaxa rosearia is a widespread native New Zealand moth, occasionally reaching pest levels, that feeds on a range of plants including plantain. Its recent appearance in large numbers in plantain crops has given rise to the name plantain moth.

Reports started to come in around early December from farmers concerned about the number of plantain moth caterpillars they were seeing in their fields and the resulting damage occurring in plantain crops, says AgResearch scientist Colin Ferguson.

“Some farmers have reported up to 90% of their crop being decimated by the insect.”

Until recently the plantain moth has caused little concern to farmers.

“It is a native of which we know virtually nothing, but its activities this summer has put it firmly on our radar,” says Mr Ferguson.

“In the past plantain used to be planted within a mixture of ryegrass, chicory and clover. Due to its drought resistant properties we are seeing more and more farmers using plantain as a monoculture: single source pasture. These plantain crops should last a number of years but it looks as though populations of the moth build up in the first year and, assuming conditions are right, it is those second year crops that are getting hit by moth populations which increase exponentially.”

Mr Ferguson says the effects of the move to plantain as a single source forage crop was compounded by the very mild conditions last winter.

“The mild conditions allowed a greater number of moth eggs to hatch and more of the caterpillars and moths survived right through the winter. Warm, dry summers, like the one we are having now, are perfect insect breeding conditions and as result the plantain moth population has just exploded in some areas.”

Normally the growth of plantain is so great that feeding by these caterpillars has very little impact on plant production. Occasionally however, severe damage is reported as a consequence of very high caterpillar numbers and may be concentrated in some paddocks while others close by are unaffected.

Why, and how, this situation arises is currently unknown but is under investigation. Crop health, plant stress and growing conditions may have a significant role to play in the severity of damage observed.

Although current information suggests the moth is widespread throughout New Zealand, Mr Ferguson has had no reports of damage to plantain south of the Manawatu.

“This may be because weather conditions do not allow the moth to breed as many times as it does in northern areas.”

There are no registered insecticides for control of plantain moth caterpillars but broad spectrum insecticides with good activity against other caterpillars may also provide control of plantain moth. AgResearch scientists strongly recommend consultation with agrichemical professionals before using these insecticides for off-label use.

“We believe that treatment in early summer, if moths are present then, would knock back the season’s first generation leading to less pressure for autumn pastures,” says Mr Ferguson.

“Further investigation into this and grazing management practices is underway. As a general guide a healthy crop will tolerate more insect damage than an unhealthy one.”

Plantain moths are small, less than 20mm wide, light brown with darker spots and a distinct darker brown band towards the end on the wings. It belongs to a group of moths commonly called carpet moths.

Caterpillars are brown and small, less than 20mm long.

Very little is known about the biology of this insect.

Generally little plant damage is attributed to this insect but it has been recorded as causing damage to Caucasian clover and is commonly found in plantain stands.

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