An unusually wet summer – even by South Island West Coast standards – could be influencing the behaviour of one of the region’s traditionally worst agricultural pests.

The numbers of porina moths caught in monitoring traps across the West Coast have been very low leading scientists at AgResearch to believe the significant rain over January could have reduced the number of moths flying compared with the previous year.

However, whether this is good news for farmers or will simply extend the moth-flying season remains to be seen. This issue is one of many that will be discussed at a series of upcoming field days near Whataroa, Hokitika and Greymouth aimed at educating farmers about porina - caterpillars that have the potential to reduce the long term quality and production of pasture.

Young porina caterpillars construct silk-lined casings on the soil surface, and as they grow they construct permanent burrows in the soil. They emerge from their burrows at night to feed, severing grass and clover leaves at the base of plants and dragging them back to their burrows where they are eaten.

Moths usually fly in large numbers during spring and early summer. They do not feed, but each moth can lay up to 3000 eggs on the pasture surface. Caterpillars and the damage they cause are mostly noticeable from April to September. At low densities, porina are direct competitors with stock for foliage and at higher densities plants are destroyed allowing inferior plant - or weed - species to establish.

AgResearch scientist Sarah Mansfield is urging farmers to take part to hear first-hand about the Sustainable Farming Fund (SFF) porina research programme specific to the West Coast.

“Until now, little has been known about the pest’s specific impact in this area,” Dr Mansfield says.

“So, the aim of these field days is to update farmers about the work we’re undertaking but to also provide fresh ideas for more efficient and effective management using current methods.”

In addition to SFF funding, the work is supported by DairyNZ, the Ministry for Business, Innovation and Employment’s Ecosystems Bioprotection programme, Westland Milk Products, Landcorp and the West Coast Pest Management Group.

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