Caterpillar land threat is assessed by farmers

In response to farmers being caught out by the invasive larvae of the porina moth last winter, a workshop funded by Beef + Lamb New Zealand was held at Te Tohe Road in the Mangaorapa district last week.

Recent field monitoring by farmers at Mangaorapa and Ashley Clinton showed that the population of porina caterpillars is high and that they reached the ideal size for control (15-20mm long) when using the lowest cost control option (an insect growth regulator).

Farmers in all areas of southern Hawke's Bay and Tararua are urged to check pasture paddocks for caterpillar activity.

In order to determine likely pasture loss, the formula is the percentage loss in pasture over the caterpillar feeding period (six to seven months), which is equal to half the population per m sq found.

Four caterpillars in a 20x20cm spade spit 30cm deep represent 100 caterpillars per m sq. They are capable of causing about 50 per cent pasture loss.

Concerned farmers set up three monitoring sites in January. This involves daily trapping and counting of moths, taking essential temperatures, plotting peak moth flight times and maturity rate of juvenile caterpillars.

High numbers of moths were recorded during mid-to-late February. Moisture was a key factor in survival and the late summer/autumn had been ideal.

The workshop was in two parts. The background biology of the pest was discussed, along with its unique southeastern North Island life cycle that involves an almost continuous flight of moths between September and April.

The role of temperature, to grow the young caterpillars and the effect the colder autumn has had on delaying caterpillar development, were discussed. Farmers were shown how to predict the time in finding the point where caterpillars are particularly vulnerable to an IGR insecticide.

In the practical session, farmers were shown the ideal way to break down turfs to locate and identify surface and early-burrowed caterpillars.

However, alarm bells were ringing for many as farmers turned up well-advanced and quite high populations of grass grub, Tasmanian grass grub and clover root weevil.

Porina caterpillars found ranged from 10-40mm long. Seventy per cent of caterpillars found were 10-20mm long.

This practical session showed the importance of correct identification and it was also generally agreed that farmers would have to be vigilant next February/March and would have to check thoroughly for grass grub and Tasmanian grass grub.