Pasture pest under probe
Fresh research by AgResearch scientists will help unlock mysteries about a serious West Coast agricultural pest and allow farmers to make better management decisions and potentially save money.
Taihape veterinarian Anthony Oswald, who's been facilitating field days for Beef + Lamb NZ, says until recently there wasn't a problem with porina in the area.
But about three years ago the Rangiwahia region got severely affected and this season areas in the Kawhatua valley and in Ohakune have seen significant porina damage. Some of his clients are badly hit and fears persist that porina could become more of a pest there.
"I've seen whole hillsides bare because of porina. When high enough in numbers they can do real damage. If you find one to two porina per 200mm spade square, they eat the same amount of grass as one ewe per hectare.
"I have heard of farmers who have found up to 20 porina caterpillars per spade square so you can imagine the effect they can have on a farmer's stocking rate. It will impact winter carrying capacity and... farm profitability.
Oswald says a key factor in limiting the damage by porina is to know when the peak flights of the moths are occurring. This is why he's getting a network of farmers in different parts of the region to monitor porina moth numbers using simple traps – made from an ice cream container, hung next to an outside light, with water in the bottom – and counting the moths trapped every few days.
"By keeping a running tally on how many are caught we can identify the peak flights of the moths and then a certain number of days from that peak flight we start looking for the caterpillars in the soil," he explains.
Oswald says mean daily temperature also plays a part in determining when to start looking for porina caterpillars in the soil. From what they have discovered this season, the peak flights seem to occur in late February/early March, which means that by May or June the caterpillars should be at the best stage to start digging to see how many are present. This information is then used to decide whether to spray or not.
Porina can be killed by spraying the chemical diflubenzuron. But Bevis says it may be necessary to use an extra one called diazinon when there are larger caterpillars present at spraying. Spraying costs $23-$30/ha.
"Farmers need to be on the alert, that's why we are trying to get people to monitor the porina moths – we will then let people know when they should start looking for the caterpillars. If numbers are high enough they can spray before the caterpillars are big enough to eat a significant amount of pasture."
