Grass grub poses threat to pasture

POTENTIAL MENACE: A Tasmanian grass grub infestation, if left untreated, has the potential to wreck pastures and affect  production.
POTENTIAL MENACE: A Tasmanian grass grub infestation, if left untreated, has the potential to wreck pastures and affect production.

Nelson farmers are being urged to check for Tasmanian grass grub, a little-known pest that can cause serious pasture damage.

While the insect has been in New Zealand for almost 100 years, it has largely been confined to the lighter soils of Northland, Waikato, Bay of Plenty, Hawke's Bay, Marlborough and coastal Canterbury.

But in the past few years it has begun showing up in Nelson.

One farmer who has battled the pest is Lloyd Faulkner, who sprayed his 19-hectare property near the village two years ago and is facing having to do it again after another infestation.

He said the night air was thick with adult beetles on the wing the weekend before last.

"On the Saturday night we had a big flight of them, and Sunday was a shocker.

"I left the door open, and I had to get the Electrolux to clean them up off the floor."

Faulkner said that left untreated, the Tasmanian grub had the potential to wreck pasture and affect a farm's production.

"They chew the top off paddocks and leave bare patches.

"They certainly whack your winter grass off, make a mess and leave you with nothing in early spring."

He said the grubs were not easy to kill, although spraying did knock their numbers back.

He thought the porina caterpillar was causing the damage to his grass until he did some digging.

"We went and sprayed for porina and nothing happened, so we did a bit of digging and found it was these little sods."

Like porina, they tunnelled, and could "shift an awful lot of dirt on to the ground".

He said it was not unusual to find 40 in one spadeful. Agriseeds suggests that more than two grubs per spade square will cause pasture loss and are economical to control.

Dennis Meade, who farms in the nearby Sherry Valley, is among others in the district who have the pest.

He only became aware of it when he heard about Faulkner's efforts to control it.

"I don't know if farmers know they have even got it."

Other farmers in the region contacted by the Nelson Mail either had not checked for the grub's presence or did not know it was present.

Farmwise consultant Brent Boyce said he had not heard how bad they were this season, but an outbreak some years ago around Tapawera cleaned out whole paddocks.

Former agricultural contactor Paul Breeding said he had sprayed about six farms, mainly around Tapawera, over the past two years for the grub.

"Last year they were prominent. It is quite dramatic when they are out - they basically cover the ground, and the damage they do is noticeable," Mr Breeding said.

He said there appeared to be a lot of the insects around this year as well. "This year could be really bad because there are a lot of people out there who haven't done much about them."

Many farmers confused the grubs with porina and used the wrong insecticide to control them, he said. However, a new synthetic pyrethroid, Sheriff 100, apparently offered improved protection. The best time to spray was in March and April.

Don Wardle, a northern South Island territory manager for chemical company Nufarm, said the grub was now quite widespread throughout the Nelson region.

"It is quite patchy, but I've seen them in Motueka and Golden Bay as well as at Tapawera, and they seem to have got worst over the last few years.

"Not a lot of guys spray for them, but that's probably because they don't realise they have got them.

"They can be quite damaging, and most guys probably don't realise how much feed they are losing, and put it down to the dry or other things."

The Tasmanian grass grub added to the confusion many farmers had about what was damaging their pastures, Wardle said.

"They look like the native grass grub but feed like a porina."


Tasmanian grass grubs are smaller than their New Zealand counterparts and have a darker, almost black head and a whiter body. Unlike the native grub, the larvae crawl rather than assume a protective C shape, and feed on grass leaves rather than roots.

Adult beetles, which fly on still, warm summer nights, are attracted to dung and lay their eggs in or near it from January to March.

The larvae feed on this before moving on to pasture. From about April onwards, they tunnel into the soil, coming out at night to feed on grasses, clovers and lucerne. They feed through to August but may go on as late as November.

New or shorter, more open pastures are more vulnerable to damage.