Southland is one of the last farming regions in the country to be affected by the weevil with some farmers reporting destruction of clover this summer in paddocks needed for dairy production and fattening lambs.
About 80 people filled the Knapdale Hall near Gore on February 4 to hear Invermay scientist Colin Ferguson talk about the weevil, and they questioned why the biological control wasn’t available commercially. He didn’t have an answer.
The Irish wasp has been released on Southland farms for the past two years, funded mainly through DairyNZ and Environment Southland, but a mild winter has seen heavy infestations of the weevil with the wasp struggling to keep up.
Farmers were told even a moderate infestation would require 30 to 60kg/ha of nitrogen to be applied annually to limit the effects of the loss of nitrogen fixing by the damaged clover.
“For my farm that is going to cost about $15,000,” one farmer said.
“Why can’t I just go out and buy the wasp instead?”
Farmers at the end of the field day queued to receive jars of 100 weevils which contained grubs of the Irish wasp, produced in a lab at Invermay. The wasp lays an egg into the weevil which develops into a grub, killing the host after about three weeks. Each wasp lays about 70 eggs in her life and there can be three to four generations, depending on the weather, each year.
With not enough jars to go around, Ferguson said farmers could collect wasp-infected weevils from the Invermay farm near Dunedin. Landcare Southland regional co-ordinator Janet Gregory quickly organised several carloads to travel to Invermay and weevils were also
available at a farm near Mataura which was an early release site.
The weevils, which at Invermay are about 70% parasitised by the wasp, are collected using a leaf vacuum with a fine mesh bag.
Farmers were told to release the parasitised weevils in paddocks where they could see clover root weevil damage.
“Look for notching on the leaves. Don’t release them in paddocks where all the clover has gone because the weevils will have flown somewhere else. A warm gully is best, as the wasps multiply faster in warm weather, and upwind of the rest of your farm so they will spread easily.”
He said the weevil was an excellent hitchhiker.
“You will probably find them in your car on the way home from here. We even think they travel on planes.”
Clover root weevils were first detected in New Zealand in 1996 but it’s believed they were here as early as 1990, Ferguson said.
They were first found near the ports of Auckland and Tauranga so it was possible they had arrived in containers.
“They weren’t considered a major pest. The weevil is throughout Europe and they don’t have any problems there but when it came here it found a country with no pests and no diseases and lots of clover.”
He said the weevil and the wasp would survive a Southland winter.
“They live in Finland so they’ll do fine here.”
To limit damage, he said farmers had to look after their clover, sowing at higher rates and keeping grass short to give plants enough sunshine.
“When renovating pastures don’t go grass to grass. Use a crop such as a brassica in between otherwise the weevils will survive on the dying clover roots and then attack the new seedlings and you will have no clover at all come up. You need to keep the clover out of the paddock for five to six months. That doesn’t mean you will get rid of them, they will simply fly somewhere else looking for clover, but it means you will have a weevil-free seed bed when you sow.”
He said paddocks seemed to recover without resowing clover.
“There appear to be enough clover seeds in the soil for them to come back once the weevil has moved on or is in fewer numbers.”
Pugging, wet weather, mowing and cultivation did nothing to stop the weevil.
“They’re tough. You can’t squash them between your fingers. And insecticides don’t really work as they have to be broad spectrum which wipes out everything else and the weevil can hide right down in the clover root and sprays usually don’t penetrate that far down.”
Coastal areas of Southland as well as Mossburn and Te Anau seemed as yet unaffected by the weevil.
“But they’ll be there. Next year these areas will probably feel their impact.
“The weevil is in charge now in Southland but the wasp will dominate here, just like it has in the North Island. It’s just going to take a couple of years.”
Coast weevils come with parasite
Populations of the clover root weevil have been found on the West Coast but the good news is they have brought the Irish wasp with them.
The wasp is not a great flyer but the weevils, even when they have been parasitised by the wasp, can still fly up to 20km and are also known to hitch rides in vehicles.
AgResearch entomologist Dr Scott Hardwick said the Irish wasp has been confirmed at Little Wanganui, Westport, Cronadun and Greymouth.
“However, we’re concerned that the weevil may be getting a jump start on the wasp further south on the West Coast. In wetter areas, clover root weevils may not fly as readily as they do in summer dry areas such as Canterbury,” he said.
“This both limits the dispersal power of the Irish wasp, as it needs to be carried into new areas as eggs and grubs inside parasitised weevils, and leads to isolated ‘hot spots’ of the weevil which are often started by them hitching a ride on vehicles.
“We’re now considering carrying out releases of the Irish wasp south of Greymouth when new populations of the weevil are found, rather than relying on it making its own way.”
West Coast farmers who suspect they may have the weevils should contact AgResearch.
