NEW ZEALAND - One of New Zealand’s most serious pasture pests is on the move, this time being reported on the South Island’s west coast as parasitic controls struggle to keep up.
Clover Root Weevils (CRW) were first discovered in New Zealand in 1996 and plagued dairy pastures until an Irish wasp was released to control it in 2006.
North Island farmers, hit first by Sitona Lepidus, have reported clover losses of between 50 and 100 per cent.
The weevil, widespread in Europe and the US, causes considerable damage as an adult, particularly to white clover, leaving circular notches on leaves.
But farmers have been advised that it is the larval stage that wreaks the most damage.
Larvae feed on the nitrogen fixing nodules and roots of white clover, reducing pasture production and atmospheric nitrogen available in the soil.
Leading experts state 300 larvae per square metre can reduce clover dry matter production by 1000 kg/hectare.
Foliar insecticides can offer respite from Clover Root Weevils but Mark McNeill - a bio control expert at AgResearch New Zealand - explains this is complicated and can rely on getting chemistry to larvae in soils of high organic matter.
He also advised that seed coatings do not adequately protect and that reinvasion can occur when plants recover after an initial infestation is dealt with.
This is why scientists responded by releasing Irish Wasps microctonus aethiopoides, a small wasp with a particular preference for CRWs.
The wasp works by laying its eggs in the CRW, rendering it immediately infertile, before the eggs hatch out of the dead weevil.
So far the plan has been successful. The wasp has quickly established itself in core release areas and is spreading at 15-20 km annually.
AgResearch entomologist Dr Scott Hardwick is delighted with the wasps impact, which is now confirmed across vast swathes of dairy land.
But, he added: “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 weevil may not fly as readily as it does in summer dry areas such as Canterbury.”
As well as climate, traffic is also affecting CRW spead.
He said: "This both limits the dispersal power of the Irish wasp, as it needs to be carried into new areas as eggs inside parasitised weevils, and leads to isolated ‘hot spots’ of CRW, which are often started by CRW hitching a ride on vehicles."
Meanwhile, farmers have had tall fescue, red clover and chicory suggested as alternatives.
Specialists have cited red clover's deep roots as an option that provides drought tolerance and offers more summer and autumn production.
New Zealand is currently in the growing season, a time when weevils lay eggs.
CRW numbers depend on rainfall and if the summer is dry then populations suffer, resulting in a resurgence of clover.
Specialists say grassland can be maintained by nitrogen applications, best done ad libitum through 'a little and often approach' to feed weevil infested clover with a reduced nitrogen supply.