Clover root weevil in the South Island
News!
2 May 2008 - Populations of the CRW parasitoid are building up remarkably rapidly after the biocontrol was first released in the Nelson region in July 2006. Sampling by AGR in April 2008 found that 50% of CRW at Richmond were parasitised, while near Canvastown 45% were parasitised, and in northern Rai Valley 75% were parasitised. The biocontrol will now be helping to reduce clover damage at these sites, and will also be spreading to other nearby locations where CRW is present.
1 May 2008 - The South Island CRW distribution map has been updated to show locations where we think CRW is currently present and absent in the South Island.
11 August 2006 - Thanks to the sharp eyes of Barbara Stuart of the Nelson branch of the NZ Landcare Trust, AgResearch has confirmed CRW is present between Pelorus and Canvastown, as well as just north of Rai Valley township, but it hasn't yet been detected in Kaituna Valley. A report of its possible presence at Linkwater remains unconfirmed.
26 July 2006 - AgResearch scientists today made the first South Island release of the clover root weevil parasitoid biocontrol agent at Richmond. The release was funded by Dairy Insight and Meat & Wool New Zealand, with logistical support from Nelson Federated Farmers, the NZ Landcare Trust, and local farmers. >>
8 June 2006 - Dairy Insight and Meat & Wool New Zealand have agreed to jointly fund a release of the clover root weevil biological control agent in the Nelson region. AgResearch has begun mass rearing the agent, a parasitoid wasp, and is aiming to make the first releases at Richmond and Rai Valley in August 2006. >>
Background
Clover root weevil (CRW), Sitona lepidus, was first discovered in 1996 in Waikato and Auckland, and by 2004 it had spread throughout the North Island. It was first discovered in the South Island early in 2006.
The first South Island specimen of CRW was caught by AgResearch in an experimental insect trap at Christchurch International Airport in late January 2006. Subsequent sampling has found two more CRW individuals (out of several thousand specimens of other weevil species) in the vicinity of the airport, suggesting that if the weevil is established nearby, its population is still very small.
In April 2006, AgResearch found a large population of CRW established in dairy pasture at Richmond, near Nelson. There were also unconfirmed reports of CRW at Dovedale and in the Rai Valley.
In May 2006, AgResearch confirmed that CRW was indeed present in the Rai Valley, but no CRW were found at Dovedale. As of August 2006, the only confirmed reports of CRW have been from Christchurch, Richmond and Rai Valley. Unfortunately, CRW is almost certain to eventually spread throughout the South Island.
The problem
The adult stage of CRW feeds on clover leaves and the larval stages feed on clover roots. Young larvae tend to feed in clover root nodules (which capture atmospheric nitrogen), while bigger larvae will feed anywhere on the root system.
Clover roots capture nitrogen from the atmosphere. Livestock eat the clover (along with the nitrogen it has captured), then return the nitrogen to the pasture in their urine and dung. This nitrogen then becomes available to increase the growth of other pasture plants such as ryegrass. Clover therefore provides 'free' nitrogen fertiliser. When CRW feeds on clover roots, they reduce the amount of nitrogen that can be provided by clover, and this must be supplemented with bagged nitrogen to maintain productivity.
Clover is also a high quality food for livestock. CRW larvae and adults reduce the clover's growth rate and survival through the physical damage they cause to the roots and leaves. Thus, the amount of high quality food available to livestock is reduced.
In the North Island, CRW damage is particularly significant in dairy pastures. North Island dairy farmers have reported substantial loss of productivity due to CRW, with reductions in nitrogen fixation by clover of 50% to 100%. The amount of nitrogen fertiliser needed to compensate for CRW varies with soil type, fertility and climate.
Clover root weevil larvae (top)
and adult (bottom)
Lifecycle
CRW adults lay tiny black eggs on the soil. Small white larvae hatch from the eggs and burrow into the top 5 cm of the soil to feed on clover roots. Small, young larvae tend to feed in clover root nodules (which capture atmospheric nitrogen), while older, bigger larvae will feed anywhere on the root system. Once the larvae reach full size, they go through a pupal stage in the soil, from which they emerge as adults.
In the North Island, both the adult and larval stages of CRW can be found in pasture throughout the year. The number of adults peaks in spring and autumn.
Invasion cycle
In the North Island, the impact of CRW has been greatest a couple of years after it first arrived in a new location. CRW numbers can rapidly increase and become restricted only by the amount of clover available. Up to 1500 weevil larvae per square metre were counted in the soils underneath Waikato pastures in 1997.
Over time, CRW populations decline to lower levels which fluctuate according to clover availability and the weather. In dry conditions during summer, CRW adults reduce their egg laying and populations decline.
Clover should recover after the initial CRW invasion, but the amount of clover is usually less than before.
