Clover root weevil in the South Island

News!

April 2009 - CRW has now been detected in the South Island at various locations in upper Takaka Valley, Richmond, Rai Valley, Blenheim, Canterbury and Otago. 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.

Most of the CRW populations found in these localities currently occur at fairly low population densities and appear to have become established only recently. Relatively high population densities of CRW have been found only at Richmond and Rai Valley (both in 2006) and more lately in upper Takaka Valley and Blenheim (both in December 2008), and at one site in Canterbury (June 2008). The most recent detection has been of a single CRW from pasture near Clinton in Otago (January 2009).

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 and September 2008 found that 50% of CRW at Richmond were parasitised, while near Canvastown 45% were parasitised, and in northern Rai Valley over 80% 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.

Preparations are underway to release the CRW parasitoid at Takaka Valley, Blenheim and Canterbury through autumn and winter 2009.

The CRW parasitoid releases in NZ have been supported by Dairy New Zealand and Meat & Wool New Zealand. In the South Island, logistical support has been provided by Nelson Federated Farmers, the NZ Landcare Trust, and local farmers.   >>

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 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.