Clover root weevil in the South Island

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 vicinty 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 have also been reports of CRW elsewhere in the Nelson region at Dovedale and in the Rai Valley, but (as of May 2006) these had not been confirmed. AgResearch was sent pictures of damaged clover from Dovedale in April 2006, but the damage did not appear to be caused by CRW. Also, a suspected CRW sent to AgResearch from Dovedale turned out to be another weevil species which isn't a clover pest. It is possible the clover damage at Dovedale is currently being caused by another less serious pest such as little lucerne weevil (Atrichonotus taeniatulus). Little lucerne weevil also appears to be responsible for clover damage recently observed at Okaramio, between Havelock and Blenheim.

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.