Dairy NZ

The most obvious sign of CRW is the distinctive notching on clover leaves caused by adult feeding. If notching is observed, further evidence of CRW can be found by digging up some clover plants to look for larvae in the top 5cm of soil near clover roots.

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.

For more information:

• Farmfact: Managing dairy farms profitably with clover root weevil (1-25)

• Go to www.pestweb.co.nz  or click here