Development of clovers that are tolerant to clover root weevil

Abstract

Clover root weevil (CRW) has become a major pest of white clover in pastures in much of the North Island. In contemporary, intensively stocked pastures, high volumes of nitrogen (N) cycling through the soil/plant/ animal system stimulate grass growth and reduce the clover content of pastures. Clover root weevil imposes further stresses on clovers. A range of white and red clover germplasm was evaluated f or CRW tolerance from 2002-2005 on Waikato dairy and sheep/beef farms. Results confirmed the importance of good agronomic adaptation if clovers are to survive the additional stress of CRW. Two recently released white clover cultivars (Grasslands Kopu II and Tribute) were amongst the top performing lines under dairy farm grazing. Under sheep/ beef grazing, spreading type red clovers also performed well. A white c lover breeding line selected for tolerance of plant-feeding nematodes was in the top group of clovers at both sites. Reducing stresses on clovers from other pasture pests will contribute to improved tolerance to CRW. A glasshouse experiment showed that CRW larval feeding can severely damage white clover nodules and reduce N fixation. The plants have considerable capacity to grow new nodules but this diverts energy away from other growth processes. Our conclusions are that welladapted, vigorous white clover cultivars are better able to cope with the additional stress imposed by the root weevil. Good pasture management and attention to fertiliser requirements can improve clover survival under root weevil herbivory. New generation red clovers show considerable potential for weevil infested areas. Coselection for tolerance/resistance to other major clover pests e.g. clover flea and root nematodes will improve plant performance under CRW stress. Keywords: clover root weevil, nitrogen fixation, plant breeding, red clover, white clover