Insect populations of six dryland pastures grown in Canterbury

Abstract

The 9 year ‘MaxClover’ experiment at Lincoln University concluded that ryegrass and white clover pastures were less persistent than cocksfoot and lucerne under dryland conditions in Canterbury. Measurements of insect pests commenced in Year 5 in response to a measured decline in sown ryegrass and white clover. The aim was to determine if there were differences in insect pressure among the different pastures. Insect pest pressure was present in all pastures from when measurements commenced until the experiment finished in Year 9. Grass grub larvae were the main pest that contributed to the decline in sown species, particularly in ryegrass/white clover, and they were found in all grass-based pasture treatments. Larval populations reached 156/m² in August 2008 in the cocksfoot/balansa clover and cocksfoot/white clover pastures. Argentine stem weevil overwintering adult populations reached 63/m² in July 2010 and were highest in cocksfoot/Subterranean clover pastures. These may have contributed to the slow decline in cocksfoot. Low populations (<5/m²) of adult clover root weevil were found in all treatments in winter 2010, with dissection finding reproductively mature adults with no indication of parasitism by Microctonus aethiopoides. Lucerne was the only host of Sitona discoideus. Results suggest insect pressure did not differ among the grass-based pastures over the duration of measurements but white clover appears to have been the main host for grass grub.