Effect of Argentine stem weevil on productivity of grasses in the Waikato

Argentine stem weevil ( Listronotus bonariensis (Kuschel)) damage to pasture grasses was studied in relation to life cycle events in the Waikato. For a grazed pasture, numbers of adults at the onset of spring oviposition were low. First generation egg and larval numbers were correspondingly low, and the larvae caused negligible pasture damage. The second generation was considerably larger, and the larvae caused extensive damage. Applications of systemic insecticides increased the yield of grazed Nui ryegrass pasture by 17% over the spring‐autumn period. Similarly in autumn‐sown plots, Manawa, Ruanui, Nui, and Ellett ryegrasses, browntop, cocksfoot, paspalum, phalaris, and tall fescue, without clover, gave 15–91% higher yields under insecticide protection during the first spring‐autumn period. Prairie grass production did not respond to insecticide application. Results show that responses in yield were caused by insecticidal control of stem weevil at a stage when the establishing grasses were particularly vulnerable.