Annual variations in the severity of attack by Argentine stem weevil on perennial ryegrass

Abstract

Experiments on the effects of Argentine stem weevil (Listronotus bonariensis (Kuschel)) predation on perennial ryegrass (Lolium perenne L.) were carried out at Palmerston North from 1978 to 1986. A tagged tiller technique was used to assess the probability that any tiller would be attacked by Argentine stem weevil larvae. After grazing by sheep, a single tiller was tagged with a plastic ring at 25 sites 15 cm apart along line transects through each plot, and examined twice weekly throughout the seasons during which Argentine stem weevil is active. Annual variation at Palmerston North was substantial, with the probability of a low endophyte 'Grasslands Nui' perennial ryegrass tiller being attacked ranging from 55% to 95%. ASW annual larvae production calculated from the probability of tiller site damage and the tiller site density varied from 800 to 3000/m'. Variations in the severity of attack of this magnitude are sufficient to explain the variability in the success of sowings of low endophyte grass. Variations in annual larvae production were not correlated with weather parameters, but may be related to natural cyclic disease infestations. Keywords Argentine stem weevil, 'Grasslands Nui' perennial ryegrass, endophyte