Damage potential of Argentine stem weevil in Lincoln dairy pasture: has biological control by microctonus hyperodae altered the balance?

To measure the impact of biological control of Argentine stem weevil, Listronotus bonariensis (ASW) by Microctonus hyperodae in dairy pasture, a research site was established in the autumn of 1997 on a dairy farm near Lincoln, where the parasitoid had been present since 1994. Existing pasture was killed with glyphosate and four ryegrass cultivar treatments consisting of three perennial (Nui, Embassy, Vedette) and one hybrid mixture (Marsden/Greenstone), each containing either low or high levels of Neotyphodium lolii endophyte, were sown by direct drilling. A third main plot treatment was low endophyte plus insecticide. Between 1997 and 1999, seasonal pasture production was measured in all plots. Herbage composition was also measured in all plots for the first 2 years but only in the Nui treatments in 1999. ASW adult and larval densities, larval damage to ryegrass, parasitism by M. hyperodae and ryegrass tiller density were measured on Nui plots in spring and again in summer, coinciding with periods of peak ASW egg laying and larval development. Between 1997 and 2000, ASW adult densities in Nui were highest in the first summer (26 January 1998) following establishment. Thereafter, densities did not rise above c. 60 ASW/m2. Densities of adults, larvae, levels of larval damage to ryegrass, and parasitism by M. hyperodae were significantly higher in summer than spring (P<0.001) but with the exception of larval damage, there were no significant differences between the low and high endophyte Nui. There were no significant differences in herbage production among the low endophyte, high endophyte, and low endophyte plus insecticide treatments for any of the ryegrass cultivar treatments. The ryegrass component of the pasture showed a similar rate of decline over time for all Nui treatments. Over a 20-month period, average levels of endophyte in the high endophyte Nui plots increased from 35 to 46%. Conversely, average levels in the lowendophyte Nui plots, remained at c. 5%. The lack of any significant differences in herbage production and the relatively stable endophyte levels are discussed in relation to the ASW population data and M. hyperodae parasitism rates obtained from the Nui plots.