Long-term Assessment of the Biological Control of Sitona discoideus by Microctonus aethiopoides and Test of a Model

Autumn densities of the pest weevil Sitona discoideus and its braconid parasitoid Microctonus aethiopoides were monitored from 1996 to 1998 on the Canterbury Plains, New Zealand. M. aethiopoides was introduced as a biological control agent in 1982 and first appeared in the study area in 1986. By 1991 around 50% of autumn weevils were parasitized and weevil density had been reduced by 75%. A model for the system at that time suggested that this level of suppression would be sustained. In agreement with the model, the recent survey confirmed that successful biological control had been maintained, with 75% suppression of weevil density but slightly lower rates of parasitism of around 35%. Weevil densities showed a significant trend longitudinally across the area surveyed, increasing from east to west, probably reflecting soil type. Weevil sex ratio was significantly biased towards females, yet the proportion of males that were parasitized was twice that of females. Percent parasitism in autumn related positively to weevil density over time and space. The scale of homogeneity, and by implication effective annual dispersal, is estimated at 12 +/- 4 km radius for weevils.

Keywords:

• Biological Control
• Sitona Discoideus
• Microctonus Aethiopoides
• Dispersal
• Population Model