Modelling the biological control of bitou bush (Chrysanthemoides monilifera: Asteraceae) by Mesoclanis polana (Tephritidae).
Bitou bush has been the subject of a biological control campaign in Australia since 1987. Several agents have been successfully established, including the bitou bush seed fly, M. polana. Recently, there has been increasing interest in assessing the impact of weed biological control programmes including bitou bush, so that the success of importations could be gauged in more meaningful terms than simply whether the agent became established, or whether it reduced seed production by a measured amount. In the case of bitou bush, the perceived 'weediness' is not related to either its fauna, or its seed bank, but rather, to its dominance of coastal vegetation communities and suppression of native vegetation. These factors are directly related to the canopy cover proportion of bitou bush in any land catena. Depending upon the life history traits of the agent and target, the disturbance regime, plant community composition, and other environmental factors, there may be very little relationship between the average bitou bush canopy cover and its seed production rate. A modelling methodology was adopted to explore the likely impact of M. polana upon bitou bush across the range of the fly in Australia. Monthly field surveys were conducted at five bitou bush populations along the New South Wales coastline, Australia, between May 2001 and April 2002. A process-based population dynamics model of bitou bush was modified to incorporate the effects of M. polana. This involved changing the time-step from weekly to daily, including a detailed life cycle module for M. polana, and creating the linkages between the life cycles of the weed and the seed fly. The model was used to assess the impact of the seed fly on the population dynamics of bitou bush under a variety of environmental conditions, and in association with the typical cultural management techniques employed against bitou bush.