The efficacy of the biological control agents Comostolopsis germana (Bitou Tip Moth) and Mesoclanis polana (Bitou Tip Fly) in controlling the invasive weed Chrysanthemoides monilifera spp. rotundata (Bitou Bush)

Biological control (biocontrol) is commonly used in Australia and generally considered to be a successful cost effective method to regulate invasive weeds. Post-release research to measure an agent’s effectiveness enables weed mangers to fine tune biological programs strategies and alter other management options, however there is limited research reporting on this type of rigorous assessment. This study seeks to quantify the efficacy of two biocontrol agents on the invasive weed Chrysanthemoides monilifera subsp. rotundata (Bitou Bush). Comostolopsis germana (Bitou Tip Moth) and Mesoclanis polana (Bitou Seed Fly) are two established agents released to control C. m. spp. rotundata. Specifically, I asked: What is the distribution of C. germana? What are the current damage levels of C. m. subsp. rotundata tips by C. germana? Is there a difference in damage levels by C. germana between the fore-dune and the hind-dune? What are the damage levels for C. m. subsp. rotundata seeds by M. polana? Is there is difference in damage levels by M. polana between the fore-dune and the hind-dune? What impact does C. germana have on the reproductive output of C. m. subsp. rotundata? Sampling was undertaken at 14 sites along a 900km stretch of the NSW coastline to determine damage levels by C. germana. Soil samples were also taken at all 14 sites and the seeds from the soil samples were counted and examined for evidence of M. polana infestation. In addition to the sampling observations a manipulative experiment was undertaken at Windang Beach using the pesticide exclusion method. Both agents were found to be distributed along the entire range of bitou bush. Tip damage by C. germana was highly variable but was consistently more effective at the fore dune position. C. germana was found to reduce flower production by 15% to 59%. Tip damage by C. germana increased from north to south. The average rate of damage to seeds by M. polana was 11%. Dune location did not affect the amount of seed damaged by M. polana and latitude did not influence seed damage. C. germana and M. polana are reducing the reproductive output of bitou bush and in conjunction with other control methods, such as aerial spraying, will have significant effect on reducing the weeds impact.