The fickle activity of a fly and a moth: variation in activity of two biocontrol agents of Chrysanthemoides monilifera

Biocontrol agents released to control exotic pests may not have the same spatial distribution as the pest species and may therefore vary in efficacy across the exotic range. These changes in distribution are unlikely to be known until species have had time to fill all preferred niches in the invasive habitat. However, studies of post-release activity of biocontrol agents rarely assess longer-term patterns of establishment in the landscape. Comostolopsis germana and Mesoclanis polana were released to control Chrysanthemoides monilifera spp. rotundata (bitou bush) between 29 and 32 years ago. We assessed their activity in foredune and hinddune habitats of coastal beaches across the major distribution of bitou bush and experimentally assessed the effectiveness of C. germana at preventing flowering and seed set. Both biocontrol agents were found to be distributed along the 870 km of coastline, representing the core area of infestation. Tip damage by C. germana was highly variable but was consistently more effective in the foredune. Comostolopsis germana was found to reduce flower production from 15 to 59% with tip damage increasing with latitude. Mesoclanis polana did not show differences in activity with latitude and only showed a marginal increase in activity in hinddunes. Comostolopsis germana and M. polana are reducing the reproductive output of bitou bush but are unlikely to be effective as a sole management strategy particularly in warmer latitudes where more seeds are released