Evaluating the impact of biological control against Mimosa pigra in Australia: Comparing litterfall before and after the introduction of biological control agents

The impact of biological control on the woody weed, Mimosa pigra was investigated by comparing litterfall in 2001–2003 with a previous study performed at the same site, in 1984–1986, before biological control agents were released, and correlating litterfall to individual agent damage. Total litterfall was similar in both studies. However, leaf litterfall was 20% higher than prior to the introduction of biological control agents whilst seed rain was 47% lower overall and 67% lower at the stand edge. Abundance of the flower-feeding weevil Coelocephalapion pigrae lagged variation in flower fall and it was estimated to destroy only 10.6% of flowers. No correlation was found between C. pigrae abundance and seed rain. Neurostrota gunniella and Carmenta mimosa abundances were both highest at the stand edge. Seed rain was negatively correlated with both N. gunniella and C. mimosa damage, declining by ca. 60% at the highest N. gunniella and C. mimosa densities, indicating that both species have a significant impact on mimosa fecundity. However, the introduction of biological control agents is not the only potential explanatory factor affecting mimosa to have changed since the mid-1980s. Alternative hypotheses and implications for the design of future evaluation experiments are discussed.