Biocontrol in action: Assessing the impact of a biocontrol agent on invasive plant seed dynamics

The management of extensive areas invaded by alien plants using only physical and chemical control methods is challenging as it frequently only provides short-term suppression. Additionally, success depends on thorough follow-up, which can be prohibitively expensive and, as such, neglected. More successful and sustainable management of widespread invasive alien plants may be achieved with biological control (biocontrol), either alone or integrated with other methods. Acacia longifolia was introduced into Portugal in the late 19th and early 20th centuries for dune stabilization and has become one of the most widespread invasive alien plants in the coastal areas, promoting significant negative impacts. To decrease A. longifolia seed production, the Australian bud-galling wasp Trichilogaster acaciaelongifoliae was introduced as a biocontrol agent in 2015. This study reports the effect of T. acaciaelongifoliae on A. longifolia seed production for over five to six years and seed bank replenishment since its introduction. Galls have significantly increased five years after the biocontrol agent release, followed by major pod reduction and consequently seed rain decline. By monitoring the same trees for several years, we have confirmed that seed rain is higher in non-galled A. longifolia stands, peaking in mid-summer. Our results further support that both the number of pods and seeds progressively decline to zero, demonstrating a 98,1 % reduction in seed rain. Additionally, the increase of the biocontrol agent induces stress on galled plants leading to phyllode abscission and, eventually, plant mortality. On the other hand, the A. longifolia seed bank continued to increase gradually between 2015 and 2024, except at one dune site, where a major wildfire in 2017, followed by tree planting, contributed to its depletion. Mechanical control reduced seed bank numbers, and this result was enhanced in areas where T. acaciaelongifoliae helped to maintain low seed input. Where the biocontrol agent has been established for more than six years, and/or has a high establishment rate, seed banks are now stabilizing. These results are encouraging for the recovery of coastal dunes invaded by A. longifolia, emphasizing the pressing need for establishing an integrated strategy and increasing investment in biocontrol to enhance invasive plant management in Europe.