Efficacy of lime, hydrogen peroxide and azamethiphos as potential control treatments against the proliferation of the invasive ascidian Didemnum vexillum

Abstract

The invasion of the colonial ascidian Didemnum vexillum (Kott, 2002) represents a significant threat to marine ecosystems. In Norway, D. vexillum was recorded for the first time in 2020 and has since continued spreading along the coast. Although the total eradication of this species appears highly unlikely, the implementation of control measures is important to slow down its proliferation and limit the impacts on important species and habitats. In this study, we performed three laboratory experiments, to separately test the effects of lime (fine and coarse quicklime particles and hydrated lime), hydrogen peroxide (H2O2; 0, 1000, 2000 and 4000 mg l-1) and Azamethiphos (0, 50, 100 and 200 µg l-1 ), on the biofouling cover of D. vexillum. No significant effect of Azamethiphos was detected on D. vexillum cover after 1 hr of exposure, while exposure to H2O2 for 1 hr only reduced its biofouling cover by 50% after exposure to the highest concentration (4000 mg l-1 ). Conversely, both fine and coarse quicklime particles appeared as suitable control options, reducing the size of the colonies by 100% and 99%, respectively, after exposure for 20 hrs. Additionally, treatment with hydrated lime induced a significant decline in D. vexillum cover by about 89% after 1 hr of exposure. Our results suggest the importance of considering the seasonal variations in environmental conditions and the biology of D. vexillum, to optimize the efficacy of the treatment. Here, we suggest using quicklime and hydrated lime between October and December in the Northern Hemisphere, to benefit from the natural degeneration of D. vexillum during winter and to more effectively eradicate potential remnants that could serve as a basis for regeneration.