study found that spraying with Fusilade Forte® had initial toxic impacts on benthic invertebrates but that the community recovered and by 12 years resembled that of natural tidal flat – there were fewer crustaceans but more molluscs, particularly gastropods (Shepherd, 2013). In Chongming Island, China, one month of spraying haloxyfop appeared to have no adverse effects on the meiofauna community (Zhao et al. , 2020). Native vegetation: Global evidence about using herbicide to eradicate invasive species from brackish/saline wetlands shows it has neutral or positive effects on the native vegetation (Taylor et al. , 2021). However, some studies found the cover of native, non-target vegetation decreased as well as the target invasive species (Whitcraft & Grewell, 2012; Tobias et al. , 2016). A recent study in Laizhou Bay, China found that the native herbs Salicornia and Suaeda increased in density after spraying Spartina with haloxyfop-R-methyl (a grass-specific herbicide) for 10 months (Wei et al. , 2023).
3. Factors that can affect outcomes
Tidal flow: Large tidal ranges lead to the replacement of water (known as tidal flushing), which can wash herbicide from plants. When Spartina is submerged, this can reduce the absorption rate of herbicides. For example, in the Yangtze Estuary in China, the large tidal range leaves S. alterniflora submerged for long periods, which shortens the absorption time of herbicide (Zhao et al. , 2020). Therefore, applying herbicide during neap tides, when plants are submerged for a shorter period, will allow more of the herbicide to be absorbed (Peng et al. , 2022). Density of Spartina vs. non-target plant population: How Spartina is distributed, and how close it is to native species, may influence the herbicide that is used. If Spartina is mixed with native species, it is more appropriate to use a grass-specific herbicide, such as haloxyfop, to project the native species. For large monospecific stands of Spartina , a broad-spectrum herbicide such as imazapyr can be used (David Melville, pers. obs.). Site accessibility: The method of herbicide application will depend on how accessible a site is to both humans and vehicles. For example, soft mud can make a site inaccessible to humans (Hassell et al. , 2014), making precise treatment more difficult. In New Zealand, helicopters have been used to apply herbicides to overcome access issues (David Melville, pers. obs.). Time of application : The control efficacy of the same herbicide may differ depending on the time of year it is applied. For example, Zhao et al. (2020) found that applying herbicide in July/August resulted in 100% mortality, while S. alterniflora was able to recover rapidly when applied in May. Use of heavy machinery : Using heavy machinery to apply herbicide (e.g. boom sprayers) can be challenging in wet, soft, intertidal sediments. Furthermore, vehicles can displace or compress any vegetation present and could have potential adverse impacts on benthic invertebrates in the sediment (Evans et al. , 1999; David Melville, pers. obs.).
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