4. Implementation
What chemicals to use: Current global practice suggests that the two most effective herbicides are haloxyfop and imazapyr, with high kill rates shown in New Zealand, USA and China (Brown & Raal, 2013; Strong & Ayres, 2016; Patten et al. , 2017; David Melville, pers. obs.). In the New River estuary, New Zealand, haloxyfop (registered as Gallant) was shown to have a 95% mortality rate on its first application, decreasing Spartina cover from 800 ha to <1 ha (Miller & Croyhers, 2004). Imazypyr was used in Willapa Bay, USA, imazypr application reduced Spartina coverage from 3,440 ha to 0.36 ha over an 11-year period (Patten et al. , 2017). Other chemicals that negatively affect Spartina (from a recent meta-analysis of 26 studies), include: imazameth, glyphosate, 2,2-dichloropropionic acid (commercially Dalapon) and cyhalofop butyl (Reynolds et al. , 2023). Given the small number of studies which have investigated the effectiveness of some individual herbicides, it is important to use caution when drawing conclusions about their ability to control Spartina at scale. Chemical application: Ways to apply herbicide include using drones, quad bikes, aircraft, and backpack-mounted sprayers. This may be followed up with hand spraying to target small patches and reduce the impact on native vegetation. For example, in the Great Brak Estuary in South Africa, glyphosate was initially applied by backpack spraying but, as Spartina cover reduced, this was switched to bottle spraying (Riddin et al. , 2016). Similarly, in Willapa Bay in Washington, USA, Spartina was removed by boom spraying imazapyr with follow-up hand and backpack spraying, which involved intensive searching on foot by multiple searchers (spaced 4 to 20 m apart along 230 km of shoreline) at least twice a year (Patten et al. , 2017). Some control efforts have employed the use of drones carrying a 20 – 30 litre tank, which are operated with a pre-programmed GPS to ensure site coverage (David Melville, pers. obs.). On harder substrates, crawler tractors with spray booms may present the most effective herbicide deployment method (David Melville, pers. obs.).
Drones can be used to apply herbicide aerially. Pictured here are drones used for Spartina removal efforts in China. [Credit: David Melville].
Dose: Wang et al. (2023) compared four herbicides, three of which removed only 25 – 35% of Spartina with the highest tested dose: glyphosate (8.0 kg/ha), cyhalofop-butyl (0.8 kg/ha) or imazameth (0.4 kg/ha). Haloxyrop was the most effective, with a dose of 0.3-0.45 kg/ha removing 95% in the first year. In Chongming Dongtan, China, one study found the highest tested dose of haloxyfop, 2.70 g/m 2 , to be the most effective, removing 100% of small patches
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