Myriophyllum aquaticum (parrot's feather)

Mechanical cutting is rarely effective because of the ability of M. aquaticum to regrow rapidly from shoot fragments (Guillarmod, 1977). However, more effective harvesting systems that remove the biomass and nutrient reserves accumulated in the emergent tissues (Sytsma and Anderson, 1993) may be an effective control measure. Ferreira and Moreira (1990) described the ecology and succession behaviour of M. aquaticum produced by disturbance-based mechanical clearance regimes in two highly mineralized, nutrient-rich canals of the Sorraia Irrigation Area of central Portugal. The shallow Guedelha channel was dominated by M. aquaticum, Polygonum hydropiper [Persicaria hydropiper], Apium nodiflorum and Veronica anagalloides. Following the removal of M. aquaticum, grasses such as Paspalum distichum became established, then coexisted with regrowing M. aquaticum, before a more diverse emergent weed community finally established itself. In the deeper Vala Real, which was dredged during winter floods, a diverse community became established during the spring, and was followed by the progressive dominance of exotic weeds, including M. aquaticum by the end of the summer. Weed removal in spring resulted in a clear canal for 3 months, after which the same pattern of colonization occurred.

Biological control of M. aquaticum remains at an early stage (Gassmann et al., 2006; Haller et al., 2006). According to Verma and Charudattan (1993), Mycoleptodiscus terrestris formulated as a mycoherbicide in alginate beads showed some toxicity to M. aquaticum. A considerable amount of research and practical management programmes utilizing, in part, insect biocontrol agents for M. aquaticum have been carried out recently in South Africa, using insects such as Lysathia (Cilliers, 1999; Olckers, 2004; Zimmermann et al., 2004). Grass carp (Ctenopharyngodon idella) apparently dislike M. aquaticum and trials in Portugal found that the fish will not consume the plant at all (M.T. Ferreira, Instituto Superior de Agronomia, Lisbon, personal communication, 1996). Similar low-preference results were found with triploid grass carp using M. aquaticum in feeding trials; the plant was tenth out of 13 macrophyte species offered, in terms of preference shown by the fish in the USA (Pine and Anderson, 1991). However, more recent work in New Zealand and Argentina has reported some success (Armellina et al., 1999; Wells et al., 2003).

Myriophyllum aquaticum is highly susceptible to 2,4-D, in spray or granular formulations (Blackburn and Weldon, 1963; Braddock, 1966), and is most effective when applied to young, actively-growing plants (Sutton and Bingham, 1970). It is also susceptible to simazine and copper (Sutton et al., 1969; Sutton and Blackburn, 1971). Endothal, diquat, chlorsulfuron and dichlobenil have been reported to control M. aquaticum (Mixon, 1974; Sikka et al., 1974; Negrisoli et al., 2003; Turgut et al., 2003; Hofstra et al., 2006). In field trials in Portugal, Monteiro and Moreira (1990) evaluated diquat, 2,4-D amine, glufosinate ammonium and glyphosate against M. aquaticum. Herbicide use resulted in 9.0-18.3 kg/m² fresh weight of M. aquaticum by 1 month after the first application, compared to 22.1 kg in untreated areas. The lowest fresh weights were produced by glufosinate ammonium followed by 2,4-D amine. By 4-5 months after the first application, treatment with 2,4-D amine and other herbicides resulted in 2.2 and 13.4-18.2 kg/m² fresh weight, respectively, whereas the control area supported 21.8 kg/m² of M. aquaticum. However, the control of M. aquaticum resulted in the spread of other aquatic weeds, in particular, Sparganium erectum, Typha spp. and Paspalum distichum, but these were considered less of a problem than the target species. Other herbicides reported to show varying degrees of control against M. aquaticum include triclopyr, glyphosate and carfentrazone ethyl (Glomski et al., 2006; Hofstra et al., 2006; Gray et al., 2007), although Foloni and Pitelli (2005) reported poor results using carfentrazone ethyl, even at the highest dose used (60 g a.i./ha).