10.1.1 Parrot’s feather Myriophyllum aquaticum

Beneficial

Chemical control using the herbicide 2,4-D

1Five laboratory studies (three replicated, controlled and two randomized, controlled) in the USA and Brazil and two replicated, randomized, field studies in Portugal reported that treatment with 2,4-D reduced growth, biomass or cover of parrot’s feather. Assessment: beneficial (effectiveness 80 %; certainty 80 %; harms 0 %).

2https://www.conservationevidence.com/actions/1606

Likely to be beneficial

Chemical control using the herbicide carfentrazone-ethyl

3Five laboratory studies (one replicated, controlled, before-and-after, three replicated, controlled and one randomized, controlled) in the USA reported that treatment with carfentrazone-ethyl reduced growth. Assessment: likely to be beneficial (effectiveness 50 %; certainty 40 %; harms 5 %).

4https://www.conservationevidence.com/actions/1676

Chemical control using the herbicide triclopyr

5Three replicated, controlled laboratory studies in the USA and New Zealand reported that treatment with triclopyr reduced growth or that cover was lower than that of plants treated with glyphosate. One replicated, controlled field study and one replicated, before-and-after field study in New Zealand reported that cover was reduced after treatment with triclopyr but one of these studies reported that cover later increased to near pre-treatment levels. Assessment: likely to be beneficial (effectiveness 60 %; certainty 55 %; harms 0 %).

6https://www.conservationevidence.com/actions/1689

Chemical control using the herbicide diquat

7Two replicated, controlled laboratory studies in the USA reported reduced growth after exposure to diquat. However, one replicated, randomized, controlled field study in Portugal reported no reduction in biomass following treatment with diquat. Assessment: likely to be beneficial (effectiveness 60 %; certainty 40 %; harms 0 %).

8https://www.conservationevidence.com/actions/1680

Chemical control using the herbicide endohall

9Two replicated, controlled laboratory studies in the USA and New Zealand reported a reduction in biomass after treatment with endothall. However, one replicated, controlled field study in New Zealand found that cover declined after treatment with endothall but later cover increased close to pre-treatment levels. Assessment: likely to be beneficial (effectiveness 50 %; certainty 40 %; harms 0 %).

10https://www.conservationevidence.com/actions/1681

Chemical control using other herbicides

11One replicated, randomized, controlled field study in Portugal and one replicated, controlled, laboratory study in the USA reported reduced growth or vegetation cover after treatment with glyphosate. Two replicated, randomized, controlled laboratory studies (one of which was randomized) in the USA have found that the herbicide imazapyr reduced growth. Four replicated, controlled (one of which was randomized) laboratory studies in the USA and New Zealand reported reduced growth after treatment with the herbicides imazamox, flumioxazin, dichlobenil and florpyrauxifen-benzyl. Two replicated, controlled (one of which was randomized) field studies in Portugal and New Zealand reported a decrease in cover after treatment with dichlobenil followed by recovery. One replicated, randomized, controlled field study in Portugal reported reduced biomass after treatment with gluphosinate-ammonium. Three replicated, controlled laboratory studies in New Zealand and the USA found no reduction in growth after treatment with clopyralid, copper chelate or fluridone. Assessment: likely to be beneficial (effectiveness 50 %; certainty 40 %; harms 0 %).

12https://www.conservationevidence.com/actions/1699

Reduction of trade through legislation and codes of conduct

13One randomized, before-and-after trial in the Netherlands reported that the implementation of a code of conduct reduced the trade of invasive aquatic plants banned from sale. One study in the USA found that despite a statewide trade ban on parrot’s feather plants, these could still be purchased in some stores. Assessment: likely to be beneficial (effectiveness 60 %; certainty 45 %; harms 0 %).

14https://www.conservationevidence.com/actions/1604

Trade-off between benefit and harms

Biological control using herbivores

15Two replicated, randomized studies in Argentina and the USA found that stocking with grass carp reduced the biomass or abundance of parrot’s feather. However, one controlled laboratory study in Portugal found that grass carp did not reduce biomass or cover of parrot’s feather. One field study in South Africa found that one Lysathia beetle species retarded the growth of parrot’s feather. Assessment: trade-offs between benefits and harms (effectiveness 50 %; certainty 40 %; harms 20 %).

16https://www.conservationevidence.com/actions/1599

Unknown effectiveness (limited evidence)

Water level drawdown

17One replicated, randomized, controlled laboratory study in the USA found that water removal to expose plants to drying during the summer led to lower survival of parrot’s feather plants than water removal during winter. Assessment: unknown effectiveness (effectiveness 60 %; certainty 30 %; harms 0 %).

18https://www.conservationevidence.com/actions/1585

Biological control using plant pathogens

19One study in South Africa found that exposure to a strain of the bacterium Xanthomonas campestris did not affect the survival of parrot’s feather. Assessment: unknown effectiveness (effectiveness 5 %; certainty 10 %; harms 0 %).

20https://www.conservationevidence.com/actions/1601

No evidence found (no assessment)

21We have captured no evidence for the following interventions:

• Mechanical harvesting or cutting
• Mechanical excavation
• Removal using water jets
• Suction dredging and diver-assisted suction removal
• Manual harvesting (hand-weeding)
• Use of lightproof barriers
• Dye application
• Biological control using fungal-based herbicides
• Use of salt
• Decontamination / preventing further spread
• Public education
• Multiple integrated measures

10.1.2 Floating pennywort Hydrocotyle ranunculoides

Beneficial

Chemical control using herbicides

22A controlled, replicated field study in the UK found that the herbicide 2,4- D amine achieved almost 100 % mortality of floating pennywort, compared with the herbicide glyphosate (applied without an adjuvant) which achieved negligible mortality. Assessment: beneficial (effectiveness 80 %; certainty 70 %).

23https://www.conservationevidence.com/actions/1127

Likely to be beneficial

Flame treatment

24A controlled, replicated study in the Netherlands found that floating pennywort plants were killed by a three second flame treatment with a three second repeat treatment 11 days later. Assessment: likely to be beneficial (effectiveness 60 %; certainty 50 %).

25https://www.conservationevidence.com/actions/1131

Physical removal

26Two studies, one in Western Australia and one in the UK, found physical removal did not completely eradicate floating pennywort. Assessment: likely to be beneficial (effectiveness 40 %; certainty 40 %).

27https://www.conservationevidence.com/actions/1126

Unknown effectiveness (limited evidence)

Combination treatment using herbicides and physical removal

28A before-and-after study in Western Australia found that a combination of cutting followed by a glyphosate chemical treatment, removed floating pennywort. Assessment: unknown effectiveness (effectiveness 70 %; certainty 35 %).

29https://www.conservationevidence.com/actions/1128

Unlikely to be beneficial

Biological control using co-evolved, host-specific herbivores

30A replicated laboratory and field study in South America found that the South American weevil fed on water pennywort but did not reduce the biomass. Assessment: unlikely to be beneficial (effectiveness 20 %; certainty 50 %).

31https://www.conservationevidence.com/actions/1123

Use of hydrogen peroxide

32A controlled, replicated study in the Netherlands found that hydrogen peroxide sprayed on potted floating pennywort plants at 30 % concentration resulted in curling and transparency of the leaves but did not kill the plants. Assessment: unlikely to be beneficial (effectiveness 10 %; certainty 60 %).

33https://www.conservationevidence.com/actions/1129

No evidence found (no assessment)

34We have captured no evidence for the following interventions:

• Biological control using fungal-based herbicides
• Biological control using native herbivores
• Environmental control (e.g. shading, reduced flow, reduction of rooting depth, or dredging)
• Excavation of banks
• Public education
• Use of liquid nitrogen.

10.1.3 Water primrose Ludwigia spp.

Likely to be beneficial

Biological control using co-evolved, host specific herbivores

35A controlled, replicated study in China, found a flea beetle caused heavy feeding destruction to the prostrate water primrose. A before-and-after study in the USA found that the introduction of flea beetles to a pond significantly reduced the abundance of large-flower primrose-willow. Assessment: likely to be beneficial (effectiveness 60 %; certainty 50 %).

36https://www.conservationevidence.com/actions/1135

Chemical control using herbicides

37A controlled, replicated laboratory study in the USA found that the herbicide triclopyr TEA applied at concentrations of 0.25 % killed 100 % of young cultivated water primrose within two months. A before-and-after field study in the UK found that the herbicide glyphosate caused 97 % mortality when mixed with a non-oil based sticking agent and 100 % mortality when combined with TopFilm. A controlled, replicated, randomized study in Venezuela, found that use of the herbicide halosulfuron-methyl (Sempra) resulted in a significant reduction in water primrose coverage without apparent toxicity to rice plants. Assessment: likely to be beneficial (effectiveness 80 %; certainty 60 %).

38https://www.conservationevidence.com/actions/1139

Combination treatment using herbicides and physical removal

39A study in the USA found that application of glyphosate and a surface active agent called Cygnet-Plus followed by removal by mechanical means killed 75 % of a long-standing population of water primrose. A study in Australia found that a combination of herbicide application, physical removal, and other actions such as promotion of native plants and mulching reduced the cover of Peruvian primrose-willow by 85–90 %. Assessment: likely to be beneficial (effectiveness 70 %; certainty 55 %).

40https://www.conservationevidence.com/actions/1140

Unlikely to be beneficial

Physical removal

41A study in the USA found that hand pulling and raking water primrose failed to reduce its abundance at one site, whereas hand-pulling from the margins of a pond eradicated a smaller population of water primrose at a second site. Assessment: unlikely to be beneficial (effectiveness 30 %; certainty 50 %).

42https://www.conservationevidence.com/actions/1138

No evidence found (no assessment)

43We have captured no evidence for the following interventions:

• Biological control using fungal-based herbicides
• Biological control using native herbivores
• Environmental control (e.g. shading, reduced flow, reduction of rooting depth, or dredging)
• Excavation of banks
• Public education
• Use of a tarpaulin
• Use of flame treatment
• Use of hydrogen peroxide
• Use of liquid nitrogen
• Use of mats placed on the bottom of the waterbody.

10.1.4 Skunk cabbage Lysichiton americanus

Likely to be beneficial

Chemical control using herbicides

44Two studies in the UK found that application of the chemical 2,4-D amine appeared to be successful in eradicating skunk cabbage stands. One of these studies also found glyphosate eradicated skunk cabbage. However, a study in the UK found that glyphosate did not eradicate skunk cabbage, but resulted in only limited reduced growth of plants. Assessment: likely to be beneficial (effectiveness 60 %; certainty 50 %).

45https://www.conservationevidence.com/actions/1102

Physical removal

46Two studies in Switzerland and the Netherlands, reported effective removal of recently established skunk cabbage plants using physical removal, one reporting removal of the entire stock within five years. A third study in Germany reported that after four years of a twice yearly full removal programme, a large number of plants still needed to be removed each year. Assessment: likely to be beneficial (effectiveness 65 %; certainty 55 %).

47https://www.conservationevidence.com/actions/1101

No evidence found (no assessment)

48We have captured no evidence for the following interventions:

• Biological control using co-evolved, host-specific herbivores
• Biological control using fungal-based herbicides
• Biological control using native herbivores
• Combination treatment using herbicides and physical removal
• Environmental control (e.g. shading, or promotion of native plants)
• Public education
• Use of a tarpaulin
• Use of flame treatment
• Use of hydrogen peroxide
• Use of liquid nitrogen.

10.1.5 New Zealand pigmyweed Crassula helmsii

Beneficial

Chemical control using herbicides

49Seven studies in the UK, including one replicated, controlled study, found that applying glyphosate reduced Crassula helmsii. Three out of four studies in the UK, including one controlled study, found that applying diquat or diquat alginate reduced or eradicated C. helmsii. One small trial found no effect of diquat on C. helmsii cover. One replicated, controlled study in the UK found dichlobenil reduced biomass of submerged C. helmsii but one small before-and-after study found no effect of dichlobenil on C. helmsii. A replicated, controlled study found that treatment with terbutryne partially reduced biomass of submerged C. helmsii and that asulam, 2,4-D amine and dalapon reduced emergent C. helmsii. Assessment: beneficial (effectiveness 78 %; certainty 75 %).

50https://www.conservationevidence.com/actions/1279

Decontamination to prevent further spread

51One controlled, replicated container trial in the UK found that submerging Crassula helmsii fragments in hot water led to higher mortality than drying out plants or a control. Assessment: beneficial (effectiveness 80 %; certainty 70 %).

52https://www.conservationevidence.com/actions/1308

Likely to be beneficial

Use lightproof barriers to control plants

53Five before-and-after studies in the UK found that covering with black sheeting or carpet eradicated or severely reduced cover of Crassula helmsii. Assessment: likely to be beneficial (effectiveness 65 %; certainty 50 %).

54https://www.conservationevidence.com/actions/1294

Use salt water to kill plants

55Two replicated, controlled container trials and two before-and-after field trials in the UK found that seawater eradicated Crassula helmsii. Assessment: likely to be beneficial (effectiveness 80 %; certainty 45 %).

56https://www.conservationevidence.com/actions/1288

Unknown effectiveness (limited evidence)

Use a combination of control methods

57One before-and-after study in the UK found that covering Crassula helmsii with carpet followed by treatment with glyphosate killed 80 % of the plant. Assessment: unknown effectiveness (effectiveness 75 %; certainty 30 %).

58https://www.conservationevidence.com/actions/1313

Unlikely to be beneficial

Use dyes to reduce light levels

59One replicated, controlled study in the UK found that applying aquatic dye, along with other treatments, did not reduce cover of Crassula helmsii. Assessment: unlikely to be beneficial (effectiveness 0 %; certainty 53 %).

60https://www.conservationevidence.com/actions/1293

Use grazing to control plants

61One of two replicated, controlled studies in the UK found that excluding grazing reduce abundance and coverage of Crassula helmsii. The other study found that ungrazed areas had higher coverage of C. helmsii than grazed plots. Assessment: unlikely to be beneficial (effectiveness 23 %; certainty 43 %).

62https://www.conservationevidence.com/actions/1301

Use hot foam to control plants

63One replicated, controlled study in the UK found that treatment with hot foam, along with other treatments, did not control Crassula helmsii. A before-and-after study in the UK found that treatment with hot foam partially destroyed C. helmsii. Assessment: unlikely to be beneficial (effectiveness 20 %; certainty 50 %).

64https://www.conservationevidence.com/actions/1286

Use hydrogen peroxide to control plants

65One controlled tank trial in the UK found that hydrogen peroxide did not control Crassula helmsii. Assessment: unlikely to be beneficial (effectiveness 0 %; certainty 50 %).

66https://www.conservationevidence.com/actions/1281

No evidence found (no assessment)

67We have captured no evidence for the following interventions:

• Alter environmental conditions to control plants (e.g. shading by succession, increasing turbidity, re-profiling or dredging)
• Biological control using fungal-based herbicides
• Biological control using herbivores
• Bury plants
• Dry out waterbodies
• Physical control using manual/mechanical control or dredging
• Plant other species to suppress growth
• Public education
• Surround with wire mesh
• Use flame throwers
• Use hot water
• Use of liquid nitrogen.