Parrot’s feather: Use of herbicides - triclopyr

Overall effectiveness category Likely to be beneficial
Number of studies: 5
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How is the evidence assessed?

Effectiveness

60%
Certainty

55%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

Chemical herbicides have been used for the localized control of parrot’s feather (e.g. Morreira et al. 1999). Herbicides licensed for use around the world use a wide diversity of different active ingredients (e.g. 2,4-D, imazapyr, carfentrazone-ethyl, diquat). However, herbicide control of aquatic vegetation is prohibited in numerous countries (e.g. no herbicides are approved for submerged species in Europe) (De Winton et al. 2013; Hussner et al. 2017). Therefore it is important to consider local regulations before using herbicide control. Legislative restrictions may be lifted for herbicides under particular conditions (Hussner et al. 2017). Impacts on non-target species should be considered prior to herbicide use.

Emerged and submerged vegetation may require different herbicides, and adjuvants may increase the efficacy of the treatment. More than one application of the herbicide is often required. The use of fungal-based herbicides to control parrot’s feather is discussed under the intervention ‘Biological control using fungal-based herbicides’.

De Winton M., Jones H., Edwards T., Özkundakci D., Wells R., McBride C., Rowe D., Hamilton D., Clayton J., Champion P. & Hofstra D. (2013) Review of Best Management Practices for Aquatic Vegetation Control in Stormwater Ponds, Wetlands, and Lakes. Auckland Council technical report, TR2013/026.

Hussner A., Stiers I., Verhofstad M.J.J.M., Bakker E.S., Grutters B.M.C., Haury J., van Valkenburg J.L.C.H., Brundu G., Newman J., Clayton J.S. & Anderson L.W.J. (2017) Management and control methods of invasive alien freshwater aquatic plants: A review. Aquatic Botany, 136, 112-137.

Moreira I., Ferreira T., Monteiro A., Catarino L. & Vasconcelos T. (1999) Aquatic weeds and their management in Portugal: insights and the international context. Hydrobiologia 415, 229–234.

Study locations

Key messages

Two replicated, controlled laboratory studies in New Zealand and the USA reported reduced growth of parrot’s feather following treatment with triclopyr.
One replicated, before-and-after and one replicated, controlled field study in New Zealandfound that cover was reduced after treatment with triclopyr. However, one of the studies noted that cover later increased to levels close to pre-treatment.
One replicated, controlled laboratory study in New Zealand found that the application of triclopyr led to a greater reduction in cover than the application of glyphosate.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A small, replicated, controlled, laboratory study conducted between 1999 and 2000 in New Zealand (Hofstra et al. 2006) found that the herbicide triclopyr reduced the growth of parrot’s feather Myriophyllum aquaticum. After 17 weeks, plants treated with triclopyr had a lower dry weight (1–2 g) than that of untreated plants (274 g). Plants were grown for approximately two months prior to herbicide application in 60 l plastic tubs. Triclopyr was sprayed onto plants in three tubs at a concentration of 2 and 4 kg/ha and plants in four tubs were left untreated.
Study and other actions tested
Referenced paper
Hofstra D.E., Champion P. & Dugdale T. (2006) Herbicide trials for the control of parrotsfeather. Journal of Aquatic Plant Management, 44, 13-18.
A replicated, controlled field study conducted between 2001 and 2002 in a wetland in the Northern Island of New Zealand (Hofstra et al. 2006) reported treatment with the herbicide triclopyr reduced vegetation cover of parrot’s feather Myriophyllum aquaticum plants soon after application, but after 28 weeks cover was similar to pre-treatment levels. Results were not subject to statistical tests. After 10 weeks and following a second herbicide application, vegetation cover of treated plants was lower (1.5%) than of untreated plants (47%). However, after 28 weeks, vegetation cover of treated plants (68–84%) was similar to that of untreated plants (97%). Authors reported that the increase in vegetation cover resulted from the encroachment of plants from outside sprayed areas rather than due to regrowth in treated plots. Triclopyr was applied at concentrations of 2 and 4 kg/ha. Each herbicide concentration was sprayed into three 5 x 5 m plots and three plots were left untreated. Herbicides were applied in early summer (December). A second application took place 51 days after the initial treatment.
Study and other actions tested
Referenced paper
Hofstra D.E., Champion P. & Dugdale T. (2006) Herbicide trials for the control of parrotsfeather. Journal of Aquatic Plant Management, 44, 13-18.
A replicated, before-and-after field study conducted between 2002 and 2003 in two drains in the Northern Island of New Zealand (Hofstra et al. 2006) reported a reduction in the cover of parrot’s feather Myriophyllum aquaticum after the application of the herbicide triclopyr. This result is not based on statistical tests. The areas occupied by parrot’s feather were greater before herbicide application (35 m² and 128 m²) than following herbicide application (22 m²and 2 m², respectively). Authors reported that native species such as Potamogeton cheesemanii and Persicaria decipiens were either not affected or recovered quickly, although no data were presented. Triclopyr was applied at a concentration of 4 kg/ha into two sections with a low density of parrot’s feather (one 3 m x 2 km and the other 1 km long, being 3 m wide in the first 500 m and 1 m wide in the remaining 500 m). Application occurred during the spring and summer of 2002 to 2003 and vegetation cover was assessed visually.
Study and other actions tested
Referenced paper
Hofstra D.E., Champion P. & Dugdale T. (2006) Herbicide trials for the control of parrotsfeather. Journal of Aquatic Plant Management, 44, 13-18.
A small, replicated, controlled, laboratory study conducted between 1999 and 2000 in New Zealand (Hofstra et al. 2006) found that the vegetation cover of parrot’s feather Myriophyllum aquaticum plants treated with triclopyr was lower than that of plants treated with the glyphosate. One year after exposure, the vegetation cover of plants treated with triclopyr ranged between 0 and 13% whereas the vegetation cover of plants treated with glyphosate was 73%. The percentage cover of untreated plants was 83%. Plants were grown for approximately two months prior to herbicide application in 60 l plastic tubs. Triclopyr was applied at concentrations of 2, 4 and 8 kg/ha whereas glyphosate was sprayed onto plants in four tubes at a concentration of 3.2 kg/ha and plants in four tubs were left untreated.
Study and other actions tested
Referenced paper
Hofstra D.E., Champion P. & Dugdale T. (2006) Herbicide trials for the control of parrotsfeather. Journal of Aquatic Plant Management, 44, 13-18.
A small, replicated, controlled, laboratory study conducted between 2007 and 2008 in the USA (Wersal & Madsen, 2010), found that the application of the herbicide triclopyr reduced growth in parrot’s feather Myriophyllum aquaticum. After six weeks, the dry weight of parrot’s feather plants treated with triclopyr was lower than that of untreated plants (3–6 vs 10 g/pot). Subsurface and foliar herbicide applications led to similar changes in biomass. Six weeks after application, triclopyr had controlled parrot’s feather by 15–70% (visual assessment with 0% corresponding to no control and 100% to complete control). Parrot’s feather shoots were propagated in 3.78 l pots and placed inside 246 l containers filled with water. Each herbicide rate (subsurface: 1.25 and 2.5 mg/l; foliar: 6.7 kg/ha) was applied to four 246 l containers, each holding four plants. Number of plants used as control is not presented and control in the context of the visual assessments was not clearly defined.
Study and other actions tested
Referenced paper
Wersal R.M. & Madsen J.D. (2010) Comparison of subsurface and foliar herbicide applications for control of parrotfeather (Myriophyllum aquaticum). Invasive Plant Science and Management, 3, 262-267.

Please cite as:

Aldridge, D., Ockendon, N., Rocha, R., Smith, R.K. & Sutherland, W.J. (2020) Some Aspects of Control of Freshwater Invasive Species. Pages 555-87 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Control of Freshwater Invasive Species

Related Actions

Actions	Effectiveness	Studies	Category
Parrot’s feather: Manual harvesting (hand-weeding) Action Link	No evidence found (no assessment)	0	Synopsis Link
Parrot’s feather: Biological control using plant pathogens Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Parrot’s feather: Biological control using herbivores Action Link	Trade-off between benefit and harms	4	Synopsis Link