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Providing evidence to improve practice

Action: Parrot’s feather: Biological control using herbivores Control of Freshwater Invasive Species

Key messages

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Supporting evidence from individual studies


A replicated, controlled, laboratory study from 1994 to 1996 in Portugal (Catarino et al. 1997) found that grass carp Ctenopharyngodon idella did not reduce the biomass or cover of parrot’s feather Myriophyllum aquaticum. Biomass and cover of parrot’s feather did not differ between ponds where grass carp were present (biomass: 0.09 kg/m2, cover: 5%) and ponds where carp were not present (0.09 kg/m2 and 5%, respectively). Grass carp favoured soft-tissue native plants relative to parrot’s feather. When presented only with parrot’s feather and water hyacinth Eichhornia crassipes, one year old grass carp consumed parrot’s feather at a daily rate of approximately 3% of their body weight but this increased to 20% by the age of two. Trials were conducted in six 660 l plastic tanks. Five tanks were stocked with carp and one control tank had no carp. Grass carp were presented with a selection of four plants with a total fresh weight similar to the biomass of fish present in the tank. After two days, the biomass of each plant species was weighed. The number of grass carp per tank was not specified.


A field study from 1995 to 1998 in a river in South Africa (Cilliers 1999), reported reduced growth of parrot’s feather Myriophyllum aquaticum following the release of a South American leaf-feeding Lysathia beetle. Three months after beetle release nearly all emergent parrot’s feather shoots had been damaged by herbivory. After three years, 30% (558 out of 1251) of parrot’s feather shoots were damaged by the Lysathia beetle. Damaged plants had lower mean shoot length (10 cm vs 19 cm) and dry weight (63 g vs 187 g/m2) compared to undamaged plants. Herbivory was reduced during winter. A total of 120 adult Lysathia beetles were released into one river site. Herbivory was quantified in ten 0.1 m2 quadrats by counting the total number of shoots and the number of shoots with feeding damage. Sampling took place at intervals of four to six weeks for three years.


A replicated, randomized, controlled study from November 1996 to February 1997 in a water channel in Argentina (Armellina et al. 1999) found that stocking with grass carp Ctenopharyngodon idella reduced the biomass of aquatic plants, including parrot’s feather Myriophyllum aquaticum. After two months, dry weight of aquatic plants was lower in plots with grass carp at both low (50 g/m2) and high stocking densities (10 g/m2) than in plots without carp (320 g/m2). The experiment was performed in a medium size water channel with an aquatic plant community dominated by Potamogeton pectinatus, M. aquaticum and Chara contraria. Aquatic plant biomass was measured four times (sampling frequency not provided) in nine 30 m-long plots separated by iron barriers with plastic nets. Carp stocking density was 100 kg/ha (low density) and 200 kg/ha (high density).


A replicated, controlled, paired sites study from July 2005 to September 2007 in a reservoir in North Carolina, USA (Garner et al. 2013) found that high-density stocking with grass carp Ctenopharyngodon idella reduced the abundance of parrot’s feather Myriophyllum aquaticum. For five out of six comparisons, the biomass of parrot’s feather was lower in areas available for grass carp (0–113 g/m2) than in areas where grass carp were excluded (0–1330 g/m2)). During the second year of the experiment no vegetation was detected in quadrats located in areas accessible to grass carp. Grass carp density was 100 fish/vegetated ha and grass carp were excluded from eight 6 m2) areas using 1.3 cm plastic mesh. Vegetation in six 6 x 1 m quadrats was sampled monthly from July to September each year.

Referenced papers

Please cite as:

Aldridge, D., Ockendon, N., Rocha, R., Smith, R.K. & Sutherland, W.J. (2019) Some aspects of control of freshwater invasive species. Pages 569-602 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.