Action: Parrot’s feather: Biological control using herbivores
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- One replicated, controlled laboratory study in Portugal found that grass carp did not reduce biomass or cover of parrot’s feather.
- Two replicated, randomized field studies in Argentina and the USA found that stocking with grass carp reduced the biomass or abundance of parrot’s feather.
- One field study in South Africa reported reduced growth of parrot’s feather following the release a South American leaf-feeding Lysathia.
Both host-specific insects and grass carp have been used for the biocontrol of parrot’s feather with some success (Moreira et al. 1999; Hill & Coetzee 2017), however, also native vertebrate herbivores and livestock can consume invasive aquatic plants and consequently can contribute to inhibit their establishment, growth and expansion (Gassman et al. 2006).
The potential of host-specific insects to act as biocontrol agents depends on their ability to cause harm to the target plant, and may also be limited by the climatic conditions required by the insect species (Gassman et al. 2006). Although several insects have been suggested as potential control agents for parrot’s feather (e.g. the stem-boring weevil Listronotus marginicollis has been found to show a feeding and host preference for parrot’s feather, often killing its terminal bud (Oberholzer et al. 2007)), none seem to be in use. Grass carp Ctenopharyngodon idella consume large amounts of vegetation, and sterile fish have been used for the management and eradication of invasive aquatic plants (Hussner et al. 2017). However, grass carp are generalist herbivores and will consume all palatable plants available to them (Dorenbosch & Bakker 2011, Pine & Anderson 1991). The introduction of non-native control agents, such as herbivorous insects and grass carp, should only be considered following in-depth studies investigating possible undesired consequences to non-target species (Hussner et al. 2017).
Herbivory by both native vertebrates and livestock is often used as a conservation measure in terrestrial habitats. Depending on the dietary preferences of the animals, herbivory can also affect the competitive advantage of aquatic species such as parrot’s feather, and consequently increase biotic resistance of freshwater ecosystems to invasive plants. As an example, herbivory by North American beavers Castor canadensis reduced the abundance of parrot’s feather by 90% (Parker et al. 2007). The impact of terrestrial herbivores may be restricted to the margins of waterbodies or marshy areas, but aquatic and amphibious herbivores such as beavers can affect plant communities further away from water margins.
Dorenbosch M. & Bakker E.S. (2011). Herbivory in omnivorous fishes: effect of plant secondary metabolites and prey stoichiometry. Freshwater Biology, 56, 1783-1797.
Gassmann A., Cock M.J.W., Shaw R. & Evans H.C. (2006) The potential for biological control of invasive alien aquatic weeds in Europe: a review. Hydrobiologia 570, 217-222.
Hill, M. P., & Coetzee, J. (2017). The biological control of aquatic weeds in South Africa: Current status and future challenges. Bothalia - African Biodiversity & Conservation, 47, 1-12.
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.
Oberholzer I.G., Mafokoane D.L. & Hill M.P. (2007) The biology and laboratory host range of the weevil, Listronotus marginicollis (Hustache)(Coleoptera: Curculionidae), a natural enemy of the invasive aquatic weed, parrot's feather, Myriophyllum aquaticum (Velloso) Verde (Haloragaceae). African Entomology, 15, 385-390.
Parker J.D., Caudill C.C. & Hay M.E. (2007). Beaver herbivory on aquatic plants. Oecologia, 151, 616-625.
Pine R.T. & Anderson L.W.J. (1991). Plant preferences of triploid grass carp. Journal of Aquatic Plant Management 29, 80-82.
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.
- Catarino L., Ferreira M. & Moreira I. (1997) Preferences of grass carp for macrophytes in Iberian drainage channels. Journal of Aquatic Plant Management, 35, 79-83
- Cilliers C. (1999) Lysathia n. sp.(Coleoptera: Chrysomelidae), a host-specific beetle for the control of the aquatic weed Myriophyllum aquaticum (Haloragaceae) in South Africa. Hydrobiologia, 415, 271-276
- Armellina A.A.D., Bezic C.R. & Gajardo O.A. (1999) Submerged macrophyte control with herbivorous fish in irrigation channels of semiarid Argentina. Hydrobiologia, 415, 265-269
- Garner A.B., Kwak T.J., Manuel K.L. & Barwick D.H. (2013) High-density grass carp stocking effects on a reservoir invasive plant and water quality. Journal of Aquatic Plant Management, 51, 27-33