Weeds in our Area (Part One Hundred and Eight)
By Bob and Ena McIntyre – Garden Route
BIOLOGICAL CONTROL CONTINUED
More Water Weeds – Myriophyllum aquaticum (Parrot’s feather)
We have come to the last of the aquatic weeds also occurring locally for which a biological control
agent has been effectively released. Our subject is Myriophyllum aquaticum (Parrot’s feather,
waterduisendblaar). The Afrikaans common name is particularly descriptive. Originating in South
America M. aquaticum is now found throughout the world. It was introduced into South Africa
around 1919 as an ornamental plant for ponds. First recorded in the Paarl area, Parrot’s feather
has spread through the south-western Cape, KwaZula-Natal, southern and eastern Mpumalanga
and is a declared Category One invader. Parrot’s feather
is a rooted aquatic plant that can grow into dense mats
and is equally at home in clear, polluted or brackish
water. The plant produces minute (1.2 mm long) cream
flowers in the axils of the leaves during May to
September. Interestingly, the plants reproduce asexually
(needing only one parent) and there are no male plants
found outside South America – fragments from rooted
plants produce new plants.
Impacts: The health risk posed by Myriophyllum
aquaticum is by far the most serious. The plants provide
an ideal breeding environment for mosquitoes and
Schistosomiasis (also known as bilharzia, bilharziosis or
snail fever) carrying snails. It is estimated that 207 million
people suffer from bilharzia worldwide. Malaria affects
approximately 515 million people annually and between
one and three million people die from the disease, these
are mostly young children in sub-Saharan Africa. This
clearly illustrates the incredible importance of minimizing any conditions that are favourable for
the breeding of the vectors of either of these diseases. As with most other water weeds Parrot’s
Feather blocks water-ways and causes problems in irrigation systems.
Control: No herbicide is registered in South Africa for use on this weed. Although herbicides with
the active ingredient diquat (dibromide) are registered in other parts of the world, this is the
active ingredient in Midstream which is registered for Salvinia molesta (kariba Weed) and
Eichhornia crassipes (water hyacinth). Apart from manuallly removing the plants from the waterbody, bio-control is the only other solution. When manually removing the plants the process must
be particularly thorough and cleared areas must be checked very regularly to spot any re-growth.
A leaf-chewer; Lysathia sp., released in 1994 established itself successfully and has had a
significant impact on containing the spread of the weed.
References: Alien Weeds and Invasive Plants by Lesley Henderson, Copyright © 2001 Agricultural
Research Council
Global Invasive Species Programme © 2004 (including illustration), Problem Plants of South Africa
by Clive Bromilow 2001. http://www.wikipedia.org and http://www.arc.agric.za.