Pontederia crassipes: Pretty for some, a nightmare for others

Tuesday, July 2, 2024
Water hyacinth plant depicting beautiful purple flower. Photo credit (D. Muir - DFFE)

By Debbie Muir

Pontederia crassipes, commonly known as water hyacinth, might be admired for its striking purple flowers, but it is also one of the most aggressive and problematic invasive plants globally. Originating from the Amazon basin, the species was first recorded in South Africa at the beginning of the 20th century and has since become a significant environmental threat in this country. Water hyacinth's rapid growth and reproduction capabilities pose severe challenges to water bodies in South Africa.

Under ideal conditions, water hyacinth can double its biomass every 5 - 10 days under ideal conditions. Each plant can produce up to 22 daughter plants, which mature, break off, and produce their own daughter plants, creating a vicious cycle. A single inflorescence with 20 flowers produces up to 3000 seeds depending on the site and time of year. Seed dispersal is also an important factor to consider as seed germination from seed banks or reservoirs takes only 3 days on average, with a combination of contributing factors such as water fluctuation, eutrophication and decomposition influencing the dispersal of seeds. The predominant dispersal strategy used by water hyacinth is via the daughter plant propagation through the formation of stolons.

Water hyacinth's rapid growth and reproduction capabilities pose severe challenges to water bodies in South Africa. It easily overwhelms small ponds are easily overwhelmed by the species, leading to desperate pond owners removing the plants and improperly disposing of them in nearby rivers or dams, exacerbating the spread.


A green field with a river and a city in the distance</p>
<p>Description automatically generated

Water hyacinth covering the Roodeplaat dam, Gauteng. (photo credit: D. Muir - DFFE)


Despite this bleak scenario, there is hope. Biological control agents, specifically insects, mites or even pathogens that feed on water hyacinth, serve as our ecological warriors. There are eight biocontrol agents known to attack water hyacinth effectively. However, their success is closely tied to water quality; the more eutrophic the water, the longer it takes for these agents to control the invasive plant effectively. Some biocontrol agents are also limited by environmental conditions such as temperature and seasonality. For example, the mirid (Eccritotarsus catarinensis) does not respond well to excessive cold or heat so releases of these biocontrol agents should be in coastal areas or areas with sufficient canopy cover, whereas the planthopper (Megamelus scutellaris) is much more adaptable and can withstand colder climates – meaning it can be released inland and in the coastal areas. This is the reason why M. scutellaris was chosen for the testing of the integrated approach at the Roodeplaat Dam in Pretoria.


A person holding a leaf with white spots on it</p>
<p>Description automatically generated

The biocontrol agent, Megamellus scutellaris, showing adults and nymphs (photo credit: Julie Coetzee)


In September 2020, the Department applied a biocontrol with sub-lethal spraying of an aquatic-safe herbicide, such as a glyphosate-based herbicide (GBH) without the carcinogenic co-formulant polyethoxylated tallow amine (POE-T), to the Roodeplaat Dam. Large numbers of the biocontrol agent, M. scutellaris, were released over three months. Initially, the water hyacinth infestation covered approximately 60% of the dam.

The intervention at Roodeplaat Dam demonstrated the potential of this method. Mass rearing stations were set up around the dam to boost biocontrol agent numbers, facilitating quicker establishment. By the third month, the water hyacinth cover had dropped below 45%.

The biocontrol populations were monitored until the water hyacinth mass dropped to below 20% at which stage a sub-lethal strip spray was applied by spraying 2 strips in the centre of the floating water hyacinth mass, ensuring the herbicide did not harm the biocontrol agents.

Care was also taken to ensure that the swaths that were sprayed were not within 5m of the shoreline to mitigate against riparian contamination and spray drift (Riparian contamination occurs when pollutants from nearby land enter rivers, streams, or other water bodies. This can harm aquatic ecosystems, affect water quality, and disrupt the natural habitat of riparian zones).

This approach not only controlled the water hyacinth but also made the plants more palatable for the biocontrol agents, accelerating their feeding and control efforts. Within three months, the water hyacinth was effectively managed without disrupting the aquatic ecosystem.

Given the success of the integrated approach on Roodeplaat Dam, the Department embarked on the process of formalising this approach as the standard policy for controlling floating macrophytes nationally. The integrated approach would then serve as the protocol to be applied on all water bodies, especially dams and rivers battling water lettuce and water hyacinth, amongst others.