Tiny army unclogs Harties Dam
The tiny planthoppers are biological control agents and natural enemies of the world’s worst aquatic weed
South Africa’s most problematic aquatic invasive plant, water hyacinth, is not clogging Hartbeespoort Dam, thanks to an army of tiny insects. The cover of water hyacinth — which has been named the world’s worst aquatic weed — on the polluted dam has plunged to below 2.5% in the past three weeks, according to Julie Coetzee, the deputy director of the Centre for Biological Control (CBC) at Rhodes University.
Biological control relies on the safe use of natural enemies of invasive species to feed on and reduce their populations. In Hartbeespoort Dam’s case, the biological control programme the CBC runs involves the deployment of host-specific Megamelus scutellaris planthoppers in their thousands to destroy the non-native weed.
The tiny planthoppers are natural enemies of water hyacinth, which is native to South America, and has thrived in nutrient-enriched waters like Hartbeespoort Dam since the 1960s. It forms dense choking mats that harms aquatic biodiversity and affects water sport activities.
The planthoppers reproduce rapidly and recover faster after periods of cooler temperatures than some of the other biocontrol agent species. Water hyacinth weevils (Neochetina eichhorniae and N. bruchi) are also in the CBC’S arsenal, as host-specific herbivores of the water hyacinth plant.
“At the beginning of spring [in 2023], the plants all came back, as we suspected they would but with the community mass-rearing [of planthoppers], once it went above 30% … it stayed at 30% and then crashed, so people could actually use the dam over December,” said Coetzee, who manages the biological control programme on aquatic weeds in South Africa.
“For the last three weeks the cover has been below 2.5%. Now, we have to prepare for the coming spring with potential seedling emergence. That’s all through the mass-rearing with our community rearing partners [who keep] the insects going over winter” until the weeds return in spring, she said, adding that there are 10 rearing stations around Hartbeespoort Dam.
The planthoppers pierce the plant’s tissue, damaging cells. This harms the petiole, which connects the leaf blade to the stem and results in waterlogging, reducing plant buoyancy and causing the tissue to rot.
A large seed bank exists in the sediment because each water hyacinth flower can produce thousands of seeds, which remain viable for many years. “We started the inundative releases in 2019 and then the first huge crash was January 2020, so every single year we’ve had the peak [of water hyacinth] and then the crash. They keep regenerating from a very big seed bank.”
The damage that the biocontrol agents cause prevents the weeds from producing new flowers, said Coetzee. “I think that in the long term, we’re going to almost deplete that seed bank and effectively, we won’t have these huge spring emergences.”
Her team plans to start taking soil cores and “then we can see if the number of seeds in these cores has decreased”.
A minor drawback of the biological control programme is that the bugs do on occasion swarm around people’s lights. This happens when the planthoppers can’t get nutrients from the plants.
On the sewage-polluted Vaal River, the CBC is doing similar biological control work to tackle the water lettuce invasion, which has infested up to 40km of the river in the Vaal Barrage area, threatening to spread downstream of the 1 200km stretch of the Vaal River.
Water lettuce, originally from South America, is a floating herb in rosettes of grey-green leaves, which form dense mats that cover a water surface.
The invasive plant clogs waterways and irrigation equipment, reduces water flow, impedes navigation, fishing and other recreational activities and provides a breeding place for mosquitos and bilharzia-carrying snails, according to Invasives South Africa.
The CBC is working with Rand Water on an integrated management plan for the invasion of water lettuce, ensuring a constant and abundant supply of the biocontrol agent, the water lettuce weevil Neohydronomus affinis, a Brazilian weevil species, Coetzee said.
Since November, it has released thousands of weevils into the Vaal River from its mass-rearing facility in Makhanda. Businesses, residents and Rand Water are similarly rearing weevils.
Huge mats of water lettuce are collapsing, Coetzee said, adding: “But there’s water hyacinth there as well and where the water lettuce is dying, the water hyacinth is taking over. So, we have to have a two-pronged approach: we have to get on top of the water lettuce and we have to get on top of the water hyacinth.”
A public outcry erupted in February over the use of glyphosate — a controversial broad-spectrum systemic herbicide — after the first chemical spray to tackle the water lettuce on the Vaal River. Last Friday, landowners proceeded with a second chemical spray, under Rand Water’s general authorisation. The bulk water supplier did not respond to the Mail & Guardian’s inquiries this week.
Last month, Water and Sanitation Minister Senzo Mchunu told News24 that Kilo Max, a glyphosatebased herbicide, would be used for the chemical sprays because it was a “safe chemical” and the “best weapon to kill the weeds”.
“Between the department of forestry, fisheries and the environment and the department of water and sanitation, they’re convinced that it’s the safest herbicide to use and it’s not carcinogenic,” Coetzee said. “They are saying that Roundup is what got all the bad press in the US. We use Kilo Max in South Africa, which has different surfactants or additives and they’re saying it’s the additives in the Roundup, which makes it toxic, not the active ingredient, which is the glyphosate.”
Ferrial Adam, the executive manager of Watercan, said: “The bottom line is it’s going to be sprayed. How do we then get them to test the water and to give us exactly what is in the spray? Without dealing with the source of these issues and the real kinds of challenges of the sewage, is this going to become an annual spraying?”
This is the concern, she said. “I understand that the residents around that area are very concerned; they are saying it’s their property and whatever, but at the risk of the environment do we then go wild and say it’s okay to spray? I’m not sure … The challenge is what do you do now? It’s so bad, it’s choking the life out of that whole part of the river.”
Water hyacinth had choked Hartbeespoort Dam for decades, she said. “Our responses are not longterm and sustainable, they’re kneejerk reactions, which then in two to three years gets us to the same point. Fix the issue that is feeding this [i.e., the sewage pollution] and be stronger on enforcement.”