Bugs sent into battle — meet the SA team using insects instead of pesticides to tackle invasive plants

Daily Maverick

a day ago

Sustenance farmers are hit the hardest by the impacts of invasive plant species. Scientists are working to change that, but they face challenges.
South African farmers spend about R1,200 per hectare on weed control, including herbicide application and other methods, according to an estimation from 2021. Their costs also include diesel for machinery, crop insurance and harvesting.
Although commercial farmers feel the costs, they often have resources to buy pesticides, herbicides and manual labour to ensure their crops do well and the land is workable.
Daily Maverick visited sustenance farmers in northern KwaZulu-Natal who were losing thousands of their goats to an illness they claim is from an invasive plant species – parthenium. In KwaNongoma, the weed is visible throughout the grazing land, and it is highly competitive with grass. It also causes rashes in children after long-term exposure, along with other effects.
Daily Maverick visited the Plant Health and Protection unit of the Agricultural Research Council (ARC-PHP) in Hilton, Pietermaritzburg, where scientists focus on ecologically sound management strategies for agricultural pests, plant diseases and invasive plants.
One of the researchers, Lorraine Strathie, said invasive plant species can be controlled by bringing insects from their country of origin to feed on them here. But if this sounds simple, it is not. The process takes devotion, trial and error and collaboration.
'For example, parthenium is indigenous to the lands around the Gulf of Mexico in North America. So in its native range, in their native range, plants have evolved for millions of years, and they have natural enemies in the form of insects which feed on them, fungi and other pathogens. So it's what we call top-down control,' she said.
'So, they reduce their reproductive output, the number of seeds they produce, and their growth rates. And these insects, many of these natural enemies, because the plants produce defensive chemicals, most of the insects and fungi that feed on them have become highly specialised to get around those chemicals and other defences. So they become highly, what we call host-specific – they only feed on one species of plant.'
Entomologists and those in related sciences dedicate years to understanding which control agents are likely to be suitable to bring to South Africa. When the insects get here they are quarantined and numerous tests and observations are done to make sure they don't have a taste for South African plants, which would add them to a long list of pests instead of control agents.
'So one of the main theories is that when a plant is taken out of its native environment and brought to another continent, the natural enemies don't come with it because it comes as a seed, and then it can grow without any kind of top-down control. And its growth rates increase, its seed output increases, and it outcompetes the local plants. Then it becomes invasive,' Strathie said.
The ARC-PHP was established in 1962 with the amalgamation of the divisions of eEntomology and plant pathology of the Department of Agriculture, Forestry and Fisheries.
Challenges
The challenge with this science is that it can take time and needs the land these agents have been placed on to not be disturbed by pesticides, herbicides and building. This requires land owners to cooperate with the team so they can check the population they have released every year to see how much it has grown, how far it has spread and the impact.
To achieve optimal biological control, studies have shown that 'integrating various biological control options that include classical biological control, mycoherbicides and suppressive plants with management tools such as chemical, physical, grazing management and cultural management is desirable'.
Dr Costas Zachariades, officer-in-charge and senior researcher in the ARC-PHP, explained that the population starts breeding by itself and feeds on the plant, and spreads by itself.
'So once we've got past that stage of what we call establishing the population, then it becomes self-sustaining, and it doesn't need any more human intervention. And so with weed control you have to keep on spraying with herbicide, uprooting manually, doing what you're doing. So there's always human intervention required on an ongoing basis, whereas with weed biocontrol, that is not needed. So it's a long-term, sustainable solution. The downsides are that it takes a long time,' Zachariades said.
It could take up to 20 years for the biocontrol agent to reach its full potential – that is, being widespread over large areas, the plant has decreased and invasive plant growth is naturally stifled. But it could be effective in three to five years, according to the scientists.
Community resistance
Affected people, such as those in agricultural communities with small-scale farmers, have been resistant to the idea, worried that the agents will start eating their crops once the weeds are gone.
Strathie emphasised that this is the core of their work in the quarantine lab, over and above growing the insect population – they do a number of tests to ensure the agent only eats that weed and nothing else.
'It's a very common perception. A lot of growers will know that some insects or pathogens can be highly host-specific,' she said.
Even in the case of lantana, which had many different hybrids and varieties, some of the insects were so specific that they might only go for a certain colour of a certain hybrid.
The agents could have certain chemical cues they required for attraction to a plant, as well as for mating and feeding. 'So the natural enemies we're using are really host-specific.'
In the lab the team showed how the insects are not only plant-specific but eat specific parts of the plant – some eat the stem, some like the leaves, some lay eggs on the flowers, thereby crippling the invasive plant's growth.
'… we ensure that whatever is released into the environment is completely safe. I mean, the science of weed biocontrol has been around for 112 years in South Africa, and South Africa is one of the leading countries in the world in this science. So it's quite a small community of weed biocontrol researchers in the country, but the science is very rigorous, we're not going to be introducing something that's going to be potentially harmful on other plants,' Strathie added.
Funding
This work appears to be significant in ensuring biodiversity, especially as civil society organisations are calling for the banning of a number of pesticides.
And this team considers itself lucky because it receives funding from the Department of Agriculture, while other teams doing similar work have stopped due to a lack of funding.
'Funding, I think, globally is challenging, and invasive species often are not recognised for the level of importance of how they impact on people's livelihoods, not only from conservation, but food production and health,' Strathie said
'And I think there's generally not a common understanding of what kind of impact they may have. And I think [that] for research it's critical, and especially this type of research needs sustained funding for long periods, and once you have a biological control agent to be able to reap the benefits of all that investment. Developing a biocontrol agent, you then need government's national support to ensure it's rolled out to its full benefit.'
Mass rearing for impact
The team explained that a mass release of the tried-and-tested control agents has made an impact in biocontrol in affected provinces such as Mpumalanga and KwaZulu-Natal.
This is made possible by the Department of Agriculture, which has been supporting the programme since 2023.
'But also previously, the Department of Forestry, Fisheries and the Environment (DFFE) were the major funders for biological control of weeds in South Africa. So there was a lot of R&D done in those decades with DFFE that we now also benefit from South Africa being able to [mass-rear] those agents,' Strathie said.
Zachariades added that they have six major weeds currently, for which they have multiple biocontrol agents being mass reared at the unit and at Brunner Plot in Pretoria, a sister institute. Which meant that, if there were no more funding to mass-rear the agents, 'we can hasten their impact by releasing more of them and more often at many sites'.
'So, for example, for parthenium insect agents, we've got three of them, and we've got something like 400 release points already, and those will just keep increasing in KZN and Mpumalanga, because those are where the weed is most prevalent at the moment. It's in other provinces as well, but those are where it's really severely invading.'
The team had also mass-reared agents for commelina, lantana pompom weed ('which is problematic on the high field at this stage') and Tecoma stans, or yellow bells ('it's become a big weed along the coast here') as well as the Mexican sunflower, all of which pose a threat to indigenous flora. DM