- Apart from mechanical and chemical control methods to tackle invasive plant species, scientists also employ classical biocontrol agents such as fungi.
- Key to the success of a classical biocontrol agent is its host specificity. Scientists subject the agent to meticulous testing to ensure that it will not harm unintended targets.
- More than 600 biocontrol agents have been introduced across the globe, but the method has had low success because it is time-consuming and cannot be used for all invasive species.
For a long time, it was largely accepted that the invasive plant Mikania micrantha was brought to India by the British during the Second World War, so that they could use the rapidly growing plants to camouflage their airfields in the northeast, thereby protecting them from attack by Japanese bomber planes. However, when Achyut Kumar Banerjee, an invasion biologist at Azim Premji University, Bhopal, was compiling herbarium sheets from various national herbaria during his Ph.D., he discovered that this was not wholly true.
“The plant was introduced multiple times into India,” he clarified, with the earliest introduction dating back to the early 1900s. “The plant was likely taken from the Caribbean islands to Kew Botanical Garden, London, for their medicinal properties, from where they were introduced to the botanical garden in Calcutta.” Since then, there were a couple of more introductions in the 1940s and then in the 1950s. Cut to the present, the weed has severely impacted India’s ecology and economy, wiping out local flora, endangered wildlife, as well as decimating yields of plantation crops such as tea, teak and rubber.
At a recent meeting about bioinvasion that Banerjee helped organise, Bibhuti Prasad Lahkar, scientist at the biodiversity conservation organisation Aaranyak, noted that the mikania weed was one of the two primary threats to the grasslands of Manas Wildlife Sanctuary, a UNESCO World Heritage Site in Assam. The site is home to rare and endemic species such as the tiger, rhinoceros, elephant, pygmy hog and hispid hare.
Controlling the mikania weed spread across the Asia-Pacific region, has proved incredibly difficult. Mechanical control by weeding and uprooting are labour intensive and no match for its alarming rate of growth. It grows several centimetres a day, earning it the nickname ‘mile-a-minute’ weed. Chemical control too doesn’t seem to work in the long-term and the herbicides are expensive and hazardous to the environment.
Fungus at the end of the tunnel
It is with this context and a sense of urgency that forest scientist K.V. Sankaran, knocked on the doors of the international not-for-profit agricultural research organisation, CABI, in the mid-1990s. Sankaran, who would go on to become Director of Kerala Forest Research Institute, was at the time in the U.K. pursuing postdoctoral research. When he learnt that CABI was in fact investigating a biocontrol agent that seemed to be effective against M. micrantha, he was immediately interested.
“Classical biological control is the most benign, sustainable and cost-effective way to control invasive alien species,” says Sankaran, echoing the views of most conservation ecologists in the world. In this mode of bioinvasion management, an exotic agent — a living organism such as an insect, or a pathogen — is introduced into a geographical region in order to control non-native weeds or pests that have turned invasive there.
One of the reasons exotic species turn invasive in a new area is because they are allowed to proliferate with abandon, without the presence of their natural predators. So, when trying to identify biocontrol agents, scientists usually turn to the natural enemy of the invasive species’ host region. Indeed, in its native range across the Americas, M. micrantha rarely behaves as a weed; their natural enemies keep their population in check. One such enemy, found by U.K. scientists, was the rust fungus Puccinia spegazzinii. It took years of testing by CABI to confirm which pathotypes of the fungus would efficiently infect and kill Indian varieties of the mikania weed.
Accidental victims
The key to the success of a classical biocontrol agent is its host specificity or the degree to which the agent is restricted to plant species or even specific tissues within a plant. Scientists must subject the agent to meticulous testing to ensure that it will not harm unintended targets; only then do governments approve the transport of exotic species into the country.
The case of the small Indian mongoose, native to south Asia, illustrates what can happen if this is not done. This species was introduced around the globe to tackle rat invasions, but in a twist of fate, turned invasive itself in several regions including Europe, Hawaii, the Caribbean islands and Japan. The mongoose has gone on to cause the decline and extinction of several species of amphibians, reptiles, birds and mammals.
Sankaran and his collaborators were able to confirm that a rust fungus isolate from Trinidad was effective against mikania in India. After completing the host specificity tests as required by the Indian Ministry of Agriculture and Farmers Welfare, they proceeded to prepare a dossier and worked towards getting approval from the Indian government to release the fungus in the field.
“We released the fungus on a limited scale in sites across Kerala and Assam,” he said. “Since the fungus cannot be grown in a culture, we had to infect live mikania plants and place pots of these infected plants among the local population. In the night-time, the fungi’s reproductive spores are released and get deposited on [mikania] leaves.” The resulting leaf petiole and stem infections would ultimately cause death, curbing the weed’s spread.
So, did these first experiments work? “The results, in general, indicate good spread of the rust from the source plant to field population of mikania weed in Kerala and Assam,” Sankaran and his colleagues reported in a paper published in 2008. The fungus spread to the native population, up to two or three metres from its original released site but died off when the atmospheric humidity dipped and temperature rose. “It could not spread further,” said Sankaran.
Identifying the right agent
For Sankaran this is not a failure but just part of the journey towards an effective biocontrol strategy to control the mikania weed. In retrospect, the scientists realised that they released the fungus too late in the year, after the monsoons. Moreover, it may have been that the Trinidadian variety was not the best to tackle the mikania in India. “Puccinia spegazzinii has been successful in controlling mikania in five of the nine countries where it was released, but most of them used a pathotype from Ecuador. It is just a matter of identifying the right pathotype and the right time,” he said.
However, there are several examples of successful biocontrol of invasive species, globally and also some from India. According to Puja Ray, an invasion biologist at Presidency University, Kolkata, one of the most successful biocontrol programmes against invasive plants has been the one against Salvinia molesta, a free-floating aquatic fern from Brazil that has turned highly invasive in many parts of the world. “Tiny weevils (Cyrtobagous salviniae) from the plant’s native range were found to have great potential,” said Ray. The exotic weevils have been successfully deployed in various countries such as Australia and Sri Lanka. Ray pointed out that they have also shown excellent control in certain weed-infested sites in south and central India.
In some scenarios, classical biocontrol is not so practical. One of these is in the case of invasive fish. “As a conservation biologist, I won’t advocate the introduction of one fish to eliminate another fish,” declared Rajeev Raghavan, a fisheries scientist from Kerala University of Fisheries. Unlike pathogens and some insects, animals such as fish tend to be more generalist in their diets, he explained. That means that there is a high probability that there will be unintended targets. He pointed to the example of a reservoir in Munnar where the only fish that remains is the invasive alien African catfish. “Biocontrol may work in countries with very little native fauna, where there is no risk of losing a lot of native endemic fish. But in India there is a big risk of introducing a new predator.”
On the other hand, biocontrol of invasive fish is sometimes possible with the help of pathogens. “In Australia, they have used Tilapia lake virus to specifically control tilapia, a fish that is considered a pest in some countries such as Australia and India,” said Raghavan, conceding that it will not be easy to find similarly super specific pathogens to use as biocontrol agents against other fish.
Not always smooth sailing
When biocontrol agents are approved and available, there are still hurdles. “We have to accept that biocontrol takes time. Herbicides — though not good for the environment — are relatively fast and sureshot,” conceded Ray. Even when people are willing to try it (implementation of available biocontrol solutions) out, it’s not always smooth sailing.
She recollected the time she was working with local farmers to help them manage the invasive carrot weed or congress grass (Parthenium hysterophorus) with the host-specific beetle Zygogramma bicolorata which is a proven biocontrol agent against the weed. “There was one farmer who would come season after season and complain that the intervention was not working. When we investigated, we found that the Zygogramma beetles somehow had jumped borders into the neighbour’s farm and were multiplying enough to control their parthenium.”
Every day there are new reports of ecologically and economically important species being decimated by invasive species. While enhanced biosecurity and prevention is crucial, established invasions also need to be dealt with. As a former consultant with the UN’s Food and Agriculture Organization and a Coordinating Lead Author of the IPBES thematic assessment of invasive alien species (IAS) and their control, Sankaran firmly believes that classical biocontrol is one of the most trusted and sustainable ways to manage invasive alien species.
The biggest challenge, according to him, is the excessive red tape that slows down or halts these programmes. “There is an unfounded fear among government agencies that the biocontrol agent will have off-target effects on economically important crops. Though we spend years testing, they are still not convinced. More than 600 biocontrol agents have been introduced across the globe to manage IAS so far and only a very few had any non-target effects,” he stated.
The final spanner in the works is climate change. “We know that organisms are trying to safeguard their existence by looking for new food in new ranges — things are getting very erratic,” said Ray. “Biocontrol agents can become non-active due to climate change,” admitted Sankaran, “however, mutation is extremely unlikely especially in the case of fungal biocontrol agents.” Both Ray and Sankaran are assured that stringent testing will protect us from dire eventualities. “The agent HAS to be extremely well studied,” said Ray.
It’s been 20 years since the promising experiments with Puccinia spegazzinii against the increasingly devastating Mikania micrantha, but there has not been much progress since. “The fungus is a silver bullet,” insists Sankaran, “but we need someone to champion the re-introduction of the biocontrol agent.”
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Banner image: Invasion biologist Puja Ray observing the invasive water hyacinth. Ray shares there are several successful biocontrol programmes against invasive plants across the world, and in India. Image courtesy of Puja Ray.
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