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Ultimate ‘sting op’ — how Indian scientists imported wasps from Africa to tackle an alien pest

The parasitoids, which arrived in a shipment from Benin last August, have been released in parts of Tamil Nadu where the Cassava Mealybug caused massive damage to tapioca crops.

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New Delhi: What did Indian agricultural researchers do when they found out that one of the world’s most destructive pests is wreaking havoc in Tamil Nadu and Kerala? They pulled off the ultimate ‘sting operation’, and introduced it to its arch nemesis — an exotic wasp they imported all the way from Africa.

The Cassava Mealybug (scientific name Phenacoccus manihoti), which is alien to India but has somehow made its way into the country, was first spotted by farmers in an experimental plot of cassava crops in Kerala’s Thrissur in April 2020. Since the pest is not native to India, it has no known natural predators here.

The cassava is a nutty flavoured tuber native to South America, from which tapioca, a starch, is extracted.

India is the fifth largest producer of cassava tubers in the world, and the crop is grown over a total area of 1.73 lakh hectares across the country. It is cultivated predominantly in Tamil Nadu and Kerala, which account for 50.6 and 42.8 per cent of the country’s production, respectively.

Soon after the presence of the Cassava Mealybug was reported in Thrissur, researchers at the Indian Council of Agricultural Research (ICAR) realised that the pest — which had caused massive crop loss and endangered the subsistence of around 200 million people in sub-Saharan Africa in the early 1970s — was already widespread in Tamil Nadu.

Various stages of damage inflicted by the Cassava Mealybug on a cassava plant | Credit: ICAR-NBAIR
Various stages of rot inflicted by the Cassava Mealybug on a cassava plant | Credit: ICAR-NBAIR

ICAR is an autonomous body responsible for coordinating agricultural education and research in India. It reports to the Union agriculture ministry’s Department of Agricultural Research and Education.

The researchers’ solution for this was to import a shipment of Anagyrus lopezi, a type of parasitic wasp, from Benin in West Africa. ICAR’s National Bureau of Agricultural Insect Resources (ICAR-NBAIR) has the mandate to import foreign biological species into the country. 

The first field release programme of the parasitoids and their distribution to cassava farmers was organised on 7 March by ICAR-NBAIR, in collaboration with the Tapioca and Castor Research Station, Tamil Nadu Agricultural University, in which over 300 tapioca farmers from six districts of Tamil Nadu participated. On average, around 250 wasps were released for every acre. 

Over the next 2-3 years, the team will carry out impact assessment tests to understand whether the operation was successful. It is also training farmers on how to mass produce the wasps in their own fields, M. Sampath Kumar, senior scientist at ICAR-NBAIR, told ThePrint.

The ‘sting operation’ was inspired by past success stories. So far, the wasp has been introduced in 26 African and four Asian countries — resulting in a substantial reduction in the population density of Cassava Mealybugs in most agricultural fields.

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How did the Cassava Mealybug reach India?

The Cassava Mealybug is an alien invasive species, which means that the pest is not originally found in India. Although the researchers are not yet sure how the bugs entered Indian farms, they do have some theories. 

“It is possible that a cut of the (cassava) plant was smuggled from Thailand and brought to Tamil Nadu, although we will have to do further studies to confirm this,” ICAR-NBAIR scientist Kumar told ThePrint. 

Since it has no natural predators in India, the pest got a chance to thrive and multiply unchecked through Tamil Nadu and Kerala. 

The Cassava Mealybug usually infests the shoot tips and the under surface of the top leaves of the plant | Credit: ICAR-NBAIR
The Cassava Mealybug usually infests the shoot tips and the under surface of the top leaves of the plant | Credit: ICAR-NBAIR

The cassava crop is also grown in Andhra Pradesh, Nagaland, Assam, Meghalaya, Karnataka, Madhya Pradesh and to some extent in Puducherry, Tripura, Mizoram and the Andaman & Nicobar Islands. 

The productivity of cassava in India is 35 tonnes per hectare — which is much higher than the world average of 10.76 tonnes per hectare, according to an ICAR-NBAIR press note. The major reason for such high yield is that the Indian ecosystem had no major biotic threats to cassava. 

However, that changed during the pandemic with the emergence of the Cassava Mealybug.

“With the pest infestation, the yield reduced to 3-5 tonnes per hectare,” Kumar said.

Researchers believe that increased globalisation and trade have made India a target for entry of many new alien insect pests. 

In the last two decades, many exotic mealybug species such as Paracoccus marginatus, Phenacoccus solenopsis, Phenacoccus madeirensis, and Pseudococcus jackbeardsleyi have been recorded in India for the first time.

All of these have caused significant economic damage to several agricultural and horticultural crops. Hence, instead of targeting the mealybugs with pesticides, the researchers decided to go a more sustainable route to tackle them.

Damage caused in Africa & measures taken to counter it

In the 1970s, the Cassava Mealybug caused large-scale damage in the cassava belt of Africa, leading to widespread famine. Around 200 million people were affected, most of them poor farmers who depended on cassava as a staple. 

Scientists working at the International Institute of Tropical Agriculture (IITA) in Nigeria, one of the research centres of the Consultative Group for International Agricultural Research (CGIAR) — a global partnership that unites international organisations engaged in research about food security — helmed the search for natural enemies to be used in biological control of the Cassava Mealybug. 

Following its discovery in Paraguay, the parasitic wasp Anagyrus lopezi was mass-reared in Nigeria and Benin. The wasps were then released in around 150 locations in two dozen African countries, in collaboration with government agencies.  

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How Anagyrus lopezi cut down mealybug populations abroad

According to CGIAR data, the impact of Anagyrus lopezi was observed over seven years in Nigeria, and demonstrated long-term control. Surveys conducted in more than a dozen countries confirmed a ten-fold drop in Cassava Mealybug population densities and damage in 95 per cent of the surveyed fields. 

When the pest infested cassava farms in Thailand, a similar operation was carried out in the Asian country in 2009.

Inspired by these successful operations, ICAR decided to try this method out in India. 

Through the months of pandemic-induced lockdowns, the research team at ICAR began the process of obtaining the required permissions from the Union Ministry of Agriculture & Farmers Welfare and its Directorate of Plant Protection, Quarantine & Storage, for importing the parasitic wasp from Thailand. 

Despite getting the permit, diplomatic issues cropped up, which meant that the scientists had to look for another source, ICAR scientist Kumar told ThePrint.

The team then approached the IITA’s sub-centre in the Republic of Benin.

The first consignment of the wasps arrived in India in July 2021, but due to a mix-up at customs, the wasps died after being stored in a quarantine facility meant for plants, Kumar said. The researchers then imported a second batch in August. 

Mandatory quarantine studies on biology, safety and host specificity of the Anagyrus lopezi were carried out at the ICAR-NBAIR quarantine facility.

The wasps being released in a cassava plantation in Tamil Nadu | Credit: ICAR-NBAIR
The wasps being released in a cassava plantation in Tamil Nadu | Credit: ICAR-NBAIR

“We have to take special care before introducing any foreign species. Before importing such species, there are detailed studies by experts to ensure that the species does not cause any harm to or compete with existing natural species in the ecosystem,” said Kumar.

“We have mandatory protocols laid out to ensure that the imported species does not become a new pest, and that it will not harm any existing species upon release,” he added, saying they have to establish protocols for mass producing the species, and keep the imported species in quarantine to ensure it does not introduce any new infections. 

The researchers organised training programmes for staff members of state agricultural universities, state horticulture departments and Krishi Vigyan Kendras (farm science centres) about mass production and field release techniques of the wasps, before the first field release on 7 March.

(Edited by Gitanjali Das)

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