- With three exotic whiteflies appearing within a span of two years in India’s coconut plantations, researchers have flagged increased bio-security risks arising from the uncontrolled exchange of plant material.
- Drought appears to favour the spread of the pests in coconut plantations while biocontrol methods and monsoon rain have helped in curbing their spread.
- Experts have called for strengthening of quarantine measures.
First reported in Central America 2004 and then in the United States, the rugose spiraling whitefly, an invasive alien insect pest, was quietly siphoning out the sap from coconut palms in southern states of India in 2016, as they reeled under a prolonged dry spell.
The rampage of the two millimeters-long whitefly, a sap-feeding bug, was subsequently subdued by its natural enemies. Two years later, in December 2018, after the devastating Kerala floods, two tinier whitefly species, also invasive, had displaced their predecessor, scientists said in a study.
In the wake of the appearance of three exotic whiteflies within a span of two years in India’s coconut plantations, the Central Plantation Crops Research Institute (CPCRI) scientists have sounded a warning bell on the increased bio-security risks arising from the uncontrolled exchange of plant material.
“The exchange of planting materials through liberalised trade and transboundary movement of such materials without proper pest risk analysis has to be dealt with strict reforms and policy frameworks,” said Chandrika Mohan of CPCRI, as farmers battle the twin pests with recommended biocontrol strategies.
Calling for the strengthening of quarantine measures, scientists also underscored framing palm management strategies in drought-prone areas in the country, underpinning the climate change link to the spread of pests such as whiteflies.
“Hotter temperatures favour the growth of sucking insects such as whiteflies. The prolonged dry spell during June to September 2016, after deficit rainfall, coupled with decreased relative humidity appears to favour the spread of the pest in coconut plantations,” said Mohan.
“So introducing the pest’s natural enemies in drought-struck areas could help check the spread,” Mohan said. Another reason that they are able to successfully cling on in India could be the prevalence of similar weather conditions in the native range of these whiteflies and in Kerala.
Sunder Moorthi who is currently with the Locust Warning Organisation (LWO) in Jodhpur and had earlier dealt with the whitefly menace, agreed that quarantine measures and monitoring need to be strengthened, including cohesion between customs officials and plant quarantine personnel on checking agri-products coming into the country.
“If a person is carrying agri-products such as fruits or vegetables or flowers that are under two kilograms then they are not scanned by customs officials. Materials that are above two kilograms in weight are brought to the notice of plant quarantine officials in international airports. But even a small amount of plant material may be carrying a potential pest,” Moorthi, who was not associated with the study, told Mongabay-India.
“There are loopholes in cross-border controls that can facilitate the passage of pests. For instance, in 2018, some West Bengal districts bordering Bangladesh saw deadly episodes of wheat blast. This was because of plant material carried by people traveling across the open border areas,” Moorthi said.
Under the Plant Quarantine (Regulation of Import into India) Order, 2003, import of seed nuts/ seedlings/pollen/tissue cultures of coconut and related species of Cocoidae is prohibited from certain countries such as Ghana, Cuba and Sri Lanka, because of the incidence of destructive pests responsible for yellowing, lethal boll rot and other diseases.
“While the Plant Quarantine Order is the Bible for international plant import regulation, each state has its own regulation. For example, the Tamil Nadu Agricultural Pests and Diseases Act, 1919 and Kerala Agricultural Pests and Diseases Act, 1957 which should help control inter-state spread,” said Moorthi, adding that these regulations should be enforced properly.
R. Sundararaj, of Forest and Wood Protection Division, Institute of Wood Science
and Technology, Bengaluru, who helped identify the whitefly with colleagues from ICAR-National Bureau of Agricultural Insects Resources (NBAIR), underlined the need to have competent people for accurate identification of the species.
“Import of ornamental palms may have also led to the introduction of the spiralling whitefly. In recent years, we are seeing more whitefly invasives. To strengthen quarantine measures, we also need people to correctly identify invasive pests,” Sundararaj told Mongabay-India.
“There are some whitefly species that are native and not potentially harmful but non-native species are impacting crops,” said Sundararaj.
The march of the whitefly through south India
Tamil Nadu, Kerala, Karnataka, and Andhra Pradesh are India’s top coconut producers with the fruit integral to the coastal states’ economic, cultural, and tourism sectors. During Chinese President Xi Jinping’s India visit in October, 2019, Prime Minister Narendra Modi and Xi Jinping sipped fresh coconut water in the coastal city of Mamallapuram in Tamil Nadu.
In 2016, these graceful coconut palms swaying along India’s coasts came under attack from never-before-seen tiny, white moth-like insects. Rugose spiraling whitefly (RSW) (Aleurodicus rugioperculatus Martin) is considered to have made its entry into India (Pollachi, Tamil Nadu, and Palakkad, Kerala) during 2016 from Florida (USA). It is one of the three whitefly species reported to attack coconut. After gaining a firm foothold in southern India, it snuck into Assam in northeast India.
Whiteflies inflict direct feeding injury on plants while sucking sap and serve as vectors of plant diseases.
“The whitefly starts attacking from the lower leaves of the coconut palm. It drains the sap from the underside of the palm leaves, producing a significant amount of honeydew which settles on the upper surface of the next lower leaf leading to the growth of black sooty mould. This is the secondary infection arising out of the whitefly infestation,” explained Mohan.
In a very short span, RSW spread across coconut plantations and also branched onto guava and banana crops. It was eventually brought down by its natural enemy, an insect parasitoid, Encarsia guadeloupae, which had already present in India. Parasitoids live on or in a host organism and ultimately kill them.
As for the black sooty mould, a scavenger beetle, Leiochrinus nilgirianus could successfully reduce the invasive potential of this non-native pest.
Mohan said: “We advised the farmers need not panic as the pest is not reported to cause economic damage in any of the crops from other countries but to refrain from using insecticides as that would destroy the natural predators of the whitefly. We augmented the survival of the natural enemy in areas where the pest had not invaded yet. The parasitoid helped check 60 to 70 percent of the pest population.”
Control strategies are recommended when more than 40 percent of the coconut palm leaves are infested. “There are about 30 to 40 fronds in a healthy coconut palm canopy. When less than five percent of lower leaflets are infected we need not go for any control methods,” she said.
“Kerala floods also helped control the pest population. The continuous rainfall disturbed the presence of the pest on the leaf undersides,” said Mohan.
However, the respite from RSW outbreak was shortlived. Just as the RSW was receding, two nesting whitefly species, Bondar’s nesting whitefly, Paraleyrodes bondari Peracchi (Bondar’s nesting whitefly) and Paraleyrodes minei Iaccarino emerged.
What stumped researchers was the fact that these two species occurred simultaneously in most of the coconut leaflets – a “rare exception” to the competitive exclusion principle or Gause’s law, which states that two species competing for the same limiting resource cannot coexist at constant population values.
They apprehend that further increase in pest population and sooty mould deposits could lead to economic setbacks.
“The parasitoid predator for the rugose spiralling whitefly was not able to control these two species. Instead, beetles and lacewings are able to check the spread of the two nesting whitefly species,” said Mohan.
The recent spate of monsoon rain, according to Salva Saidutly, Plant Protection Officer (Entomology), Central Integrated Pest Management Centre, Ernakulam office in Kerala, also helped rein in the RSW and Bondar’s whitefly expansion in Kerala.
“Continuous rainfall dislodged the whitefly from underneath the coconut palm fronds. We conducted training sessions with farmers to boost the growth of natural predators to control the whitefly infestation in the state,” Saidutly told Mongabay-India.
Banner image: Coconut Trees, Theni, Tamil Nadu, India. Photo by SnapMeUp/Wikimedia Commons.