From ornament to outlaw: The unchecked march of the Mexican Devil

A native of the tropical and subtropical regions of Mexico and Central America, Ageratina adenophora—commonly called Crofton weed or Mexican Devil—was first introduced to Africa and Asia in the 19th century as an ornamental plant. It soon escaped gardens and spread rapidly along roadsides, slopes, riverbanks, and farmlands. The plant is now found in over 40 countries, including India, Nepal, China, Australia, and parts of Africa. The National Biodiversity Authority and the Union Ministry of Environment, Forest and Climate Change have both identified it as an invasive alien species.

Spread and distribution

Although claims exist that it was first sighted in the Doon Valley in the 1930s, this remains unverified. The Flora of Kanjilal (FRI, 1969), regarded as a reference for foresters, makes no mention of it. Researchers now believe the species entered India in the early 1970s, probably from New Zealand, where it was first noticed in the late 1950s. The Nilgiri Hills and Darjeeling were among the earliest Indian regions affected. Since then, it has spread across Uttarakhand, Sikkim, the Western Ghats, Kerala, Andhra Pradesh, and Tamil Nadu, advancing by nearly 20 km each year. Once restricted to below 2,500 metres, recent field studies show it thriving even at 2,700 metres.

Ecological impact

The weed grows up to three metres tall in moist tracts. Its purplish stems root easily and form dense thickets. Each plant produces around 100,000 seeds every year, dispersed effortlessly by the wind or water. The species not only competes for sunlight and soil nutrients but also releases toxic (allelopathic) chemicals that inhibit the growth of native flora. “In some areas of the central Himalayas, almost three-fourths of the forest undergrowth has been replaced by Crofton weed,” says Akhil Joshi, a forest researcher. “It doesn’t allow regeneration of native herbs and grasses, and once established, it is extremely difficult to remove.” Locally known in Nepal as banmara—literally ‘forest killer’—the weed thrives well even in disturbed ecosystems and human altered environment. Livestock generally avoid it, and in horses, it can cause the fatal “blowing disease.”

The situation in Uttarakhand

The Central Himalayan region, especially around Nainital and Bhimtal, has seen a sharp rise in its spread over the last decade. Areas once dominated by Lantana are now blanketed by Mexican Devil. “The Gangikharak slope near Nainital, once a biodiversity hotspot, is now covered entirely by Crofton weed,” says Vinod Pande, a retired forest officer. He attributes its expansion to repeated forest fires and official indifference. “The Forest Department has neither proper data nor a concrete plan for eradication,” he says. Pande also links the spread to the suspension of traditional forest management known as silviculture after the 1980s. “In the name of protection, forests were left unattended, and invasive species quietly took over.”

Control measures and challenges

Efforts to control the weed worldwide have seen limited results. Even countries like China and US states like Hawaii have achieved only partial containment. In parts of Australia, it has forced farmers to abandon land. Mechanical removal remains the most effective though labour-intensive approach. The Benog Bird Sanctuary near Mussoorie, Vinod Pande informs, succeeded in getting the native herbaceous growth reappear once again after three consecutive years of uprooting the plant. Chemical control using glyphosate and dicamba has had modest success but poses ecological risks. Biological control trials have used the gall-forming fly (Procecidochares utilis) and fungal pathogens. “The gall fly worked well in Hawaii and parts of China,” explains a scientist. “But its success depends on local climatic conditions and host dynamics.”

Community awareness and possible solutions

Given the limited success of control methods, experts recommend ecological interventions. Planting fast-growing native trees such as Alnus nepalensis (Alder) could create shade that suppresses the sun-loving weed. “We must assess forest health not merely by canopy cover but by the biodiversity beneath,” notes Pande. Local communities, too, are finding uses for it. “During the Maoist insurgency in Nepal, people found that its sap could heal wounds and is an exceptionally quick blood coagulant,” recalls a migrant labourer  from Nepal. “Now, an NGO in Gupta Kashi is using it to make natural dyes.” Researchers have proposed converting it into compost, biodegradable herbicides, or biochar to improve soil fertility.

Conclusion

The unchecked spread of Ageratina adenophora across Himalayan ecosystems reflects a larger pattern of ecological neglect. Its expansion, aided by human disturbances and recurring fires, threatens native biodiversity. The battle against it will demand not only scientific solutions but also social awareness and collective responsibility. The Mexican Devil’s advance is nature’s way of reminding us that neglect can often grow into an uncontrollable force. 

Rajshekhar Pant is an author, freelance journalist, and documentary filmmaker based in Nainital

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth