Rubber plantations in Kerala have been under threat since a beetle-fungus alliance has been attacking trees, causing severe leaf fall and rapid drying. Researchers at the Kerala Forest Research Institute in Thrissur recently identified the parasite to be the ambrosia beetle (Euplatypus parallelus).
In their new study, published in Current Science, the beetle has been reported to share a mutualistic relationship with two fungal species, Fusarium ambrosia and Fusarium solani.
This is the first report of F. solani in association with the adult ambrosia beetles.
Fungi in the galleries
Earlier, farmers working in the rubber plantations in the Irrity-Kannur region of Kerala noticed latex oozing from the barks of rubber trees. Under the Tree Health Helpline project, they alerted researchers from the Institute, insect ecologist Jithu Unni Krishnan said.
Ambrosia beetles get their name from the ambrosia fungi that call the beetle their home. The name ‘ambrosia’ is not taxonomic but ecological. These beetles are native to Central and South America. They were first reported in India in the cashew trees of Ponda, Goa, in 2012.
These beetles attack dead or infected trees, although they’re also known to attack stressed trees. At times, the stressed trees release ethanol, a volatile compound that the ambrosia beetles can sense and attack. The beetles don’t feed on the woody bark of trees; the fungi do. The beetles bore tunnels called galleries in the bark, carry fungi into the galleries, and farm the fungi to concentrate nutrients. The beetles and their larvae feed on nutrient-rich fungal mycelia. The fungi also release enzymes that weaken the wood, allowing beetles to penetrate deeper.
In other insect hosts, the fungi are present in sacs called mycangia. In the present study, however, the team didn’t find mycangia in the ambrosia beetle. Krishnan said that it’s of interest to study how the fungal species interact with each other while coexisting in a beetle without the mycangia.
Long time to heal
The beetle-fungus association harms trees in many ways. Aside from weakening the structure, the duo causes severe leaf fall, trunk drying, and in some cases even tree death. The infection also affects total latex production from rubber trees, causing economic and agricultural losses. To combat the infection, experts follow specific methods, such as using antifungal agents, removing the infected part of trees, burning or chipping away any part that displays holes, and preventive measures such as using traps for ambrosia beetles.
Further, once a tree is infected, it takes a long time to heal. “The systemic infections progress through the plant xylem, blocking the xylem vessels. Moreover, proliferation of the fungus inside the xylem leads to sporulation, which leads [it] to secrete several enzymes, weakening the wood strength and showing death in distinct parts of the tree,” Amey Redkar, reader at the National Centre for Biological Sciences, Bengaluru, an independent expert working on Fusarium oxysporum, also a plant pathogen, said.
Controlling a fungal infection in plants is difficult. The fungi reside in deeper parts of an infected plant, where insecticides or fungicides often don’t reach. “Once [fungi] have progressed systemically, it’s too late to save a plant from it. Many Fusarium sp. spread either through soil or can also be carried by insect vectors,” Redkar added. “As these pathogens have adapted to outcompete neighboring microbes to reach their vascular niche, they have evolved to modulate the surrounding microbiome.”
Future team-ups
A few species of ambrosia beetles, including Euplatypus parallelus, are invasive and threaten horticulture and silviculture worldwide. Being flying insects, they can access a variety of trees. “It can infest to my understanding more than 80 species of broadleaf trees, including cashew, teak, coconut, and coffee,” Krishnan said.
While the beetle has coevolved with its fungal partners, it may in future associate with other pathogenic fungi, posing a bigger threat to plantations. Likewise, Fusaria sp. are very virulent and known to expand their host range. “The concern is how many indigenous virulent pathogenic fungi are likely to associate with this insect and thereby broaden the host range and impact of this insect,” Krishnan said.
Fusarium fungi infect beetles as well as other organisms, including spiders, frogs, and humans. These fungiare opportunistic pathogens in humans, meaning they can affect those with a compromised immunity, posing a significant health risk to workers in rubber plantations, as well as other plants and animals within an ecosystem.
Much to lose
The destructive power of the beetle-fungi association and the risk of other pathogenic fungi teaming up with the beetles together portend alarm. According to experts, the possibility calls for an action plan to mitigate and prevent further attacks. Since the number of invasive ambrosia species is increasing as well, Krishnan said policymakers and researchers should step up, collaborate, and provide solutions to manage infections.
India is the world’s sixth-largest producer of rubber and the second-highest in terms of productivity. Kerala produces 90% and accounts for 72% of India’s rubber cultivation area.
While there are reports of successful phytosanitary measures in coniferous trees, the same measures may not work on broad-leaf trees such as rubber and teak. The infection also makes other economically significant plants, such as coffee, cashew, mango, and coconut, vulnerable to infections. To prepare, experts recommend management strategies be designed based on the geographic location of the plantations.
“Things that are applicable to other parts of the world may not be applicable to Kerala or South India,” Krishnan said.
Redkar added that sustainable treatments, such as using antagonistic fungi that can compete with the pathogenic onesor using microbial consortia with a diversity of bacterial species that can live inside plants, can offer promising results in mitigating fungal infections in plants.
Rohini Karandikar is a science communicator, educator and facilitator. She currently works at TNQ Foundation as a consultant.
Published – July 23, 2025 05:30 am IST
