Island pinhole borer, nature’s unexpected ally, combats invasive Senna trees in Wayanad 

It could be a blessing in disguise that Senna spectabilis, a fast-spreading invasive alien tree species in Wayanad forests, is now under massive attack from island pinhole borer (Xyleborus perforans). This insect, which burrows tiny pin-sized holes into the tree trunks, has become a serious threat to the invasive plant species that damages wildlife habitats.

Xyleborus perforans is called a “pinhole borer” as it creates tiny, pin-sized holes in wood by burrowing into it to lay eggs. The females of the species carry fungal spores with them. Both the larvae and adults feed on the fungus they cultivate within the tree trunk, and it is this combination of insect activity and fungal infection that eventually kills the tree.

T.V. Sajeev, who coordinates the Nodal Centre for Biological Invasions at the Kerala Forest Research Institute (KFRI), Peechi, notes that the insect’s massive infestation was first spotted at Thakarapadi in the Muthanga Forest Range and is now rapidly spreading across the region. The insect attacks live trees, and infested trees don’t survive for long. Interestingly, Xyleborus perforans is native to India.

Dr. Sajeev conducted field observations in the Muthanga Range and collected the insects, which were identified using molecular tools by Arathy M.S., a postgraduate student at St. Joseph’s College in Irinjalakuda, who is now working on her dissertation work at the KFRI.

“The borer is a tiny pest, measuring just over 2 mm in length, but it infests trees in large numbers. All life stages of the insect can be found in the tunnels it creates. It completes its life cycle in 16-18 days, and they either construct new galleries in the same tree or move to nearby ones, “ Dr. Sajeev noted.

Diminished food supply

Senna spectabilis, also known as manjakonna, originally introduced to India as a shade tree for coffee plantations and as a source of firewood, has become a major environmental threat. Native to Central and South America, it has spread rapidly across the Nilgiri Biosphere, severely damaging wildlife habitats. This invasion has led to a dramatic decline in floral diversity, reducing the food supply for herbivores. The loss of natural habitats is believed to be a key factor behind the increasing number of wild animals straying from the forest in search of food and water.

In Wayanad alone, it’s estimated that Senna trees have overtaken 123.86 km² of forest. The problem is also growing in Karnataka and Tamil Nadu. In response, various methods such as debarking, cutting, and removal have been tested with varying success. Recently, the Kerala Forest department has struck a deal with Kerala Paper Products to convert the invasive trees into pulpwood for paper production.

Fire hazard

“While this is a welcome development in the efforts to manage Senna spectabilis, the population build-up of the insect and the response of its natural enemies need careful monitoring,” Dr. Sajeev pointed out.

One immediate concern is the increased fire hazard posed by the large number of trees that have dried up as a result of the insect infestation, he added.