KATHMANDU, JANUARY 30
Following reports of widespread defoliation in sal (Shorea robusta) forests in Dailekh district, the Forest Research and Training Centre (FRTC), under the Ministry of Forests and Environment, has conducted a field investigation to determine the causes.
A three-member team led by Sunita Ulak, Undersecretary at FRTC, included Dr. Prem Budha, an entomologist at Tribhuvan University, and FRTC researcher Pratik Pandey. The study was carried out in coordination with the Ministry of Industry, Tourism, Forests and Environment of Karnali Province, the Karnali Province Forest Directorate, and the Forest Division Office in Dailekh.
Local accounts from Kimugaun Community Forest indicate that a similar outbreak occurred approximately three years ago but subsided naturally following timely winter rainfall.
Scope of the Damage
According to Ulak, Nepal's sal forests host a wide range of insect species, including defoliators, sap-sucking insects, leaf miners, and wood borers. Among them, defoliators-primarily moths and butterflies in their larval (caterpillar) stage-pose the greatest threat, as they feed intensively on sal leaves, weakening tree health and reducing forest productivity.
Preliminary observations show that sal forests in Kimugaun, Chupra, Kotila, Sristhan, and Belpata have been severely affected. The outbreak is believed to have been triggered by a prolonged dry season and insufficient winter rainfall, which created favorable conditions for insect proliferation.
"Due to delayed reporting of the outbreak, we were unable to collect live larvae or adult insects, making species-level identification difficult," Ulak said.
Early Observations
Local communities reported that the insects first appeared around five months ago (around Aswin) and gradually began feeding on sal leaves. At present, foliage in most affected areas has been almost entirely destroyed, Ulak noted following her field visit.
Delayed communication with authorities allowed the infestation to progress to an advanced stage, complicating both management efforts and scientific assessment. This highlights the critical need for early detection and timely reporting in future outbreaks.
"We observed extensive damage caused by hairy leaf-eating larvae, particularly in the western and southern forest blocks," Ulak added.
Although no live larvae, pupae, or adult insects were found during the survey, large quantities of exuviae (molted skins) and pupal cases were recorded. Preliminary findings suggest the infestation is likely caused by a single species or a small group of closely related species.
The larvae are believed to belong to the Erebidae and Lasiocampidae families; however, laboratory analysis will be required for definitive taxonomic identification, lead investigator Dr Budha said.
Probable Causes and Climatic Influence
Speaking to The Himalayan Times, Dr. Prem Budha stated that there is no conclusive evidence directly linking the outbreak to climate change, though climatic factors appear to have played a contributing role.
"Prolonged drought, inadequate winter rainfall, and seasonal irregularities seem to have created conditions favorable for insect population growth," Budha said. He noted that southern and western forest areas were more vulnerable due to their warmer microclimate.
Historical records show a similar outbreak occurred three years ago, but insect populations declined naturally following winter rains, indicating that precipitation plays a key role in regulating pest cycles. In some heavily affected zones, sal trees have already begun early budding, suggesting the forest is undergoing natural regeneration despite the damage, Budha added.
Expert Insights
Dr. Budha explained that the outbreak is caused by moth larvae classified as defoliators. "Sal forests host between 300 and 400 species of leaf-eating insects, but only one or two species typically trigger large-scale outbreaks," he said. "Our field observations confirmed intense larval activity, with abundant molted skins and pupal cases."
Insects in sal forests are categorized based on the type of damage they cause, including leaf chewers, sap suckers, leaf borers, flower and fruit feeders, and wood borers. "However, the current outbreak is clearly driven by defoliators that feed directly on leaves," he said.
Adult moths typically emerge during Chaitra–Baisakh (March–April), laying eggs on sal leaves. The larvae undergo five to six molting stages before pupating and eventually emerging as adult moths.
Management Measures
Ulak warned that similar outbreaks could occur in other sal forest regions and urged community forest user groups to remain vigilant and report infestations at an early stage.
She recommended light forest cleaning and pruning during Bhadra–Ashoj (August–October) to reduce infestation risks. She also suggested scientific control measures such as light traps during the adult emergence period to limit pest populations.
Dr. Budha concluded that the defoliation crisis in Dailekh is driven by multiple factors, including seasonal conditions, insect life cycles, and broader climatic trends. He emphasized the importance of early intervention, community participation, and long-term scientific monitoring to prevent future outbreaks.
''We plan to return to the affected areas next season to conduct further studies and determine the precise cause of the infestation," Ulak said.
