Invasion status and impacts of parthenium weed (Parthenium hysterophorus) in West-Central region of Bhutan

Parthenium weed (<i>Parthenium hysterophorus</i> L.) is an invasive alien plant species that is spreading rapidly all over the world. With globalization and climate change, it will continue to spread, posing threats to agriculture, biodiversity and the environment. To determine its invasion and impacts in Bhutan, an investigation was undertaken in West-Central Bhutan using a questionnaire survey, complemented by MaxEnt modelling. Most farmers (19–30%) recall having seen parthenium for the last 11–15 years, or more. Infestations of parthenium differed significantly (<i>P</i> < 0.05) across three districts for dryland, orchards, fallow land and pastures, but not in rice fields and on roadsides. The most intensely invaded land types were fallow land, pastures, and roadsides, with farmers perceiving it as 'somewhat common'(score ~ 3) and 'common' (score ~ 4) weed. Farmers' perceptions of the invasiveness of parthenium weed were high at 77% (Wangdue), 80% (Punakha), and 88% (Tsirang), considering it as at least 'somewhat invasive', or higher. Only 13–25% of respondents scored the weed as 'highly invasive'. Parthenium invasion significantly (<i>P</i> < 0.01 to 0.001) escalated the cost of production, reduced fodder quality, supressed native species and caused allergies to the locals in the study districts. Further, MaxEnt modelling of parthenium distribution was undertaken, using the parthenium presence-only data and bioclimatic variables, under 2 CO2 emission scenarios of the Intergovernmental Panel on Climate Change (IPCC), i.e. Representative Concentration Pathways (RCP2.6 and 8.5). The projections showed discernible spatial changes in climate suitability over 2070, mostly in the mid- and high-altitude zones, indicating aggressive incursion into Bhutan's important agro-ecological landscapes. The findings, highlight the urgent need to initiate parthenium management strategies for food production and natural resources.