Trade and transport of goods is widely accepted as a primary pathway for the introduction and dispersal of invasive species. However, understanding commodity flows remains a challenge owing to its complex nature, unavailability of quality data and lack of systematic modeling methods. A robust network-based approach is proposed to model seasonal flow of agricultural produce and examine its role in pest spread. It is applied to study the spread of Tuta absoluta, a devastating pest of tomato in Nepal. Further, the long-term establishment potential of the pest and its economic impact on the country are assessed. Preliminary analyses indicate that T. absoluta will invade most major tomato production regions within a year of introduction and the economic impact of invasion could range from $17-25 million. The proposed approach is generic and particularly suited for data-poor scenarios.