What factors regulate insect populations and their movement in the tropics? We censused butterflies flying across the Panama Canal at Barro Colorado Island (BCI) for 16 years to address two questions. What environmental factors determine the date on which the number of migrating butterflies peaked? What factors determine the quantity of migrants crossing the Panama Canal on the peak date? The number of Aphrissa butterflies was greatest four weeks after the onset of wet season. Wet season initiates the production of new foliage on hostplants, which probably cues females to lay eggs. This new generation grows into the butterflies that migrate across the isthmus from the Atlantic to Pacific coasts. Favorable winds do not synchronize the migration, but clear weather is certainly a factor. Larsen proposed the El Niņo migration hypothesis to explain the association of butterfly outbreaks and migrations in seasonally dry forests and desert regions of the world. We find that El Niņo also explains butterfly migrations in lowland wet forests spanning the isthmus of Panama. The number of migrating butterflies Aphrissa crossing the Panama Canal on the peak day of each year was determined by the Pacific sea surface temperature (SST) anomaly in April-June. This SST anomaly was also a determinant of soil moisture content in April on BCI, which lies on the migration flyway. One year was a notable exception. Although an El Niņo year, the number of migrants was low in 1993, yet the soil was atypically moist. Thus both global and local environmental factors influence the number of migrants traversing the Panama Canal.