Monarch Butterfies in Sonora and Adjacent Northwestern Mexico

Since 1979, observations of monarch butter y (Danaus plexippus L.) in Sonora, Mexico have been low. There are 10 records of monarchs breeding in Sonora on Asclepias curassavica, A. lemmonii, A. linaria, and A. subulata (Apocynaceae).

In July-August, monarchs from the Canelo-Hereford area in Arizona y to the Río San Pedro, Sonora, south to the Cananea area, and then in the Río Sonora through central Sonora to Hermosillo and the Gulf of California. From the Río Sonora, they move up into the Sky Island Mountain ranges. In Hermosillo, they thrive in urban gardens. Higher humidity along the coast allows them to disperse in arid habitats from Bahía de Kino north to Punta Chueca and south to Guaymas. Individuals observed in northwestern Sonora may be from the Colorado River population in western Arizona-southeastern California.

The population in tropical southern Sonora and western Sinaloa with winter feeding and breeding may be resident, reflecting the continuously blooming and abundance of the native A. curassavica in large areas of tropical deciduous forest converted into agricultural fields.

Monarchs tagged in southeastern Arizona recovered in Michoacán (Morris et al. 2015; Billings 2019) likely traveled eastward to the Janos area in northwestern Chihuahua on the east side of the Sierra Madre Occidental to join the southward migration across the Mexican Plateau.

From the relatively few monarch records in Sonora, the few observations in the Sierra Madre Occidental in Sonora and Chihuahua, and winter feeding-breeding populations in tropical southern Sonora and western Sinaloa, we conclude that the Sonoran population is not connected to the eastern migration to overwintering sites in Michoacán. Like the Rocky Mountains, the Sierra Madre Occidental is more a barrier to migration than a corridor.