Monarch Butterfly Field Notes from Mexico by Dr. Bill Calvert

Field Notes from Texas by Dr. Bill Calvert

March 30, 2004

The return of the monarchs seems delayed this year. Not so much in that they are not as far north as they usually are this time of the year - they seldom are north of the Red River by March 28th--but they have not traveled very far east and the numbers reported are much lower than previous years.

The best guess is that the low numbers are due one or both of two factors:

1) Unseasonably cool and wet weather in Mexico and the lower Rio Grande Valley and presumably in between has delayed their departure from the overwintering grounds and retarded their progress northward.

2) It is also likely that the population size is down from previous years. Two devastating storms hit the colonies in January. The Rosario colony was particularly devastated.

I last visited Chincua on March 15th. On this date the colony was healthy and intact and sequestered in the canyon bottom. Cool, cloudy weather seemed to hold them in place. There was little mass cascading as is common at break up time and no evidence of a rapid progression down the canyon. (That is, there was no evidence of bud colonies forming along the canyon between the main colony and the canyon mouth.) Admittedly I did not have a lot of opportunity to search in this area.

I last visited Pelon on March 17th. The situation was somewhat different from Chincua. The weather was also cloudy and cool, but many butterflies were present at the canyon mouth. There was cascading and the main colony and a bud colony had all but disappeared.

These very different observations suggest that the colonies located in different settings behave differently at breakup. Colonies such as Chincua nestled in the dense forested canyon on the north face of the Chincua massif may delay their departure until the ambient really warms up. In contrast the Pelon colony is located on the warmer south face of the massif. Breakup here likely occurs earlier than in the deep, cool canyons at Chincua.