the mystery of Vanessa unidirectional migration

Paul Cherubini monarch at
Tue May 1 01:27:00 EDT 2001

Chris J. Durden wrote:
> I think the whole story can be compared to the much better known locust
> migrations out of Africa. I think the reasons are the same. A good year in
> marginal habitat exhausts larval food supply. The population must move out.

I think there is a roughly northward PL dispersal every year, but lepidopterists 
simply don't take notice of the movement except during population outbreaks.

One thing we know for sure is that in September, PL's are ALWAYS
abundant (nectaring) in blooming alfalfa fields in the upper Midwest 
(Minnesota, Iowa, Dakotas, etc) and probably into southern Canada as well. 
Since PL's don't overwinter up at these northern latitudes the abundant
September PL's must be derived from butterflies that moved northward 
from the southern states in the spring.

Paul Cherubini, Placerville, CA


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