Is there a "Premigration" Migration of Monarchs?
Chip Taylor - August 2002
" For the last 5 years or so I have been referring to a premigration migration, a migration of apparently reproductive monarchs that moves south beginning in late July and continuing, in Kansas at least, until mid August. I have posted a number of observations of monarchs seen moving south during this period to Dplex-L. A few others have posted similar observations and we sometimes get calls from people wondering why the monarchs are migrating so early. Monarchs generally disappear or only maintain low population densities in most areas south of 37 degrees North from mid May to August.
However, the region south of 37 degrees is frequently repopulated with monarchs in August and September (before the true migration) arrives in a pattern consistent with the idea that recolonization occurs via butterflies from the north rather than from a resurgence of the local population.
this is the case, how can we explain the behavior of monarchs moving
before the true migration which generally occurs a month later? Why do
these monarchs leave and what environmental cues are they sensitive to
trigger this migratory behavior? And, if some leave the north at this
why don't they all do it?
An alternative for this butterfly is to head south shortly after emergence. This might be more advantageous than producing offspring that might be too late for the migration. However, moving south in late July and early August does not guarantee success. It is often hot and dry during this period and the milkweeds are usually in poor condition having flowered and beginning to senesce (grow old). On the other hand, late summer rains south of 37 degrees N can rejuvenate milkweeds such as A. viridis, A. asperula, and A. oenotheroides, providing sources of food for monarch larvae.
Thus, although a southward migration prior to the real migration might not be successful, the probability of success could be high enough to be favored by selection.
so, this still leaves the question as to the environmental information
that the butterflies might use as a cue to initiate the migration. At
time we don't know what information the butterflies use but the
information, in terms of sun angle, during this period is similar to
that during the period when the first generation monarchs are moving