Is there a "Premigration" Migration of Monarchs?

Chip Taylor - August 2002

Texas Monarch Watch

" 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. 

If this is the case, how can we explain the behavior of monarchs moving south before the true migration which generally occurs a month later? Why do these monarchs leave and what environmental cues are they sensitive to that trigger this migratory behavior? And, if some leave the north at this time, why don't they all do it?

Given the way we think natural selection acts in these cases, it is standard to ask why it might be selectively advantageous for some monarchs to move south ahead of the true migration. One possibility is that there is a portion of the population that emerges in late July and early August too late to successfully reproduce at their respective latitudes. For example, let's suppose that a monarch emerges in Minneapolis on the 1st of August. This butterfly requires 45 degree days before it is sexually mature. This could mean that it would take 4-5 days before this butterfly would mate. If this monarch were a female and she started laying eggs on the 6th of August, continuing to lay eggs for the next three weeks, her first offspring would mature in 30-35 days (6-11 Sept) about halfway through the fall migration at this latitude. The mid and late offspring from this female would emerge at the end of the migration or later and would be subject to killing frosts that can start in mid to late September in Minneapolis. 

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. 

If so, this still leaves the question as to the environmental information that the butterflies might use as a cue to initiate the migration. At this time we don't know what information the butterflies use but the celestial information, in terms of sun angle, during this period is similar to that during the period when the first generation monarchs are moving north in May.

If the scenario I've outlined above is true, the size of the premigration migration is likely to be a function of the number of butterflies emerging in this critical period and this in turn is related to the time of arrival of monarchs in the early summer and the overall size of the population. It also follows that the reproductive success of the butterflies moving south is dependent on the condition of the milkweeds and the availability of nectar sources to sustain the adults and both of these are dependent on soil moisture and rainfall. It could be that there are years when the premigration migrants produce enough progeny to contribute significantly to the overwintering population.

Is there a premigration migration? You can help by reporting to or to Dplex-L ( what you see monarchs doing in your area during the period from the 15th of July to 15 August."

31 Aug, 2009 - Mike Quinn / / Texas EntomologyTexas Monarchs