Monarch Annual Cycle: Migrations and the number of generations

The monarch annual cycle is quite extraordinary since it involves an overwintering phase that follows a long fall migration, a remigration in the spring and a succession of generations before the start of the next fall migration. This pattern is even more complex since it involves overlapping generations and a total of 4 migrations. Yes, 4 migrations. Further, monarchs breed over a wide range of latitudes with differing temperatures and periods of production. It can be very confusing and there is a lot of misinformation about these patterns in documentaries about monarchs and on many websites. I’ve been trying to sort out the migrations and generations since 1992, and the summary below fits the data that I’ve seen to date. Please be aware, I’m describing overall patterns – average conditions – to which there are exceptions if you look all the data that’s available.

MIGRATIONS
Let’s deal with the migrations first since there is a widespread perception that monarchs take three generations/migrations to reach Canada. That’s not the case. Recolonization of the breeding area requires only two migrations. That’s well-established in the literature and you can see the pattern in the first sightings data posted to Journey North each year. Here is a quick summary:

The first migration
Monarchs leave the overwintering sites starting in late February, a process that can continue into the first week of April. This migration starts to reach Texas in the second week of March. The returning female monarchs move N and NE mating, laying eggs until they die. Most are dead by 1 May with the effective northern limits generally around 37°N.

The second migration
Offspring from the returning migration reach the adult stage mostly in mid to late April and, after a few days needed to fully mature, they begin to migrate N and NE. This generation – the first generation of the season – makes the second migration. These butterflies continue migrating until sometime late in the first week of June. At that point, all directional flight ceases, ending the second migration. By the end of the first week of June, this second migration, or wave of monarchs, has reached most of the areas containing milkweeds, and recolonization is complete or as complete as it can be for a given year.

The third migration
There is no directional movement from the first week of June until sometime in the fourth week of July. In other words, there is a six or seven-week interval during which all flight and reproduction is local. Directional flight for SOME but not ALL monarchs begins in the fourth week of July. These monarchs move S and SW and have been observed to recolonize southern areas from Georgia to Texas that had contained few or no monarchs for the previous two or more months. I had called this migration the “pre-migration migration” in previous discussions. That’s an awkward and confusing term so hereafter I’m going to refer to this migration as the “midsummer migration.” Although we know this migration occurs, we don’t know exactly when it starts or ends or the geographic origin of the butterflies. Judging by observations of directional flight and the arrival of fresh monarchs in areas from which monarchs have been absent, it appears that this migration starts in the fourth week of July and ends in the beginning of the third week of August. How long this migration takes to pass through a particular location is not clear. Reproduction follows the arrival of these butterflies from the North creating some confusion as to the number of generations produced in the South. I’ll get to that later.

The fourth migration
It’s the fourth migration that is well known. It starts at 50°N (Winnipeg) at the end of the first week of August and continues through the fall with the last monarchs arriving at the overwintering sites in early December. This migration is the longest both in terms of distance and time. It also differs from the other three migrations due to the fact that butterflies in this migration do not mate and reproduce during the migration. Nor do they reproduce at the overwintering sites in Mexico.

That monarchs reproduce in Mexico during the winter is another misconception frequently repeated in the media. The overall pattern is this: three migrations involving fast-moving butterflies that mate and reproduce as they expand the range seasonally with an outcome that generally increases monarch numbers and one fall migration that contracts the geographic range during which the population declines due to morbidity and mortality. In effect, there is an eight-month period of reproduction (March–October) that overlaps with an eight-month interval (August–March) during which the population declines. Simply put, there are two migrations going south in summer/fall and two migrations going north in the spring.

GENERATIONS
When thinking about the duration of a developing generation, it is common to add up the intervals needed for monarch immatures to complete each stage from egg through pupation — and that interval can vary from about 25 to over 50 days depending on temperatures. Egg to adult intervals are generally cited as about 30 days. That’s a good average, but it doesn’t account for a period of maturation females need before mating and egg-laying begins. That interval is also temperature sensitive but probably averages 5 days for monarchs at the northern latitudes. My point is that it’s better to think in terms of egg to egg intervals rather than egg to adult intervals when counting generations, e.g. 35 vs 30 days.

As discussed, first-generation monarchs are offspring from monarchs returning from Mexico. These first-generation offspring mate and lay eggs while moving north. At a particular point, new eggs could be deposited by passing monarchs for several weeks, in effect spreading out the second generation over time and space that later leads to overlapping generations. That’s complicated, and we won’t go there since our purpose is to characterize the general pattern of the number of generations. To do so, it helps to create a simple scenario for different latitudes.

Let’s start with eggs laid by a first-generation female on the 20th of May in northern Iowa. Using the egg to egg interval of 35 days, the second-generation females would begin laying eggs on the 25th of June. Offspring from these eggs constitute the third-generation, and they would be expected to begin egg-laying in another 35 days or around the 30th of July. As the eggs from this third-generation complete development and reach the adult stage, they constitute the fourth or fall migratory generation.

However, if we start our scenario later in Iowa, or start further north, we lose a generation. For example, eggs laid in the Winnipeg area by newly-arriving first generation females, let’s say on the 1st of June, probably wouldn’t reach the adult stage and begin egg-laying until 8 July or later, due to a longer egg to egg interval resulting from lower temperatures at that latitude. Eggs laid by those second-generation females become third generation adults in August that migrate. The message here is that most of the migratory butterflies originating from north of 40°N latitude are third and fourth generation butterflies.

But what about further south? Are there 5th and 6th generation migrants? Probably, but it’s complicated and the numbers are likely few. To explain what might be happening, we have to consider the extent to which there are continuously breeding populations in the South and the impact of the midsummer migrants. There appear to be habitat islands associated with major cities where monarchs survive in low numbers through the summer. It’s conceivable that some of these lineages produce 5th and 6th generation migrants. However, the biggest number of migrants originating from the South in September and October are probably progeny of midsummer migrants that were third generation butterflies. If this is indeed the case, then most of the monarchs originating from the South are fourth generation rather than 5th generation monarchs.

The above description applies to the eastern monarch population. It’s possible, maybe even probable, that what I’ve characterized here for eastern monarchs also applies to the western monarch population. There are many seasonal parallels but there are too many unknowns about western monarchs to know if the annual cycle differs significantly from that of their eastern counterpart.

SUMMARY

• Four migrations annually – two northward and two southward.
• Most butterflies in the fall migration represent 3rd and 4th generations.
• Three migrations involve butterflies that are reproductive. Butterflies in the fall migration are non-reproductive.
• Monarchs DO NOT reproduce at overwintering sites in Mexico.