Monarch Population Status

Stage 4 (1 May–9 June) Update

As I indicated in the Status of the Population post to the Monarch Watch Blog on 2 May, I partition the annual cycle into 6 stages in an effort to understand the interannual variation in monarch numbers. This stage–specific model breaks down as follows: 1) overwintering (late Oct–early April); 2) return migration through Mexico (late Feb–April); 3) breeding in the US in March and April; 4) recolonization of the regions north of 37N (May–early June); 5) summer breeding north of 37N (June–August); and 6) fall migration (August–Oct). While there is some overlap, each stage is intended to capture the dominant activity during that period. For example, in Stage 3 (March–April) monarchs continue arriving from Mexico through most of March; later, in the second half of April, some first-generation monarchs are beginning to move north. Nevertheless, the most important action during that stage – the one that has the greatest impact on the population – is the reproductive success of the monarchs that returned to the South Region from Mexico.

In the earlier posting, I provided an estimate based on the conditions observed through Stage 3. Specifically, I said “my current prediction is that the overwintering population will be in the range of 4–5 hectares. At this writing, I see the population as trending toward 4 hectares.”

We are now at the end of Stage 4 (1 May–9 June). It is during this stage that the majority of first–generation monarchs originating from the South Region (primarily Texas and Oklahoma) migrate north to colonize the areas north of 37N (40N more precisely defines the southern limit of the northern breeding range in terms of numbers of monarchs generated that join the fall migration yet 37N is more inclusive). This stage ends on the 9th of June. The end date is somewhat arbitrary. We know that monarchs stop directional, hence migratory, flight sometime in early June but are not sure when. However, directional flight appears to stop before the 9th. We haven’t been able to detect directional flight after the 6th of June in Lawrence, KS. Data are needed on this feature of the annual cycle. Sightings recorded after this date are assumed to represent butterflies that had arrived in the area before the 10th. In fact, there is little evidence in the first sightings data reported to Journey North of an expansion of the northern distribution of monarchs after 9 June.

Recolonization is all about timing and numbers. In 2012, the monarchs arrived in the northern breeding areas too soon and in 2013 too late, and in both years the population declined. This observation tells us there is a “sweet spot” or optimal set of dates to arrive in the northern breeding areas. Our original assessment was that the optimal period was between 11–30 May. It now appears that this should be shortened to 16–30 May. The following is how the first sightings reported to Journey North break out this year vs 2018.

Table. First sightings north of 37N and east of 110W.

So, what can be said about these data? First, the number of first sightings for these two years are the highest in the Journey North first sightings records. These numbers probably reflect both excellent returns and a larger number of people willing to report first sightings. In 2018, the higher percentage of first sightings from 1–15 May combined with favorable temperatures for larval development immediately after arrival in the northern area may have aided population growth. The higher number of first sightings this year, especially in the north central region (Michigan, Indiana, Ohio, Ontario), is also a positive. Overall, both years are exceptional and quite similar, and since we know that the first sightings of 2018 were a precursor to an overwintering population of 6.05 hectares, the highest number recorded since 2006, what do the 2019 numbers tell us about the overwintering numbers in 2019–2020? Should we expect another population of 6 hectares? Maybe, but it’s too early to say. We still have to see how Stage 5 works out. Right now, in most of the breeding area, the prospects for a normal summer and a reasonably robust population look quite good. The exception is the northeast (east of Toronto in Canada, and most of eastern New York, Pennsylvania and north through New England. The colonization of those areas by first generation monarchs was scanty with low temperatures for the first half of June. Further, a colder than normal summer is predicted for most of that region which will retard population development. The migration in the east this fall will be on the low side relative to good years.

After Stage 4, I said the overwintering population was likely to be between 4–5 hectares and trending toward 4. Stage 5 recolonization has been excellent with respect to both timing and numbers, with the exception of the northeast. Based on the recolonization data and the long–term temperature forecasts for the Upper Midwest and the north central region, my prediction is that the 2019–2020 overwintering population will be at least 5 hectares and could trend toward 6 hectares if the summer temperatures from 80W (western Pennsylvania) to the west (105W) average at least a degree above the long–term averages.

I’ll update this prediction, if necessary, for the premigration newsletter and will summarize the outcome of Stage 5 (June–August) in early September.

I wish to thank Janis Lentz for her assistance in summarizing data for this article.