Monarch Population Status

“Recalculating! Recalculating!” That’s what I hear from the GPS system in my car every time I make a wrong turn. The satellite that is tracking my position has detected that I’ve deviated from the course that my system plotted for me when I entered my destination at the start of the trip. The start of the monarch trip that I plotted for this season directed me to a significant increase in the population. Unfortunately, all the data I have been following since posting my optimistic Monarch Population Status report via our Blog on the 6th of May has told me to recalculate the expectations for the fall migration and the overwintering population. As of this writing (16 July), it appears that the fall migration and the overwintering numbers will be similar to those seen last year (1.13 hectares). A substantial increase in the number of migrants and the area of the forests in Mexico occupied by overwintering monarchs is highly unlikely. I was expecting much better.

My earlier prediction was based on 1) the rate of arrival of overwintering monarchs in Texas and Oklahoma, 2) the temperatures in that region in March and April, and 3) the 45-day forecast that predicted favorable temperatures in May and early June. Although the number of monarchs overwintering in Mexico last year was relatively small by historical standards, the number of monarchs returning from Mexico was encouraging. They also arrived later than usual but moved rapidly into Texas with many monarchs reproducing north of the fire ant zone – an area of intense predation on monarch eggs and larvae by those voracious ants. In the past (1996, 2005, 2010), similar arrival patterns and temperatures have resulted in an increase in the population. But, then came May and the bottom fell out. Those favorable temperatures predicted in the 45-day forecast didn’t materialize. Rather, it rained and rained and rained with lower than predicted temperatures. The news reports during May and June were filled with stories of record rains and flooding throughout the South Region (Texas, Oklahoma, Kansas). Moisture-laden air masses swept in from the Pacific across the Southwest – typical of what is often seen as a result of an El Niño in the Pacific and an El Niño did form in late winter (see “Record Strong El Niño Ahead?“). Similarly, moisture swept into Texas and Oklahoma from the Gulf and in Kansas some moisture arrived from the northwest. The forecast didn’t account for these events.

Due to the lower temperatures encountered by the first generation monarchs moving north and northeast during May and early June, the arrival in the northern breeding area was delayed and less than optimal. Monarchs reached Minnesota and portions of western Wisconsin late and in modest numbers, the arrivals east of central Wisconsin were delayed even more and the numbers of first sightings reported to Journey North was quite low. It was worse for the Northeast with few monarchs being seen until well into June. My monarch recalculations now tell me that the number of monarchs in the fall migration will be similar to those seen and tagged last year from the eastern Dakotas to perhaps western Michigan. Lower numbers will be seen from eastern Michigan to western Pennsylvania and still lower numbers will be found in the northeast. And the migration will be late. This will be the third later than normal (as defined by migrations from 1992-2012) in as many years. Why the migration has been late the last three years is not clear. Do these late migrations reflect long-term changes in the weather patterns that drive the monarch numbers or are they due to a series of chance events? Time will tell. Whatever the case, these late migrations show us that the migration and population growth throughout the breeding season are driven by the temperatures and rainfall that occur from the moment monarchs leave the overwintering sites (late February through early April) to the time the last monarchs arrive at the overwintering sites (December).

That said, the numbers of monarchs we’re seeing now are also a reflection of the amount and quality of milkweed and nectar plant habitat that remains for the breeding population. Loss of habitat is a topic I’ve covered many times. For a discussion of this topic and what needs to be done to address these losses, please read my Monarch Butterfly Recovery Plan memo.