Observations from Dr. Chip Taylor

Monarch Population Growth in 2019

March 12, 2019

Questions: Will the population increase again this year? Last season the population increased from 2.48 hectares to 6.05. If the monarchs wintered well, that is with a normal rate of mortality and morbidity, that should result in a relatively large (compared to recent years) returning population with the potential to yield another large migration and overwintering population. Right? May and maybe not.

To forecast what might happen we have to know what happens during March and April in the South Region and then what happens in May as the first generation moves northward. Timing and numbers are critical to understanding the prospects that the population will grow or decline. 

OK, so what is happening now? There is good news, some questions and perhaps more good news.

The first bit of good news is the sighting of 27 monarchs in TX in the first 10 days of March. That’s the highest number for this period since Journey North started keeping records in 2000. Some of these records indicate that monarchs are well into TX having reached Austin and beyond. The question is how many monarchs are already in TX and how many are still moving northward through MX? It is probable that most of the monarchs are still en route and we know that substantial numbers still remain at some of the colonies. 

I raise this issue because the monarch highways through MX are going to be shut down by cold weather from 15-21 March. This shutdown could mean that once the weather warms in NE MX, monarchs could flood into TX after the 21st. The effect would be late arrivals for the majority of monarchs coming out of MX and that would not be a good thing. The peak arrival in TX is usually from 21-25 March and often from 26-30 March. Later arrivals are often associated with lower population numbers. 

The good news comes in twos. First, the weather for monarchs that have already reached TX will be favorable for feeding, mating, egg laying, larval growth rate and movement to the NE. Secondly, the long range forecast, if accurate, indicates that most of the returning monarchs will lay the majority of their eggs in TX, and the southern half of Oklahoma. Restricting egg production to TX and OK is a good thing since it will minimize the average age to first reproduction of the progeny of the returning monarchs. The population has increased each year egg distribution has been limited to the lower portion of the South Region. 

Returning monarchs that have survived to mid April in OK will be able to continue moving NE once the daytime highs reach the lower 70s. A few of these may reach KS but the number reaching NE KS, where I am, will be few, if any, unless the forecast is wrong and the  numbers returning to TX are higher than in recent years. 

The bottom line — I’m cautiously optimistic that the population will get off to a good start this breeding season. I’m keeping my fingers crossed that my key assumptions - good returning numbers, restricted egg laying, and favorable weather conditions - will be validated in time.