Monarch Watch Blog

Monarch Population Status 2/2

Saturday, May 5th, 2018 at 8:30 am by Chip Taylor
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For monarchs every year is the same and yet every year is different. There are similar factors that affect the growth of the population every year, but the combinations differ. The average March temperature in Texas this year was 5.3ºF above normal. This high temperature continues a pattern. March temperatures in Texas have been 2ºF above normal for 11/18 years. Higher than normal temperatures in Texas have usually been associated with rapid movement of the returning monarchs beyond the borders of Texas with 2012 and 2017 being the extreme examples of movement northward. That didn’t happen this year due to a multitude of factors from a split in the polar vortex, sudden stratospheric warming in mid-winter and a persistent blocking high centered over Greenland in March and April, all of which had the effect of sending a series of cold air masses over the mid-continental region as far south as North Texas. It was simply too cold for monarchs to move northward during late March and much of April with the result that egg laying by returning monarchs was confined to Texas in the central corridor – and that’s a good thing.

In most ‘normal’ years, the returning monarchs are near the end of life in mid-April with few surviving to 1 May. That’s likely to be the case this year given the warmer than usual temperatures in Texas. While warmer than normal temperatures probably shorten the lives of returning adults, they have the effect of speeding up development of larvae and pupae and that is better than having females lay eggs further north where temperatures are lower and development of the immatures takes longer. The bottom line for this year is that what is normally a negative, when in combination with weather conditions that confined monarchs to Texas, has actually been beneficial. So, this year the high March temperatures are likely to lead to a growth in the population rather than a decline. That expectation is based on several assumptions. First, that the returning monarchs laid a lot of eggs in Texas and that many of the new larvae will mature to the adult stage and begin moving north in late April and through May and early June. That may be the case. There have been a number of reports both to Monarch Watch and Journey North of large numbers of eggs and larvae on milkweeds from Houston to North Texas and many other locations.

The next assumption deals with the conditions first generation monarchs will encounter as they move north. Because the movement northward is temperature sensitive, I checked the expected temperatures for a number of locations in the northern breeding area from 1 May to mid-June. An additional assumption is that temperatures in the high 60s and low 70s favor movement northward. With that in mind, conditions will be mostly favorable for northward movement through the 20th of May followed by about 10 days of lower than normal temperatures that should slow down the advance. Temperatures will again be favorable during the first half of June. Overall, even though there is some uncertainty in the forecasts and about the size of the first generation moving north from Texas, I’m still expecting the population to increase this year.

My self-interruption while trying to finish the text started on the 10th of March (see Monarch Population Status 1/2) concerned my quest to understand and trace the movement of monarchs north from the overwintering colonies. Based on some guesswork, a small amount of prior knowledge from working on neo-tropical African bees (Africanized bees) in northern Mexico and inference as to the temperatures preferred by migratory monarchs, I traced out two prospective pathways returning monarchs might use to reach the milkweed-rich areas of Texas. The target areas are north and east of the counties in South Texas in which milkweed diversity and abundance is low as shown in Figure 1. There is surprisingly little milkweed in South Texas with most areas having only scattered populations of one species – Asclepias oenotheroides (Zizotes milkweed).

South Texas milkweed map
Figure 1. Counties in South Texas with limited diversity and abundance of milkweeds.

After laying out the prospective pathways on a Google Earth image of northern Mexico (Figure 2), I consulted the records posted to Journey North for Mexico for this year to see if they approximated my imagined pathways.

Prospective monarch pathways
Figure 2. Prospective monarch pathways.

There is a modestly good fit for much of the interior pathway and that appears to correlate to some extent with the distribution of human population centers. The coastal pathway is not supported by sightings and that is likely due to the lack of population centers along that route. The pathways are represented by narrow lines in this image, but the real pathways, as suggested by some of the outlying observations, are likely to be rather broad. Both routes are crudely drawn. The interior pathways probably range in elevation from 800′ to 3500′, with most monarchs flying at elevations from 1200-2400′ in the mountains. As to distances, I measured the pathways and came up with the following minimal distances from the colonies to areas with abundant milkweeds in Texas (Figure 1).

Coastal pathway – from Angangueo to the vicinity of Corpus Christi, TX = at least 635 miles.

Interior pathway – from Angangueo to an area between Del Rio and Eagle Pass and then to just SW of San Antonio = at least 800 miles. If monarchs were to take the western interior path, the distances would be even greater.

Throughout March I monitored the temperatures along the routes of this northern exodus with the use of Looking at these images, I quickly formed the opinion that one or both pathways could be ‘shut down’ by high temperatures. This idea became a preoccupation when I learned of reports of 30 trees still covered with monarchs at El Rosario as of the morning of the 3rd of April. This was a very late date for monarchs to still be at El Rosario and the prospect that it could become too hot for them to successfully reach breeding areas in Texas seemed likely. On the 7th of April I learned that most of these monarchs had left on the afternoon of the 3rd. That set me on a path to create a record of the temperatures along the pathways from the 7th through the 20th of April. Under favorable spring conditions, monarchs appear to be able to advance about 50 miles per day, so by stopping the observations on the 20th, that gave them about 16.5 days to reach milkweed areas in Texas by taking the interior pathway ( +/- 800 miles).

As far as I have been able to determine, directional flight (i.e. migration) shuts down completely once temperatures exceed the low 80s. The preferred temperatures for sustained directional flight are in the low 70s. In contrast to the fall, during which monarchs deploy a great deal of gliding and soaring, in the spring, most of the migration involves powered flight. Powered flight generates high thoracic and head temperatures requiring monarchs to slow down or even stop flight to avoid overheating. Hence, the idea that the pathways could be blocked during periods when the temperatures reached the 80s or higher. To document these conditions, I took screen shots of the temperatures along the pathway areas represented at for 10AM, 1PM and 4PM each day. Each set of images is accompanied with my assessment as to the likelihood that the conditions favored migratory flight. These images are provided in Figures 3-16.

Temperature map
Figure 3. Saturday 7 April 2018 10AM, 1PM, 4PM High temperatures block coastal route.

Temperature map
Figure 4. Sunday 8 April 2018 10AM, 1PM, 4PM Temperatures favorable.

Temperature map
Figure 5. Monday 9 April 2018 10AM, 1 PM, 4 PM High temperatures block coastal route.

Temperature map
Figure 6. Tuesday 10 April 2018 10 AM, 1PM, 4 PM Unfavorably low temperatures.

Temperature map
Figure 7. Wednesday 11 April 2018 10AM, 1PM, 4PM Temperatures and winds favorable.

Temperature map
Figure 8. Thursday 12 April 2018 10AM, 1PM, 4PM Temperatures unfavorable.

Temperature map
Figure 9. Friday 13 April 2018 10AM, 1PM, 4PM Temperatures unfavorable.

Temperature map
Figure 10. Saturday 14 April 2018 10AM, 1PM, 4PM Temperatures favorable, headwinds.

Temperature map
Figure 11. Sunday 15 April 2018 10AM, 1PM, 4PM Temperatures and winds favorable.

Temperature map
Figure 12. Monday 16 April 2018 10AM, 1PM, 4PM Temperatures mostly favorable.

Temperature map
Figure 13. Tuesday 17 April 2018 10AM, 1PM, 4PM Temperatures unfavorable.

Temperature map
Figure 14. Wednesday 18 April 2018 10AM, 1PM, 4PM Temperatures unfavorable.

Temperature map
Figure 15. Thursday 19 April 2018 10AM, 1PM, 4PM Temperatures favorable.

Temperature map
Figure 16. Friday 20 April 2018 10AM, 1PM, 4PM Temperatures favorable.

The bottom line here is that I don’t know whether the monarchs that left El Rosario on the afternoon of 3 April made it to the milkweed area in Texas or not. There were no reports of late monarchs moving through Mexico in mid-April and only one late report from Texas (Beeville, 15 April near the north end of the coastal pathway) in an area through which these monarchs might have passed. What is clear in these images is that moving north from the colonies late in the season is not easy and could result in considerable attrition among the migrants due to high temperatures and perhaps the lack of nectar in times of drought.

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