Monarch Watch Blog

Western Monarchs

Thursday, January 6th, 2022 at 12:40 pm by Chip Taylor
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My father was a Certified Public Accountant, and had I demonstrated a facility for numbers, I might have become a CPA as well. Instead, I became a biologist which I naively assumed would mean that I wouldn’t have to work with numbers – at least not a lot. As it turns out, understanding monarchs, or more accurately, attempting to understand monarchs, requires a lot of number crunching. But that only works if we have some numbers to work with, and if the numbers are reasonably accurate, which brings me to a monarch puzzle. Namely, how did the Western monarch population grow from an overwintering number of 1,914 in 2020 to over 200,000 in 2021? Both numbers are based on Xerces sponsored Thanksgiving Day Counts. Each count involves an examination and estimation of the number of monarchs at 261 known overwintering locations and the search for new sites which are often on private land. At many sites, the number of monarchs is so low, the count is an actual count rather than an estimate. That was mostly the case last year and the Thanksgiving number was followed by an end of the year accounting that produced only 1049 monarchs. The decline from 1914 to 1049 was not unexpected. From the end of a breeding season in September and October (depending on location), the population declines until the end of the first reproductive period in April.

So, from a January number of 1049, how many females would we expect to be alive in March at the beginning normal reproduction in California? We can expect attrition to continue from January to March, but let’s assume that 70% survive. That’s probably high, but let’s go with it. That leaves 734 individuals. Since the sex ratio shifts in Mexico from being dominated by males at the beginning of the winter season to being female dominated at the end of the season, let’s assume that applies to these numbers. At best, that would mean that 55-65% of the survivors would be females. Let’s say it’s 60%, or 441 females. The next question is – Is it possible for 441 females to produce a population of 200,000 monarchs in 3-4 generations? I don’t think so. I’ve done the math several times, and you can as well*. So, if we can’t see a way to account for a fall population of 200,000 from the overwintering numbers, it means we are missing something, and the next question is what could that be? There seem to be three possibilities, 1) monarchs originating from Mexico recolonized the breeding areas in the West in March and April, 2) there were thousands of overwintering monarchs that clustered at sites that have never been discovered or 3) that some monarchs that breed along the coast during the winter months responded to seasonal cues in March and moved inland where they gave rise to a first generation of monarchs that colonized the summer breeding ranges in the West, particularly those areas to the east and north of California.

To determine if there is any support for the idea that the West was recolonized by monarchs from Mexico, I examined the first sightings records posted to Journey North and all the images of monarchs posted to iNaturalist for March and April. There is nothing on either site that supports the hypothesis that monarchs returned from Mexico in good numbers this past spring. If they did, this influx was missed by all those who are inclined to report monarch sightings.

The possibility that there are coastal or even inland overwintering sites in California that have never been discovered is real. New sites are discovered from time to time. Given the length of the coastal area, and the possibility that some areas are relatively inaccessible, there is the possibility that thousands of monarchs overwintered at locations that weren’t part of the surveys in 2020. There are reasons to think that the conditions for establishing overwintering clusters were different in 2020. The mean temperatures in California for both September and October 2020 were the highest in the record. October was +6.7F above the long-term average. It is during these months that monarchs migrate toward the coast with numbers beginning to show up at the overwintering sites in mid-October. Monarchs don’t migrate when the temperatures are in the 90s and often not when in the mid 80s. Rather, they seek cooler sites. This behavior might have led them to cluster in atypical locations. Under these conditions, monarchs that are unable to avoid extreme high temperatures often become reproductive and drop out of the migration. Both of these things could have happened in September-October 2020. Although it’s possible, even likely, that there are alternative overwintering sites in California, there is simply no evidence that such sites played a role in the number of monarchs that survived the winter in 2020-2021.

The third explanation, that offspring of winter breeding monarchs moved inland to reproduce in March and April is a possibility but, again, there is no precedent for such an occurrence. However, given the widespread reproduction this past winter reported to the Monarch Larval Monitoring Project (view summary of California data by year), this possibility shouldn’t be discounted. The fact is, we know little about the population dynamics of monarchs that reproduce during the winter months in California. So, what is the possibility that some recently emerged offspring of coastal breeding monarchs do move inland in March to reproduce? To answer that question, we need to know how monarchs respond to seasonal cues that might initiate migratory behavior. To dive into that topic, let’s look at the daylength/celestial conditions that occur during the migration northward in Mexico to see if they apply to California. We know that mating increases at the overwintering sites in Mexico in mid-February after the sun angle at solar noon (SASN) increases to 57 degrees. The date of the migration northward from the colonies is temperature dependent but usually starts around the 1st of March. The SASN at that time is 63 degrees. The possibility that the northward migration from Mexico is associated changes in the SASN is supported by the graphic (Fig 1) David Gibo and I put together based on first sightings assembled by Elizabeth Howard and her team at Journey North years ago**. The 63 SASN for a beginning of the departure from Mexico doesn’t seem to match a tracking of 56 SASN once the monarchs reach Texas. However, it is possible that the cues to leave actually began weeks earlier when the SASN reached 56 or 57 degrees. Preparing for departure from the colonies may in fact take several weeks since it is still cool at the Mexican sites, especially at night, which could delay the physiological and behavioral responses to the changing conditions. Once monarchs leave the overwintering colonies, they encounter consistently higher temperatures and move northward about 50-55 miles a day, which is faster than the pace of the SASN in degrees. The SASN advances about one degree, or 69 miles, every three days***. The difference in the respective paces allows the butterflies to soon track an SASN of 56 degrees – if the weather allows.

As to prepping to leave the coastal areas for the interior, cool conditions along the coast and the nearby foothills could result in a delay in physiological and behavioral development and could determine when monarchs move inland. It may help, when thinking of these dynamics, to remember that overwintering monarchs have a low metabolic rate which increases once they respond sufficiently to seasonal conditions to become reproductive. Further, when thinking about the possibility that breeding monarchs along the coast might move inland, that may only apply to newly emerged monarchs that have yet to become reproductive. From all we have been able to ascertain, once late summer monarchs become reproductive, they do not migrate even if exposed to seasonal changes that result in migration by younger cohorts. In effect, once reproductive, older adults appear to be insensitive or “blind” to seasonal cues. Mating is delayed in monarchs with most mating occurring on days 3-5 post emergence. That coincides with ovary development and the production of juvenile hormone. My guess is that the conditions experienced during these first 3-5 days determine whether monarchs migrate. If I’m correct, newly emerged monarchs that result from continuous breeding along the coast, if exposed to seasonal cues that signal migration, would do so. There is no a priori reason to expect that that would not happen. Still, there are reasons to be skeptical about whether this scenario played out last March and April. We know too little about winter breeding along the coast. We need to know the where and when new adults are produced as well as their numbers.

Some may also wonder whether conditions seen in Mexico apply to California. They probably do. The following is a summary of the first day that the SASN reaches 56 degrees in California locations along with cities in Texas and Oklahoma. These dates and latitudes are represented in Fig. 1. These comparisons suggest that, at least in terms of daylength associated celestial cues, Western and Eastern monarchs are exposed to similar conditions.

Table 1. March dates SASN reaches 56 degrees in California, Texas and Oklahoma

DateLocation 1Location 2
17 MarchSan Diego, CA (32°42'56")Dallas, TX (32°48'20")
21 MarchVentura, CA (34°15'17")Ardmore, OK (34°9'9")
23 MarchPismo Beach, CA (35°9'12")Okmulgee, OK (35°38'5")
27 MarchPacific Grove, CA (36°37'13")Enid, OK (36°24'8")

JN data

Figure 1. A few monarchs from Mexico appear to reach Texas during the first week of March when the SASN is close to 56 degrees. As monarchs continue to arrive from Mexico and move further north in Texas and into Oklahoma, they appear to be tracking an SASN of 56 until the third week of April. By that date, most have died and there is a drop in sightings until the first-generation monarchs begin moving north in late April and May.


So, how can we explain the difference in the overwintering numbers from 2020 to 2021? Two things seem unlikely. It doesn’t seem possible that the few hundred females that survived from last year’s overwintering population could account for over 200,000 monarchs this year. Nor is there any evidence that monarchs arrived in the inner West in March and April from Mexico. That leaves us with the possibility that there were unfound overwintering sites with thousands, perhaps 10’s of thousands of monarchs, and the possibility that offspring of coastal breeding monarchs moved inland and generated a first generation that colonized the breeding areas in the inner West and the northwestern states. The problem is there is no evidence for either – although both may have occurred. There is at least one other approach. We can use what is known about monarch population dynamics to bracket the possible number of females it took to produce more than 200,000 adults – a lot more in fact – since we need to account for attrition during the migration. Those calculations will have to wait until we get the final numbers, but see the notes below.


*Here are a few of the details that are required to calculate population increases. First, you have to know, or estimate, the number of females at the start of a generation. Then, you need to estimate the mean number of eggs produced per female (250-350). That is followed by an estimate of the survival to the pupal stage (0.01 or more) and then the survival of pupae to adult (0.76). Since not all adults reproduce, you need another estimate of those numbers (0.85). The last multiplier is the proportion of females which is generally close to 0.45.

Here is an example calculation:

1000 x 250 = 250,000 x 0.01 = 2500 x 0.76 = 1900 x 0.85 = 1615 x 0.45 = 727. Hmmm, those numbers don’t work since you end up with fewer females than we started with. If we increase the egg to pupal survival to 0.03 we get the following.

1000 x 250 = 250,000 x 0.03 = 7500 x 0.76 = 5700 x 0.85 = 4845 x 0.45 = 2180. That yields a 2.2-fold increase. Even after 4 generations that rate of increase isn’t sufficient to generate more than 200,000 migrants.

Perhaps I’m missing something or underestimating egg numbers or survival, but if these calculations are even close, they suggest that thousands of females were required to produce this year’s overwintering numbers. That said, we haven’t accounted for the attrition during the migration made by the first generation as it moves into the inner west and the northwestern states. Two things could happen. The number of females surviving to reproduce would be reduced, and the mean number of eggs per female would decline as well, not only due to a reduction in the size of the cohort, but because the search time for host plants is likely to increase, especially in the West. If there is either a significant loss of the first-generation females during colonization or significant reduction in mean egg number, it would be difficult for the population to produce an overwintering population of >200K. The keys to population growth are the number of females at the start of each generation and, in the case of monarchs West and East, the size of the first generation and its success in reaching the main summer breeding grounds. Again, those considerations suggest that a relatively large number of females had to be involved in the establishment of the first-generation last year.

The calculations I’ve used assume constant rates of mortality from one generation to the next. However, the incidence of parasitism by tachinid flies and O. e. tends to increase as the season progresses. Such increases would reduce both the proportion of larvae reaching the pupal stage and surviving to the adult stage. In the case of the adults with O. e., their fitness to reproduce would be compromised. We also need to recognize that realized fecundity, i.e., the mean eggs per female per generation, varies with temperature, nectar availability, and the distribution, abundance and quality of milkweeds. These factors also affect population growth.

** In subsequent years, especially in 2012 and 2017, it became apparent that high March temperatures and strong southwesterly winds allowed monarchs to advance much faster than expected for SASN. It was cooler in March in Texas from 1997 to 2000 than it has been in recent years.

*** Actually, the SASN advance averages less than 69 miles per day over the entire migration, but three days per degree of latitude is a useful reference point for discussion.

Relevant literature

Altizer, S., Oberhauser, K. S., and L.P. Brower. 2000. Associations between host migration and the prevalence of a protozoan parasite in natural populations of monarch butterflies Ecological Entomology 25(2):125 – 139 DOI:10.1046/j.1365-2311.2000.00246.x

Brower, L.P. and R.M. Pyle. 2004. The interchange of migratory monarchs between Mexico and the Western United States, and the importance of floral corridors in the fall and spring migrations. In Conserving Migratory Pollinators and Nectar Corridors in Western North America, edited by G. Nabhan, 144-166. Tucson: University of Arizona Press.

Grant, T. J., D. T. T. Flockhart, T. R. Blader, R. L. Hellmich, G. M. Pitman, S. Tyner, D. R. Norris, and S. P.Bradbury. 2020. Estimating arthropod survival probability from field counts: a case study with monarch butterflies. Ecosphere 11(4):e03082. 10.1002/ecs2.3082

James, D.G.; Schaefer,M.C.; Krimmer Easton, K.; Carl, A. First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California. Insects 2021, 12, 946. insects12100946

Nail, K. R., C. Stenoien, and K. S. Oberhauser. 2015. Immature monarch survival: effects of site characteristics, density, and time. Annals of the Entomological Society of America 108:680–690.

Oberhauser, K., R. Wiederholt, J. E. Diffendorfer, D.Semmens, L. Ries, W. E. Thogmartin, L. Lopez-Hoffman, and B. Semmens. 2017. A trans-nationalmonarch butterfly population model and implica-tions for regional conservation priorities. EcologicalEntomology 42:51–60.

Semmens, B. X., D. J. Semmens, W. E. Thogmartin, R.Wiederholt, L. Lopez-Hoffman, J. E. Diffendorfer, J.M. Pleasants, K. S. Oberhauser, and O. R. Taylor.2016. Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus). Scientific Reports6:23265.

Taylor OR Jr, Lovett JP, Gibo DL, Weiser EL, Thogmartin WE, Semmens DJ, Diffendorfer JE, Pleasants JM, Pecoraro SD and Grundel R (2019) Is the Timing, Pace, and Success of the Monarch Migration Associated With Sun Angle? Front. Ecol. Evol. 7:442. doi: 10.3389/fevo.2019.0044

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