Monarch Watch Blog

Monarch Population Status

Thursday, May 2nd, 2019 at 8:35 am by Chip Taylor
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Stage 3 (March–April) Update

As most of you know, I endeavor to predict how the monarch population is developing each year. While I always fail to correctly predict the exact number of hectares measured at the overwintering sites, I’m getting better at it with each iteration. Last year it was evident as early as March that the population was going to increase substantially from the relatively low numbers measured in 2017 (2.48 hectares). After watching the population develop through March and April by tracking both first sightings reported to Journey North and then following the colonization of the northern breeding areas (>37N) in May and early June, it was even clearer that the migratory and overwintering population was going to be much larger than recorded in 2017. I then followed the conditions during the summer months, and again, all signs were positive and it was clear that the overwintering hectares would be at least as great as those of 2008 (5.06 hectares) and perhaps larger. My final prediction was that the overwinter population was going to be greater than 5 hectares, but I couldn’t say how much greater. I predicted the direction of change correctly and was close on the magnitude of change but missed the mark. The actual total was 6.05 hectares. Which brings me to why this is the Stage 3 Update.

In March I conducted a webinar of monarch population development through the sponsorship of the Monarch Joint Venture and the cooperation and facilities of the US Fish and Wildlife Service. In that webinar I introduced the idea of developing a Stage Specific Model for Monarch Population Development. This verbal model is based on my attempts to understand the interannual variation in monarch numbers. The data used to derive these predictions are based on the conditions associated with the growth of the monarch populations each year from 1996 to the present. These data include patterns associated with first sightings, rates and times of colonization and physical conditions, mostly temperatures, from March through October. There is a narrative for each year. Comparing and contrasting the responses of the population from year to year and particularly for years with extreme conditions has shaped this approach.

To understand the monarch annual cycle, I break down the year into 6 stages: 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). Although overwintering mortality varies from year to year, the available mortality data for this stage are too fragmentary to be used in the model. Similarly, there are no estimates of the mortality experienced by monarchs from the time they leave the colonies to when they reach the milkweed rich areas of Texas. Mortality during this 600-800mile journey could be significant due to spring droughts, high temperatures and strong winds. Lacking these measures, we are left with first sightings in the US, mostly in Texas and Oklahoma, as a surrogate for both the relative survival and fitness of the overwintering population and the return migration. While first sightings have a number of limitations, the number of sightings as well as the temporal and spatial distribution of these sightings when examined in the context of the physical conditions and comparisons across years can be quite useful. I will deal with first sightings and my methods in greater depth at a later date. In the meantime, if interested, you might check out the webinar to see some useful year to year comparisons:

Getting back to Stage 3, it’s the critical period that sets the stage for what could happen through the rest of the season. Briefly, the key factors in Stage 3 are 1) the returning number of monarchs 2) their condition upon arrival, i.e. their structural and physiological conditions 3) the spatial and temporal distribution of the new arrivals through March and April 4) the temperatures that determine #3 and 5) the number of and spatial and temporal distribution of eggs as determined by 1-4. The distribution and abundance of fire ants could also be a factor, but there doesn’t seem to be any way to assess this threat though it is known that fire ant numbers decrease under drought conditions and increase when precipitation favors plant and insect growth.

Now that Stage 3 is at an end, we can compare the conditions during this past March and April with those of last year and previous years to get a rough approximation as to what to expect during the fall migration and next winter in Mexico. First, I can say unequivocally that the fall migration will not be as robust as last year and that the overwintering number of hectares will be less than 6.05 hectares. The conditions for Stages 3, 4 and 5 last year were all favorable for population growth. In Stage 3, the mean temperatures for March in Texas were 5.4 F above normal, yet due to cold temperatures that hovered over Oklahoma and north Texas the majority of returning monarchs were effectively confined to Texas through March and early April. These conditions have two outcomes, 1) most of the eggs laid by the returning monarchs were laid in Texas and 2) due to the higher temperatures, the larvae and pupae developed rapidly thus exposing the developing matures to lower rates of predation and parasitism. Another consequence of these conditions was a low mean age to first reproduction (egg to egg interval) for the first generation. A key factor in population growth is age to first reproduction with the fastest growing populations having the shortest age to first reproduction. The higher mean temperatures during Stage 4 (May and early June) facilitated recolonization of the summer monarch breeding areas north of 37N. As the first-generation monarchs arrived in the northern areas in the first part of June, the temperatures were again favorable for both flight and egg laying – more so than in most years. The good starting numbers and conditions in the northern breeding areas resulting from the conditions in Stages 3 and 4 were followed by warmer than normal summer temperatures in the Upper Midwest. Although 2F higher than normal, these temperatures were not in the range that results in drought conditions and lower life expectancy of adults and therefore a reduction in realized fecundity. The bottom line on 2018 is that the conditions in Stages 3-5 were the best since 2001 and may actually have been better than recorded for 2001.

So, what can we expect this coming fall and winter? Based on first sightings, temperatures, probable egg distributions, generation length and early May conditions, 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. However, higher than average temperatures for May and June-August in the Upper Midwest could result in a population that is closer to 5 hectares.

The positive conditions in Stage 3 this year include the highest number of first sighting recorded in the central flyway since Journey North has been recording these data. That seems to tell us that the relatively large population of 2018–2019 wintered well, in spite of reports that the butterflies were abnormally active during the winter, and navigated from the colony sites back to Texas without experiencing high mortality. Temperatures in Texas were lower than in 2018 which had the effect of again confining most of the egg laying by returning monarchs to Texas but also increased exposure to predators and parasites and increased generation length. Overall, the first generation produced in Texas and southern Oklahoma could be larger than in 2018 due to the larger number of returning monarchs. While that could be true, the predators and parasites could be more abundant this year. Unfortunately, we have no measures of the year to year variation in egg and larval mortality resulting from these causes. Due to the lower temperatures, the average date of departure from Texas and Oklahoma for first generation monarchs should be a bit later this year. That may not be a bad thing since the forecasts indicate that conditions for recolonizing the northern breeding areas won’t be favorable until the second half of May.

The bottom line for Stage 3 this year is that we can expect monarchs to have another good year, not as good as 2018, but a good year – assuming near average temperatures of Stages 4 (May–early June) and 5 (summer).

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