Monarch Population Status

Abstract
Monarchs are off to a good start and the population should increase modestly from now until the fall migration, assuming normal temperatures for the remainder of the breeding season. While an increase is likely, we still don’t have a good estimate of the size of the returning monarch population. If the population of females returning from Mexico has been low, the number of first generation monarchs heading north through May and the first half of June will also be low relative to other years and will limit population growth through the rest of the season. First sightings for March and April, as recorded by Journey North, show a population more restricted temporarily and spatially than recorded for any of the previous 10 years. The size of the overwintering population last year was 1.92 hectares (“Monarch Population Status” January 2010) and, with a modest increase this summer, the population might get back to this number. If the conditions for the rest of the summer are highly favorable, a winter population of 4 hectares is possible but that doesn’t seem likely at this point. In any case, the winter population of 2010 is certain to be below the long-term average of 7.44 hectares.

Analysis
I spend a lot of time fretting over the status of the monarch population and I’m always searching for factors or data that will help me understand the past as a way of predicting the future trends in monarch numbers. Since the status report of 2 March, I’ve been pouring over the first sightings records reported on the Journey North website. Like many of you I’ve looked at the Journey North first sightings maps from time to time and occasionally at the individual records but I’ve haven’t attempted to analyze these data.

I’ve looked at the data in some detail over the last several weeks and there are some neat patterns that I will explain in the future. My specific task at this time was to see if the first sightings data contained any clues as to the size of the returning population. I was concerned about this issue since there were no quantitative estimates of the numbers of monarchs that survived the winter storms. To answer this question I looked at the total number of first sightings reported in years with low and high numbers of returning monarchs. Unfortunately, the numbers of first sightings are not related to population size. There are several problems with working with the raw numbers. The numbers of first sightings has increased over the years as the network of monarch observers has increased making it difficult to compare one year with another. There are other problems such as trying to account for the density of observers, which is higher in and around cities compared to the surrounding countryside. In addition, it is difficult to determine if weather conditions such as cold fronts, and rainy periods in particular, affect the number of observations. And, publicity could play a role in the number of observations reported. In years when the population is low, monarchs tend to get a lot of publicity and general awareness of the low numbers might lead to an increase in the reported sightings. Further, since the number of observations does not correlate with the numbers of returning monarchs, does this mean that there is a finite number of people who will report what they see? Given all of these issues with the data, is there still a way it can be used? The short answer is yes, but caution is required. One thing we can do is look for patterns in the data and use proportions of monarchs seen in a give place or by a given date to compare one year with another.

I’m not going to dive deeply into the data but I will show you briefly how this year compares with others – and this returning population has been most unique. One way to look at the data is to ask how many of the first sightings reported in March* come from Texas and how many are reported from areas outside of TX. On average about 78% of the March first sightings are from Texas, this year an astonishing 96.8% of the first sighting were from sites within Texas. April was also distinct in this regard, since 8 of 22 (36.4%) observations from 22-30 April** were reported from Texas. In 7 of the last eleven years no Texas sightings have been reported during this period and the remaining years had 2, 2 and 1 Texas sightings. Overall, combining March and April sightings, 73% were from Texas, more than 20% higher than the long-term average. The data clearly shows that monarchs were limited to Texas this spring more than in any of the previous 10 years. What does this mean? Was the dispersal of monarchs limited this spring because of the lower than average temperatures or because the population is low or some combination of both? The answer is probably the latter; a combination of low numbers of returning monarchs and lower temperatures. Further analysis is needed.

As I pointed out in the addendum to the last status report, low numbers of returning monarchs could limit the amount of increase in the population we might expect in the breeding season even under the most favorable conditions. Assuming the returning numbers have been low, and there is no evidence to the contrary, what might we expect for the fall migration and the overwintering population in Mexico. Actually, the report based on the first sightings is not all doom and gloom. The conditions for growth in the monarch population in Texas have been exceptionally favorable the last two months. The temperatures have been moderate and due to adequate soil moisture, the milkweeds and nectar sources have been abundant. In addition, the fire ants have been scarce having not recovered from the prolonged drought of last year. So, small population or not, the monarchs should be off to a good start. Whether the population will just replace itself this year, increase above last year’s numbers or decline once again will be determined by 1) the number of first generation monarchs that come N/NE out of Texas over the next 6 weeks and, of course, 2) the weather conditions throughout the northern breeding range over the remainder of the summer. The May and June first sightings reported to Journey North do not appear to be as informative about future trends in the population as the March and April records but, we will be following them intensely just the same.

*All sightings from Florida and from west of the Rockies have been excluded from this analysis.

**April records used in this analysis were inclusive of 1-21April since an examination of the individual reports show than an increasing number of the sightings after the 21st of April clearly involve first generation monarchs.

Our Amazon Earnings – April 2010

Our second biggest month of all-time!

As you may already know, you can help support Monarch Watch with each purchase at Amazon.com and Endless.com (Amazon’s specialized Shoe and Handbag store). Monarch Watch earns a small referral fee equal to 4-15% of the item total when you use the links available on our site to visit these online stores.

In April, the following items were ordered in support of Monarch Watch:

Since February 2009: 833 items ordered and $1346.02 for Monarch Watch!

Thank you to everyone who contributed to these numbers – remember to stop by our site first whenever you shop online!

Complete details are available at monarchwatch.org/amazon

Please help us by spreading the word to friends, family, coworkers, and any other Amazon.com or Endless.com shoppers you can think of – thank you for your continued support!

PollinatorLIVE: A Distance Learning Adventure

Dr. Chip Taylor, Director of Monarch Watch, will participate in a webcast on May 12 from 1:15 to 2:30 p.m. ET and discuss tagging monarchs, the current status of the population, Monarch Waystations and more.

Dr. Taylor is participating in the webcast as part of PollinatorLIVE: A Distance Learning Adventure. For more information and to register, go to pollinatorlive.pwnet.org. PollinatorLIVE is continuing last year’s distance learning adventure that focused on monarch butterflies. This year’s FREE distance learning adventure will feature pollinators, citizen science projects, and gardening.

Update: If you missed the live webcast you can now check it out online at

pollinatorlive.pwnet.org/webcasts/webstream.php

Monarch Watch Spring Open House & Plant Fundraiser

It’s that time of year again!

Monarch Watch Spring Open House & Plant Fundraiser
Saturday, May 8th 2010 8am-3pm
KU’s West Campus, Lawrence, KS

This free public event usually attracts 500-1000 visitors and we expect the same this time around.

Nearly 4,000 butterfly plants (both annuals and perennials), including seedlings of several milkweed species, will be available (modest contributions are suggested). A list of available plants is posted at the bottom of the Open House page.

We will also provide refreshments, lots of show & tell, videos and games for children, tours of our garden and lab space, iChat videoconferencing demonstrations, and, of course, monarch butterflies!

You may have heard about our “Monarchs In Space” project but you can learn more about this program on Saturday and see the actual monarchs that traveled from Monarch Watch to the International Space Station and back to Monarch Watch, a mere 40 million miles – that’s quite a migration!

We’d love to see you here so mark your calendars! If you can’t make it in person, be sure to check us out online – we plan to post photos and broadcast some LIVE video throughout the day.

Complete details, including a map and directions, are available online at

monarchwatch.org/openhouse

If you have any questions please feel free to drop us a line anytime!

Grants Available to Create Monarch Waystations

On the 12th of May I will be part of the Pollinator Live webcast at the National Zoo. I will provide more details on how to connect with this broadcast in the near future but you can get some details from the Pollinator Live website.

My role will be to talk about monarchs and to take everyone to “butterfly school” much as I do when we have our tagging day at the Baker Wetlands in Lawrence each September. However, we will touch on other pollinators as well and at some point while trying to explain insect vision you may see me pretend to be a praying mantis.

But, the real point of this communication is to tell you that Kristy Liercke, one of the organizers of this event, wanted me tell you that there are small grants available for people who wish to start Monarch Waystations / pollinator gardens at schools.

The maximum grant amount is $500 and the deadline to apply is December 3, 2010.

In her words she asked me to

“spread the word that the project has grant money for gardens. So if you have people needing a little money to start Monarch Waystations, please have them apply.”

Complete details (including additional grant opportunities) are available at:
Project Learning Tree GreenWorks! Pollinator Garden Grants

Our Amazon Earnings – Mar ’10

Our second biggest month of all-time!

As you may already know, you can help support Monarch Watch with each purchase at Amazon.com and Endless.com (Amazon’s specialized Shoe and Handbag store). Monarch Watch earns a small referral fee equal to 4-15% of the item total when you use the links available on our site to visit these online stores.

In March, the following items were ordered in support of Monarch Watch:

Since February 2009: 742 items ordered and $1116.14 for Monarch Watch!

Thank you to everyone who contributed to these numbers – remember to stop by our site first whenever you shop online!

Complete details are available at monarchwatch.org/amazon

Please help us by spreading the word to friends, family, coworkers, and any other Amazon.com or Endless.com shoppers you can think of – thank you for your continued support!

Monarchs in Space: The Return

As most of you know, we were able to send monarchs into space via the shuttle Atlantis on the 16th of November 2009 to be delivered to the International Space Station (ISS) where they lived out their lives. These monarchs are now back in Kansas and will become part of a permanent display at Monarch Watch. The entire venture was a most curious migration, three monarch larvae were sent from Kansas to Florida, delivered to the shuttle Atlantis as part of the payload for mission STS-129 that took them to the ISS. Aboard the ISS they fed on the artificial diet created by Monarch Watch, molting from 4th into 5th instars and into chrysalises, finally becoming adults.

The shuttle Endeavour returned our monarchs to Earth on the 21st of February and from there the monarch habitat was shipped to BioServe Space Technologies at the University of Colorado in Boulder – the specialists who coordinate biologically based science experiments for NASA and with whom we worked to get our monarchs into space. BioServe sent the monarchs back to us on the 4th of March 2010, thus completing a most amazing journey. The entire venture, including our engagement with numerous schools, teacher evaluations of the project, photos, videos, and a summary of what we have learned from this project can be found on our Monarchs in Space page.

The entire adventure gave rise to some curious questions and answers that have parallels with those asked about monarchs all the time. So, the next time someone asks we can truthfully say:

Q: What is the longest distance monarchs have traveled?
A: 40 million miles*

Q: How far have monarchs traveled in one day?
A: 421,371 miles

Q: How fast have monarchs traveled?
A: More than 17,000 miles per hour

Of course, the questions aren’t framed correctly and this is the case for many questions that come from students and the public.

In the classroom the unexpected answer to a poorly worded question can often be used to get students to ask questions that are more incisive and to spark a discussion. Experts will often reframe poorly-worded questions from the public. For example, “How many miles can monarchs cover in one day?”, might become “Assuming you want to know the maximum distance monarchs have been recorded to move from one day to the next, the answer is more than 150 miles.” Or, to provide an answer that is more revealing to the audience, experts may turn the question back on the questioner and ask, “Do you mean by powered flight or do you want to know how far they can advance on weather fronts in a day or two or do you want to know the average speed of flight or average rate of progression of the migration?” The answers to these questions lead to a better understanding of the dynamics of the migration. If you don’t know the answers to these and related questions, you might refer to the Monarch Watch Press Briefings. These questions often turn up in press interviews and we’ve summarized the answers in this text.

So, will there be another Monarchs in Space project?” We don’t know – but wow, this sure was fun! We do have plans to recreate this project here on Earth and will soon have monarch kits designed to allow students to follow the development of monarch larvae in their classrooms and to compare their results with those established for monarchs aboard the International Space Station. Stay tuned!

* The three monarchs left the Earth on 16 November 2009 at 1:28p EST and returned approximately 2337 hours later, on 21 February 2010 at 10:20p EST. At an average speed of 17,210 mph that would make the Lawrence-to-Lawrence journey approximately 40.2 million miles. The Lawrence to Florida and Florida to Colorado to Lawrence portion of the trip doesn’t add much to the overall total.

Our Amazon Earnings – Feb ’10

As you may already know, you can help support Monarch Watch with each purchase at Amazon.com and Endless.com (Amazon’s specialized Shoe and Handbag store). Monarch Watch earns a small referral fee equal to 4-15% of the item total when you use the links available on our site to visit these online stores.

In February, the following items were ordered in support of Monarch Watch:

Historic total (since February 2009): 618 items ordered and $919.55 for Monarch Watch!

Thank you to everyone who contributed to these numbers – remember to stop by our site first whenever you shop online!

Complete details are available at monarchwatch.org/amazon

Please help us by spreading the word to friends, family, coworkers, and any other Amazon.com or Endless.com shoppers you can think of – thank you for your continued support!

Post-Storm Status of the Monarch Colonies in Mexico

In previous blog posts we have described the conditions during the breeding season that resulted in a small migratory population, provided the report from Eduardo Rendón (WWFMexico) indicating that the overwintering population was at an all time low (1.92 hectares), and expressed concern about the impact of January and February storms on the overwintering colonies.

Anticipating that it would be difficult for Eduardo and others to assess the colonies due to the devastation of Angangueo and the surrounding area, I asked those who were visiting the monarch colonies to report what on what they have seen. The reports below are from scientists, tours leaders, and tourists. Collectively, these reports provide a general view of the health of the colonies but it remains difficult to assess the extent of mortality due to the winter conditions and the size of the population that will be returning northward.

These reports deal with the following colonies (number that follows indictes the area, in hectares, in December): Herrada (0.06), Sierra Chincua (0.47), El Rosario (0.50), and Cerro Pelon (0.53). There is no information on the remaining three colonies, all were small with a combined area of 0.36 hectares.

A Brief Summary

The small colony at Herrada appears to have been decimated by the winter storms and it would be surprising if more than 30% of the butterflies survived at this location. Although significant mortality was reported by one observer at Cerro Pelon, this colony seems to have survived the winter quite well and should send a good proportion of the original population northward over the next 2-3 weeks. The status of Chincua and El Rosario relative to their original populations is difficult to assess. Chincua appears to have far fewer butterflies than in mid-January and the area shows a considerable amount of damage. At El Rosario monarchs are clustered near the entrance to the trail and are flying and mating with the gusto monarchs typically display at this time in the season but the population is so spread out that it is difficult to determine if the colony is intact or just a small remnant of the original 0.50 hectare colony. We are anxiously waiting for additional reports.

The following reports have been edited for clarity and brevity.

Received from Donald Planey late on the 26th of February:

We just got back from Chincua today. We were supposed to do it yesterday, but we got lost so we just ended up going to El Rosario via Ocampo yesterday*. Here’s what I saw both days:

As far as I could tell at Ocampo (El Rosario), the butterflies were doing great. Unfortunately, we couldn’t see all of it, because the people who operated the trail closed it off after 150-200 meters (5-10 mins from the ticket booth) due to damage to the trail from the storm. However, what we did see was encouraging. There were so many butterflies that I’m not even sure how to estimate their number. At the very least there were thousands and thousands, if not hundreds of thousands. The air was thick with them, and there was hardly a surface that wasn’t covered in monarchs. Most of the pine trees were completely covered, and were even drooping due to the weight. Now, I’d just like to clarify again that we weren’t able to see the entire trail, but what we did see looked great.**

There weren’t as many butterflies at Chincua, but at the same time there was definitely a lot less damage from the storm, not to mention that none of the trail needed to be blocked off by the people managing the trails. We were able to see a section of the forest (at least several hundred square meters) that was also covered in butterflies, but not as densely as in El Rosario. My dad and I agreed that it looked like there simply aren’t as many butterflies at Chincua storm or no storm.

* In a subsequent communication Don confirmed that this road is now open. He added this note: “Although the Ocampo route to El Rosario was open, we were among the only people we saw there the entire time, besides a couple of a Mexican tourists. It looks like the people who depend on this are probably going to get hit hard economically.”

** It sounds like this is the same area visited by Paul and Phill Justice of Rocamar Tours on the 19th as described in another blog post (“Driving Through Mexico After the Storms“).

Received 26 February from Paul and Phill Justice, Rocamar Tours:

Based on numerous visits each year over the past ten years, our visits to Chincua and Cerro Pelon over the past couple of days have been good at best. The butterflies at Chincua are dispersed over a fairly large area so the area available for viewing by visitors is quite limited resulting in a good showing at best. It continues to be unseasonably cold resulting in not near as many butterflies in flight as we have experienced other years. There is considerable wind damage with trees down throughout area, but no tree damage within the sanctuary itself. There are not many more butterflies dead than we would normally expect to see at this time of year.

We reported on Rosario last week. The condition of the sanctuary was quite dismal with very few butterflies and fewer visitors. It was the worst we had seen it.

We visited Cerro Pelon yesterday. The butterflies were 45 minutes from Macheros on horse-back. There were a fair number of butterflies streaming down the valley and onto the lower trail all the way into the village about mid-morning. The streaming would be about half what we would normally see on the trail. By comparison, last year the last week of February and first week of March, the number of butterflies in the air was spectacular and some of the best showings we have ever seen. In the sanctuary itself there appears to be a good number of butterflies in the trees over an extensive area. The fact that the weather continues to be unseasonably cool results in fewer butterflies in the air, even at the ideal time of day. There may very well be a similar number of butterflies in the sanctuary this year, however most are still on the trees. There are very few dead butterflies on the forest floor and no visible wind or rain damage anywhere in the area. The vigilantes told us that even though there are more open areas with flowers and nectar on the other side of the mountain, it has been far too cold for the butterflies to move over to that area. There are virtually no butterflies on the other side (or the “exit side”) of the mountain.

Received 26 February from Bill Calvert and Bonnie Chase:

We visited two Monarch colonies – Chincua on 23 February and Pelon on 24 February.

Chincua: We arrived late ca. 3:00 PM. We found a small largely inactive group strung out along 100m of drainage leading down from the Mojonera Alta into the Arroyo La Plancha. The clusters were small and high indicating that they had moved there from some other location. These are the type of clusters that appear late in the season. We estimate that at their widest, the area that the butterflies occupied was 50 to 30 meters across. But it was difficult to tell because the ejidotarios did not allow us very close to the colony. Some dead butterflies were scattered about, but not so many where we were. Some of the dead ones were greasy indicating that they had frozen or had been preyed upon; others showed no greasy appearance and had flattened abdomens that felt empty when squeezed. These had likely starved. The low vegetation showed frost damage – curled and browned leaves.

Pelon (or Macheros): At the approach to the community of Macheros we found butterflies streaming down (and up) the mountain. There was considerable flight activity – a behavior that we usually encounter later in the season. After an hour’s horse-back ride up the steep slopes of the Cerro Pelon we arrived (ca. 2:30) at the Pelon colony. Here we found a very densely packed colony also strung out along 80m of canyon leading down from the Gota de Agua. This aggregation was not wide (maybe 30 m), being confined by the steep canyon sides. The forest guards explained that this was the largest of three groups that were spread out along the canyon. The day was considerably warmer than the previous day at Chincua. There was much activity – flying, basking and roosting. Our impression was that there were many more butterflies here than at Chincua. A number of dead were strewn about the forest floor, but nothing like in past seasons after major storms. We detected no frost damage to vegetation at the butterfly colony or its approaches.

25 February Herrada (nr. Valle de Bravo). The following account was created from a phone conversation with Tom Emmel.

This colony measured 0.06 hectares in December – perhaps 20-25 trees. Tom Emmel and Court Whelan visited the site on the afternoon of 24 February.

Hundreds of monarchs were crossing the road leading to Herrada from Valle de Bravo. Police were present and driving speeds were reduced to 15km/hr. The horse ride to the colony took about 50 minutes. The colony was located at the top of a west facing valley. The N slope of valley had three trees whose trunks were covered with monarchs while the S slope contained 4 trees with clusters of monarchs. Lots of monarchs were in the air with many seen streaming down the mountain to the west evidently in search of water. Many were seen taking water at a moist area. There were relatively few dead and only one mating pair was seen. The clusters on the trees were located at heights from 12-15 feet at the bottom to 30-35 feet, with bare trunk and branches above 35 feet. Since a large proportion of the butterflies were in flight, the colony was probably larger than the 7 trees seen – perhaps as many as 10 trees assuming all came back to roost.

The colony was located at N 19 11 15.8; W 99 57 40.4 – at an elevation of 10,452.

24 February. A report by phone from Tom Emmel:

Tom Emmel (Expedition Travel, McGuire Center, U. FL) and his entourage visited Cerro Pelon on Saturday 20 February. Their route to the monarchs took nearly an hour. Tom estimated that the area occupied by monarchs to be roughly 3 hectares (note that a guide told Trecia the area was about 1 hectare). The colony was located at 9,072 feet (N 19.22,46.5, W 100.16,4.2). The colony was nothing short of spectacular and Tom declared it was the best viewing experience he has had in 25 yrs of visiting the monarch colonies. There was little evidence of wind damage at Pelon and in the surrounding area. Tom heard nothing about a second colony. There was no evidence of high mortality at this site.

Tom and his party visited Sierra Chincua on the 21st. The butterflies were located to the east at 10,374 feet (N 19.41,11.6, W 100.17,47.0). The number of occupied trees was only about 10% of the number seen in January. The colony is highly scattered along the east side of Chincua and small clusters and groups of occupied trees were found here and there for over 1.5 km as the group descended the trail. There were no signs of massive mortality but the colony had moved and scattered due to the storms. Wind damage was evident throughout the area. The guides maintained that only 10% of the butterflies had died as a result of the storms. Due to the scattered distribution of the living butterflies, it will be very difficult to accurately assess the mortality at this site.

There is still no clear picture of the condition of the colony at El Rosario.

In case anyone is wondering – it is roughly 22 miles from Chincua to Pelon – about two hours of air-time for a monarch.

23 February. The following is a summary of several reports, including that of Trecia Neal from Monarchs Across Georgia.

Visitors to Chincua are being taken to a colony that is only about 20 mins by horse from the head of the trail. One visitor said she only saw 10 trees with monarch clusters, while a second visitor (last Wednesday) only saw 4-5 trees with monarchs, however, the conditions were cold and rainy so the full extent of the colony may not have been evident. Both were under the impression that the colony they saw was the only one at Chincua.

To give you some perspective there was only one colony at Chincua in December and it measured 0.47 hectares (WWFMX report). The tree density at Chincua is usually given as 350 trees per hectare so the colony would have occupied roughly 160 trees at that time.

The Chincua and Rosario colonies were hit by a severe storm with hail and 40-50 miles/hr winds on 14-15 January that evidently had the effect of spreading out the colonies. (An earlier storm may also have helped to disperse the colonies). On 17 January Tom Emmel (Expedition Travel, McGuire Center, U. FL) estimated that the Chincua colony occupied 1.7 hectares (equivalent to almost 600 trees). So, reports of the relatively small number of trees presently occupied at Chincua is shocking. Lets’ hope there is another colony and that the guides were simply directing visitors to the most accessible site.

Although a long time resident in Zitacuaro, Pablo Span, reported seeing (2 February) more dead monarchs at Cerro Pelon than he had ever seen before, Trecia Neal (Monarchs Across Georgia) reported that as of Friday (19 February) the monarchs at Pelon were doing well.

Quoting Trecia – “The butterflies were about 40 minutes up the trail by horse…back where they usually where down the trail into the trees…not up in the meadow area where they have been for the last two years. They were magnificent! I did notice quite a few dead butterflies here, and noticed that the sex ratio was about 2:1 male to female. There was a lot of flying that day, but it was still cool and the butterflies were still shivering to generate heat. They looked to be in quite good condition. We sat at the end of the trail in a swarm of butterflies for about an hour, and I didn’t notice very many tattered butterflies at all, in fact, many of them seemed to be in pristine condition! The guide that we spoke to said that they estimated the colony to be about 1 hectare in size! He said that there was another large group up a little higher, and that they had been moving down the mountain.”

In December there was a single colony at Pelon that measured 0.53 hectares according to the WWFMexico report prepared by Euardo Rendon and his colleagues. Evidently this colony split, moved and spread out over a larger area. It is not uncommon for colonies to spread out over a broader area as the season progresses.

Weather, Storms, and Monarchs

On 13 February 2010 we downloaded the data from our electronic WeatherHawk weather station located in the Sierra Chincua, in the Monarch Butterfly Biosphere Reserve. This instrument is at nearly the same elevation as the Chincua butterfly colonies, and 2 – 4 km east of where the colonies form each year. We provide here a preliminary summary of the severe storm that caused extensive landslides, destruction of homes and tragic loss of human lives in Rosario, Angangueo, and other parts of eastern Michoacán.

Anecdotal accounts on the internet have suggested variable butterfly mortality, from 0% to 80% in different colonies. We have not made direct measures of butterfly mortality in this storm, but our weather records, combined with empirical data from Anderson and Brower (1996), Brower et al. (2004, 2009) and Fink et al. (in prep.) give insight into the interplay between local microclimate and butterfly survival. It is clear that butterflies suffered less than the local citizens …but only by a hair’s breadth.

As is typical of the region’s dry season weather pattern, from 17 to 30 January 2010 no rain fell in the area. Light rain began at about 11 pm on 31 January and fell steadily from 6 am to 11 pm on 1 February, accumulating 3.9 cm (1.5 inches)*. Rain began again at 7 pm on 2 February and was continuous and heavy until midnight on 4-5 February. The total precipitation on 4-5 February was 32.0 cm (12.6 inches); the entire storm produced 36.0 cm (14.2 inches).

We know that this heavy storm must have soaked the butterflies. We also know that in January 2002, a combination of heavy precipitation followed by an early morning temperature plunge resulted in 80% mortality of monarch butterflies (Brower et al. 2004). Butterfly survival and mortality depend not on low temperature and not on precipitation, but on the interaction of the two, because wet butterflies have significantly less freeze resistance than do dry butterflies.

During the recent storm, from 31 January through midnight on 4-5 February, the hourly temperature at our weather station ranged between 1.9 and 8.4 °C; at these temperatures no butterflies suffer freeze mortality. Because the lowest air temperature occurs close to sunrise, the morning temperatures on 5 February were crucial in determining the butterflies’ fate. On this morning, the minimum temperature at our weather station reached -3.02°C at 7:42 am. This was fortunate for the butterflies for two reasons. First, our experimental data tell us that wet, wintering monarchs have sufficient freeze resistance for few if any to be killed at this temperature. For wet butterflies, the crucial range of temperatures is -4 to -6°C: in mid-winter, the majority of butterflies can survive many hours of exposure to -4°C, but almost all will be killed by just a few minutes of exposure to -6°C (Fink et al. in prep). Second, because the oyamel fir canopy buffers the forest temperature, most butterflies in the colonies would have experienced a minimum temperature that we estimate to have been about 4°C warmer inside the forest (Brower et al., 2009).

During the day on 5 February the skies were clear. From late morning through mid-afternoon the temperature was 9 to 12 °C and the relative humidity was very low, between 9% and 11%. These conditions would give the butterflies the opportunity to dry their bodies and wings. This is fortunate, because the next night — 5-6 February — was clear and the temperature dropped to -5.96°C at the weather station. Depending on their specific locations within the forest, if they had not had the opportunity to dry, some monarchs would likely have frozen.

The weather data, therefore, tell us that this major storm very likely did not cause major butterfly mortality. If the skies had cleared earlier on the night of 4-5 February and a morning temperature plunge had occurred while the butterflies were still wet, the outcome might have been dramatically different.

It is well to remember that any thinning of the oyamel forest results in less temperature buffering, lowering temperatures in the forest and therefore increasing the probability of butterfly mortality during the clearing that follows storms. Vigilance is needed to prevent tree removal because the butterflies are overwintering on a microclimatic knife-edge.

Additional news of concern: the Sierra Chincua was reported to have had 15 cm of snow which was melted by rain on 18 February 2010. If conditions similar to those described above prevail, damage to butterflies will be minimal. However, where colonies have formed in and adjacent to thinned forests, the chances of butterfly mortality are much higher.

As of 20 February we are still receiving contradictory anecdotal statements on the extent of the mortality caused in several colonies by all these storms. We will keep you informed.

References:
Anderson, J.B. & Brower, L.P. (1996) Freeze-protection of overwintering monarch butterflies in Mexico: critical role of the forest as a blanket and an umbrella. Ecological Entomology, 21, 107-116.

Fink, L.S., Brower, L.P., Helton, T., and C. Kisiel. In prep. The ecology of cold-hardiness in overwintering monarch butterflies.

Brower, L.P., Kust, D.R., Rendon-Salinas, E., Serrano, E.G., Kust, K.R., Miller, J., Fernandez del Rey, C., & Pape, K. (2004). Catastrophic winter storm mortality of monarch butterflies in Mexico during January 2002. In The Monarch Butterfly. Biology and Conservation (eds K.S. Oberhauser & M.J. Solensky), pp. 151-166. Cornell University Press, Ithaca.

Brower, L.P., Williams, E.H., Slayback, D.A., Fink, L.S., Ramirez, M.I., Zubieta, R.R., Limon Garcia, M.I., Gier, P., Lear, J.A., & Van Hook, T. (2009) Oyamel fir tree trunks provide thermal advantages for overwintering monarch butterflies in Mexico. Insect Conservation and Diversity, 2, 163-175.

*This account does not mention the hailstorm that occurred from 5AM-8AM in Angangueo on 1 February 2010. Estela Romero, our colleague who lives in Angangueo, reported to us that 4-6 cm of hail accumulated during this period, an event that was also recorded on video by others. During this same time period 12 mm of precipitation was recorded at the Chincua Weather Station. We are not certain at this time whether it also hailed at Chincua, an event that might have knocked many butterflies from the trees. Nor is it clear whether hail would have been effectively recorded by our instrument. What we can say is that Angangueo received more than 12 mm of precipitation from 5-8 AM on 1 February. The amount of water in 4-6 cm of hail is uncertain but could have been as much as 2.8-4.3 cm.