Modest numbers of monarchs have already arrived at the overwintering sites in Mexico and they will continue to arrive through the end of the month. Preliminary reports indicate that the monarchs are spread out over an area of four hectares or so at this time but we expect this area to shrink in the coming weeks as the masses of monarchs become more consolidated.

Our predictions regarding the size of the overwintering monarch population still stand (visit the link above for details); official measurements will be done in late December and we will report the findings in an update after the first of the year.

Stay tuned!

The leading edge of the migration has now reached northern Texas. As many of you know, we attempt to follow the monarch population closely. Based on our experience, and ongoing data analysis of monarch numbers, we offer opinions/projections on what to expect in the near future based on our understanding of how the monarch populations have been affected by patterns of temperature and rainfall in the preceding months.

Late in the spring I started predicting a small migration this fall. In the Premigration Newsletter sent out with the Monarch Watch Tagging Kits, I predicted that overwintering population in Mexico would be similar in size to that of the low populations recorded in 2004 (2.19 hectares) and 2009 (1.92 hectares). It was clear that the monarch numbers in New England and recorded at Cape May would be low this fall, and that the numbers originating in the central region would be slightly better than those of the eastern Dakotas through Wisconsin but still low relative to long term numbers. The New England/Cape May projection appears to be correct as the numbers are down in this region. I was wrong about the central region (Ontario, MI, OH, IN, IL) – fewer monarchs appear to have been produced in this area than I expected. Wisconsin numbers also appear to be down.

The surprise is the eastern Dakotas and western MN. This area seems to be the source of a large number of the monarchs moving through the lower midwest at this time. Nevertheless, the overall numbers are down. But, it gets worse. The migration is just beginning to navigate a 1000 miles of hell – a nearly flowerless/nectarless and waterless expanse of central KS, OK, TX, and NE MX (see Drought Monitor).

It is too late for rains to change the situation in TX and northern MX. Monarchs will make it to the overwintering sites but their numbers will be significantly reduced by these conditions. My expectation is that that the overwintering numbers will be the lowest ever (previous low 1.92 hectares) and that the arriving butterflies will be in relatively poor shape with low fat reserves. If the average condition (mass) of the overwintering monarchs is lower than average, mortality during the winter could also be high. Other scenarios could include low returning numbers next spring with a reduced reproductive capacity due to low fat reserves. Keep your fingers crossed that there are no winter storms in MX that could make matters worse.

It will be interesting to see how monarchs cope with the lack of nectar and water as they move through TX. Monarchs, like most insects, have hygroreceptors (sense organs that are sensitive to humidity gradients); therefore, when conditions are extremely dry, we might expect monarchs to seek out the darkest and most humid habitats. If this plays out, most monarchs will accumulate in drainages, along rivers, move in an out of forests, and concentrate around other water sources.

As I pointed out in the Premigration Newsletter (and the August Population Status blog article), there is a new reality, or expectation, regarding the size of the overwintering population in MX. It now appears that winter populations will be in the range of 2-6 hectares (down from the long term average of 7.24) with 6 hectares being reached only during the most favorable conditions. In the near term, the average overwintering population will be close to 3 hectares. As we pointed out recently (Brower et al. 2011), the decline is related to the loss of habitat, particularly the rapid adoption of herbicide tolerant (HT) crops. The majority of these crops are planted within the summer (June-August) breeding area for the monarch population. In spite of weed control methods prior to 1996, when HT crops were first introduced, milkweed persisted in these croplands at a low level where they provided an excellent resource for monarchs. With the planting of HT engineered corn and soy followed by the use of glyphosate to control weeds, milkweed has been almost completely eliminated from these crops. At present, the total area of HT crops is larger than that of any state except TX and AK, or about 4 times the state of IL). The decline in the monarch population first became noticeable in 2004 when the percentage of HT corn and soy acreage exceeded 50% of all acreage for these crops.

Low monarch numbers in MX this winter and in the future means that the integrity of the overwintering sites is now more important than ever and that planting milkweeds in gardens and incorporating these plants in restoration projects either as seeds or plugs should receive the highest priority.

Monarch Waystations – Create, Conserve, & Protect Monarch Habitats

Bring Back The Monarchs – It’s not too late…

We started tagging in a down year for monarchs but we didn’t realize it then. Only later did we recognize that the dust veil created by the eruption of Mount Pinatubo had led to a significant reduction in summer temperatures and monarch numbers as well. A lot has happened since 1992. We’ve seen the overwintering population in Mexico increase each year from 1994 to 1996, only to crash inexplicably in 1997. We have seen ups and downs in overwintering numbers – but mostly downs since 2003. In fact, the population has been below the long-term average over the last seven years. Ominously, the four lowest populations recorded to date occurred in the last 11 years. The downward trend is now statistically significant (Brower, et al. 2011) and it is clear that we have entered a new era of monarch numbers.

The great migrations of the 90s are a thing of the past. In the future, we can expect overwintering populations in Mexico of 2-6 hectares. The main reason for the decline is loss of habitat. Monarch habitat has been reduced by at least 140 million acres in the last 10 years – about a fifth of the total breeding area available to monarchs has been lost. At least 100 million acres of habitat has been lost due to the adoption of herbicide resistant corn and soybeans. The herbicide tolerant (HT) crops allow growers to spray their crops with herbicides without affecting the crops. The result has been the near elimination of milkweeds in these row crops and a reduction in monarch numbers – monarch production in these fields was higher when measured in 2000 than in roadsides, old fields, conservation reserve lands and other habitats (Oberhauser, et al PNAS 2001). The adoption of HT crops began slowly in 1996 but has been increasing rapidly since 2003. By 2010 80.7% of the corn and soybeans planted in the United States (161 million acres) were herbicide tolerant. Since these crops are used in rotation, it is likely that milkweeds have been eliminated in more than 81% of the total acreage.

So, where does this leave us and what does this mean for tagging? It means that we will have another year like the last seven and a year not unlike 1992 when we started the program. Specifically, we can expect a low year, perhaps not as low as 2009 (1.92 hectares) or 2004 (2.19 hectares) but close to these numbers. The migration should be particularly low in the New England area and the numbers at Cape May will be low as well. The central region (Ontario, MI, OH, IN, IL) will see a modest migration and could produce more monarchs than the area defined by the eastern Dakotas, MN, WI, and IA. Even though the population will be down from historical highs, there will still be plenty of monarchs to tag. And, as always, you and your fellow taggers will not only have fun but you will also contribute to our knowledge of the monarch migration.

In addition to watching the pattern of the returning butterflies, I monitor other conditions – temperature, rainfall, drought, abundance of fire ants, etc., as they play out each month of the breeding season. As you’ve heard from me before, Texas is key. For the monarchs to have a good year, the conditions in Texas for the first generation have to be favorable. If they are, the population grows, as it did last year. If unfavorable, as they have been in a number of years such as 2004, the population declines. Conditions in Texas this spring have been hot and dry – a significant drought. Milkweeds have been abundant and nectar seems to have been available in most locations but due to high winds and temperatures monarchs just kept moving. The result is that monarchs are not off to a good start and the prospects that the population will rebound in the summer months are getting slimmer each day.

At the end of March, I was saying that the population this coming winter would be no greater than 5 hectares due to conditions in Texas. In contrast, at the end of March in 2010 it was quite clear that the population was going to increase and the only question was by how much; it more than doubled from 1.92 to 4.02. It now appears that the 5-hectare prediction was too optimistic. Four hectares (4.02 last year) is possible but not too likely. If the long-range forecasts for the northern breeding areas are accurate, and they have been recently, the prospect for producing a large monarch population in and north of the corn belt is not great. In fact, the population could drop back to 2009 numbers (1.92 hectares), if the summer is as cold as forecast.

In a week or more – weather permitting – first generation monarchs from Texas should begin migrating through this area to colonize the northern breeding areas. The numbers reaching these northern habitats is largely a function of reproduction in Texas and the weather conditions in May. Reproduction is Texas has yet to play out in numbers, but if past seasons are a good measure, the number moving northward should be less than expected based on the size of the overwintering population. Below normal temperatures are projected for May, which, if true, would limit the number of monarchs reaching the milkweed patches throughout the northern breeding area and ultimately the size of the fall migratory population. While we can hope that the long-range forecasts are wrong, and that reproduction will be higher than I’m envisioning, the prospects – at this date – favor a migration that will result in an overwintering population of 2-4 hectares.

We can’t do anything about the physical conditions that drive the monarch population but we can provide the milkweed and nectar resources they need – PLANT MILKWEED!

Attempts to find out how the monarchs fared following these winter storms were unsatisfactory. No measurements were made of the numbers of monarchs killed by the storms. Accounts by visitors to the colonies after the storms suggested that substantial numbers of monarchs died during these events. Reports from reliable observers suggested that as much as 80% of the butterflies died at some colonies while others suggested moderate mortality at one large colony of perhaps 20%. Overall we estimated that at least 50% of the monarchs died during the winter months, recognizing that this value could have been low.

Our next concern was the number of butterflies coming north in late February and March. Would the number of surviving butterflies be sufficient to establish a first generation in Texas and the South large enough to lead to a recolonization of the northern breeding areas and a recovery of the population?

Fortunately, the conditions encountered by the monarchs that reached Texas were favorable. Rainfall over the previous 6 months signaled an end to the long drought and produced an abundance of milkweed. The fire ants were down due to the drought and temperatures were lower than normal which seems to favor production of first generation monarchs. The result, in spite of the low number of returning monarchs, was a substantial first generation. These butterflies colonized much of the northern breeding area from late April to mid-June; however, not all areas were well colonized.

The numbers of monarchs reaching the northeastern area (New England and New York) seemed to be well below average, leading to the expectation that the fall migration in this region will be low. Rainfall and temperatures in the central area (Ohio, Indiana, Tennessee, etc.) were not ideal as monarchs arrived from the south and the fall population in this region is likely to be below average except for those areas receiving butterflies from eastern Ontario, which reported many arriving monarchs in May and early June. Based on the numbers of arriving monarchs and the abundance of eggs and larvae found by observers, the fall migration in the east north central region (eastern Dakotas to Michigan) should be above average – assuming that temperatures do not reach the 100s before the end of August.

In summary, it appears that the monarchs are making a modest recovery and I expect the overwintering population will measure close to 3 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.

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.

This small population was of some concern at the outset and the development of El Niño conditions in the Pacific late last summer was unsettling. Such conditions have been known to be associated with moisture-laden systems that have lead to catastrophic mortality at the overwintering sites. The freezing weather that followed heavy rains in the winters of 2002 and 2004 killed roughly 80% and 70% of the populations in those years. Were something similar to happen this winter, the returning population might be only the equivalent of 0.6 hectares, or less.

We have no prior experience with such a small returning population and it is uncertain whether monarchs in such numbers could recover in one year. Assuming that 80% of the overwintering monarchs died in 2002, and that there was some additional mortality due to normal causes, the returning population that year was probably equivalent to 1.4-1.6 hectares. So, a surviving population of 0.6 or less would certainly be unprecedented, and seemingly vulnerable unfavorable conditions as they move northward into the breeding areas in March and April.

The priorities in the monarch area involve dealing with refugees, the massive cleanup, and the damage to the infrastructure of the region due to recent disastrous storms. News of the monarchs and how well they have survived the storms of January and February are fragmentary and confusing. One of the smallest colonies, Herrada, initially only 0.06 hectares, was hit hard by high winds, rainfall, and freezing conditions. The number of surviving butterflies is said to be quite low but no precise estimate of mortality or survival is available. Similarly, masses of dead butterflies were reported at Cerro Pelon, yet other post-storm reports indicate strong flights.

The colonies at Sierra Chincua and El Rosario are reportedly intact with good flights reported for Chincua. But these reports mean little, since 80% of the butterflies could have been killed while the surviving 20% put on a show good enough to impress the tourists. Detailed assessments are needed of the living and the dead. I’ve been told that such studies are planned and should be completed by the end of the month. Let’s hope that in spite of the winds, hail, rain, and freezes, the number surviving will be enough to repopulate our fields and gardens this summer!

ADDENDUM – 21 February 2010
Unable to wait for an official report on the numbers of monarchs that have survived the onslaughts of the repeated storms at the overwintering sites, I have been soliciting reports from those that lead tours and tourists that have visited the monarch colonies in recent days. The reports are fragmentary but none have been positive and it is keeping me up nights. It seems certain the returning population will be less than a 1-hectare equivalent – perhaps 0.6 hectares or even much less.

The returning numbers make a difference because, even though monarchs have a relatively high reproductive rate, there is a limit in how much recovery we might expect in one year. Let’s look at what happened again after the 80% loss in the winter of 2002. The returning population the following spring was probably close to 1.4 hectares and the following winter the population was 7.64 hectares, a 5.45-fold increase. This is the highest increase seen from one year to the next in the 16 years we have good data on the sizes of the overwintering colonies.

However, even if we suppose that monarchs are capable of a 10-fold increase in one year, once the returning population drops significantly below 0.6 hectares (ha), the chances of a recovery in one year diminish substantially. A returning population of 0.6 ha, with favorable conditions, might get us back to overwintering numbers of 5-6 hectares but a returning population of 0.3 ha would only get back to 2-3 hectares – again with favorable conditions. In both cases, these numbers would be below the long-time average of 7.44 hectares.

Given that we have yet to see a 10-fold increase in monarch numbers in one year, my thinking now is that it will take monarchs at least two, and perhaps more, years to recover from the effects of the last breeding season and the winter of 2009-2010. All of us interested in monarchs are going to have to work especially hard to keep this migration going.

E. Rendón Salinas, C. A. Valera-Bermejo, S. Rodríguez-Mejía, Y F. Martínez-Meza. Monitoreo de las colonias de hibernación de la mariposa Monarca: superficie forestal de ocupación en diciembre de 2009. WWF y Reserva de la Biosfera Mariposa Monarca. 8 pp.

The news is not good.

Only 7 colonies were found and the total area occupied by monarchs at the overwintering sites in December was 1.92 hectares (ha). The three largest colonies (El Capulin (Cerro Pelon) at 0.53ha, El Rosario at 0.50ha, and Cerro Prieto (Chincua) at 0.47ha) constitute 78% of the total area. The totals for both hectares and numbers of colonies are at an all-time low. Good records of the numbers of colonies and area occupied go back to 1992 and there is less complete data for most years going back to the late 1970s. The numbers this year appear to be lower than observed for any year since the overwintering colonies became known to science in 1975. The lowest previous total, 2.19 hectares, was reported in 2004. This decline continues a trend that started in the late 1990s. In the decade of the 90s the mean area occupied by monarch colonies was close to 9 hectares. The mean for the last 10 years, through the 2009 migration, is now below 5 hectares per year; the three lowest monarch overwintering populations were reported in this decade.

In the Monarch Population Status posting to the Monarch Watch Blog on the 20th of October I summarized the conditions monarchs confronted during the breeding season and predicted that: “the total hectares will be in the range of 2 to 3.5 when all colonies are measured in December.” More recently, based on the relative success of taggers this fall, I was anticipating that the total hectares would be closer to 3 hectares. Unfortunately, the final number is below my most pessimistic expectation.

As many of you know my predictions are based on a combination of interpretations of the effects of temperature on monarch populations, observations reported to us throughout the breeding season and early in the migration and my 18 years of experience in following the fall migration closely. Of these, temperatures are the best predictor.

Without going into great detail and wishing not to repeat the October report, here is a brief summary of the reasons for the low overwintering numbers for the winter of 2009-2010.

1.) High temperatures in Texas in March and early April limited production of first generation monarchs. It is these monarchs that recolonize the northern breeding range and fewer monarchs moving north/northeast out of Texas from late April to June impacts the rest of the breeding season.

2.) Conditions were less than ideal for the first generation monarchs as they moved north in May and early June.

3.) Upon arrival in Minnesota, monarchs encountered drought conditions that limited reproductive success of first generation in that area.

4.) As the summer progressed, cool and cool, rainy conditions prevailed in many areas, limiting reproduction and slowing development of larvae.

5.) Colder than normal conditions prevailed for most of the western two-thirds of the northern breeding area from mid June into early September.

In many respects, the conditions during the monarch breeding season in 2009 were a repeat of the conditions seen in 2004 that contributed to the previous low overwintering population number of 2.19 hectares.

In spite of the recent cold snap that reached into Mexico, there have been no indications of weather-related mortality at the overwintering sites. Let’s hope that normal winter conditions prevail during the next 7-8 weeks. Even if there should be some mortality this winter, our experience with the disaster of 2002, in which an estimated 80% of the population died as the result of a January storm, suggests that the monarchs can recover. The number of monarchs surviving is critical. IF at least 1 hectare of monarchs survives to move north and IF they encounter normal conditions as they move north through Mexico and Texas, the population can rebound in one breeding season.

Let’s start with the 2008/2009 overwintering population. The area occupied by monarchs at all the overwintering sites measured about 5.5 hectares – a modest population just a bit lower than the mean of 6 hectares for this decade. The butterflies wintered well, aside from some excessive mortality at Cerro Pelon, a colony that insists on forming at a badly cut over site. I spent four days from 7AM to 7PM working with a Disney film crew at El Rosario in early March and was impressed by two things: the low number of dead monarchs beneath the colony and the excellent condition of the butterflies for so late in the season. The monarchs were already on the move northward at this time and it seemed likely they would arrive in Texas in good condition and high numbers; however, the drought in Texas and the higher than normal temperatures in March and early April were not welcoming. These conditions took their toll on the arriving monarchs. The butterflies are 6-8 months old when they arrive in March and these are the kinds of conditions that reduce lifespan, egg laying, and therefore what ecologists refer to as “realized fecundity.” So, Texas in March was beyond optimum and therefore on the right side of the curve.

Because of these conditions, the numbers of first generation monarchs heading northwards from late April through the first two weeks of June was modest if not lower than normal. Unfortunately, this wave of butterflies moving northward encountered long periods of unfavorable weather. Further, those reaching parts of the East North Central climatic region (IA, MN, WI, MI), particularly Minnesota, encountered an unusual spring drought. Droughts sometimes reduce realized fecundity, especially if the effect is to reduce nectar availability and the availability of host plants, and this may have been the case in Minnesota in late May and early June.

The overall result for the first generation moving into the summer breeding range was that they got there late and the numbers arriving were low – then came the cold and wet weather. Cold and wet, other extremes, also reduce realized fecundity. Egg laying is restricted when it is too cold or raining and eggs deferred are seldom compensated for by more egg laying when conditions improve.

The summer breeding area for the monarchs consists of three climatic regions, the East North Central, Central, and North East (and there is the monarch production area in Ontario with weather that is usually midway between the East North Central and the North East). The summer temperatures (June-August) were colder than normal in all three regions. Precipitation was a bit lower than normal for the East North Central region but was above normal in both the Central and North East. Above normal precipitation isn’t necessarily bad but long rainy periods that reduce egg laying can reduce realized fecundity as I mentioned above.

The underlying reason for the cold summer was once again the position of the jet stream, which uncharacteristically settled over the middle part of the continent for most of the summer, just as it did in 2004. The poor monarch production in the cold summer of 2004 – a mere 2.19 hectares, the lowest population ever recorded in Mexico – prompted me to brush up on my meager knowledge of climates and causes of extremes resulting in my attempt to educate others about what I had learned (see “Teaching with Monarchs” from a January 2005 update). Will we see a low population in Mexico this winter? Yes, and my guess is that the total hectares will be in the range of 2 to 3.5 when all colonies are measured in December.

So, how cold was it in the summer of 2009? The total number of monarch degree days (in F) is shown for selected cities in the mid portion of the continent from 2003 through 2009 in Table 1. Note that for most cities the number of monarch degree days was about the same in 2004 and 2009.

Table 1. Monarch degree days (in F) for selected mid-continental cites from Dallas in the south to the limit of milkweeds and monarch reproduction in Winnipeg.*

*Thanks to Janis Lentz for patiently calculating the monarch degree days.

The most recent previous cold summer was in 1992, the summer during which the dust veil from the eruption of Mount Pinatubo in the Philippines cooled the entire planet. Having a cold summer is one thing, but where it is cold and how extreme the cold was is another. It seems reasonable from all we know about monarch summer production that the greatest monarch production area is in the East North Central climatic region. The mean summer temperatures for this region from 1895-2009 are shown in Figure 1. A glance at the right of the figure shows the three cold summers mentioned above but if you look closely, you will see that you have to go back to 1928 to find a summer as cold as 2009 (aside from 1992 and 2004).

So, let’s revisit the Three Bears Theory of Ecology and that temperature curve. Aside from the winter conditions, which seemed to be optimal, the monarch population appears to have been either to the left or right of the most favorable temperature and precipitation conditions since leaving the overwintering colonies beginning in late February. On the bright side, the drought has eased in Texas and there is an abundance of flowers to sustain the monarchs as they pass through to Mexico.

Figure 1. Mean summer (June – August) temperatures for the East North Central climatic region from 1895-2009.

The overwintering population this winter will probably be less than 4 hectares and could be much less if the long term drought that has persisted in much of Texas and northeastern Mexico doesn’t abate. Monarchs are highly dependent on nectar and sources of water to fuel the migration through this region in October and November and, if the drought persists and nectar resources are scarce during this period, the number of monarchs reaching the overwintering sites will be reduced. There have been some recent rains in Texas but it is not yet clear whether these rains have been sufficient or widespread enough to provide the fall flowers needed by the monarchs.

The last two strong migrations in recent history occurred in 2005 and 2003. It appears that strong monarch migrations occur when the temperatures and rainfall are favorable during the breeding season, including the two periods of migration in the spring and early summer.

This year, the temperatures were a bit higher than normal during the March–April period but perhaps more important were the conditions during the period from late April through mid June. Temperatures in May and June were below normal in many areas, and Minnesota (one of the big production areas for monarchs) experienced below normal rainfall and moderate drought conditions. Overall, the production of monarchs in the south appears to have been moderate, followed by conditions that limited the numbers and reproductive success of the monarchs reaching the northern breeding areas.

Nevertheless, if you have been tagging over the last three years, you should expect to see and be able to tag a reasonable number of monarchs this fall.

Stay tuned for updates as the season progresses…