Monarch Watch Blog

Monarch Population Status

Thursday, January 8th, 2009 at 12:42 pm by Chip Taylor
Filed under Monarch Population Status | 5 Comments »

It is the season at Monarch Watch when the mailbox is full of tagging datasheets. Many of the datasheets are arriving with returned tags along with short notes lamenting the lack of monarchs this past fall and requests for an explanation as to why the fall 2008 monarch population was so low.

The lack of monarchs was apparent all through the season and not just in the fall. On July 18th, again at the urging of many concerned Monarch Watchers, I posed my midseason assessment of the status of the population (“Where Are The Monarchs?”). My interpretation at that time was that the conditions during the period from late April to early June, when the first generation monarchs were moving north from the breeding areas in Texas and other southern states, limited the number of monarchs reaching the summer breeding range. Specifically, I said:

May, the moving month for first generation monarchs, was cold – throughout the entire northern breeding range. It was also a period of frequent storms and heavy rains, particularly during the second half of May. Early June also saw heavy rains, especially in the east north central and central portions of the country.” Cold and rain were thought to a) limit dispersal, b) reduce egg laying, and c) increase mortality of adults resulting in a reduction of the potential fecundity of this generation. After assessing all of the factors my conclusion was that “unless you are all missing something out there, the number of reproducing adults over the next three weeks will be low, to be followed by a relatively small migratory generation.

Unfortunately, our collective observations were correct and a relatively small fall population has come to pass.

One issue that I raised in the July assessment was that the number of first generation monarchs coming out of Texas and other areas of the South might have been lower than predicted by temperatures and observations of the numbers of larvae found on milkweeds. Drought, fire ants, herbicides (especially those applied to the milkweed rich pastures in central Texas), and loss of habitat could certainly reduce the numbers of first generation monarchs. The quality and quantity of monarch habitats as well as land management practices need to be assessed for the South. Indeed, we need a national assessment of monarch/milkweed habitats throughout all breeding areas to better understand what needs to be done to conserve the monarch migration. I will be writing more on this topic in the coming months.

As the observations of the migration came in via email, Dplex-L, the Monarch Forums, and other sources, I tried to arrive at a guess as to the size of the overwintering population in Mexico. Based on previous experience it was obvious to me the overwintering population was certain to be less than five hectares. But how much less? The previous models I’ve developed were not too helpful, since we have only seen one other year (1997) when the April-May conditions seemed to limit the summer breeding population. So, I’ve just had to make a flat out guess, albeit one based on my memory of migrations over the last 16 years, and my guess is that the overwintering population will be 3-4 hectares with an expectation that the final number will be closer to the low end of this range.

An early December report in a Mexican newspaper indicated that authorities expected the overwintering population to be 4.8 hectares. However, it was not clear whether this figure reflected actual measurements of the colonies or was an estimate. As you may remember, the overwintering colonies are measured twice in December and it is the measures taken in the second half of the month that are used to calculate the final estimate for the year. Two measures of each colony are taken because many butterflies are on the move early in the month and the colonies are still in the process of consolidating. Once the colonies are well established, cooler weather tends to cause the colonies to contract and the area occupied by monarchs diminishes by 10-20%. So, the expectation of 4.8 hectares reported to Mexican newspapers could well be reduced if it was, in fact, based on early December measures of the colonies.

Tagging Datasheets – a Reminder

If you have not sent in your tagging data sheets, PLEASE DO SO NOW before these data are lost. We will be heading for Mexico later this winter to buy tags. Each year we return having bought many tags without data due to the fact that some of our participants have not returned datasheets. Although most of the money for the recovered tags comes out of my pocket, I’m most concerned about the lost data. Every recovery provides valuable information and you may never learn if the monarchs you, or your group, caught and tagged were recovered in Mexico, so please send us those datasheets! Thank you for your interest and participation in our Monarch Tagging Program.

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  1. 5 Responses to “Monarch Population Status”

  2. By janet miault on Jan 23, 2009

    Hi, You are so very right about the weather and the decline in the migration last summer(2008).I live in Thunder Bay, On.Can. and I did not get one Monarch in my garden last summer. Don’t think I wasn’t watching and waiting. I checked my milkweed daily for signs of life. In 2007 I raised in my greenhouse over 80 monarchs of which 50 were released. I lost the others to lack of food I think, they did not fully develop and dropped off from their pupae stage. I ordered tags that year but they came way too late in the season for us in Thunder Bay to tag them. I must of kept and fed them for a good 2wks in the greenhouse before I just had to released them. Our season is short and by the time the tags get mailed to us here, the monarchs are long gone. I think if you want tagging to be done north of the border the tags will have to be sent to us much earlier, at least a month. I would like to hear from others about this. I tried my best to mark the monarchs I raised and released with a black marker, an X on the rt. wing. So if any one happened to find one with this marking, it was born in Thunder Bay On. Can. 2007.
    In the summer of 2007 I lived and breathed Monarchs from sun up to sun down, counting and dating all movements. Thanks for keeping us all informed and up to date on our little friends. Looking forward to seeing the results on 2008 migration.

  3. By Dennis Seevers on Jan 24, 2009

    I just discovered this website and this topic covers a concern on mine in regards to the monarch’s status. Over the last 2 years I was an over the road truck driver and as some of you may know, the truck traffic in NA has increased incrementally over the past 10-15 years, killing hundreds of thousands on migrating monarchs. It is of my opinion, that this accumulative negative affect on the migrating population along with the Genetically Modified crop seed being planted, which affects the milkweed,
    may be too overwhelming for them to continue. The species demise is then noticablly reduced when a significant weather system affects the remaining seasonal migrates.
    My question is in regards to the over-wintering population data. How many years has this been taking and how accurate is it? Is there enough to find a similar instance in the past where a cold system may have effected the migrates in the southern plains as it did last year? If so, a rebound in population could be expected if this is the key factor to the low monarch survey. This years survey should give us some important answers. Thank you.

  4. By Johanna Vienneau on Feb 1, 2009

    I have enjoyed tagging for several years in New Hampshire. This past year seemed a bit unusual to me in the following way: even though I found huge numbers of caterpillars and brought many into my classroom to raise, it was harder to find adult butterflies to tag. I visited several prime clover-filled fields, where in the past I could catch and tag butterflies until I grew tired. But ot this past fall, the butterflies were rare. I gentleman I met just the other day, that lives near a milkweed field, said it was a poor year. I assume he meant he did not see many adults. In my classroom, more larvae than usual died from tachinid [fly] parasitism. Last year the fields were crowded with larvae, like this year, and I think the parasite population is building also. But the other unusual thing for me is some of the adults that hatched in my classroom were weak and one even died before we released it. We sometimes keep adults for a few days but always enjoy feeding them apple juice. I recall one adult that hatched in the afternoon,so could not be released until the next day. But in the morning it was dead. I understand that there is a very contagious disease that can survive on cages. But not seeing as many wild adults points towards an bigger problem.

  5. By Mrs. Palermo on Sep 14, 2009

    I am an elementary school teacher in northern Vermont and am very concerned about the severe lack of monarchs this summer. Two years ago one student brought in over 60 caterpillars from one night of hunting. This year I have not found one. In school I am known for my love of butterflies and I have asked all students to look for me. Noone has found any. Reading about all the weather, and man-made situations (poisons and traffic by trucking) makes us very sad. Monarchs are one of the number one life cycles most recognized by my students. What shall we do? I encourage students to let milkweed grow around their homes. But this time of year the local farmers are haying and fields have been cut. However, many people have come back and said that there been an increase in slug activity on the milkweed plants. Just wanted to know others thoughts.

  6. By Sheryl on Sep 21, 2009

    I have planted in my gardens lantana, butterfly bush and two kinds of milkweed. I also have hummer feeders, which the Monarchs partake in, as well.
    I counted eleven caterpillars, maybe twelve. Of those, five were found in chrysalis formation (three still on the milkweed). Two that attached in a different location have “hatched”: one has left, and the other is still hanging around. It broke out early this morning, and it is now after nine (PM). The others are darkening, so I suspect we will have full blown Monarchs tomorrow.
    I live in east central Kansas. The weather two days ago was warm (when Monarch #1 left), today cool, windy and rainy. That may have some impact on the fact Monarch #2 hasn’t done much. Tomorrow is supposed to be in the sixties and overcast, the following days warm again. Hopefully all five and other six will be winging their way south and arrive safely.

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