Monarch Watch Blog

Monarch Population Status

Monday, September 10th, 2018 at 1:59 pm by Chip Taylor
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It’s been more than a few years since we have seen a monarch migration as promising as the one that is taking place at this time (early September). The first fall roosts were reported to Journey North on the 11 August in Sauk Centre, Minnesota (45.7 N) and reports of additional roosts soon followed. There were multiple reports from Sauk Centre with the highest numbers reported on 30 and 31 August. Most of the roosts have been reported in the western portion of the Upper Midwest (eastern Dakotas to central Minnesota), but a number of others have been reported in eastern Ontario, Michigan, New York, and Ohio. Data for all the roost reports to Journey North can be found at: Journey North Monarch Roost Report 2018. A screen shot of the location of the roosts through 3 September is shown in Figure 1. To follow the roosts and therefore the migration visit Journey North Monarch Roost Map 2018. While the numbers of reported roosts through 2 September is encouraging, and is a signal of a robust migration, the estimated numbers of monarchs are also higher than seen in several years with 17 different roosts estimated to contain 1000 or more monarchs.

Figure 1. Distribution of overnight monarchs roosts reported to Journey North through 2 September 2018.

Tagging 2018

Interest in tagging has grown. Last year, a year with a moderate population (2.48 hectares) one that I was actually expecting to be larger, we issued over 320,000 tags. That was largest number of tags issued in one year to date. The returned data sheets have not all been tallied, but we are closing in on the final numbers and it appears that over 140,000 butterflies were tagged last season. That may well have been the highest proportion of tags applied of those issued since we started tagging in 1992. The highest number of tags applied in the past was estimated to be >106,000 in 2001. Requests for tags are off the charts again this year and due to the high demand we ordered extra tags. Unfortunately, they will all be spoken for by the time you read this. In fact, data sheets in both digital and paper form are being received at this time from those who tagged in the most northerly locations. Domestic recoveries, that is those within the US or Canada, are also being reported. Most, perhaps 90%, of these recoveries are of butterflies tagged within a mile or two of where they were observed. These reports tell us little about the migration. As a consequence, we assign less attention to these recoveries than those from Mexico, but there are some gems among these domestic recoveries and we will get to these in time. Please understand it takes us months to record all the records involved with the tagging program with our relatively small staff and the help of volunteers.

Overwintering numbers and fall conditions

In previous posts I’ve pointed out that the sequence of events and temperatures that determine how the monarch population grows through the season has been better this year than for any year since 2001. Based on this I’ve been projecting an overwintering population of 5 hectares, perhaps a bit more depending on the level of monarch production in the western portion of the Upper Midwest (longitudes 95W – 100W). Reporting from states in that region is typically minimal, but we recently received reports of substantial numbers of monarchs from western Kansas and the central Dakotas suggesting a good migration is underway through that area.

The last overwintering population of 5 hectares occurred in 2008 and this migration should reach that number, but there is still a big unknown. Will the fall conditions favor survival during the migration? We don’t really know how to answer this question. We can look to weather forecasts and nectar availability as inferred from drought indices, but we don’t really know how monarchs are affected negatively by weather events or nectar scarcity. While the long-range forecasts suggest average or slightly below average temperatures from the Upper Midwest through Texas, drought conditions in key areas could present problems for migrating monarchs, particularly in Texas, as shown in Figure 2. Most of the migration through Texas occurs in October so should sufficient rains occur, nectar scarcity would not be an issue. Monarchs still have to pass through northern Mexico, a traverse of another 600 miles or more depending on the routes taken. Unfortunately, there is no up to date information on the drought conditions in this region. The 31 July report from the North American Drought Monitor website suggested that the region was not abnormally dry, but that was before August, which is typically a dry month in that region.

Figure 2. Drought monitor showing increasing drought severity based on color gradients as of 28 August 2018.

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