By Sahotra Sarkar, Victor Sanchez-Cordero, Ana Bruton
Monarch butterflies on their way to Mexico began appearing over the skies of central Texas over a week ago. The swarms seem fewer than last year but that may be because cold and stormy weather funneled the fliers over Austin last October unlike this year. In spite of the high density of insects seen here last year, we later found out that the overall 2019-20 winter population was barely half of what the previous winter population had been (2018-2019).
As an earlier blog post noted, the decline of the overwintering monarch populations in Mexico is a matter of grave conservation concern. In the 1990s there were over 400 million insects; in recent years numbers have sometimes barely been a tenth of that. Monarch butterflies undertake the longest known annual insect migration in the world. If the winter population disappears, this migration will come to a halt and, though there are plenty of nonmigratory monarch populations in various parts of the world, an endangered biological phenomenon will become extinct.
To counteract the factors leading to the decline of monarch population, we must monitor what happens to them throughout the year. There are two competing (though not mutually exclusive) hypotheses. The more popular one is that they are declining because of the decimation of milkweed populations by herbicides in their breeding grounds during spring and early summer. Milkweed is important because it is the only food monarch caterpillars can consume. A less studied hypothesis is that the populations are declining because ever larger numbers of butterflies are dying during the fall migration.
In order to determine the likelihood of the migration mortality hypothesis conservation biologists are trying to figure out whether the butterflies are getting sufficient nectar to fuel their trip (and nectar is all that they consume) and whether they have safe places to roost at night. To get this information, researchers at the University of Texas and the National Autonomous University of Mexico (UNAM) have devised a project to track monarchs during the fall migration.
Part of this project has consisted of designing an app that anyone can use to track the butterflies (here are the IOS and Android versions along with a YouTube tutorial on how to use the app). Readers of this blog can make a very important contribution to monarch conservation. Birders can download the apps on their phones and take a picture of a monarch whenever they seen them. (If the pictures include flowers they are feeding on or plants on which they are resting, including that would be most useful.) The app will automatically transmit the images and GPS location to a database maintained by the researchers.
Once a good number of images come in, we will know what plants the monarchs most feed on. If it turns out to be case (as we suspect) that drought conditions lead to fewer plants, and that in turn leads to fewer overwintering insects, we will have established an important factor in their decline. We can then develop a plan to plant these species every fall in Texas to help the butterflies get to Mexico. This strategy will complement that of planting milkweed in spring to feed the caterpillars though, we should note, that there is no evidence that there is insufficient milkweed in Texas for the relatively small number of caterpillars that are found here during the journey north. What matters most in Texas is the journey south and monitoring the southern migration is of utmost importance for us.