Desert Refuge: What did we learn from your monarch and milkweed observations this year?

Desert Refuge Project

Hello USA-NPN,

Our Desert Refuge: Monarchs and Milkweeds in Arizona project is coming to a close for southern and central Arizona. If you are still seeing monarchs, you are welcome to continue to record data, as these observations are valuable to our understanding of monarchs in the state. However, we asked for a winter time commitment to better understand overwintering behavior, and you all provided that! For those in northern Arizona, where the milkweeds and monarchs are now becoming active, we would love to have you start recording data!

Below, we take a look at what we learned from your data from the past nine months, and share next steps for this project. Thank you so much for taking part in this effort to help us better understand overwintering behavior of monarchs and the milkweeds on which they depend!

Photo below of rush (desert milkweed) flowers or flower buds and open flowers at the University of Arizona. We can also see flower visitation by an adult monarch.

What you reported this year

This year, 43 observers reported data on both monarchs and milkweed. 16 other observers reported only on monarchs, and seven tracked only milkweed. The map below shows locations where observers reported monarchs (Danaus, orange x) or milkweeds (Asclepias, blue square), or both (orange x on top of blue square) across the state. You can explore this visualization here.

We are so grateful to both our backyard observers and the Local Phenology Programs who have contributed data to Desert Refuge! The table below shows all of the records that our LPP partners have contributed since last September at the start of the overwintering season. Desert Botanical Garden finished off the year at the top of the list with 2,135 observations, followed by Reid Park Zoo and Master Gardeners. Thank you all for your efforts!

The majority of our observers are located in the two metro areas of Phoenix and Tucson. We saw some differences in the activity of monarchs and milkweeds in these two locations this year. The map below shows observers reporting on monarchs and most common species of milkweed in the Phoenix area. You can explore this visualization here.

In the Phoenix area, observers reported adult monarchs throughout the fall months and December, though there were few reports in 2024. Observers reported monarch caterpillars starting in mid October through mid February.

Looking more closely at your reports in the Phoenix area, we can see that across all milkweed species, your reports of young leaves spiked in mid-October, and continued increasing in 2024. Your reports of open flowers peaked in mid December and then started increasing again in early-March. Your reports of adult monarchs peaked early in the fall, while your "yes" reports of caterpillars were highest in mid-November.

The map below shows Tucson-area observers reporting on monarchs and the most commonly observed milkweeds. You can explore this visualization here.

Tucson observers also reported adult monarchs throughout the year. Observers started reporting monarch caterpillars around the same time as observers in Phoenix. While observers in the Phoenix area stopped reporting caterpillars in February, observers in Tucson continued to report caterpillars throughout the spring up through the end of April.

Looking more closely at your Tucson-area reports, your reports of young leaves stayed fairly consistent all winter, while open flowers peaked in early September and again in May. Your reports of adult monarchs were low at the beginning of the season, but increased until a peak in mid-April. Caterpillars were more likely to be recorded between mid-October and the end of April.

How does this year compare to previous years?

We started Desert Refuge with a hypothesis that monarchs spending the winter in Arizona stopped breeding during the coldest parts of our winter (roughly December-February). This is seen in other locations where monarchs winter, and we had seen some anecdotal evidence of this. If this hypothesis were true, we would expect to see monarch adult activity the entire winter, but caterpillar sightings only in the fall and late winter. Our data in the first season, with a handful of test participants, showed this pattern. However, the second year and third year (this year), we saw a different pattern. Monarch caterpillars and adults were observed all winter!

Why do we see caterpillars all winter and why was there a difference in the first year? Simple answer is, we don’t know and it could be a lot of different variables. There are lots of things to consider like weather patterns that could either have allowed monarchs to mate all year, or for the caterpillar stage to be slowed down. But, there is one factor we can rule out, thanks to your hard work. We know it is not because we are missing observations. Participants are recording data even when there isn’t a caterpillar. The graph below shows the same caterpillar data, but with the negative observations (no caterpillar observed) shown in grey. We had excellent coverage in all three years.

What will next year look like? We don’t know, and that is why we would love to have you come back! And, please spread the word, especially to friends and family in the northern part of the state, where milkweeds are just starting to put out leaves and flowers.

Thank you for your contribution to this important project!