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

Desert Refuge Project

Hello USA-NPN,

As things heat up in Central and Southern Arizona, the Desert Refuge: Monarchs and Milkweeds in Arizona campaign is coming to a close for this season. 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!

Monarch (Danaus plexippus) by Marleigh Fletcher via iNaturalist (CC-BY-NC) (cropped)

Tag monarchs with SWMS!

As you look forward to fall, consider participating in both Nature’s Notebook and Southwest Monarch Study (SWMS). SWMS will provide you with tags (small stickers that go on the wing of a monarch) and training on how to use them. Tagging monarchs in the fall will help us track the migration, but you need to be prepared with tags. If you are interested in tagging, learn more and sign up at swmonarchs.org.

Tagged monarch via SWMS

What you reported this year

This year, 27 observers reported data on both monarchs and milkweed. 17 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.

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. Reid Park Zoo finished off the year at the top of the list with 2,920 observations, followed by Desert Botanical Garden 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.

In the Phoenix area, observers reported caterpillars in 2024 mostly from mid-October to mid-November and very few reports in 2025. Observers reported monarch adults in 2024 starting in mid October through mid December, with a few reports in 2025.

Looking more closely at your reports in the Phoenix area, we can see that across all milkweed species, your reports of young leaves rose in mid-October, and continued increasing in 2025 until March. Your reports of open flowers peaked in mid-November and then started increasing again in early-March. Your reports of adult monarchs peaked early in early October, 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.

Tucson observers started reporting monarch caterpillars later than observers in Phoenix, with reports beginning in mid-November. However, Tucson observers saw many more caterpillars in 2025 than Phoenix observers. As for adults, Tucson observers began reporting adults about a month earlier than Phoenix observers, and also saw more adults in 2025.

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 saw a slight peak in mid-October, and remained low the rest of the season. Caterpillars had a slight peak in late-December and decreased from there.

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, we've noticed a different pattern in the years since—monarch caterpillars and adults were observed all winter!

Caterpillar Activity

Monarch Adult Activity

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 four years.

Caterpillar Activity Including Negative Observations

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!