The impacts of warming temperatures on the diet quality of Monarch butterflies and what we can do to help

By Katherine Peel, with great thanks to Dr. Heather Kharouba, Jenna Boomhower, and Dr. Greg Mitchell for their work on the project

This summer, with lots of help from lab-mate, Jenna Boomhower, I embarked on an ambitious project: quantifying the impacts that warming temperatures have on nectar quality, and the subsequent impacts that a change in nectar quality has on Monarch butterflies.

The project was inspired by the significant decline in the size of the eastern migratory Monarch butterfly populations over the last few decades, of which, climate change is one of the main drivers.

In designing this project, we focused on the often-understudied indirect effects of climate change on Monarchs. It can be a challenge to disentangle the direct (for example, changes in Monarch body size or melanism as a direct result of increased temperatures) and indirect (for example, warming-induced changes in milkweed quality or nectar quality that impacts Monarchs) effects of climate change, but the results may have important conservation implications.

Since Monarchs have such a remarkable migration to overwinter in Mexico, they require lots of high-quality nectar, with lots of sugar, to pack on fat as energy stores to fuel their travels. Increased temperatures, as predicted due to climate change, may decrease the quality of nectar that is available to Monarchs.

We wanted to find out if this prediction holds true in field experiments, and if so, what are the subsequent impacts of this decreased nectar quality on Monarchs? We also wanted to know if there is a difference in nectar quality after warming among plant species. If there is a flowering plant (or more than one!) that maintains high nectar quality as temperatures increase, then we can recommend that it be used more widely in habitat restoration projects such as those managed by the National Capital Commission and the Canadian Wildlife Federation.

Katherine with one of the mini-greenhouses in the FWG’s Butterfly Meadow.

To answer these questions, with help and funding from Dr. Greg Mitchell and Environment and Climate Change Canada, Dr. Heather Kharouba designed 15 miniature portable custom-made open-top chamber (OTC) greenhouses that would be placed over flowering plants in the field. The open tops allow for significant alteration of temperature while minimizing unwanted effects such as modification of precipitation and humidity, and exclusion of pollinators.

There were a series of requirements that each flowering plant species needed to meet to be considered for the experiment. These requirements included: the height of the plant, the flowering time, and evidence that they are used by and provide benefits to Monarchs. The plants also had to be available in large populations around FWG so that we could set up the warming chambers without taking over too many of the available plants.

The three plant species I selected for use in the experiment: Solidago canadensis (Canada Goldenrod), Monarda fistulosa (Wild Bergamot), and Symphyotrichum novae-angliae (New England Aster).

The OTCs were placed over the selected flowering plants in early June and each OTC was paired with a nearby control enclosure. Once flowering began, I extracted nectar from a subset of the flowers in each enclosure and measured the concentration of sucrose using a handheld refractometer. At this time, we also introduced Monarch butterflies into the enclosures to feed on the nectar. Every couple of days, we would collect the Monarchs and record their weight so that we could keep track of how their masses changed.

Once finished their feeding trial in the enclosures, most of our Monarchs were released into the wild while a select few were retained for fat analysis. This analysis will allow us to gain a better understanding of how Monarchs are converting sugar into energy and storing fat in their bodies.

Field work includes much more than just research! Jenna and I spent many, many hours designing and building enclosures for the experiment. Here we are after successfully building our first control enclosure in the garden, pictured behind us. 

While we were conducting this experiment, Jenna was running her own experiment looking at the nectar quality of native vs. non-native plants, building off a study completed by Manon Veselovsky last summer. You can learn more about the background of that experiment from last year’s post by Dr. Heather Kharouba: The role of non-native nectar in the diet of Monarch butterflies.

The Monarchs used in both experiments were reared in controlled chambers at the University of Ottawa from five female and three male adults collected from FWG and surrounding areas. At our busiest, Jenna and I were spending upwards of 5 hours a day in the lab cleaning out the caterpillar containers and replenishing their milkweed sources. The final count of Monarchs that Jenna and I raised was close to 400!

Starting with five female and three male Monarch butterflies collected from FWG and surrounding areas, Katherine and Jenna reared almost 400 of their offspring.

Once the hungry caterpillars completed their life cycle, the new adults were placed in the greenhouses at the FWG to feed on nectar from goldenrods, bergamot, or asters.

Throughout the summer, our work at FWG received lots of attention. We loved chatting with visitors at the garden about our work, especially those who frequented the area and got to know our project well. One of our most-asked questions was “What can I do to help the Monarchs?”

Although one of the goals of this project is to inform high-level habitat restoration decisions, there is a lot of positive action that can be taken on an individual or neighbourhood scale. If you have a backyard or community garden, planting native milkweed to feed the caterpillars, and native nectar plants to feed the adults is a great way to support the population. Avoid using pesticides in your property to help protect Monarchs and other insect pollinators.

In the coming months, I’ll be analyzing all the data collected this summer, including nectar concentrations, monarch masses, and fat storage and acquisition. Stay tuned for another post in the spring where I will share my results! Until then, if you’d like to learn more about the experiment, listen to our CBC Radio interview: Monarch butterfly response to climate change (Ottawa Morning, 30 August 2023). Visit the Kharouba Lab website, and follow us on Instagram @kharouba_lab to stay in touch!