UConn student studies the relationship between milkweed density and parasitic infection in monarch butterflies

Rachel Grella performed a study with Monarch butterflies and Milkweed density. Photo by John Barnard/Pexels.

Rachel Grella, a seventh-semester University of Connecticut ecology and evolutionary biology (EEB) major, has been conducting research on how milkweed plant density impacts the parasitic infection rate on monarch butterflies. 

Ophryocystis elektroscirrha, also known as OE, is a protozoan parasite which debilitates monarch butterflies. OE spores are often found on the abdomen of adult monarchs. When a monarch lands to lay an egg, these spores often rub off onto the leaf. When these new eggs hatch, the caterpillars end up eating the leaf, digesting the spores.  

Severely infected monarchs can carry millions of spores, according to Monarch Joint Venture, and the cycle continues through their eggs, infecting new generations of butterflies.  

“When dormant spores are scattered onto eggs or milkweed leaves by infected adults, monarch larvae consume the spores and these parasites then replicate inside the larvae and pupae,” according to the website.  

Grella, who received funding for this project through the UConn SURF program, said the parasite isn’t detrimental until the caterpillar goes through metamorphosis. Severely infected individuals often emerge with curled wings, which prohibits them from flying, often resulting in death. Even mildly infected monarchs are not safe from OE’s effects, as they live shorter lives and can’t fly as well. 

“It doesn’t really have many negative effects until they’re in the chrysalis, so if they’re heavily infected, they will emerge with their wings completely deformed,” Grella said over Zoom.  

Through her experiment Grella and her mentor, Ph.D. candidate Michael LaScaleia, found there was no effect of milkweed density on the infection rate of monarchs. 

“We also looked to see if the density of milkweed impacted the probability of finding a caterpillar. We found that in lower densities, you’re more likely to find more caterpillars. For example, we had one area with three plants and we found ten eggs on that plant which was completely unexpected,” Grella said.  

She began her research at the beginning of the summer with a group of local Edwin O. Smith High School students who assisted in field work and rearing. After 10 locations with varying densities around campus were chosen, primarily in the UConn Forest, her team set up grids of bamboo rods topped with visible flagging tape around the milkweed plants.  

Milkweed plant in focus. Photo by Hanna Tomany/Pexels.

“We made all of these grids, counted the milkweed per grid. The spores most commonly spread when a monarch lands to lay an egg. But, they can also be spread by hand, so every day we wore gloves and cleaned them with bleach wipes after touching a plant,” Grella said. “We walked around checking every single plant in a patch, often doing multiple locations per day. We collected all of the caterpillars and eggs off, brought them back to the lab and then we reared them.” 

Grella was grateful to have the help of the high schoolers, because her research spanned all over campus, requiring a lot of work for one person. 

“Field work is very fun, I enjoy it a lot, but it’s also very strenuous,” Grella said. “The high schoolers helped me so much, especially in caterpillar collecting. They are really nice kids and cared about the project immensely. It was nice to see high schoolers interested in science, especially spending part of their summer break helping out a college kid.” 

Her passion for this project stemmed from a personal experience with an infected monarch, which led her down a rabbit hole of research.  

“My inspiration for the project came from a walk to class back in the fall of 2019. I saw a monarch on the ground with crumpled wings and I was so upset,” Grella said. “I texted my friends asking what I should do with it and they said it was best to let nature take its course. I started researching what caused its wings to crumple and how to prevent it, which led to my project.” 

Caterpillars in the experiment were raised on their host milkweed, meaning the leaves from the plant the monarchs were found on.  

“We ran into an issue where one plant had 10 caterpillars on it, so we didn’t have enough host plants to go around. We ended up taking milkweed from a plant not associated with the project and washed it in a bleach solution to kill any spores. After they devoured the host plant, we raised them on the washed leaves,” Grella said. 

A Monarch Caterpillar. Photo by Phil Kallahar/Pexels.

After about two weeks from hatching, the monarchs form their chrysalis. They remain in the chrysalis for eight to 15 days and they gain their signature orange and back wing color the day before they are born. Once the butterflies emerged and their wings hardened, Grella would sample them for OE spores. 

“You gently hold them by the abdomen and then you take one of your other fingers to lift up the abdomen and tap it with a clear piece of tape,” Grella said. “Then, you put it on a microscope slide and you can view the spores. I had over 70 caterpillars that we took care of, so it was really fun.” 

During her research, only a couple of Grella’s caterpillars had the OE spores and only one was seriously affected, which reflects a wider trend in Connecticut where she said the rate of OE infection is relatively low. 


  1. The title screen shows the non native Butterfly Bush, and the close up a Lilac flower cluster. Please correct to shots of Milkweeds – there are a number of species to choose from!!

  2. NICE ARTICLE by UConn student! And to the editor who included a lilac flower in the story and labeled it a “milkweed,” please do better!

    • I use a 3% solution, based on 7.50% of sodium chlorite in the bleach. If you use a different percentage bleach, your need to adjust. 1 tablespoon of bleach with 2 cups of water and the ratio is 32:1. I also bleach my eggs.

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