Higher temperatures may make monarch butterflies more vulnerable to parasites, according to new research from the University of Georgia.
Monarch infections with the parasite Ophryocystis elektroscirrha have skyrocketed, more than tripling since 2002. Discovered in the 1960s, the parasite can cause smaller wingspans, lower weight and shorter lifespans in adult monarchs. Infection can also affect the insect's ability to complete its annual migration.
The study found that monarchs exposed to elevated temperatures were 22% less tolerant of infection.
"What does tolerance of an infection mean? If you have the flu, it's the difference between you being in the hospital or just having a stuffy nose," said Sonia Altizer, lead author of the study and entomology department head in the UGA College of Agricultural and Environmental Sciences. "Monarchs took a bigger hit if they were infected when temperatures were higher."
Parasite infections continue to climb among monarchs. Rising temps won't help.
Milkweeds are the only plants on which monarchs can lay their eggs and their caterpillars can feed.
In an effort to "save the monarchs," some people have planted milkweed in their yards. But they often plant nonnative, tropical milkweed, which can grow year-round in areas with mild climates. This extended growing season enables monarchs to delay or even skip migrating in the winter, giving the parasites more time to infect the butterflies that stick around.
Tropical milkweed is also very toxic compared to most native milkweeds. Monarchs can tolerate ingesting those toxins - to an extent. The butterflies can then concentrate the toxins in their bodies, an ability that is thought to help monarchs defend against parasite infection.
Previous studies found that higher temperatures can increase the level of toxins in milkweed. Earlier research also suggests that the parasites cannot tolerate extreme heat. These lab-based findings offered hope to researchers that warmer weather may have a silver lining for the threatened species by reducing infection.
However, those experiments exposed the parasites and plants to constant high temperatures.
The present study took a more realistic approach, exposing infected and healthy butterflies to fluctuating temperatures in a natural setting. And the findings were less encouraging. The researchers raised parasite-exposed and uninfected monarchs on nonnative tropical or native swamp milkweeds in either ambient or elevated temperatures in the field.
"We thought that the parasites would do worse in the hotter treatments. We thought fewer monarchs would be infected in the hotter treatments. We thought that monarchs that fed on more toxic milkweed would also have less infection than monarchs that fed on the non-toxic milkweed," Altizer said.
None of that happened.
Hotter weather helps parasites successfully infect monarchs
Under the hotter temperatures, the protective effect of the toxic milkweed disappeared.
Most of the butterflies that were exposed to the parasite got infected. The parasites actually did better under the hotter temperatures and infected more monarchs than the researchers anticipated.
The researchers found that the toxins in tropical milkweed were slightly higher in warmer temperatures, and that could be part of the problem.
"Even monarchs have limits. They can tolerate the toxins up to a point, but there are side effects."
This could mean that a warmer world might be a sicker world for monarchs."
-Sonia Altizer, College of Agricultural & Environmental Sciences
The toxins can slow development and damage cells. Sometimes, the potent toxins are excreted by monarchs. That means those monarchs lose some of the protection they may have gained from eating the toxic milkweed.
"This experiment showed that warmer temperatures cause these plants to lose their medicinal effect for monarchs. That means in areas where the temperature is warming, we might see infections increase disproportionately," Altizer said. "I think this could mean that a warmer world might be a sicker world for monarchs."
Published in Ecological Entomology, the study was led by Isabella Ragonese as part of her doctoral dissertation in the UGA Odum School of Ecology and Christopher Brandon as part of his undergraduate honors thesis. Ragonese is now a postdoctoral researcher at the University of Massachusetts Amherst, and Brandon is attending graduate school at Colorado State University.
Additional co-authors include Joselyne Chavez, Jacobus de Roode and UGA ecology associate professor Richard Hall.
