Extreme heat proven to impact health outcomes for monarch butterflies

A recent study found that extreme heat makes monarch butterflies more vulnerable to a debilitating parasite.
Infection rates of this parasite have more than tripled over the past two decades.
Higher temperatures also reduce the medicinal, anti-parasitic effects of the milkweed plants that monarchs consume.
Extreme heat, one of many symptoms of climate change, is making monarch butterflies vulnerable to parasites, according to a recently published study.
The study published in the Royal Entomological Society Journal highlights how climate warming can impact plant quality, which in turn shapes the nutrition and immune defence of insect herbivores.
Sonia Altizer, professor of entomology at the University of Georgia, has been studying monarch butterflies and their migration for the past 30 years.
“I’ve really focused a lot of my research on a debilitating protozoan parasite that infects butterflies,” said Altizer. “There are a lot of diseases that infect butterflies and other insects. The parasite that we’ve studied can infect caterpillars when they inadvertently eat the protozoan spores that are scattered on milkweed plants. Then they become infected.”
Altizer said once infected, they grow smaller and don’t live as long, or fly as well.
“This can make a big impact for an animal that has to migrate thousands of kilometers every year,” she said. “These butterflies that are infected have difficulty completing the migration. In recent years, the prevalence of this parasite has more than tripled over the past two decades.”
Climate warming has pervasive and profound effects on host-parasite interactions, and understanding the pathways through which climate change shapes parasitism is of crucial importance for conservation agriculture, according to the study.
“Historically, only about 2-5% of monarchs, on average, across North America were infected with this protozoan. Since 2002, that percentage has jumped up to 10-15%. So we’re interested in what’s causing infections to increase in these butterflies. Is it related to climate? Is it related to habitat change? Is something else going on?”
The study found that monarchs exposed to elevated temperatures were 22% less tolerant of infection. The parasites actually did better under the hotter temperatures and infected more monarchs than the researchers anticipated.
“Another thing that was surprising to us was that the role of the milkweed was really important,” said Altizer. “Monarch butterflies will only feed on plants in the milkweed family.