Global warming acts as a 'tailwind for diseases.' Unexpected results revealed by Monarch research

Global Warming Makes "Butterflies More Susceptible to Disease"—The Main Culprit is Parasites

"With global warming, parasites might weaken in the heat and decrease."

Such "slight hope" existed in research concerning the monarch butterfly. However, a new study from the University of Georgia directly contradicts this expectation.

In conclusion, in warmer environments, monarchs' resistance to infection decreases, and parasites actually infect more successfully. Moreover, there is a possibility that the "medicinal herbs" meant to help monarchs may become ineffective due to the heat. Phys.org

Parasite OE Has Increased "More Than Threefold" Since 2002

The problematic parasite is Ophryocystis elektroscirrha (OE). Known since the 1960s, it has been reported to cause adult butterflies to have smaller wings, reduced weight, and shorter lifespans, affecting long-distance migration. The more troubling aspect is the recent increase in infections. The article notes that infections have "surged" more than threefold since 2002. Phys.org

The Pitfall of "Planting Milkweed": The Complex Relationship Between Toxicity and Immunity

Monarch larvae can only eat milkweed. Therefore, more people have started planting milkweed in their gardens to "save the monarchs." However, the commonly planted type is the non-native tropical milkweed. In warm regions, it grows year-round, allowing monarchs to continue breeding in winter, which increases the time parasites can "linger." Phys.org

On the other hand, tropical milkweed is highly toxic, and it has been suggested that monarchs may use this toxin (cardenolides) to defend against parasites. This has led to a complicated debate: "Even if it's non-native, if it acts as medicine, shouldn't it be planted?" The potential for defense against parasites has been pointed out. Phys.org

Key Aspect of the Study: "Realistic Temperature Fluctuations" Instead of "Constant High Temperature"

Previous studies included experiments where plants and parasites were kept at "constant high temperatures," leading to the outlook that "heat is unfavorable for parasites" and "increased toxicity might enhance defense." Phys.org

However, the current study is characterized by testing under conditions closer to reality, with temperature fluctuations similar to outdoor conditions. According to UGA's explanation, infected and non-infected individuals were raised on tropical (non-native) or swamp (native) milkweed, and compared under normal and high-temperature conditions. UGA Today

Results Were "All Wrong": The Hotter It Is, the More Parasites Prevail, and Defense Effects Disappear

Researchers initially predicted this:

• Parasites should weaken as it gets hotter

• Individuals that consumed highly toxic milkweed should have reduced infections

However, in reality, none of these occurred. Lead author Sonia Altizer candidly stated in the article, "None of it happened." Phys.org

There are three main points:

1. Under high-temperature conditions, "tolerance" to infection decreased by 22%. Phys.org

2. The "protective effect" of highly toxic tropical milkweed disappeared under high temperatures. Phys.org

3. Parasites actually infected more successfully under high-temperature conditions, with more individuals infected than expected. Phys.org

Furthermore, although the toxin level in tropical milkweed slightly increased with rising temperatures, it did not necessarily lead to "enhanced medicinal effects." Toxins can slow growth or damage cells, and if monarchs expel the toxins, the defensive benefit may be lost. Phys.org

The researchers' words are significant: "A warmer world might become a 'sicker world' for monarchs." Phys.org

From "Planting Can Save" to "Planting Incorrectly Can Increase"

This brings the discussion back to the gardeners.

Planting milkweed doesn't end with just planting. With global warming, shorter winters, and longer-lasting plants, monarchs may continue breeding without migrating, leading to parasite accumulation—this scenario has been discussed before. Phys.org

In California, there has been intense debate over tropical milkweed, and according to a UCANR explanation, in 2022, California authorities designated it as a "noxious weed," leading to sales restrictions in multiple counties. UC Agriculture and Natural Resources

However, within the same explanation, researchers caution that "tropical milkweed is not the top priority issue," emphasizing the importance of avoiding pesticides and planting native species and diverse nectar sources, rather than simply labeling it as a "villain." UC Agriculture and Natural Resources

Social Media Reactions: The Milkweed Debate is Not "Polarized" but a "Conditional Struggle"

The Phys.org article itself had zero comments at the time of publication (at least the page shows "Load comments (0)"), but the topic has been a hot discussion on social media for some time. Phys.org

Reaction 1: Cautious Group "If There's a Native Option, Don't Take the Risk"

In Reddit's monarch communities, there are prominent voices against tropical milkweed, suggesting it might disrupt the "migration switch (dormancy/movement)." Even without a definitive conclusion, they argue for erring on the side of caution if native options are available. Reddit

This study serves as a "tailwind" for the cautious group, as it shows that the expectation of "strong toxins = defense" may collapse under high temperatures. Phys.org

Reaction 2: Realists "It's Not Black and White. There Are Situations Where It Can Be Used with Management"

In the same thread, there are opinions that "non-native doesn't mean immediate NG," emphasizing the importance of management (e.g., cutting back in the fall) and education, suggesting distributing "care guides" rather than outright bans. Reddit

This connects with explanations from UGA and Monarch Joint Venture. While plants may have a "medicinal" aspect, they do not "cure" infections. If toxins alleviate symptoms (spore load) and extend the lifespan of infected individuals, it could also extend the time for spreading—creating a conflict between individual and group benefits. Monarch Joint Venture

Reaction 3: Hardliners "Remove Non-Natives. Avoid Pesticides First"

Another Reddit post features stronger language, advising to "dispose of tropical milkweed," "learn about native species and buy from trusted nurseries," and "beware of neonicotinoids and other chemicals," providing "practical advice." Reddit

This study does not "fully endorse" the hardliners' claims, but it does serve as a warning that "the more global warming progresses, the less likely non-native toxic plants will conveniently work." Phys.org

So What Should We Do?—From a "Plant" to a "Management" Discussion

This study does not present a hopeless scenario. Instead, it clarifies the focus of countermeasures.

• Prioritize native milkweed suitable for the region (opt for safety if alternatives exist) Monarch Joint Venture##HTML_TAG_