What’s causing the decline in monarch butterfly populations?

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Written by Mariecor Agravante
on Mar 27, 2020image

Habitat loss

For monarchs, habitat entails food, water and shelter, says the National Wildlife Federation (NWF) and the World Wildlife Fund (WWF). Specific to monarchs is their habitat corridor, a trek of thousands of miles from Central America’s warm regions, where they overwinter, to areas across the United States and southern Canada, where they stay for spring and summer.

In recent decades, population surveys reveal monarchs declining because of deforestation in Mexico, loss of grasslands in the Great Plains’ Corn Belt — which the Center for Biological Diversity calls “the heart of the monarch’s range” — and loss of native milkweed plants in the U.S. Such habitat losses negatively impact monarch populations as they breed, migrate and overwinter.

Habitat loss stems mainly from the deforestation of overwintering areas, climate change‘s fluctuating weather patterns, developmental sprawl, plus the conversion of U.S. grasslands into ranches and farmlands. This conversion to farmland for corn and soy has spurred the Center for Biological Diversity’s admonishment against the overuse of herbicides. These harmful chemicals poison a key player in monarch habitats, their host plant, the milkweed.

Problems with milkweed

Milkweed is vital to monarchs. They are host plants, upon which females lay eggs. Once hatched, caterpillars enjoy milkweed as a food source while they grow and develop into adulthood, a process that happens in the first month of a monarch’s lifespan. And, as adults, the butterflies feed on milkweed nectar. Several generations of offspring spawn on milkweed during spring and summer months before migration to overwintering sites even begins.

According to the NWF, “Monarch caterpillars feed exclusively on the leaves of milkweed, the only host plant for this iconic butterfly species. As such, milkweed is critical for the survival of monarchs. Without it, they cannot complete their life cycle and their populations decline.”

Interestingly, milkweed has the toxin cardenolide, which accumulates in caterpillars feeding on milkweed. When these caterpillars become adults, the cardenolides remain, protecting them from predation. Birds and predators veer away, signaled off by the toxin’s presence in the monarchs’ bright wings.

Unfortunately, milkweed loss is increasing in the destabilized landscape. Milkweed has lost considerable ground to urbanization, shifting land management practices, climate change and even herbicide misuse, like that of Roundup.

Alarming still are reports by Science magazine and Entomology Today that well-meaning gardeners have been planting the wrong species of milkweed. There are over 100 milkweed species, and not all are good for monarchs. Sadly, the tropical milkweed species Asclepias curassavica is heavily marketed because it is easier to obtain. But this invasive species is not well-suited for monarchs, yet remains the species good-intentioned gardeners are planting rather than the native milkweed species the monarchs are better adapted to. This invasive milkweed is now recognized by the Ecological Society of America as an ecological trap for monarch butterflies.

What dangers do these “wrong” species of milkweed pose for monarchs? For one, they harbor parasites, such as the protozoan parasite Ophryocystis elektroscirrha (OE), that are harmful to the monarch butterfly. These parasites debilitate monarchs, weakening them via “wing deformities, smaller body size, reduced flight performance, and shorter adult lifespans,” Entomology Today explained. Should these issues with milkweed persist unmitigated, their repercussions would continue to exacerbate the monarch butterfly population crisis.

a monarch butterfly resting on an orange flowered plant

Pesticide, insecticide and fungicide misuse

While media attention has spotlighted herbicides as a culprit, equally important is the fact that monarch butterflies are also vulnerable to pesticidesneonicotinoid insecticides and fungicides. For instance, a Purdue University Department of Entomology study, published last summer 2019 in Frontiers in Ecology and Evolution, revealed that non-target pesticides, insecticides and fungicides have wreaked havoc on monarch butterflies, even at their larval stage.

As the study elucidated, “agricultural intensification and a corresponding rise in pesticide use has been an environmental concern” that adversely affects beneficial pollinators, like the monarch butterfly. Exposure to these pesticides, insecticides and fungicides, can be from “direct contact with contaminated surfaces or spray droplets, residues remaining on the soil, and consumption via food resources such as leaves, nectar or pollen.” Just as vexing are pesticides, insecticides and fungicides “applied by aircraft.” The study emphasized the “evidence of lower abundance and/or diversity of butterflies.”

Climate change

The WWF affirms that “monarchs are highly sensitive to weather and climate. They depend on environmental cues (temperature in particular) to trigger reproduction, migration, and hibernation.” Their decline is also attributed to “the effects of an increasing frequency of extreme weather events such as drought and severe storms, and extremes in hot and cold temperatures.” No wonder then that the Environmental Defense Fund‘s Director of Conservation Studies, David Wolfe, has lamented that “The iconic and beloved North American monarch butterfly is one of the species that has difficulty adjusting to our new climate-stressed world. Its population has declined 95 percent in the last 20 years

Yet another way climate change adversely affects monarch butterflies is by disrupting their migration. These butterflies can travel between 50 and 100 miles a day, but when extreme weather sets in during migration, the entire cluster or roost is vulnerable.

“Every year, a new generation of these butterflies follows the same path forged by generations before them. The only thing guiding them on this migration is temperature telling them when they need to travel – like a biological trigger setting them in flight,” Wolfe explained. “But in recent years, the monarch’s fall south migration from Canada has been delayed by as much as six weeks due to warmer-than-normal temperatures that failed to trigger the butterflies’ instincts to move south. By the time the temperature cooled enough to trigger the migration, it’s been too cold in the Midwest and many monarchs died on their trip south.”

Even more worrisome, the Xerces Society, a nonprofit environmental group focused on invertebrates, has reported that warmer temperatures from climate change increase the toxicity of tropical milkweed by increasing cardenolide concentrations. Monarch caterpillars are only tolerant up to a threshold.

EcoWatch explained, “warmer temperatures increase the cardenolides in A. curassavica [the tropical milkweed species] to the point where they poison monarch larvae, delaying larval growth and stunting adult forewings. Native milkweed is not similarly impacted.” Hence, as invasive milkweed persists, they further harm monarch populations as temperatures rise in our current climate crisis.

a monarch butterfly on a green-leafed plant

Diseases, parasites and fungal pathogens

Emory University emphasizes that climate change affects pathogen development, parasite survival rates, disease transmission processes. What would monarch populations be susceptible to?

Bacterial and viral infections — like bacillus thuringiensis (BT), pseudomonas, the nuclear polyhedrosis virus (NPV) — are not unheard of, often turning an infected caterpillar or chrysalis into a darkened or black hue. Parasite attacks can come from tachinid flies or wasps (chalcid, trichogramma). Plus, fungal pathogens in the genus Cordyceps also attack. Each of these factors cause harm to monarch butterfly populations.

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