Science Matters

(11/2023) Honeybees are threatening the environment.

You read that right. The beloved little honeybee can actually be a bad guy. As it happens, so many people are keeping these insects, which are not native to North America, that they are outcompeting many species of native bees and other insects that depend on nectar, such as butterflies. As a result, there is too little nectar and pollen to go around, and native species are suffering.

In other counterintuitive insect news, the International Union for the Conservation of Nature has reversed its conclusion that the monarch butterfly is an endangered species. Turns out the charismatic creature is only "vulnerable" to extinction, a less severe category of concern. The IUCN, the world organization that maintains the global list of endangered species, says it may lower the level of alarm still further, depending on new evidence expected soon.

Let’s look at these one at a time. First honeybees.

In recent years there have been countless news reports about honeybee colonies dying from mysterious causes. Sometimes the die-offs are blamed on the parasitic varroa mite, which attacks honeybee larvae. Sometimes they are blamed on widespread use of certain pesticides in farmlands and suburban lawns. Sometimes die-offs are blamed on poor nutrition, owing to the loss of wildflowers that are suitable nectar sources. Or the cause is a combination of these. All these things may be true.

In response, many people are trying to help by becoming beekeepers, establishing hives in suburban back yards, city lots and on urban rooftops. It’s an interesting hobby, and thousands of people are learning a great deal about these fascinating insects.

But, according to ecologists with the Xerces Society for Invertebrate Conservation, 28 percent of native bumblebee species in North America are considered threatened, and at least 40 percent of native pollinators, including some butterfly species, "may face extinction in the coming decades." The Xerces Society is a professional group of entomologists and others who work to save threatened invertebrates. (Xerces is the genus name of an American butterfly that went extinct decades ago.) The declines are blamed on there being too many honeybees.

The group cites several facts to support its cause. First is that native plants and native pollinators have co-evolved to be adapted to each other. Until the early 20th century, North American food crops that required other organisms to ferry their pollen were served by native insects. (By the way, our main foodstuffs, including all grain crops, are wind pollinated and soybeans are self-pollinated. They never rely on insect pollination.)

The Xerces group also notes that honeybees are poor pollinators. They carry pollen in compact "pollen baskets" on their legs where it cannot easily reach other flowers. By contrast, native bees pick up pollen all over their bodies, making it easier to brush off as the bee visits the next flower. So more native bees means better pollination.

The combination of commercial and hobbyist hives has pushed bee densities so high that native bees are being driven out. One typical honeybee hive collects as much pollen as could support 100,000 native bees, which are solitary. A study in Montreal found that as the number of honeybees went up, the species diversity of native bees went down.

But you ask, what about reports that honeybees are threatened and in decline?

According to the U.N.’s Food and Agriculture Organization, the number of beehives around the world has risen nearly 26 percent in the last decade. There are more honeybees on the planet than ever before. That’s because commercial beekeepers routinely raise more than enough new hives to replace those that succumb to the mites and pesticides and loss of wildflowers. Honeybees are a commercially managed form of livestock that industry keeps in good supply, without regard to what that does to native pollinators.

Now, what about another insect that we have all been raised to cherish—the monarch butterfly. As most of us know, this amazing creature flies thousands of miles south to winter in the forests of Mexico. Come spring, it migrates north again, spawning a series of generations to reach as far as Canada.

Scientists generally agree that the number of monarchs in Mexican forests declined precipitously starting half a century ago, mainly because of logging and, more controversially, because herbicide use was growing as farmers adopted "no-till" practices. Herbicides were inadvertently killing the milkweed plants that monarch caterpillars feed on. But there is now evidence that the decline stabilized around 2014 at an estimated 55 million individual butterflies. There have been small drops in some years, but they were followed by increased reproduction the following year.

Last year IUCN researchers declared the species "endangered" based on an alarming assessment of monarch population trends derived from annual "counts" of the wintering population from 1990 to 2020. Of course, nobody is counting every individual. As I’ve seen for myself in the mountains of the Mexican state of Michoacan, evergreen fir forests turn orange with millions of monarchs cluster tightly on fir branches, so many that limbs are bent down. So various statistical models are used to extrapolate from sample counts.

In 2022, those methods led researchers to say that the total may have dropped by anywhere between 22 percent and 72 percent over a ten-year period. Hardly a precise estimate, but it met IUCN’s criterion for the "endangered" classification.

This year, however, an IUCN panel ruled that the statistical methods previously used were flawed. A better and more complex method showed that the population reached an inflection point in 2013. In that year monarch numbers appear to have bottomed out. They stopped declining rapidly and may even have begun to increase. The panel said the data support the less worrisome "vulnerable" designation. A new count is planned for this winter. Its results may tip the threat level either way.

A separate analysis by Andy Davis, an ecology researcher at the University of Georgia, suggests that the high populations many of us saw decades ago were a result of forest clearing in the 1800s. The resulting open landscapes allowed milkweeds and monarchs to explode to anomalously high levels. Then through the 20th century as forests grew back and farmland increased, monarch populations declined to levels more typical of bygone times. If Davis is right, the lower populations we see today are ones that have sustained themselves through centuries.

Be it butterflies or bees, ecological research is notoriously difficult. And if the species at issue are popular favorites, there is understandable emotional pressure from the public on scientists who try to stick to the evidence.

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