Weevils, long-nosed beetles, are unsung heroes of pollination

Butterflies, bees, and even bats are celebrated as pollinators: creatures that travel from flower to flower to feed, and in the process, help fertilize the plants by spreading pollen. But some of nature’s most diverse pollinators often go unnoticed, even by scientists: long-snouted beetles called weevils. A new study in the journal Peer Community Journal provides a deep dive into the more than 300 species of weevils, including ones whose entire life cycles are interwoven with a specific plant that they help pollinate.

“Even people who work on pollination don't usually consider weevils as one of the main pollinators, and people who work on weevils don't usually consider pollination as something relevant to the group,” says Bruno de Medeiros, an assistant curator of insects at Chicago’s Field Museum and the senior author of the study. “There are lots of important things that people are missing because of preconceptions.”

There are about 400,000 species of beetles that scientists have identified, making them the largest group of animals in the world. And the largest group of beetles are the weevils. “There are 60,000 species of weevils that we know about, which is about the same as the number of all vertebrate animals put together,” says de Medeiros. The new study, which de Medeiros co-authored with Julien Haran and Gael Kergoat at France’s Université de Montpellier, is a review of hundreds of previously published descriptions of interactions between weevils and plants, to better understand their role as pollinators.

Weevils are sometimes considered pests; they can sometimes be found in pantries eating pasta and grains, and around the turn of the 20th century, boll weevils disrupted the American South’s cotton economy by feeding on cotton buds. However, many species are beneficial to plants, especially as pollinators.

“In this study, we focused on brood-site pollinators-- insects that use the same plants they pollinate as breeding sites for their larvae,” says de Medeiros. “It is a special kind of pollination interaction because it is usually associated with high specialization: because the insects spend their whole life cycle in the plant, they often only pollinate that plant. And because the plants have very reliable pollinators, they mostly use those pollinators.”

Brood-site pollination is a little like a more extreme version of the relationship between Monarch butterflies and milkweed, which is the only plant that Monarch caterpillars eat and the site where the butterflies lay their eggs. But brood-site pollinators, unlike Monarchs, take the relationship a step further: adult Monarchs feed on the nectar of many different flowers, but brood-site pollinators, including many species of weevils, rely only on their one plant partner as a source of food and a site for egg-laying.

“This kind of pollination interaction is generally thought to be rare or unusual,” says de Medeiros. “In this study, we show that there are hundreds of weevil species and plants for which this has been documented already, and many, many more yet to be discovered.”

These closely-linked relationships mean that the plants and weevils need each other to flourish. “Oil palm, which is used to make peanut butter and Nutella, was not a viable industry until someone figured out that the weevils found with them were their pollinators,” says de Medeiros. “And because people had an incorrect preconception that weevils were not pollinators, it took much, much longer than it could have taken.”

He says that these sorts of misconceptions are one of the motivations for the new study. “We are highlighting a group of insects that most people want to see killed, and we're showing that they can actually be pretty important for maintaining ecosystems and products that we care about,” he says. “We hope that by summarizing what we know and providing some pointers on what we should be paying attention to, we can help other researchers and the public to better appreciate the role of weevils as pollinators, especially in the tropics.”

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