Bark Beetles Take Advantage of Global Warming

Diversity of ambrosia beetles from tropical forests. These fabulous shapes and colors can only be seen with a microscope. The beetles are tiny, no bigger than 3 mm. Credit: © Jiri Hulcr, courtesy of Jiri Hulcr

The ambrosia beetle, a type of bark beetle only slightly larger than a grain of rice, chews its way steadily into a broadleaf tree. It tunnels, determined, into the heartwood, pushing sawdust out through boreholes as it digs deeper. The tree, if it is alive to begin with, will ultimately die from the damage to its water-conducting tissue. But the beetle’s intent is not to kill. Rather, its goal is to cultivate life, to create a nursery for the fungus on which it depends.


This behavior, fungus farming, has evolved many times over in ambrosia beetles, and remarkably, each time during a period of global warming, according to new research. The findings, published in the journal BMC Evolutionary Biology, indicate that current climate-warming trends could spell disaster for North America’s broadleaf forests. “It is likely that a warmer climate will allow more ambrosia beetles native to tropical South America and Asia to become established and flourish in temperate forests,” explained study author Anthony Cognato, a researcher in the Department of Entomology at Michigan State University. “In the U.S., we have discovered at least one species of an established non-native ambrosia beetle every year for the last five years.”

Significant numbers of ornamental trees grown in the southeastern portion of the United States have been lost to recent ambrosia beetle attacks. Native forests are at risk, too. For instance, large numbers of redbay, a tree native to the Atlantic Coastal Plain (from Virginia to Texas), have been felled by fungi cultivated by the redbay ambrosia beetle, a species introduced from Asia. The redbay beetle also has the potential to devastate avocado plantations. By some accounts, the species has been advancing east to west across the United States at a rate of 20 miles per year.

Once a fungus farmer, always a fungus farmer

Cognato and his colleague Bjarte Jordal, from the Natural History Museum, University Museum of Bergen in Norway, wanted to estimate the timing of fungus farming origins in different species of ambrosia beetles. Determining when the behavior evolved in different species would provide insight into whether there were common ecological or climatic factors associated with the beetles’ transition to fungus farming.

Fungus farming is a truly unique phenomenon in nature. Just three groups of insects—ants, termites, and weevils (which include ambrosia beetles)—have evolved this ability, despite the ubiquity of fungus in forest habitats and the consequent opportunity for interaction. Among ants and termites, fungus farming evolved just once, which is a major reason why its repeated emergence in ambrosia beetles is so intriguing.

The granulated ambrosia beetle inside a tree. The white surface is the garden of fungus, which the beetle cultivates for food and which also sometimes kills trees. To the right are the beetle's eggs. Credit: © Jiri Hulcr, courtesy of Jiri Hulcr

To a certain degree, each species of ambrosia beetle farms a different species of fungus. But according to Jordal, this is an oversimplification, because while many ambrosia beetles may farm one or two species, a single beetle may cultivate more than 20 different species. He prefers to think of ambrosia beetles as farmers of “communities” of fungi, with each type of beetle focusing on one fungus in particular but having the ability to change its preference for which and how many species it farms.

Variation in preference, in fact, may be the norm among ambrosia beetles. “The prime fungus may vary through time or geographically,” Jordal said. “It is also likely that widespread [beetle] species change their prime fungus as they use different host trees in different forests.”

To identify common factors associated with the evolution of fungus farming in ambrosia beetles, Jordal and Cognato conducted a series of phylogenetic analyses. They performed these analyses, which assess the evolutionary relatedness of species based on similarities in their DNA, for every lineage of ambrosia beetle and their close relatives. They looked specifically at five independently evolving genes and created a phylogenetic tree with branch lengths that approximated the time elapsed since each lineage diverged.

The results of the analyses reveal that the degree of specialization for fungus farming in ambrosia beetles is remarkably strong, so much so that there is no going back—the beetles are totally dependent on the fungi. This dependency is apparent in their alimentary canals, which are anatomically specialized to support their fungal diet, and in the larvae of some species, which do not develop properly in the absence of exposure to fungal hormones.

White stacks of sawdust pushed out of the tree are a sign of an ambrosia beetle attack. Credit: © Jiri Hulcr, courtesy of Jiri Hulcr

The impact of climate warming on beetle evolution

From the phylogenetic tree, Jordal and Cognato determined that the transition to fungus farming in ambrosia beetles corresponded with two periods of global warming that occurred about 50 million and 21 million years ago. Each of these periods was characterized by a broad distribution of tropical forests, with the extension of these forests into higher latitudes.

The change toward a warmer and more moist climate likely accelerated diversification of ambrosia beetles during these periods. “Any extensive droughts or restriction of wet tropical or temperate forest would put ambrosia beetles under strong pressure for survival,” Jordal said.

Another period of ambrosia beetle diversification may be on the way, particularly if warming causes coniferous forests to be replaced by broadleaf forests. Climatic warming has already placed conifers under dire threat, for other reasons. Species of pine in particular have been left highly susceptible to attack by insects such as the mountain pine beetle. While not a fungus farming species, the mountain pine beetle is a type of bark beetle, and it infects trees with blue-stain fungus. The fungus triggers a defense response in the tree, but when the attack is severe, the tree’s defenses are overwhelmed.

Since the 1990s, the mountain pine beetle has devastated more than 4 million acres of forest in Colorado and Wyoming and more than 44 million acres in British Columbia. The outbreak is the worst ever recorded and has been linked to climate change and an extended warm season in western North America. The increase in warm days has accelerated the mountain pine beetle’s life cycle, enabling it to produce two broods each year, rather than the traditional single brood.

About Science Up Front

A regular Britannica Blog feature written by the encyclopedia’s senior editor of biomedical sciences Kara Rogers, Science Up Front goes behind the headlines to bring researchers’ stories of discovery centerstage. Begun in 2009 to highlight the ingenious work of pioneering scientists and to bring greater accuracy to science reporting, Rogers goes straight to the source, exploring the latest advances in science through first-hand interviews with researchers.


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