When Forest Fires Flare, Woodboring Beetles Rush In
By Meredith Swett Walker
“As a further observation on Melanophila beetles, I may add that at football games in the California Memorial Stadium at Berkeley they occasionally swarm in sufficient numbers to annoy patrons by alighting on the clothing or even biting the neck or hands. It is possible that in this case the beetles are attracted by the smoke of some twenty thousand (more or less) cigarettes which on still days sometimes hangs like a haze over the stadium during a ‘big’ game.”– E. Gorton Linsley, Journal of Economic Entomology, April 1943
Many beetles in the genus Melanophila, along with other woodboring beetle species in the Cerambycidae and Buprestidae families, are attracted to the smoke and heat from fires because they lay their eggs in dead and dying trees. In natural settings, smoke usually signals a forest fire—a feast for the beetles’ future larvae who feed on the phloem of the burnt trunks. In the case described above, the smoke instead led the beetles to an aggregation of nicotine-addicted football fans. Now a changing climate may further complicate life for woodboring beetles.
The forecast for forests in the western U.S. calls for more burnt trees due to increased fire activity and a longer fire season. In addition, droughts stress trees, making them more susceptible to outbreaks of bark beetles and more likely to die. In their death throes, these trees give off chemicals, like ethanol, that are also attractive to some woodboring beetles.
More dead trees might seem an obvious boon to woodboring beetles, but research by scientists from The Institute for Bird Populations (IBP) and the U.S. Forest Service suggest that the effects of these changing forest conditions on the beetles may not be so straightforward. Their report was published last week in the journal Fire Ecology.
Woodboring beetles play an important ecological role in forests by promoting decomposition of dead wood, recycling of nutrients, and reduction of fuel loads. Their larvae are also an important food resource for bird species that specialize in recently burned forest habitats such as the Black-backed Woodpecker (Picoides arcticus), a species of management interest in California. That’s how researchers at IBP started studying these six-legged, rather than feathered, forest dwellers.
IBP researchers had been studying the woodpecker for several years. Along the way, they met Danny Cluck, an entomologist with the U.S. Forest Service and co-author of the study. “I asked to come out when they were tracking woodpeckers just to observe the birds and watch them feed,” says Cluck. “I am especially interested in woodpeckers since I see so many of them and their excavations on trees while I am studying or surveying for bark and woodboring beetles.”
Most research on woodboring beetles has focused on economically destructive or invasive species such as the emerald ash borer (Agrilus planipennis) and the Asian longhorned beetle (Anoplophora glabripennis). Less is known about the ecology of native, more economically benign woodboring beetle species. Given their important ecological role, more information about how they colonize and use trees damaged by fire and bark-beetle outbreaks could benefit forest managers—and ornithologists.
The researchers examined woodboring beetle populations at 16 forest sites in California affected by wildfire or bark-beetle outbreaks. They modeled the response of the beetles to disturbance type (fire or bark-beetle outbreak), fire severity and timing, forest composition and structure, as well as tree characteristics.
In contrast to a previous study, the researchers found that woodboring beetle activity was often similar in stands of trees killed by wildfire and those killed by bark-beetle outbreaks. However, on a smaller scale—sampling areas on individual tree trunks—woodborer larvae were more common when bark-beetles were absent, suggesting that the two guilds of beetles compete for resources.
Perhaps the most interesting finding was the relationship between when a fire started, or the ignition date, and beetle abundance. Specifically, beetles were more abundant in burned areas where the fire had started in the middle of the fire season, rather than early or late in the season. This result fit one of the researcher’s hypotheses, that beetles would more readily colonize burns that occurred mid-fire season because they likely evolved to disperse when newly burned sites are typically most abundant.
Still, the strength of the relationship surprised author Chris Ray, Ph.D., a research ecologist with IBP, because ignition date is a crude approximation of when a given location actually burned. Forest fires can burn for many weeks, so the ignition date of a given fire can be much earlier than the date that a specific location within that fire burned. “We ended up finding evidence for this peak in beetle abundance [in burned stands with a mid-season ignition date] across different life stages and across different species,” says Ray.
The effect of ignition date was also persistent. For instance, a stand that burned late in the season still had fewer woodborers a few years later. This suggests that the initial colonization by woodboring beetles immediately after the fire is very important. According to Bob Wilkerson, a biologist with IBP, the effect of ignition date also matched their observations of the woodpeckers. “We were not finding a lot of Black-backed Woodpeckers at late season fire sites,” he says.
With fire-season lengthening due a changing climate, more fires may occur either too late, or too early, for woodboring beetles to colonize them in high numbers. Woodborers will still colonize these burned stands, but not at the level seen at sites that burn mid-season. In other words, not all fires are created equal for woodboring beetles— or the woodpeckers that feed on them.
The researchers caution that their study was limited to a small sample of the fires and bark-beetle outbreaks that nature (or accident) provided within a single decade and in one region of the country. The relationships they found will need to be tested in other regions and across a wider diversity of burned and bark-beetle outbreak sites. Still, this work suggests possible strategies for forest managers looking to maximize the ecological processes fostered by woodboring beetles, as well as habitat for Black-backed Woodpeckers.
Meredith Swett Walker serves as communications specialist for The Institute for Bird Populations, a nonprofit based in Point Reyes Station, California, that enables science-based conservation of species and habitats by studying the abundance, demography, and ecology of birds and other wildlife. Email: email@example.com.