Why are bark beetles attracted to heat-stressed trees? Alcohol, new study says

Close-up image of a red turpentine beetle (Dendroctonus valens).

Foresters have long known that trees under stress from fire injury are vulnerable to bark beetle attacks. Now, Rick Kelsey and Doug Westlind, researchers with the Pacific Northwest Research Station, have developed a model that explains how physiological changes cause heat stress in woody tissues, even after exposure to less-than-lethal fire temperatures, and produce a chemical signal that attracts some bark beetles.

When heat disrupts normal cell functions, the tree produces ethanol as a short-term survival strategy.

And if enough ethanol accumulates, mixes with volatile organic compounds in the tree’s resin, and is released to the atmosphere, the combination has proved to be a strong attractant for red turpentine beetles.

A man in protective hat, vest inspects a tall, hanging series of cones.
Retired Forest Service scientist Rick Kelsey collects bark beetles captured in funnel traps following a prescribed fire in Oregon. Understanding the interplay between tree response to heat stress and certain insects can help forest managers design fuel-reduction treatments to achieve specific outcomes. USDA Forest Service photo.

Kelsey and Westlind showed that ethanol interacts synergistically with 3-carene, a dominant ponderosa pine resin monoterpene. In a trapping study, red turpentine beetles were more attracted to lures combining ethanol and 3-carene than lures with ethanol or 3-carene alone.

Understanding ecosystem responses to fire can help managers characterize forest health and plan for post-fire management.

The results also hold promise for developing simple ethanol detection methods for monitoring tree stress.

Real-time feedback on ethanol levels could help forest managers quickly assess which trees to cull after a fire, and which to leave in place.

Learn more in the USDA Forest Service Pacific Northwest Research Station’s Science Findings 217, at https://www.fs.usda.gov/treesearch/pubs/58195.

Ethanol dissipation mechanisms: diffusion, sapflow, and metabolism with relative rates at ambient conditions. Ethanol is metabolized by alcohol dehydrogenase to acetaldehyde (Zanon et al. 2007), which is converted by aldehyde dehydrogenase to acetate, which is converted by acetyl-CoA synthase into acetyl-CoA (MacDonald and Kimmerer 1993, Gass et al. 2005). The latter can enter the tricarboxylic acid or glyoxylate cycles or be used to synthesize lipids depending on heat damage to membranes and enzymes. Ethanol dissipation mechanisms: diffusion, sapflow, and metabolism with relative rates at ambient conditions. Ethanol is metabolized by alcohol dehydrogenase to acetaldehyde (Zanon et al. 2007), which is converted by aldehyde dehydrogenase to acetate, which is converted by acetyl-CoA synthase into acetyl-CoA (MacDonald and Kimmerer 1993, Gass et al. 2005). The latter can enter the tricarboxylic acid or glyoxylate cycles or be used to synthesize lipids depending on heat damage to membranes and enzymes.
As ethanol accumulates in the tree, it immediately begins to dissipate via (1) diffusion, (2) sapflow, and (3) metabolism. Each process is affected differently by the heat-stress mechanism the tissues and whole tree experience. USDA Forest Service illustration (originally published at https://academic.oup.com/bioscience/article/67/5/443/3746565).

Source information: USDA Forest Service Pacific Northwest Research Station (Science Findings 217). The research station – headquartered in Portland, Ore. – generates and communicates scientific knowledge to help people make informed choices about natural resources and the environment. The station has 11 laboratories and centers located in Alaska, Washington, and Oregon and about 300 employees. Learn more at https://www.fs.usda.gov/pnw/

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