Forests, Carbon, and the Listening Insect

I take the discussions/questions cited below as
an indication of how uncertain is the hope of
controlling insect impact on forests, how
uncertain is the hope of sequestration by
forests, and as another indication that we face
way serious challenge to holding temp increases
to 2C, or even 3C.

“Crutchfield is more skeptical that such efforts
would keep temperatures low enough. According to
his models, a continued increase in global
temperature is likely, and the beetles’ current
reaction to this   ‘early stage of warming’  does
not bode well for future forest health.”

“But, ‘there is a possibility that you could have
an acoustic signal to break up or slow down a
beetle infestation,’ Crutchfield says. In
preliminary field work, he and Dunn played
ultrasonic noise to interfere with the beetles’
sense in this acoustic range. The tests, he says,
were effective.

“‘Again, the bioacoustic idea is still a
hypothesis, one that has to be carefully tested
in a lab.’ Right now, though, Crutchfield adds,
‘it is the only alternative I see.”

Science News
Vol.174 #5

Forests, Carbon, and the Listening Insect

The ultrasonic din of dying trees inspires a new kind of
research to save forests from beetle attacks – and battle climate change

It turns out that a tree doesn’t have to fall in
the forest to make a sound. Upright trees make
plenty of sounds, even though human ears can’t
hear them. But few aside from botanists would
have known about the voices of the trees if two
guys had not pounded an old meat thermometer
turned ultrasonic microphone into a beetle-infested

When they did, composer David Dunn and physicist
Jim Crutchfield heard “sounds that went on,
uninterrupted, for long periods of time. It was a
constant ultrasound, and it didn’t matter where
you were, the sound was there,” Crutchfield says.
“It was bizarre.” The cacophony came from a tree
besieged by drought – and from a frenzy of
tree-invading beetles.

The duo’s investigation began after Crutchfield’s
New Mexican piñon pine trees came under attack.
“I had to cut down 100 trees on my lot,” he says,
“and I wanted to know what killed them.”

It was not the drought that ultimately destroyed
the pines but the invasion of a specific type of
bark beetle and its accompanying fungus.
Crutchfield’s neighbors turned to pesticides to
thwart the insect attack, but had no luck. The
trees still died.

The same destruction has been happening – en
masse – elsewhere as well. The spruce bark beetle
has already taken a Connecticut-sized bite out of
Alaskan pine forest. And bark beetle outbreaks
have desolated thousands of square kilometers of
western North American forests, incidentally
releasing thousands of tons of carbon into the
atmosphere. The additional carbon is a concern
because of its link to climate change.

But it was the tree deaths and the failure of the
pesticides that first led Crutchfield, who models
complex chaotic systems at the University of
California, Davis, and Dunn to propose a radical
solution to dampen beetle infestations: They want
to play deceptive ultrasound to confuse the
tree-devouring bugs, luring them away from
vulnerable forests and keeping the insects from
spreading to new territories.

Crutchfield says the noise could perhaps even
stop the beetles from inadvertently adding so
much carbon to the air that humans’ contributions
to global warming would become irrelevant.

The big crunch

The idea to use ultrasound as a beetle-defense
tactic began percolating in the pair’s minds
about four years ago. As Crutchfield’s trees were
dying, Dunn, who is president of the Art and
Science Laboratory in Santa Fe, N.M., was
fabricating a device to listen to the ultrasonic
sounds of nature. The environmental sound
recordist had decided to create a high-frequency
recorder while working at the Detroit Zoo, where
he learned that endangered Tanzanian frogs used
ultrasonic calls to find mates.

Dunn and Crutchfield got together to eavesdrop on
pine trees and their invading beetles, which led
the composer to an idea. He wondered if the
beetles could, in any way, detect the ultrasound
coming from the trees. As the pines’ liquid-transporting
cells dehydrate,the trees’ water columns cave in,
creating ultrasonic pops. Scientists believe that
extended periods of dehydration and drought cause the
water cells to implode and give off the pops,
which are near the 100 to 300 kilohertz range. By
comparison, the highestfrequency a human can hear
is 20 kilohertz.

Crutchfield suggests that the pops may help
possible beetle invaders sense whether a tree is
ripe for attack and whether they should chomp
through to its inner living layers to lay eggs.
Healthy trees have a defense against this
invasion, he says. By secreting asticky, toxic
ooze, or resin, that flows into the holes beetles
bore into the bark, the pines can
“pitch out” the insects.

Drought-stressed trees affected by warming
temperatures and consequent moisture loss,
though, have trouble making the defensive resin
to push the beetles out of the bark. And,
water-deprived trees, Crutchfield says, generate
more ultrasonic pops. So aschirps – the beetles’
cries –  as well as crunches on the bark. Beetles
passing by might hear the crackles, crunches and
pops and drop by to get a bite of their own, the
physicist says.

“This hypothesis, namely that trees send signals
to beetles, needs a lot of experimental work to
back it up,” Crutchfield concedes. “It’s
difficult to studybecause, to take one example,
no one knows what the insect’s hearing mechanism
is or even if it is responding to ultrasound.”

Wood-boring beetles do seem to sense which trees
are more vulnerable to attack, but scientists do
not yet know how. The mountain pine beetle reacts
strongly to chemicals called pheromones that are
produced by the insects during their attacks, as
well as to kairomones, aromatic compoundsproduced
by the trees, says James Powell, a mathematician
from Utah State University in Logan who models the
dynamics of beetle invasions in
pine forests.

Most entomologists think beetles rely primarily
on pheromones, not sound, to communicate, mate
and possibly even rally the troops for invasion.
Powell has studied the interaction between beetles
and pine forests in the Rockies, and he says countering
beetle outbreaks with chemicals
is hit or miss. Some experiments in Scandinavia
have actually shown that using pheromones to lure
beetles away from vulnerable trees can exacerbate
beetle invasion, Crutchfield says.

Crutchfield and Dunn published their “bioacoustic
ecology hypothesis” in 2006 as a working paper on
the website of the Santa Fe Institute, a center
forinterdisciplinary research. A revised version
will appear in an upcoming Leonardo, a journal
highlighting work of artists using science- and
technology-based media.

Despite all the existing research on beetle
communication, Crutchfield says, entomologists
don’t yet know how far each species’s chemical or
soundsignals can travel. It’s also not known what
actually kills a tree after a beetle infestation.
Richard Hofstetter, Crutchfield’s collaborator
and an ecologist at Northern Arizona University
in Flagstaff, has a beetle farm and looks at the
interactions between beetles and the blue stain
fungus Ophiostoma minus, which hitchhikes with
the beetles into trees. Hofstetter studies
whether it’s the beetles’ feeding frenzy, the
associated fungal invasion or some combination of
the two that kills a tree.

More important for testing the bio-acoustic
hypothesis, however, is figuring out the full
range of sound signals that the beetles can
produce and detect. For that, Crutchfield turned
to Jayne Yack, a biologist at Carleton University
in Ottawa, Canada.

Listening to bugs

Yack eavesdrops on insects, typically butterflies
and moths, to decode their sound signals. Now she
has turned her attention to bark beetles. But
these insects, she says, are harder to study
because they live under tree wood and are small –
sometimes as small as the head of a matchstick.

“We really know very little about how they talk
to each other and what signals they send,” Yack
says. But “these guys are highly acoustic. They
talk toeach other all the time, and so they have
to have acoustic organs.”

Scientists do know that the male or female in
certain wood-boring beetle species has an organ
called a pars striden, which looks like a set of
ridges on the back or underside of the insect’s
head. An insect can “play” these organs by
curling up and rubbing its head back and forth
against the middle of its body, Yack says.

The beetles seem to make other auditory calls,
too. Although Yack is not sure exactly what the
sounds mean. She speculates that the calls could
signal aggression if one beetle violates
another’s territory when the insects are mating.
Or the signals, which she says can only travel
about 10 centimeters, could alert incoming
beetles that they need to spread out while laying
eggs in a host tree. If beetle pairs disperse and
lay eggs all over the tree, Yack says, there
might be less competition for resources and space
– at least for the beetles.

Whether the beetles have organs to sense sounds
and, if so, whether they use those organs to find
drought-stressed trees is still unknown, she
says. Yack’s team is now trying to determine if
the beetles have sound receptors called tympanals,
which can pick up ultrasonic vibration. “We have
good candidates for these receptors,” she says, but
notes that the team has yet to confirm that the
insects have the organs. She expects to submit her
research on the question for publication soon.

Next, Yack plans to test if the beetles respond
to a recording of the pine trees’ ultrasonic
vibrations. “We will put electrodes in certain
regions of the beetles’ nervous system, play
sounds and see if the nervous system reacts,” she

While Yack’s research focuses on understanding
insects’ sensory worlds, her work could have
important practical implications. If scientists
learn what sounds, coupled with what chemicals,
beetles use to signal mating cycles and impending
invasions, she says, the knowledge could lead to
new waysto control the beetles.

A frenzied loop

Invasive beetles are beginning to move not only
northward but also upward, to elevations above
their usual habitats, Powell says. Certain
species of wood-boring beetles, such as the mountain pine
beetle and piñon beetles, are native to
lower-altitude forest regions. The bugs help
forests thrive by eating old trees and letting
new ones grow, he notes. But as warmer
temperatures and less rainfall lead to drought,
the beetle population is pushed out of check.
More trees become vulnerable to beetles, which
spread to higher latitudes and altitudes. Tree
species found in these regions not native to the
beetle invasions may not be as successful at
pitching out beetles and their fungus, Powell

Beetle attacks on these new, drought-stressed
species could devastate forests and start to add
large amounts of carbon dioxide into the air,
says Werner Kurz, an ecologist at the Canadian
Forest Service’s Pacific Forestry Centre in
Victoria, Canada. Infested pines ultimately take
in less CO2 than healthy trees. And dead trees
take in none – as the wood breaks down, it
releases carbon. So every time a tree is damaged
or dies because of a beetle infestation, the bugs
indirectly contribute more carbon to the

Based on a model of the beetles’ effect on
western Canadian pine forests, Kurz and his
colleagues predict that in about 20 years, beetle
outbreaks could kill enough trees to release
greenhouse gases with a warming effect equivalent
to about 990 million metric tons of CO2. In a
single year, the beetles could add 73 million
tons worth of these gases – equivalent to about
10 percent of Canada’s total human-caused
emissions for one year, Kurz and his colleagues
reported in the April 24 Nature.

Those extra gases, if not removed, could lead to
further increases in temperatures, which could
trigger more beetle population growth, movement
to new locales and damage to trees. “We call this
a feedback loop,” a positive one, he says,
because more beetles lead to more CO2.

“The numbers the Canadians have are shocking, and
that is just one class of beetle and one class of
trees,” Crutchfield says. “If other insects are
doing the same thing – I am especially thinking
of moths infecting deciduous, boreal forests in
Siberia – imagine the damage.” Echoing
anthropogenic climate change, the beetles could
have their own “entomogenic climate change,” he

Buzzing the beetles

Kurz believes that recognition of the insects’
potential impact on the environment could lead to
new efforts to control the mountain pine beetle
attacks in Canada. Managing beetle outbreaks at
their outset, planting new trees and using the
dead and eaten ones for wood products or energy,
he says, are a few ways humans can keep forests,
traditional carbon sinks, from ever becoming
carbon sources.

Crutchfield is more skeptical that such efforts
would keep temperatures low enough. According to
his models, a continued increase in global
temperature is likely, and the beetles’ current
reaction to this “early stage of warming” does
not bode well for future forest health.

What adds to his concern, he says, is the fact
that current countermeasures against the beetles
are less than effective. But, “there is a
possibility that you could have an acoustic
signal to break up or slow down a beetle
infestation,” Crutchfield says. In preliminary
field work, he and Dunn played ultrasonic noise
to interfere with the beetles’ sense in this
acoustic range. The tests, he says, were

“Again, the bioacoustic idea is still a
hypothesis, one that has to be carefully tested
in a lab.” Right now, though, Crutchfield adds,
“it is the only alternative I see.”

The method is one that Powell admits might work.
“The idea that beetles create sound passes muster
with me, so using ultrasound to confuse beetles
is possible, at least at short distances,” he says.
And should ultrasonic beetle-blocking turn out to
be a dud, Crutchfield says, “We better hope that
the beetle-climate feedback loop does not kick


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