14 THE SCIENTIST | the-scientist.com
ago, she was presented with a problem:
it’s hard to know what soil insects are
munching on, or even how many of them
are in the ground, without digging up
the soil and destroying the very ecosys-
tem she wanted to monitor.
This dilemma isn’t unique to Görres’
research; monitoring soil insects is noto-
riously difficult. “There is so little work
being done with soil insects, and it’s
because it’s really hard,” says US Depart-
ment of Agriculture researcher Richard
Mankin, who has spent much of his career
studying the subterranean critters. For
farmers whose fields are threatened by
soil pests such as white grubs, the vora-
cious larvae of beetles in the Scarabaeidae
family that frequently devastate harvests,
the only way to confirm an infestation is
to excavate their fields.
One approach to monitoring soil
insects that scientists such as Mankin
and David Chesmore, an acoustics
researcher at the University of York,
have explored is to listen for the sounds
the larvae make. Acoustic monitoring is
common among ornithologists survey-
ing rare bird species, wildlife biologistsfollowing hard-to-track fauna such as
chimpanzees, and entomologists study-
ing above-ground insects such as crick-
ets and cicadas. Soil scientists have spent
decades trying to detect white grubs and
other crop pests by the sounds they make
while moving or feeding, but “sounds in
soil don’t travel far,” says Chesmore. As
a result, most of the noise that reaches
underground microphones, such as the
ones Chesmore has spent his career
developing, are quiet and indistinct.
Still, Görres needed a way to moni-
tor soil insects for her research, so she
teamed up with Chesmore to devise
a way to record the sounds of white
grubs in the lab. The pair’s experiment
consisted simply of “boxes with soil
with some larvae in them,” as Görres
describes it, with stick-shaped micro-
phones designed by Chesmore pushed
down into the dirt. Listening to the ini-
tial recordings, Görres heard not only
feeding sounds but a series of unex-
pected chirps. “I heard these strange
sounds, so I went to David and he said,
‘Oh my god, those are stridulations.’ We
didn’t expect that at all.”Stridulations are most commonly
associated with above-ground insects
such as crickets, which produce the
noises by rubbing their legs over a hard-
ened, comb-like organ on their bod-
ies. But these sounds are rarely heard
in the soil, and haven’t been well stud-
ied in white grubs. For Görres, it was
a lucky break that completely changed
the trajectory of her research. “When I
started out, I wanted to measure green-
house gas emissions. Acoustics was sup-
posed to be just a very minor part of the
project, and then it turned into a whole
research area itself.”
Follow-up recordings and analyses
that Görres and Chesmore published this
summer demonstrate for the first time
that two species of white grubs, Melolon-
tha melolontha and M. hippocastani,
can be monitored by their stridulations.
Importantly, the researchers were able
to distinguish between the two species
just by listening to the patterns of their
sounds—M. hippocastani stridulations
came in short, staccato bursts, while M.
melolontha stridulations were slightly
more elongated. They also found that
the average stridulation rate per larva
increased with increasing abundance of
grubs in the container of soil (Sci Rep,
9:10115, 2019).
The ability to differentiate species
within the soil based on their stridula-
tions could be a major step forward both
for researchers and for farmers, notes
Kyle Wickings, a soil ecologist at Cornell
University. An infestation of one partic-
ular species of white grub may become
problematic for farmers once there are
100 or more larvae per cubic meter of
soil, while another species may threaten
crops at an abundance of only 10 larvae
per cubic meter, he explains. Using strid-
ulations to “not only detect the presence
of pests, but also to tell [species] apart
and relate that to a management decision
is a pretty big deal.”
Görres is hopeful that placing micro-
phones in agricultural fields could one
day offer farmers a cheaper and less
destructive method to monitor infesta-
tions. Wickings says he supports that ANDRZEJ KRAUZE