LOVE is in the leaves. A tiny insect sits on a bit of foliage, serenading an
unseen partner. But don鈥檛 expect to hear the honeyed tones of a classical ballad
floating through the air. This particular little creature, the
unromantic-sounding southern green stink bug, is a bit unconventional. For a
start, Juliet woos Romeo. And she does it with the stink bug equivalent of heavy
metal. Her instrument is her body, and you really can feel the beat.
Vibrations are used by many insects, including some cicadas, ants and
beetles, to attract sexual partners. But the female stink bug鈥檚 elaborate
courtship song is more than just a come-on. It also serves as a detailed road
map, guiding males to an impending tryst. It鈥檚 the first time any insect has
been found to use vibrations to signal its whereabouts to a potential mate, and
biologist Andrej Cokl believes thousands of other species might be doing the
same thing.
Stink bugs have colonised every continent bar Antarctica and are one of the
most significant agricultural pests worldwide, attacking crops such as soybean
and cotton. Their courtship begins when males release pheromones to attract
females to the plant where they are feeding. But following scent trails isn鈥檛
easy, and the females usually end up on another part of the plant, hidden by the
foliage. Despite this, Cokl, an expert in insect communication at the National
Institute of Biology in Ljubljana, Slovenia, noticed that the males do not have
to rely on a chance meeting at the local leaf cafe. They beat a path directly to
the females, somehow taking all the right turns at every junction.
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He knew that females start vibrating when they land, and suspected they might
be tapping out directions to potential suitors. 鈥淭he sense of vibration is a
very common and ancient sense among arthropods,鈥� says Cokl. Spiders, crabs and
nocturnal scorpions are known to use vibrations to detect prey, while
leaf-cutter ants have been shown to send vibrations through plants to call
others to come and help harvest the most desirable leaves.
To find out if stink bugs were indeed navigating with vibes, Cokl set up a
series of blind dates in his lab. He separately reared sexually mature males and
females on romantic meals of peanuts, sunflower seeds and green beans. Along
with his Ljubljana colleague Meta Virant-Doberlet and Andrew McDowell from the
University of Queensland, he then kitted out Cyperus plants with tiny
vibrators on several of their leaves.
With these turned off, both male and female bugs placed on the stem
immediately moved up the plant to the leaves above. Females behaved in much the
same way when the vibrators were activated. But on a plant buzzing with a
variety of vibrations, the males began to respond with vibrational courtship
songs of their own. What鈥檚 more, most males opted to visit the leaves vibrating
with a short 100-hertz pulse every five seconds鈥攁 rhythm that closely
matches the one females produce.
Next, Cokl and his colleagues moved their bugs to the more complex branching
environment of a bean plant. Without any added vibrations, most of the insects
just moved up to the top of the stem. When the researchers played a direct
recording of the female courtship call through one leaf, the females again
鈥渢urned their noses up鈥� and ignored the vibes. But the males swung into action.
They began searching for the females, singing their courtship song all the
while.
At each junction, a male needs to decide whether to go up or down, or to move
left or right to a side branch. He seems to work out the correct route to a
love-match by placing various combinations of legs and antennae on the stem and
branches. This might seem easy, but just how the males do it isn鈥檛 clear. 鈥淭o
recognise this cue a lot of neuronal processing has to take place in male
brains,鈥� says Cokl. 鈥淲hat is even more striking is that males find the direction
on a crossing with a very short distance between their legs鈥攐ne centimetre
or less.鈥�
In theory, bugs can work out that a signal is coming from, say, the left
because it gets picked up by the left leg first. More exact information could be
gleaned from the fact that the strength of the signal weakens the farther it has
to travel. So the right leg may also receive a weaker signal than the left.
In practice, however, any weakening is relatively small. What鈥檚 more, the
vibrations move through plants at speeds of between 30 and 100 metres a second.
It is unclear how stink bugs, with their limited neural capacity, can
discriminate between stimuli across just a centimetre, which means detecting the
difference between two sensory inputs less than a thousandth of a second
apart.
鈥淭his is one of the questions we shall focus our attention on in the very
near future,鈥� says Cokl. Arthropods are particularly sensitive to time
differences between sensory inputs, points out Flavio Roces of the University of
W眉rzburg in Germany. 鈥淚 think a good candidate as a cue for detecting
directionality would be the difference of arrival time of the vibration in two
different legs,鈥� says the man who discovered the leaf-cutter ant鈥檚 predilection
for vibration. 鈥淪ince a number of back-up or redundant sensory systems are known
in insects, it would not be a surprise that mechanoreceptors on the antenna also
participate in signal catching.鈥�
Another mystery is exactly how female stink bugs make their enticing love
songs. Cokl has seen the bugs moving an abdominal plate attached to the thorax
by a small membrane in time with the rhythms oscillating through plants. 鈥淏ut we
do not know exactly how this signal is produced,鈥� he says.
Work planned for next year should reveal a lot more about the habits of the
southern green stink bug. And what could be more romantic than discovering that
plants the world over are humming to the tune of love?
