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10/18/2006
Predators
In past columns, I’ve mentioned seeing a praying mantis in an
elevated parking lot at our local mall and then a picture in
National Geographic of a mantis chewing on a hummingbird.
More recently, just off the green on the first hole of our
municipal golf course, I saw my second praying mantis of the
year. I’ve never seen two mantises in one year. I couldn’t tell
whether this one was praying, preying or applauding my putt for
a par on that hole. A couple of days later, I found that Brian
Trumbore had marked for my attention a very brief item on the
praying mantis in the October Smithsonian magazine.
The female mantis is not a very sociable critter, often
cannibalizing a male mantis with which it has mated. The
Smithsonian item referred to the work of Jonathan Lelito and
William Brown at the State University of New York (SUNY) at
Fredonia. In the August issue of The American Naturalist, they
published the results of a study aimed at deciding whether the
male mantis accepts this fate as the price of sexual dalliance or
does he try to avoid his demise? Were I a male mantis, I
certainly would opt for the latter!
The hungrier the female, the more likely she is to bite off the
male’s head. The SUNY researchers found that a male mantis
can tell when a female is hungry. When a male approaches a
hungry female, it approaches more cautiously, takes longer to
court the gal and mounts her from a greater distance than if she
were well fed. After mating, the male hangs on as much as three
times longer than with a satiated female. This apparently gives
the male time to await his chance to quickly dismount and get
away with head intact. The males are not suicidal!
Eating pesky insects, hummingbirds and even each other, the
mantis is a predator to be reckoned with. When it comes to
human predators, there’s the FBI’s Ten Most Wanted List. But
did you know about the U.S. Department of Agriculture’s Top
Ten Weeds List? At least one these top ten weeds is a true
predator. This is Cuscuta pentagona, more commonly known as
dodder. A relative of the morning glory, dodder is also known as
strangleweed or witches’ shoelaces, indicative of its fearsome
nature. Dodder is a parasitic vine that overwhelms its plant host
in a mass of swirling yellowish tendrils or stems that somewhat
resembles a cobweb of sturdy threads. There was a good picture
of the dodder in action in an article by Sarah Everts in the
October 2 issue of Chemical and Engineering News (C&EN).
The C&EN article alerted me to what may be a landmark paper
by Justin Runyon, Mark Mescher and Consuelo DeMoraes at
Pennsylvania State University in the September 29 issue of
Science. The Penn State researchers have shown that dodder
somehow “sniffs” volatile chemical compounds elicited by its
favorite tomato and other plants. Like a bloodhound, the dodder
follows the scent. As a parasite, the dodder has to quickly find
and settle on a host in order to gain sustenance.
When a dodder seedling sprouts, there’s only enough energy
stored in the seed for the dodder to grow and travel a few
centimeters. It doesn’t have any leaves and isn’t good at
photosynthesis like most familiar plants. It can’t grow by
utilizing sunlight, CO2 and water. It grows by attaching itself to
a plant and sucking out stuff from inside the plant. When the
dodder seed sprouts, the tendril heads for a host plant and wraps
itself around the plant. It looks rather like a growing strand of
spaghetti heading out from the seed and wrapping itself around
the plant stem. Once the dodder is attached to a host plant, it
picks up the nutrients it needs to keep growing and entwining
itself haphazardly around the rest of the plant.
Dodder has to find a host plant to survive, but how does it locate
the plant? At Penn State, the researchers placed a dodder
seedling in a vial of water at the center of a disc of filter paper.
As they put it, they then allowed the seedling to “forage” for 4
days, tracing its circuitous growth on the filter paper and marking
the spot where the growing tip ended up. They repeated this 30
times each under three different sets of conditions: (1) a pot of
moist soil was placed near an edge of the paper disc; (2) a 20-day
old tomato plant was substituted for the moist soil; (3) a mix of
volatile chemicals found in tomato plants was released from a
rubber septum near an edge of the disc.
The filter paper discs were marked into four equal sections or
quadrants; one quadrant centered on the pot of moist soil, potted
tomato plant or the volatile chemicals emitted from the septum.
If the seedlings grew randomly, out of 30 seedlings you’d expect
a quarter of them, roughly 7 or 8, to end up in each quadrant.
With the tomato plant, 15 ended up in the quadrant closest to the
tomato plant. With the septum releasing tomato plant volatile
compounds, 17 headed for the septum. With the moist soil, only
6 were in the quadrant closest to the soil. Bottom line – half or
more of the seedlings grew towards the tomato plant or its smelly
equivalent while only a fifth of the seedlings grew towards the
moist soil. Does dodder have a nose?
The researchers also ran other experiments with 10-day old
tomato plants (17), vials of red or green colored water (5, 5) and
an artificial tomato plant (5) as the targets. The numbers in
parentheses are the number of seedlings out of 30 ending up in
the closest quadrant to the target. Again, dodder favored a real
tomato plant over the faux targets. Lest there be any possible
visual clues that the clever dodder might use, the Penn State
workers ran an experiment in which the odor from a bunch of
tomato plants was transmitted through a plastic pipe with a 90-
degree angle bend in it. The dodder seedlings still headed
preferentially towards the end of the pipe, following the tomato
plant odor.
Dodder prefers tomato to wheat plants, given a choice. The
researchers ran numerous experiments of the same type using
individual volatile compounds found in tomato plants and in
wheat. They found that tomato and wheat plants shared at least
one volatile compound. However, they also found that wheat
had one volatile that appeared to have a repellant effect on the
dodder seedlings. More of the seedlings ended up in the
quadrant opposite to the quadrant centered on the septum. This
finding raises the possibility of controlling the dodder if a
repellant volatile compound can be used to fend off dodder
attacks on farmers’ crops. Perhaps other weeds on the Top Ten
Weed List could also be controlled in this manner.
How does the dodder sense the attractive volatile compounds and
know how to travel in the right direction? The whole field of
plant communication is ripe for innovative research to figure out
the plant’s equivalent of a nose. I believe that I’ve mentioned in
a past column research that has shown, for example, that if
certain plants are attacked by a pest, the attacked plant may emit
some sort of chemical signal that’s picked up by another plant.
The latter plant then starts producing a chemical that tends to
repel the pest or the plant takes some action that makes it less
attractive to the pest. This may be in the form of attracting
another insect that is a sworn enemy of the pest.
Maybe a tomato plant will learn about the wheat volatile that
repels dodder and start making the compound on its own.
Allen F. Bortrum
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