News: After more than 100 years apart, webworms devastate New Zealand parsnips.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Thu, 31 Jan 2008 17:56:12 GMT
Public release date: 30-Jan-2008
After more than 100 years apart, webworms devastate New Zealand
parsnips
What could be lower than the lowly parsnip, a root once prized for its
portable starchiness but which was long ago displaced by the more
palatable potato? Perhaps only the parsnip webworm gets less respect.
An age-old enemy of the parsnip, the webworm is one of very few
insects able to overcome the plant?s chemical defenses. The tenacious
parsnip webworm has followed the weedy version of the parsnip in its
transit from its ancestral home in Eurasia to Europe, North America
and ? most recently ? New Zealand.
The long association of the parsnip (Pastinaca sativa) and parsnip
webworm (Depressaria pastinacella) offers a unique window on the
complex interaction of plant and insect enemies, according to a study
appearing this week in the Proceedings of the National Academy of
Sciences. And the recent appearance of parsnip webworms in New
Zealand, more than 100 years after the parsnip first arrived there,
offers the best view yet of how these species influence one another.
The research team, led by University of Illinois entomology professor
and department head May Berenbaum, made two key findings. First, the
researchers found, the New Zealand parsnips had significantly lower
levels of certain chemical defenses than parsnips growing in Europe
and North America, where webworms are a constant threat. Second, the
New Zealand parsnip webworms were dramatically affecting the plant?s
ability to reproduce. The webworm caterpillars eat the parsnip flowers
and burrow into their stalks.
?In certain populations affected by webworms, 75 percent of the plants
were completely devoid of any reproductive parts,? said Art Zangerl, a
senior research scientist in the department of entomology and
co-author on the paper. ?The affected plants were contributing zero
fitness, which is really dramatic. We don?t often see that.?
Fitness is a measure of a species? ability to successfully reproduce.
Environmental factors that reduce the fitness of an organism ? by, for
example, destroying all of its offspring ? can influence the course of
its evolutionary trajectory. Survivors less susceptible to that
environmental factor, or selective agent, enjoy a reproductive
advantage, contributing more offspring, and more of their genetic
attributes, to subsequent generations.
In New Zealand, the newly arrived parsnip webworms are a major
selective agent, Zangerl said, wiping out a majority of the flowering
parsnips.
The altered chemical defenses of New Zealand parsnips are probably
allowing the webworms to feast on most of the plants in any given
locale, Berenbaum said.
The parsnip?s chemical defenses normally include a good dose of
furanocoumarins, a class of organic compounds that can be toxic to
insects that eat the plant. While the parsnip webworm has evolved to
tolerate large doses of furanocoumarins in its diet (it can eat up to
5 percent of its body weight of these toxins) the chemicals do limit
its capacity to inflict damage.
What isn?t clear is whether the absence of parsnip webworms in New
Zealand for more than 100 years allowed the parsnips to let down their
guard, Berenbaum said.
?Parsnips have been in New Zealand since the 19th century,? she said.
Absent an aggressive enemy like the webworm, the parsnip had no reason
to keep producing large amounts of furanocoumarins.
?It could be simply that the parsnips have had 100 years to relax,?
Berenbaum said.
Other factors may explain the lower levels of certain furanocoumarins,
however, she said. It could be that the parsnips that were first
brought to New Zealand had less of these chemicals to begin with. Or
perhaps the soil or climate influenced their evolution.
The appearance of parsnip webworms in New Zealand offers an appealing
research opportunity, Berenbaum said. The researchers will be able to
measure any changes in plant chemistry that result from the webworm
infestation.
?Here, we?re looking at one variable, and it?s the insect,? she said.
?The soil is essentially the same as it was 10 years ago. The climate
is more or less the same.
?The neighboring plants are the same. The only variable is the insect
and we have shown that the insect is a selective agent.?
Berenbaum and Zangerl have spent several decades studying the
co-evolution of the parsnip and its webworm.
In the late 1990s, they studied pre-1900 museum specimens of parsnips
collected in the U.S. They found that the furanocoumarin compound
sphondin ? produced in high levels in parsnips growing in the U.S.
today ? occurred at low levels or not at all in the oldest museum
specimens. Early colonists brought parsnips to the New World in the
early 17th century, but the parsnip webworms did not arrive for
another two centuries. The webworms have difficulty metabolizing
sphondin.
This suggested that the plants ramped up production of sphondin in
response to the webworm infestation, the researchers concluded.
These findings have implications for those hoping to manage invasive
weeds by importing the insects that attack them in their native land,
Berenbaum said. While such strategies may appear to be effective
initially, the plants may be able to adjust to the insect threats over
time by upping their chemical defenses. Only time, and more data
collected in New Zealand, will determine if this occurs, and if it
does, how quickly the plants can respond, she said.
###
M.C. Stanley, of the University of Auckland, New Zealand, was a
co-author on the study.
--
Bob.
.
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