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Garlic and mustard are common
ingredients that can be found in American households.
But garlic mustard? Well, that’s a different story.
Garlic mustard, Alliaria petiolata,
is considered one of the most problematic invaders of
temperate forests in North America. According to legend,
it was brought here from Europe in the 1860s as a
culinary herb, but unfortunately, it doesn’t taste very
good. Since then, garlic mustard has spread to 34 U.S.
states and 4 Canadian provinces.
“Garlic mustard is an invasive plant
that gets a lot of attention,” says ecologist Adam
Davis, who has been studying the weed for years. “It’s
very noticeable and hard to eradicate because of its
seed bank.”
The term “seed bank” refers to seeds in
the soil that are dormant but capable of germinating.
Garlic mustard seeds can remain viable for more than 10
years. A single plant can produce hundreds of seeds,
which scatter as far as several meters from the parent.
“You can spend a lot of time and money
pulling garlic mustard up or spraying it with
pesticides, but it’ll just come back the next year,”
says Davis. “That’s why it’s such a problem. It’s very
resilient.”
Flowers of garlic mustard
produce up to several thousand seeds per plant,
making it difficult to control. |
A member of the mustard family
Brassicaceae, garlic mustard got its name because its
leaves, when crushed, smell like garlic. Garlic mustard
is a biennial plant, meaning it takes 2 years to
complete its life cycle. During its first year, the
plant is in the form of a rosette with kidney-shaped
leaves that remain green throughout the winter. In its
second year, the plant matures and produces small, white
flowers, each with four petals in the shape of a cross.
The mature plants either self-pollinate or are
pollinated by insects, producing seeds that fall to the
ground and enter the soil.
Garlic mustard is cold hardy and shade
tolerant, enabling it to grow early in spring when most
plants are not able to grow. It also secretes
allelochemicals into the soil. Allelochemicals are
chemical compounds a plant introduces into the growing
environment to suppress growth of another plant. “It’s
kind of like chemical warfare against the native
plants,” says Davis.
The insects and fungi that feed on
garlic mustard in its native habitat are not present in
North America, increasing the weed’s seed productivity
and allowing it to outcompete native plants.
In Urbana, Illinois, ecologist
Adam Davis records the position of garlic
mustard rosettes (clusters of green leaves low
to the ground) on a sheet of transparent mylar.
The locations are recorded in June and October
of the first year and in June the following year
to estimate rosette survival rates. Once garlic
mustard rosette locations are recorded, they are
then converted into digital coordinates in a GIS
(geographical information system) program,
permitting spatial analysis of rosette survival.
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A Model Solution
To better understand garlic mustard and
find a suitable biocontrol, Davis—in collaboration with
colleagues at Michigan State University, Cornell
University, the University of Illinois, and the Centre
for Agricultural Biosciences International (CABI) in
Switzerland—created a computer model that simulates the
weed’s life cycle.
“In part, we wanted to answer
ecologists’ criticisms that biocontrol can potentially
cause as many problems as it solves because of
unintended consequences,” says Davis. “We were looking
for a way to choose agents that are most likely to
succeed while reducing their potential for harm to
native plants and environments. Ideally, we want to try
to release only one organism, if possible.”
Through this model, Davis was able to
predict the type and severity of damage that would be
needed to reduce garlic mustard’s population growth
rates. Davis performed an analysis using computer code
that enabled him to change one variable at a time while
keeping all the others constant, allowing him to probe
the life cycle for the plant’s weak point. He found that
in order to make an impact, a biocontrol agent has to
reduce garlic mustard’s survival in the rosette stage
and its ability to reproduce in the adult stage.
Well before Davis created the life-cycle
model, CABI scientists began looking for and testing
potential biocontrol agents to tackle garlic mustard.
They collected data on the amount of damage each insect
could inflict on the garlic mustard population. From a
list of more than 70 natural enemies found to be feeding
on garlic mustard in Europe, four Ceutorhynchus
weevils were selected as the most promising control
agents.
Combining the feeding information
collected by CABI scientists and the demographic
information of garlic mustard in North America, Davis
used the computerized life-cycle model to assess each
weevil’s ability to inflict damage on the weed and
inhibit its growth. One weevil, C. scrobicollis,
came out on top.
High Hopes for Little Insect
The tiny C. scrobicollis has a
life cycle of 1 year and produces one batch of offspring
per lifetime. It lays its eggs on garlic mustard’s leaf
stems in the fall. When the eggs hatch in the spring,
the larvae feed on the weed’s root crown, the area from
which the rosette’s leaves grow and where nutrients are
stored.
By feeding on the root crown, C.
scrobicollis stops the flow of nutrients and water
from the roots to the rest of the plant. It also damages
the meristem, the area of the plant where new growth
takes place. As a result, garlic mustard produces fewer
seeds or, in areas with high weevil populations, dies
prematurely in early spring without producing any seeds.
C. scrobicollis also appears to
be monophagous, meaning it eats just one thing: garlic
mustard. That means scientists won’t have to worry about
any unintended consequences when using this insect as a
biocontrol agent.
During preliminary testing, CABI
scientists believed C. scrobicollis was the
best candidate to control garlic mustard. Putting the
weevil’s feeding data through Davis’s life-cycle model
confirmed their beliefs and created a stronger case for
the permit process.
“The model gave teeth to the permit
application to release this weevil in the United
States,” says Davis. “It provided a peek into the future
as to the impact the weevil could have on the garlic
mustard population here.”
C. scrobicollis is currently in
quarantine at the University of Minnesota. If all goes
well, this beneficial weevil may soon be roaming North
America to find a nice garlic mustard meal. |
