Q&A: BIOLOGICAL CONTROLS for Purple Loosestrife
(Lythrum salicaria )
Why worry about the control of purple loosestrife?
Purple loosestrife, a wetland plant that is not native to North America,
is very aggressive, and if given a favorable site, will dominate
that site within a few years, squeezing out native vegetation. There are
very few native species which find loosestrife useful for forage or habitat.
In Europe, where purple loosestrife is native, there are more than 120
insect predators of the plant. Few exist in North America, and none achieve
control of the plant.
What kinds of control methods have been tried?
Cutting doesn't work; burning doesn't work. Digging large plants is
difficult, but useful for small infestations. Flooding the areas where
loosestrife is prevalent, to a depth of at least 16 inches for a period
of at least seven weeks does affect theviability of loosestrife; however
this treatment also negatively affects other species of plants. Careful
removal of the flowering stalks before seed dispersal, and disposal in
black plastic for refuse removal may be a temporary control for individual
plants. Chemical control can be partially achieved using Rodeo (glyphosate)
in wetlands, but spraying is expensive, requires a licensed applicator
and permit process, and can harm native species. Chemical sprays will not
reach the seeds which may germinate, so repeated chemical applications
are necessary. Chemical herbicides have potential negative effects
on other species (including humans). In short, these methods are too costly,
not species specific and don't work long-term.
Are there biological methods to help control purple loosestrife?
Since 1992, researchers have been rearing and and releasing in a few
states two European beetles, Galerucella pusilla and Galerucella
calmariensis , which eat only purple loosestrife plants. Trained biologists
have released them in all Canadian provinces and twenty-seven other states;
Vermont and New Hampshire began in 1997. The European root-mining weevil,
transversovittatus, is also being reared for release, but it has limited
availability. Two weevils which feed only on flowers and seeds of L.
salicaria have been approved, but are not yet commonly released. Assuming
that biological controls survive, it is expected that it will take five
to seven years to achieve control of purple loosestrife.
Are there any success stories for the bio-control of introduced species?
A few early efforts at bio-control had disastrous results, such as
the weasels introduced to control rabbits in New Zealand, or the mongoose
released on Guam to control rats. These bio-disasters have introduced considerable
caution into the scientific community, and a healthy skepticism on the
part of the public. Some successes in biological control in the weed world
are alligator weed in Florida and southern US, Klamath weed control in
Northern California, tansy ragwort control in Oregon, and leafy spurge
control in the great plains where hundreds of thousands of acres
have been returned to productive rangeland and pasture. There is hope for
biological control for other invasive problem plants such as phragmites,
saltcedar and garlic mustard. While there have been some careless bio-control
programs (see next question), the program designed for purple loosestrife
is considered a "model of a well-conceived, meticulous bio-control endeavor"
(DeVine, 1998, page 240)
Is Rhinocyllus Conicus (a weevil) on native thistles a case
of bio-control gone wrong?
Rhinocyllus Conicus was introduced to control musk thistle,
a serious pest plant for farmers. This weevil is known to attack 4 different
thistle genera in Europe (Carduus, Cirsium, Silybum, Onopordum)
and its attack on native thistles (though much slower) in the genus Cirsium
was predicted (Louda,1997). Some claim that at the time of introduction
in the 60's the attitude was "only a dead thistle is a good thistle", whether
native or introduced, and that what went wrong was that societal values
have changed since the 60's. Any such introduction would not be permitted
today, according to David Ragsdale, (Professor of Entomology,University
of Minnesota) since the screening process, a protocol required by the US
Department of Agriculture's Animal and Plant Health Inspection Service,
APHIS, has been tightened significantly since that time. Though it is not
risk-free, one must make an evaluation based on what a do-nothing policy
or a policy based on broad spectrum herbicide would do to native (or rare)
species sharing the same habitat.
How are insects for biological control selected today?
In the case of purple loosestrife, researchers identified insects which
feed exclusively on the plant. Approximately 15 species of insects were
tested for host specificity against 41 species of North American native
plants and 7 species of agricultural plants. Test plants included 17 members
of the family Lythraceae, including five other species of Lythrum.
They were tested first at the International Institute of Biological Control
in Europe, and then under quarantine conditions in the US. (Blossey 1994;
Kok 1992). The insects were tested on both their feeding preferences and
their ability to survive. Although the insects showed some ability to feed
upon a few native plants, these plants were not preferred if Lythrum
salicaria were nearby. In no case, were the insects able to complete
their life cycle on any of the native plants (Blossy, 1994). According
to an article by Elizabeth McGowan in Brooklyn Botanical Garden's Plant
and Garden News, today bio-controls are chosen primarily for their specificity
for the target plant. The testing process takes at least five years, and
even if given approval the insects may fail to become established.
Are there native controls for the Galerucella beetles?
Yes, spiders will prey on these beetles and ladybugs prey on the beetles'
eggs . Other generalist predators exist.
What will the beetles do when they run out of purple loosestrife?
They won't run out. Biologists no longer expect total eradication of
an introduced, invasive species. The hope is to have purple loosestrife
reach some sort of equilibrium in the wetlands, where it is present but
does not dominate. Experimenters hope for a 90% reduction in abundance
(but not eradication) of purple loosestrife.
Will the beetles threaten other native species?
In our area, there may several native species of Lythrum: Lythrum
alatum (winged loosestrife), L. lineare and a close relative,
verticillatas (swamp loosestrife or waterwillow). All were tested and
rejected by the insects in the field (although some were nibbled at in
the no-choice caged test.) (Blossey, 1994)
Can the beetles evolve, or change their preferences?
According to David Ragsdale, Professor of Entomology,University of
Minnesota insects are "wired differently than a long-lived animal like
a mammal or bird; in the latter, it is an adaptive advantage to broaden
one's food source in case the principal food is scarce or not available."
In the case of insects, whose reproductive lives might be measured in hours
(such as mayflies) or at most a few weeks (like Galerucella), it
is an adaptive advantage to narrowly select a host rather than spending
time feeding on a wide variety of hosts -- indeed, the more hosts, the
more exposed they are to predation and success of eggs deposited on unsuitable
hosts is limited, effectively eliminating from the gene pool those individuals
that engage in this behavior. There is no case where an introduced insect
has either exterminated the target weed, or unexpectedly switched hosts
to become a serious pest of other plants. (Harris, 1988) Although Bernd
Blossey (Director of Biological Control of Non-Indigenous Plant Species
Program, Department of Natural Resources, Cornell University) can not categorically
exclude the possibility, he states that the probability for such a shift
is in the same order of magnitude as the monarch butterfly switching from
milkweeds to a different host plant (personal communication)
What happens after beetles are released?
In all states, the release sites must be approved in advance by the
responsible state agency. These sites must meet certain criteria such as
minimal flooding, minimal human activity, no mowing, permission of property
owner, fairly high density of plants in an easily monitored area. The monitoring
for all release sites must be done annually, about the same time each year.
Bernd Blossey at Cornell has devised a protocol which is widely used for
the detailed observation of a few test sites in each state, which will
be monitored over a period of many years.
What has been the experience with beetles elsewhere?
In Minnesota, there has been a vigorous campaign to rear and release
beetles, with the help of citizen volunteers. This program is now in its
fourth year, and has more than 80 volunteer groups rearing insects, and
releasing them in sites approved by the state, and then annually monitoring
these sites according to a state approved protocol. David Ragsdale reports
that this is a great public relations and education project. Not a single
volunteer group has dropped out in the four year period, and these have
been cooperative and reliable in the reporting required by the state. Since
they are responsible for beetle rearing, site selection and monitoring,
volunteers really take ownership of their release sites. With volunteer
help, more than 400 release sites have been established, many more than
state employees could do on their own, and now they can observe major success
in about a dozen of the sites -- though it is still a bit early in the
campaign to gauge results. Michigan and New York state also have citizen
volunteers involved in rearing beetles. Several sites in New York, Canada,
and Oregon have shown significant reduction in loosestrife coverage. Donna
Ellis (University of Connecticut) reports that of the state's ten release
locations since 1996, one release site at Storrs had significant reduction
in plant flowering by 1998. In 2001, both New Hampshire and Vermont
plan to have citizens involved in rearing beetles and monitoring bio-control
Shouldn't we just let nature take its course?
You must provide the answer to this question.
Compiled by Barbara McIlroy, January 2000. The above information has
been gathered from e-mail with Bernd Blossey (at Cornell), David Ragsdale
(U of Minnesota) and Donna Ellis (U of Connecticut), as well as some of
the references cited on this website's reference