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The Purple Menace
Biological Control of Purple Loosestrife
(Lythrum salicaria)
By Ann Widmer
Meet purple loosestrife…
•Ornamental
•Seeds came in soil used as
ship ballast 19th century
•Medicinal uses
Highly invasive
Highly invasive
Purple Loosestrife infested areas
Map courtesy of The Nature Conservancy
Problems associated with purple
loosestrife invasion
• Forms dense monocultures. Leads to the
reduction of wetland plant diversity.
• Alters decomposition rates and nutrient
cycling
• Reduces pollination and seed output of
related native Lythrum alatum
• Reduces habitat for specialized wetland bird
species
Traditional methods of control
• Spraying herbicides
• Pulling by hand
• Mowing
• Burning
• Water level alteration
Biological Control
Theory behind biological control
• A species introduced outside of its native
range escapes the diseases, parasites, and
predators that have coevolved with it
• These factors had limited the “competitive
vigor” of that species in its area of origin
• Biological control involves reuniting a pest
species with one or more of these
controlling factors or “enemies”
The goal of biological control
To reduce the population to an acceptable level… not
to exterminate the species
The trick is to find an enemy
that is highly host specific, but
can still survive when its target
has been reduced to acceptable
levels
Selecting a biological control
agent
Risks of biological control
• Introduced biocontrol agent attacks native
vegetation
• Biocontrol agent evolves over time to be
less host specific
-- Different from other methods of control
because it cannot be discontinued once the
biocontrol agent is released.
The process of investigation
1. Study the invasive weed – origin, potential
for damage, existing natural enemies, and
possible uses (panel of scientists from
different disciplines)
The process of investigation
2. Search for the weed’s enemies from the
center of its natural range. Select the most
damaging and host specific for further
ecological studies.
The process of investigation
3. Host specificity tests with horticultural,
crop, and native plants related to the weed.
If it does not survive in the absence of its
target, it is considered safe for trial in the
U.S.
The process of investigation
4. Upon federal approval for importation the
candidate agent is put in quarantine and
screened for its own parasites.
The process of investigation
5. Behavior, survival, and reproduction of the
agent is studied in field cages in areas
where the target species is found.
Release the agent
Galerucella calmariensis
4 insect species were approved for
release to control purple loosestrife
in the U.S. and Canada
Galerucella (=Pyrrhalta) calmariensis
Galerucella (=Pyrrhalta) pusilla
Hylobius transversovittatus
Nanophyes marmoratus
Different insects attack different
parts of the plant
Who’s involved in release and
monitoring?
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State agencies
Federal agencies
Universities
Schools
Private citizens
Has biological control been
successful?
Mixed Results
• Are the insects effective at controlling
purple loosestrife?
• Is purple loosestrife replaced by a diverse
native wetland plant community?
• Are the insects host specific?
Are the insects effective at
controlling purple loosestrife?
Insect establishment
– Galerucella calmariensis most successful
– Multiple reports of G. pusilla not establishing
– Hylobius transversovittatus populations
difficult to monitor
– Nanophyes brevis not widely released
Are the insects effective at
controlling purple loosestrife?
Defoliation
– Early release sites show purple loosestrife
biomass reduced by as much as 95%
– Different programs have recorded different
levels of success from 10%-100% defoliation
of plants
Are the insects effective at
controlling purple loosestrife?
Seed production
– Within 5 years, average stem height reduced by
73-85%
– Stunted and malformed shoot tips
– Reductions in length of flower stalk and
number of flower buds
– Even just 10% defoliation shown to reduce seed
production
Is purple loosestrife replaced by a
diverse native wetland plant
community?
• In some areas, native plants are making a
comeback
• In other areas, another invasive species has
expanded where purple loosestrife has
declined
• Purple loosestrife litter may inhibit the
growth of some native plant seedlings
Are the insects host specific?
• The insects cannot complete their lifecycle
without purple loosestrife
• Spill-over effects
Conclusions
Conclusions
• Released biocontrol insects are defoliating
purple loosestrife stands
• Damaged purple loosestrife stands produce
fewer seeds, so fewer purple loosestrife
seeds are in the seedbank.
• No lasting damage has been done to native
vegetation
Biological control, in combination with
traditional methods of weed control, shows the
potential to control purple loosestrife in North
America
Something to think about
The risks of biological control in the case of purple
loosestrife were justified because of...
• the magnitude of damage to wetlands caused by
purple loosestrife
• the ineffectiveness of traditional methods of weed
control
• the overwhelming potential for seed dispersal
• the availability of host-specific insect species.
Something to think about
Is it actually worth the risk?
Where do you draw the line?
What other pests warrant biological control?