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 investigation3. 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 transversovittatusNanophyes marmoratus

Different insects attack different parts of the plant

Who’s involved in release and monitoring?

• 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?