Who knew that the way to control an invasive species was to import another one, or two or three?

Canadian flag in snowBefore we became deeply worried about Phragmites australis, the feather-topped scrawny-stalked, wet-footed invasive, we were concerned about purple loosestrife. Lythrum salicaria, a pretty but aggressive plant that was masterminding a takeover of our wetlands.

A native to Europe and Asia, purple loosestrife most probably arrived via soil used in the ballast of ships and released in North America in the early 19th Century. Each plant is covered in flowering stems, which can spit out 2.7 million seeds a year that are then easily dispersed by water, wind and wildlife. Its dense matted root system shrinks nutrients in wetlands and ditches, degrading habitat for wetland critters and pushing out native plants. Farmland and canals were choking on the stuff, when people were happily carting these plants home for installation in their gardens. Purple loosestrife had swept across Canada and the U.S.

And then suddenly, it seemingly vanished from people’s consciousness and the landscape. What happened?

Purple Loosestrife became one of the most successful case studies of biocontrol, the introduction of more invasives to chill out the target invasive.

400px-LythrumSalicariaBigBiocontrol may raise the hackles of the we-shouldn’t-play-God faction but consider the alternatives. Pesticides and herbicides provides only temporary, geographically limited solutions and they come with a host of side effects like poisonings, cancer-causing agents, other health impacts, environmental impacts, and encouraging the genetic resistance to them in the pest species. And chemical solutions are expensive, and keep on being expensive.

In contrast, natural enemies can be self-sustaining and self-dispersing agents. They don’t require repeated applications and they hold out the hope that they will adjust their population size to that of their target species, and moreover they are unlikely to cause cancer. Once introduced, they do their job and that’s the end of it.

Still, biocontrol has been the subject of considerable controversy. Biologists have been getting much better at it, as the rules and regs for its application have become increasingly restrictive and uniform when once they were chaotic, subject to change and lacking any clearly understood national or bi-national models.

Asian Carp, introduced to deal with algae in fish farms, is an example of biocontrol gone badly, badly wrong.

Invasives typically arrive without their co-evolved predators, parasites and pathogens or in the case of weeds, herbivores. Without these checks, populations can spread rapidly.  Biocontrol requires locating the natural enemies of these target pests, evaluating the likely results of importing them, confining them for a period in quarantine and testing whether they would be likely to harm non-target species.

None of this is easy. Evaluating the ecological impacts of introducing a species requires a great deal of data that has to be garnered from the host country over many years. An understanding of the life history, dispersal, phylogeny and behavioural effects are necessary. The difficulties in getting this information can warp the data such that a disproportionate amount of info can be  learned about the particular predator/target species relative to the broader category of predator/non-target species.

Moreover, quarantine conditions can hardly replicate the complexity of interactions in the natural environment. Confining critters to petri dishes or cages obviously changes their behaviour. Under these conditions, predators have been known to go after sub-optimal prey (a form of stress eating perhaps), which can exaggerate the impact an introduced predator species might have in the wild, creating false positives.

Potential consequences for non-target species are difficult to evaluate, population dynamic data difficult to come by. The closer taxonomically the non-target species is to the target species, the greater the risk of unintended consequences. And even comprehensive studies are unlikely to identify how or whether the predator species will undergo evolutionary or adaptation changes as a result of being introduced to a new environment.  Unfortunately,  looking for trouble after the introduction of a biocontrol program can be extremely difficult. Throw in the unknown impacts of climate change, and getting a successful program looks doubtful.

How extraordinary then is the success of the biocontrol program for purple loosestrife. Research began in 1995, and by 1992, the Canadian and U.S. governments had approved the release of two European leaf-eating beetles, Galerucella calmariensis and Galerucella pusilla plus a root feeding weevil, Hylobius tranversovitattus.

Following a period of inactivity in the fall and winter, the leaf-eating bugs feed on the leaves as they emerge in the spring. They mate and lay their eggs on the leaves. After hatching, the larvae move on to eating the tips, which prevents them from growing or producing flowers.

The root-feeding weevil either survive the winter as dormant adults in the leaf litter or as larvae in the roots of the purple loosestrife. Overwintering adults feed on leaf and stem tissue. Eggs are deposited in the stem just above ground level at the rate of one or two a day. A single female will live two or three years and lay about 300 eggs.  Overwintering larvae begin to feed as the soil warms up. They then complete their development, pupate and emerge as adults in mid to late summer although some larvae may take more than one full summer to complete their development inside the root.

The purple loosestrife program has been deemed a remarkable success.  Vast stands have been wiped out without the introduction of chemicals or unintended impacts on non-target species.

Unfortunately phragmites doesn’t lend itself as easily to biocontrol, being too similar to native species, that would run the risk of becoming the prey of a predator species that was introduced. Phragmites should not be composted, root fragments (rhizomes) and seeds should be disposed of in the garbage, and stands of it should be avoided so as to reduce the risk of spreading seeds. This is a no-fun plant.

Separating critters into native and invasive categories implies a before and an after, which in turn leads us to expect that if only we could pluck out the invasives, we will find the world the way it was. But nature is always dynamic and disturbances are ongoing and inevitable.

In the old days, biocontrol was primarily directed at ridding agricultural or other “useful” land of pests. Now we have moved on to wanting to save the natural environment from introduced species. This is much more difficult to do, and layers on more morally taxing issues about how much meddling we’re prepared to do.  Fortunately, biologists know much more about the complexity of the natural world than they used to.  This is important work and we should provide  them with the means to do it.

Biological Control of Purple Loosestrife Program, Illinois Natural History Survey http://wwx.inhs.illinois.edu/research/loosestrife/

Biological Control of Purple Loosestrife, Minnesota Department of Natural Resources http://www.dnr.state.mn.us/invasives/aquaticplants/purpleloosestrife/biocontrol.html

Biological Control of Purple Loosestrife, Richard A.Malecki, Bernd Blossey, Stephen D Hight etc. http://www.dnr.state.mn.us/invasives/aquaticplants/purpleloosestrife/biocontrol.html

Invasive Phragmites, Ontario Invading Species Awareness Program http://www.invadingspecies.com/invaders/plants-terrestrial/invasive-phragmites/

2 thoughts on “Who knew that the way to control an invasive species was to import another one, or two or three?

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