When the ditches are full of purple loosestrife, in bloom during July and August, it is a treat to drive along the highways. It is happily a common sight since the seeds are carried far and wide by the ditch waters. –Katherine McKenzie, Wildflowers of the Midwest
What makes loosestrife such a successful competitor? Two factors are mainly responsible according to a 1987 publication of the U.S. Fish and Wildlife Service, Spread, Impact and Control of Purple Loosestrife in North American Wetlands.
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First, purple loosestrife has no natural predators here, having escaped the insects that feed on it when it emigrated from its homelands in Europe and Asia. The tiny seed may have stuck to the hooves of pigs and cattle, the wool of sheep, or the hay used to feed these animals onboard; seed may have contaminated the rock and sand loaded aboard ships as ballast, then dumped on New World shores to make room for lumber, cotton, and tobacco. Loosestrife found freedom here, and was even cultivated as a medicinal plant.
The threat to diversity is obvious when an invasive alien plant, or a shopping mall, extinguishes some tiny, threatened forb. But it’s also true that every time some event reduces the population of an indigenous plant or animal, it shrinks that species’ ability to respond to change: to survive. All organisms are bequeathed a genotype richness, a consequence of their own unique evolution, that’s not always expressed in any one individual but held in reserve. The ability to respond to stresses is limited by evolutionary potential, and when populations are reduced, the genetic horizons are narrowed. For species survival Noah’s ark strategies just won’t work.
The favored strategy for controlling purple loosestrife has focused on reducing disturbance stresses on wetlands, which allow loosestrife seedlings to spring up from dormant seed stocks, and responding quickly to new colonizations. But the vegetative management guidelines put out by the Illinois Nature Preserves Commission warn: “The chances of success are best with the smallest infestations. Early diagnosis is critical.” They advise that the habitat be scoured for the first signs of invasion, and that individual plants be pulled out with the roots intact and removed from the site in plastic bags so that seeds aren’t dropped.
They are waiting still. Although the beetles left signs of foraging activity, they’ve disappeared. With good luck, that’s because they’ve survived but gone into summer dormancy and incubation.
