Biological invasions are slow-motion disasters. They start small: a few insects hitchhiking on some imported nursery stock, a few seeds escaping from somebody’s backyard and taking root in a ditch, a field, or a woodlot. The spread is slow at first. Small populations, even if they are very fecund, can produce only small numbers of seeds or young. Sometimes the invading species spends decades as a small, inoffensive part of the biota before suddenly becoming a major pest. It could be that the shift to pest status is the consequence of a genetic mutation that alters the behavior or ecological tolerances of a species, enabling it to push aside competitors and spread quickly onto new ground.
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The seeds of L. salicaria can be carried by water, by wind, or in mud clinging to the feet of waterfowl. Purple loosestrife stems that break off from a parent plant can take root and grow into vigorous new individuals. Once established in a marsh, purple loosestrife grows so densely that it is difficult for large birds such as ducks to move through a stand.
The native vegetation retreats before the invader. The cattails whose roots support muskrats, the smartweeds whose seeds feed rails, all the bulrushes, arrowheads, cordgrass, wild rice, and other plants of a healthy wetland die out. All the leaf beetles and weevils and butterflies and leafhoppers that depend on those plants disappear too, leaving less food for marsh wrens and leopard frogs. The yellow-headed blackbirds find that loosestrife stems are not strong enough to support their nests, so they leave.
A preliminary survey of European purple loosestrife populations turned up about 100 species of insect that could feed on it. Of these, about a dozen were found only on purple loosestrife plants. Five of these, all beetles, were regarded as sufficiently specialized to undergo further testing. One insect was a weevil whose larva lives in–and eats–the roots of loosestrife. Two were closely related leaf-eating beetles, and the other two were extremely tiny weevils that live in the flowering heads of the plants.
There was no evidence of overwintering by those first beetles, so Schwegman approached the Illinois Natural History Survey for help. The survey has the facilities needed to grow large numbers of Galerucella beetles, and large numbers would obviously be needed. In 1995 beetles from the survey were released at Powderhorn. They were placed on purple loosestrife plants, which were then covered for a week with netting to keep the insects on the plants. Again no beetles lived through the winter.
