There's a dark side to biological weed control -- as the attack on non-target plant species by the Eurasian weevil in North America shows
eliminating weeds using conventional techniques is difficult. But alternative techniques like biological weed control may prove costlier. In 1968, Canadian and us state agencies released the Eurasian weevil (Rhinocyllus conicus) in the countryside to control some introduced thistle species. But the results were more than they had bargained for. Scientists recently discovered that the weevil had caused extensive damage to native thistle species as well. The findings place a big question mark on the clean-kill image of biological weed control, according to S M Louda and fellow researchers at the University of Nebraska's School of Biological Sciences in the us (Science, Vol 277, No 5330)
While the adult beetle feeds on grains, nuts and fruits, the larvae feed on immature seeds, including those of the thistle. According to estimates, there was an 86 per cent reduction of seeds in Platte thistle (Cirsium canescens) in Sandhill Prairie Preserve of Nebraska, and 72 per cent in wavy leaf thistle (Cirsium undulatum) in Mesa Verde National Park, Colorado. Scientists in the us are worried that the weevil may attack endangered thistle species.
A biological weedicide works by interfering with one or more stages in the life-cycle of a weed. Theory says that the population of a species increases as long as it has sufficient food and can produce viable offspring that can adapt to environmental conditions. But this simple food chain model of biological control ignores the fact that there may be other species involved directly or indirectly in the food web.
Competition between native and introduced species may disrupt the balance. Louda and colleagues found that the number of native picture-winged flies had declined sharply in areas where the weevil had attacked the native thistle. The flies also feed on thistle flower heads of Platte and wavy leaf thistle, which meant that these plants had been further affected by competition between the two insect species. The native flies lost out in the competition. Even though the weevil may not have had a greater preference for the native thistle over the alien, its burgeoning populations spilled over and attacked the native thistle. This combined attack on the native plants may spell their doom, say researchers.
Most investigations of the use of biological control today are concentrating on the ecological aspects. The most crucial area is understanding food chain interconnections, including introduction of alien species such as weeds, as the study showed. Biological control depends on short food chains -- which do not branch out and involve other organisms. But introduced species become involved in more complex natural food webs. Though biological agents to control weeds and pests may be safer for humans than chemicals, the other changes they bring about in the ecosystem may prove costlier in the long run.
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