Even well-intentioned approaches such as biological pest control can have their pitfalls. In the US, European weevils are threatening native species of thistle, and plans to release a flea beetle have been shelved until it is clear how nature will react to the new organism. For decades, biologists have been trying to drive out the devil with the Beelzebub: they control weeds and other pests that plague crops and ruin farmland by exposing non-native organisms, often insects. However, more recent studies show that a weevil introduced to North America, which was supposed to decimate the immigrant musk thistle, also damages relatively harmless thistles of another genus. These results have prompted the researchers to discontinue another field experiment with another alien insect that was also intended to be used to control the musk thistle.
The Musk Thistle came to North America in the mid-19th century. The Eurasian weevil Rhinocyllus conicus was released in 1968 for the first time, further releases followed. The insect larva eats into the thistle heads and feeds on the seeds there. Paul E. Boldt of the US Department of Agriculture Grassland, Soil and Water Research Laboratory in Temple, Texas, estimates that rhinocyllus saves farmers hundreds of millions of dollars in herbicide costs.
But in what Peter B. McEvoy of Oregon State University calls a "dogged" piece of research, Svatana M. Louda and her colleagues showed that the Rhinocyllus larvae also separated from the seeds of the flower heads five feed on native thistles. However, these are comparatively harmless companion plants from the genus Cirsium. In one location, the thistle weevil can reduce the seed production of a Cirsium species by up to 86%. Louda, who published her findings in Science in August 1997, fears that the European weevil could next infest a related and ecologically very similar North American thistle, which is officially listed as an endangered species. The weevil has spread rapidly over the past decade and now appears to be displacing populations of a native thistle seed-feeding insect as well.
Louda's results are the subject of long-standing controversy. In a report released in 1995, the now defunct US Office of Technology Assessment wrote that adverse ecological effects of biological control programs "may not have been noticed" because no one had systematically looked for them. Despite the lack of follow-up studies, different non-native species are often released, one after the other, to combat the same target organism. "There is no theory that would show that this is wise," says Donald R. Strong of the University of California at Davis. The situation is getting serious as the number of applications for biological pest control is increasing rapidly.
Test results from the 1960s show that Rhinocyllus prefers the musk thistle, but "the weevil was known to feed on species other than the target species," says James Nechols of Kansas State University. Boldt adds that researchers are more cautious now than they were 30 years ago in an effort to protect native species from harm. The US State Department of Agriculture (USDA) announces stricter regulations on biological pest control, but has met with resistance from proponents who fear annoying additional regulations.
Last spring, after USDA approval, Boldt began field studies to control musk thistle with a new non-native animal, the flea beetle Psylliodes chalcomera. The dietary breadth of this flea beetle had previously been tested in cages on 55 plant species, including some native Cirsium thistles, Boldt notes. These studies showed that adult flea beetles ate and laid eggs on a Cirsium species, but that the generally more voracious larvae "just nibbled on it a little". Encouraged by these results, Boldt released several hundred beetles in Texas, and Nichols may have inadvertently allowed a few in Kansas to escape when a storm raged over the test cages.
Nichols surmises that this beetle is likely to do less damage than Rhinocyllus to other than the desired thistle. But Strong has doubts about the evaluation procedures that led to the approval of the flea beetle project, as he believes the procedure can be influenced politically.
He notes that the original literature and permit dates do not appear to belong to the same insect. In any case, since the publication of Louda's results, Boldt and Nechols have refrained from further releases of flea beetles until they have more reliable information. Boldt explained that the insect was not tested on rare thistles because seeds for these experiments were difficult to obtain.
Louda's findings will likely be carefully reviewed at the USDA. Efforts are now being made there to find new compromises in regulating the introduction of further pest control organisms. According to Strong, carnivorous insects in particular have it easy. He suspects that some ladybugs introduced to kill other insects may have wiped out other local native ladybug populations. "It's terrifying," notes Strong, "and there's no public dialogue."
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