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Klamath weed (aka St. John's wort)

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964398.  Sat Jan 19, 2013 10:15 am Reply with quote

Extract from Silent Spring (1962) by Rachel Carson, pg 67-69

An outstanding example in the field of controlling unwanted plants is the handling of the Klamath-weed problem in California. Although the Klamath weed, or goatweed, is a native of Europe (where it is called St. John's wort), it accompanied man in his westward migrations, first appearing in the United States in 1793 near Lancaster, Pennsylvania. By 1900 it had reached California in the vicinity of the Klamath River, hence the name locally given to it. By 1929 it had occupied about 100,000 acres or rangeland, and by 1952 it had invaded some two and a half million acres.

Klamath weed, quite unlike some native plants as sagebrush, has no place in the ecology of the region, and no animals or other plants require its presence. On the contrary, wherever it appeared livestock became 'scabby, sore-mouthed, and un-thrifty' from feeding on this toxic plant. Land values declined accordingly, for the Klamath weed was considered to hold the first mortgage.

In Europe the Klamath weed, or St. John's wort, has never become a problem because along with the plant there have developed various species of insects; these feed on it so extensively that its abundance is severely limited. In particular, two species of beetles in Southern France, pea-sized and of metallic colour, have their whole beings so adapted to the presence of the weed that they feed and reproduce only upon it.

It was an event of historic importance when the first shipments of these beetles were brought to the United States in 1944, for this was the first attempt in North America to control a plant with a plant-eating insect. By 1948 both species had become so well established that no further importations were needed. Their colonies and redistributing them at the rate of millions a year. Within small areas the beetles accomplish their own dispersion, moving on as soon as the Klamath weed dies out and locating new stands with great precision. And as the beetles thin out the weed, desirable range plants that have been crowded out are able to return.

A ten-year survey completed in 1959 showed that control of the Klamath weed had been 'more effective than hoped for even by enthusiasts', with the weed reduced to a mere 1 per cent of its former abundance. This token infestation is harmless and is actually needed to maintain a population of beetles as protection against a future increase in the weed.

Another extraordinarily successful and economic example of weed control may be found in Australia. With the colonists' usual taste for carrying plants or animals into a new country, a Captain Arthur Phillip had brought various species of cactus into Australia about 1787, intending to use them in culturing cochineal insects for dye. Some of the cacti or prickly pears escaped from his gardens and by 1925 about twenty species could be found growing wild. Having no natural controls in this new territory, they spread prodigiously, eventually occupying about sixty million acres. At least half of this land was so densely covered as to be useless.

In 1920 Australian entomologists were sent to North and South America to study insect enemies of the prickly pears in their native habitat. After trials of several species, three billion eggs of an Argentine moth were released in Australia in 1930. Seven years later the last dense growth of the prickly pear had been destroyed and the once uninhabitable areas reopened to settlement and grazing. The whole operation had cost less then a penny per acre. In contrast, the unsatisfactory attempts at chemical control in earlier years had cost about 10 pounds per acre.

Both of these examples suggest that extremely effective control of many kinds of unwanted vegetation might be achieved by paying more attention to the role of plant-eating insects. The science of range management has largely ignored this possibility, although these insects are perhaps the most selective of all grazers and their highly restricted diets could easily be turned to man's advantage.


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