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This last attribute of invading exotic species, the fact that they usually have no 

 natural enemies or antagonists vet attacking them in their new home, deserves spe- 

 cial explanation, because one or the solutions exploits this situation. Exotic species 

 that have been accidentally or purposefully introduced into this country, or by es- 

 cape from experimentation, share one important characteristic: they lack the patho- 

 gens, predators, parasites, or competitors that are found naturally associated with 

 them in their place of origin. All organisms have competitors and natural enemies 

 that limit abundance and reproductive capacity. However, when a species is dis- 

 placed from this place of origin and established in a place where it has never oc- 

 curred, these competitors and natural enemies rarely accompany the new establish- 

 ment. Consequently, this species which innately is good at colonizing, establishing, 

 and distributing itself also has the added biological and ecological advantage of 

 being free of living organisms that have evolved with it in its natural community 

 of organisms and which can constrain its reproduction and growth. 



Aspects of species without natural enemies and competitors 



The Good. Our livestock animals, crops, and ornamental plants that were intro- 

 duced from abroad and which we cherish and which do not cause harm have limited 

 numbers of pests attacking them. Their natural pests were left behind and the only 

 pests they have are those that have adapted to them or "adopted" them in their new 

 home. For desirable species, this strategy actually works to our advantage. Citrus, 

 cherries, apples, peanuts, wheat, grain sorghum, horses, cattle, sheep, and many 

 other species are grown in the U.S. away from their natural environment. Many of 

 the severe pests that threaten these species were left in the place of origin. Simi- 

 larly, this happens in other coimtries. For example, sunflower, native to North 

 America, can be grown in Russia with very few pests or diseases. Cassava native 

 to South America, can be grown with few pests or diseases in Africa. This does not 

 mean plants and animals m new places do not have pests because local species can 

 evolve and adapt to exploit the presence of new resources to their reproductive ad- 

 vantage. However, this adaptation usually is a slow process; if it ever happens it 

 usually takes decades or centuries. 



The Bad. Exotic pests of our exotic livestock and crops get introduced into this 

 country at later times. Codling moth was introduced from Asia and now is a serious 

 pest of apples and other fruits in the western U.S. Cherry bark tortrix, recently be- 

 came introduced from Asia and threatens cherry production in the U.S. The brown 

 citrus aphid has made its way to just off the snores of Florida and carries with it 

 severe strains of trestazia virus of citrus. Russian wheat aphid (introduced in 1987) 

 and greenbug (introduced in 1865) both came from the middle east and have done 

 severe damage to wheat. Subsequently, greenbug evolved in a host shift to attack 

 grain sorghum. Our exotic crops and livestock are constantly in danger of reunifica- 

 tion with old pests by accidental introductions. 



Solutions 



Solutions should seek to conserve our biodiversity, to protect agriculture, and to 

 avoid human health problems. The solution for the control of these exotic biota are 

 many faceted and depend upon the nature of the organism and the conditions of 

 their establishment. The solutions involve either eradication or some environ- 

 mentally sound way to suppress the species population or reduce it competitive and 

 exclusionary advantage. 



Eradication: Eradication is a normal and appropriate response when pests are 

 first introduced or if they occur in limited discrete areas after colonization. Verte- 

 brate species usually are easily eradicated if discovered before widespread distribu- 

 tion has occurred. Immediate and well designed eradication efforts have been used 

 effectively to remove newly introduced species such as the Asian gypsy moth from 

 the Pacific Northwest, Mediterranean fruit fly from Florida, and others. An eradi- 

 cation program has successfully removed the screw worm fly from the U.S. and ap- 

 pears to have the potential for removing the pest from all of North America. 



Other eradication programs such as the Mediterranean fruit fly in California, the 

 boll weevil in the U.S., Mexican fruit fly in California and Texas, citrus black fly. 

 North American grasshoppers, gypsy moth, citrus canker have either been ulti- 

 mately judged to be impractical and abandoned as outright failures, or have evolved 

 into extended quasi-regulatory programs fostering heavy pesticide usage on an an- 

 nual basis with no end in sight. 



Population management: Eradication of exotic species is a relatively new approach 

 to addressing this problem. Many exotic species gained entry into this country and 

 became "naturalized" species. Their value often is a matter of judgment, a balance 

 of views from many special interests. Some are beneficial and we nave adjusted to 

 the damaging effects of others. For example, horses are part of Americana, but boll 



