Introduced Tree Diseases and Insects 



447 



control work, conducted in coopera- 

 tion with the States, is to prevent the 

 westward and southward spread of the 

 insect. The discovery of the extreme 

 toxicity of DDT to the gypsy moth and 

 the development of airplane spraying 

 of forested areas have furnished effec- 

 tive means of control to aid in the 

 program. Timely applications of DDT 

 by airplane will kill the gypsy moth, 

 and prevent defoliation, subsequent 

 growth retardation, and possible death 

 of trees. 



CHESTNUT BLIGHT has caused the 

 complete destruction of our commer- 

 cial chestnut from Canada to the Gulf 

 States. This record is not approached 

 by that of any other disease or insect. 

 First reported in New York City in 

 1904, the disease spread rapidly. 



For many years roots of killed trees 

 continue to send up sprouts, but these 

 sprouts are usually killed before they 

 are more than a few inches in diameter. 

 Unfortunately, search for 40 years has 

 not resulted in the discovery of a single 

 American chestnut tree with sufficient 

 resistance to be of practical value. 

 Blight has reduced millions of acres of 

 forest land to a lower productive status 

 for an indefinite period, because the 

 native tree species replacing the chest- 

 nut are usually less valuable. It also 

 has deprived us of cherished tasty nuts 

 and has taken from wildlife a food. 



Experimental plantings with blight- 

 resistant Asiatic chestnuts and with 

 hybrids of these and the American 

 chestnut indicate that on suitable sites 

 they will produce small telephone poles 

 and abundant sweet nuts. Most of these 

 resistant selections, however, are less 

 straight-stemmed, less frost-resistant, 

 and more particular in their soil re- 

 quirements than the American chest- 

 nut. Some State forestry and game 

 departments are beginning to grow re- 

 sistant Chinese chestnuts for planting 

 in farm wood lots. 



Chestnut blight was found in com- 

 mercial orchards and in ornamental 

 chestnut plantings of the Pacific coast. 

 Prompt eradication measures by State 



and Federal agencies almost com- 

 pletely eliminated the disease. The sus- 

 ceptible orchards of the West, however, 

 are not safe, because of the danger of 

 shipment of infected chestnut trees 

 from the East. 



Chestnut blight illustrates how an 

 introduced pest can upset a phase of 

 the national economy. The American 

 chestnut has been the main source of 

 our domestic tannin used in the manu- 

 facture of leather, and dead trees still 

 are extensively used. Tannin, a stra- 

 tegic material especially vital in time 

 of war, is extracted from the chipped- 

 up chestnut wood. The chips are then 

 used for paper or board pulp. This ex- 

 tensive industry, at present supplying 

 most of our domestic tannin, faces its 

 end when the supply of dead trees gives 

 out. 



The chestnut blight fungus is also 

 seriously damaging the post oak, a 

 widely distributed tree in the eastern 

 half of the country with a forest stand 

 of about 5 billion board feet. It kills 

 some trees rather slowly but has not 

 damaged others that have been ex- 

 posed for long periods. So far no other 

 kind of oak has been seriously damaged 

 by the chestnut blight fungus. 



THE SMALLER EUROPEAN ELM BARK 

 BEETLE is an example of an introduced 

 insect that was of little importance 

 until it became associated with the in- 

 troduced so-called Dutch elm disease 

 fungus. That insect is known to have 

 been established near Boston as early 

 as 1904. It did little damage and was 

 not considered a primary pest for about 

 a quarter of a century. About 1930, 

 when the Dutch elm disease fungus 

 reached this country, the importance of 

 the European elm bark beetle changed ; 

 it proved to be a carrier and trans- 

 mitter of the fungus. The relationship 

 worked to the advantage of the bark 

 beetle. American elms inoculated by 

 contaminated beetles develop disease 

 symptoms, are partially or completely 

 killed by the disease, and provide suit- 

 able breeding material on which in- 

 creasing populations of beetles develop. 



