452 



(Eichh.), and the smaller European 

 elm bark beetle, Scolytus multistriatus 

 (Marsh.). The latter is the more im- 

 portant. The beetles, widespread in the 

 eastern half of the United States, are 

 present in many places where the Dutch 

 elm disease is not yet known to occur 

 an ominous warning. 



The adult beetles feed in parts of 

 living elm trees, but they breed only in 

 recently cut, dead, or dying elms. Liv- 

 ing elm trees are seldom injured by 

 only the feeding of the adult, but when 

 the beetles are contaminated with the 

 disease organism they become of eco- 

 nomic importance. When the Dutch 

 elm disease fungus occurs in elm ma- 

 terial in which these insects breed, the 

 fungus may stick to the beetles and be 

 carried to healthy elms or other breed- 

 ing material. 



The adults of the smaller European 

 elm bark beetle emerge from infested 

 wood and fly to nearby living elm trees, 

 where they feed in the smaller twig 

 crotches. The adults of the native elm 

 bark beetle hibernate in the outer bark 

 of living elm trees. In the spring they 

 bore into the bark and feed on it. When 

 the feeding injuries penetrate through 

 the bark to the wood, the disease organ- 

 ism may be introduced into the vascu- 

 lar system of healthy elm trees. Beetles 

 of both species may fly several miles 

 in search of suitable breeding places, 

 and thus may transport the disease or- 

 ganism from one locality to another. 



All the elms commonly planted as 

 shade trees are susceptible to Dutch 

 elm disease. The degree of suscepti- 

 bility varies both within and between 

 species, however. The American elm, 

 which predominates in many shade- 

 tree plantings, is among the more sus- 

 ceptible species. The Chinese elm 

 (Ulmus parvifolia) and the Siberian 

 elm (Ulmus pumila) resist Dutch elm 

 disease, but they have undesirable 

 characteristics that limit their use in 

 shade-tree plantings. 



The variation in susceptibility of dif- 

 ferent species and varieties has stimu- 

 lated attempts in Europe and the 

 United States to breed and select su- 



Yearboo\ of Agriculture 1949 



perior types of elms that are resistant 

 to the fungus. Scientists in Europe dis- 

 covered the Christine Buisman elm, a 

 selection of Ulmus carpinifolia, which 

 has proved highly resistant in both 

 Europe and America. After inoculat- 

 ing thousands of American elm seed- 

 lings, American scientists have found 

 two resistant American elms. Other 

 recent selections and hybrids seem 

 promising, and elms that combine re- 

 sistance to the disease and high quality 

 should be available soon through com- 

 mercial nurseries. 



Because the Dutch elm disease or- 

 ganism is spread by insects, the loss of 

 elm trees from this disease can be 

 prevented by controlling the insect car- 

 riers. This can be done by the preven- 

 tion of breeding in recently cut, dead, 

 or dying elm trees and by the preven- 

 tion of feeding on living elm trees. 



Breeding may be prevented by burn- 

 ing or spraying all infested or likely- 

 to-be infested elm wood. If a spray is 

 to be used, the entire bark surface must 

 be thoroughly covered with No. 2 fuel 

 oil containing 1 percent of DDT. This 

 spray is for dead material only, because 

 it will injure living trees. 



Feeding by bark beetles in living 

 trees can be controlled by completely 

 covering the bark surface with an 

 emulsion-type spray containing 2 per- 

 cent of DDT. Such sprays have pro- 

 duced residues that remained effective 

 for more than 3 months. This method 

 of control can be applied to individual 

 trees. Further experimentation with 

 these DDT sprays is necessary before 

 we can make recommendations for 

 their general use. 



R. U. SWINGLE is a senior patholo- 

 gist in the Division of Forest Pathology, 

 Bureau of Plant Industry, Soils, and 

 Agricultural Engineering. 



R. R. WRITTEN is a senior entomolo- 

 gist in the Division of Forest Insect 

 Investigations, Bureau of Entomology 

 and Plant Quarantine. 



E. G. BREWER is in charge of Dutch 

 elm disease control, Bureau of Ento- 

 mology and Plant Quarantine. 



