14 MASS. EXPERIMENT STATION BULLETIN 343 



bodies of migrating beetles. As these mites crawl about the elm bark, they 

 may be instrumental in local spreading of the fungus over the tree. 



Trees weakened by drought, leaf-infesting fungi or insects, or other causes, 

 may become the breeding places of fungus laden beetles, and thus contract 

 the disease. Such trees will, in turn, attract more carrier beetles. 



The number of generations produced by the beetle in a season varies with the 

 climate. In warmer regions, there may be two or more generations annually; 

 in cooler regions, only one and perhaps a partial second. The number of 

 generations produced in a given area may be influenced by variation in en- 

 vironmental factors. In that case, there would be an overlapping of gener- 

 ations, and beetles would be emerging throughout the warmer seasons of 

 the year instead of all appearing at approximately the same time. 



It is apparent that the life history of this beetle constitutes a vicious cycle, 

 which produces a larger number of diseased trees each succeeding year, if it is 

 not checked. 



The Native Elm Bark Beetle 



(Hylurgopinus rufipes) 



It has been shown in experiments that this beetle also can transmit the 

 Dutch elm disease fungus to elm wood and cause trees to become infected. 

 To what extent this occurs in nature is not definitely known. The habits of 

 this beetle are very similar to those of Scolytus multistriatus. It breeds in 

 elm logs and dead or weakened elm trees. Young adults usually dig short 

 tunnels into the bark on the trunks and branches of healthy elms before 

 attacking elm logs and dead or weakened elm trees for breeding (Figure 6, 

 lower left). 



In central Massachusetts, this beetle has one complete generation and a 

 partial second. Farther south it may have two. It is far more widely dis- 

 tributed than the introduced Scolytus multistriatus. It is present in all parts 

 of Massachusetts and has been found throughout most of the range of the 

 American elm. 



The beetle is about the same size as Scolytus multistriatus, but differs from 

 it in that it has a dull, uniform, dark brown color (Figure 5, B). Its egg 

 gallery is dug more or less across the grain, and is two-branched, instead of 

 being a single straight tunnel parallel to the grain (Figure 6, lower right). 



Some Suspected Insect Vectors 



Some other insects which bore in elm trees may prove to be carriers. At 

 present, however, they are not known to be of any consequence. 



One of these is the elm borer, Saperda tridentata Oliv. This is a beetle 

 about one-half inch long, colored gray with red stripes. The larva is nearly 

 an inch in length. It makes a long winding tunnel beneath the bark. The 

 adult usually feeds on elm leaves before laying eggs in a shallow puncture 

 in the bark. 



Two closely related beetles known as the elm snout beetles, Magdalis barbita 

 Say., and Magdalis armicollis Say., which have habits somewhat similar to 

 Saperda tridentata Oliv., are also regarded with suspicion. The adults of 

 of these two beetles are black and brown respectively and are about one-fourth 

 of an inch long. The larvae or grubs resemble those of Scolytus mtdtistriatus. 



The buffalo tree hopper, Ceresa bubalus Fabr., and several other insects 

 which attack elms are likewise suspected of being possible vectors. 



