Yearbook^ of Agriculture 1949 



Resin midge resistance studies were 

 carried on from 1930 to 1940 at the 

 Institute of Forest Genetics, near Plac- 

 erville, California. This undescribed 

 species of resin midge (Retinidiplosis 

 sp.) caused considerable damage to 

 young planted ponderosa pines at the 

 Institute and to natural reproduction 

 throughout much of the western pine 

 region during this period. The feeding 

 habits of the larvae cause resin-filled 

 pits in the thin bark of the stems and 

 twigs, and these pits result in growth 

 deformities and dwarfing of the trees. 

 In time the heavily injured trees die. 



A study of the stem characteristics 

 of many trees revealed that the heavily 

 infested trees were those that produced 

 new shoots covered with a sticky, 

 resinous film, a growth character of 

 certain trees. Noninfested trees were 

 those that produced new shoots with 

 dry, smooth bark. The data collected 

 showed that only 11.2 percent of the 

 sticky-stemmed trees escaped injury, 

 while 93.4 percent of the smooth- 

 stemmed trees escaped injury entirely 

 or were only lightly attacked. The 

 next phase of these investigations will 

 be to determine whether dry, smooth 

 bark is inheritable and, if so, to pro- 

 duce trees with this characteristic for 

 reforestation purposes. 



RESISTANCE TO WEEVIL: Resistance 

 in pine to another parasitic insect, a 

 weevil, was obtained by crossing a re- 

 sistant with a susceptible species. The 

 insect (Cylindrocopturus eatoni) is the 

 most important enemy of young 

 planted pines in the brush fields of 

 northern California where, in some 

 areas, it killed 90 percent of the trees 

 within 10 years after planting. It also 

 killed natural reproduction that was 

 restocking burned-over pine areas. The 

 trees are killed by the larval mines that 

 extend through the phloem and cam- 

 bium areas and later into the wood. In 

 nature, the weevil's preferred hosts are 

 ponderosa pine and Jeffrey pine. A 

 number of other species of pines, such 

 as Coulter pine and sugar pine, appear 

 to be immune to its attacks. 



Studies were begun at the Institute 

 of Forest Genetics in 1946 to determine 

 whether a resistant variety of pine 

 could be developed that would have 

 the same desirable wood qualities as 

 ponderosa and Jeffrey and at the same 

 time survive weevil infestations dur- 

 ing the early years of growth. A num- 

 ber of species, hybrids, and varieties of 

 pines were tested by forcing the attacks 

 of the weevil on them under cage con- 

 trol. Among the trees tested was a new 

 hybrid pine first produced by genet- 

 icists at the Institute in 1939 by cross- 

 ing Jeffrey pine with a natural hybrid 

 of Coulter pine. 



The tests confirmed field observa- 

 tions that ponderosa and Jeffrey pines 

 were generally susceptible to the weevil 

 although some trees proved to be resis- 

 tant. The Coulter pine was uniformly 

 resistant as was also the Jeffrey-Coulter 

 hybrid. Here we have indications of 

 resistance to insect attack being in- 

 herited as a dominant character, not 

 only in the first generation hybrid 

 but also in the backcross of this hybrid 

 on the susceptible Jeffrey pine. 



ELMS RESISTANT TO THE DUTCH 



ELM DISEASE : The Dutch elm disease, 

 which was discovered in this country 

 in 1930, now threatens all native and 

 European elm species in the United 

 States. Elm bark beetles spread the 

 disease. 



Efforts to control the disease include 

 destruction of infected trees, pruning 

 infected limbs, and destroying, de- 

 barking, or spraying elm logs. 



The American elm is an important 

 forest and shade tree, native to parts 

 of all States from the Great Plains 

 eastward to the Atlantic coast. Two 

 minor species, also native, are the rock 

 elm and the slippery elm, both suscep- 

 tible to the Dutch elm disease. The 

 Siberian elm is resistant. 



In 1937, scientists in the Division of 

 Forest Pathology began breeding and 

 selecting elms for resistance to the 

 Dutch elm disease. Thirty-five thou- 

 sand elm seedlings, collected in the 

 Great Plains and Northeastern and 



