BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 
25 
mum hardiness during the entire winter. In March resistance to cold 
begins to decrease and continues to do so until minimum hardiness 
occurs the first part of May. During the winter, larvae developing in 
lodgepole and limber pines will survive at air temperatures from 5 to 
12 Fahrenheit degrees lower than those in ponderosa pine, but during 
spring and fall the cold hardiness of larvae in all three hosts is simi- 
lar. In mid-October mortality of larvae removed from the bark of 
ponderosa pine begins at 10° and is completed at — l c . By mid- 
December, when the larvae reach maximum cold hardiness, mortality 
begins at approximately —13° and is complete at —31°. During 
May and June the hardiness is about the same as in mid-October. 
CONTROL OF BARK BEETLES THAT TRANSMIT THE DUTCH ELM DISEASE 
In connection with the Dutch elm disease project there has been 
some demand for a method that would protect elm wood from attack 
by bark beetles and other insects. Tests were conducted in which 
various materials were applied to the bark of elm logs. Treated 
and untreated logs were then placed in woodland where elms infested 
with various insects were present. Orthodichlorobenzene. mono- 
chloronaphthalene. or naphthalene, mixed with fuel oil or kerosene, 
were the most effective substances used. When applied in May 
they effectively repelled attack by the elm bark beetles Scolytwp 
multistriatus (Marsh.) and Hylurgopinus ?*ufipes (Eich.) and cer- 
tain cerambycid beetles during the remainder of the year. Applied 
from September to April, inclusive, they reduced considerably the 
attack during the following season of insect activity. 
Other experiments were conducted to test the value of various 
mixtures in killing the immature stages of Scolytus multistriatus and 
Hylwrgopinus ruftpes. The mixtures were sprayed on the bark of 
logs infested with the bark beetles. Twelve different combinations 
were effective when applied to logs that had become infested with 
beetles 3% to 7 weeks previously. Variations in the moisture con- 
tent of the bark had no apparent effect on the results of the treat- 
ments. The most outstanding feature of the experiments was the 
excellent results obtained with emulsified sprays in which water was 
used in place of fuel oil as a carrier for the lethal material. 
An experiment was begun to determine the extent to which the 
population of the smaller European elm bark beetle {Scolytus mirf- 
twtriatus) can be reduced in a certain area by means of trap trees. 
Three woodland plots, each 2 miles square, in which elm trees are 
common were established. The elms in the areas were counted, their 
diameters at breast height recorded, and their locations indicated 
on maps. Two percent of the trees in the 6- to 8-inch diameter 
class were treated with sodium chlorate during the summer. Pre- 
vious tests had shown that this treatment killed the trees and that 
they became specially suitable for bark beetle attack. From 6 to 7 
weeks after treatment the trees were felled, barked, and appropriate 
notes made. All insect-infested material was then destroyed. The 
experiments will have to be continued in subsequent years to deter- 
mine whether the bark beetle population can be satisfactorily reduced 
by chemically treating certain trees and destroying them after they 
become infested. 
