THE HALL SCALE ERADICATION PROJECT Q 
TREATMENT 
OIL SPRAYS 
At the beginning of the project in 1941, all infested properties were 
sprayed with oil as soon as discovered. The early program included a 
winter spray containing 3 to 3.5 percent of oil, and spring and late summer 
sprays containing 1.75 to 2 percent. Sometimes a second spring spray was 
applied to heavily infested trees. Light-medium or medium oil with a 
minimum unsulfonated residue of 92 percent was used. It was first planned 
as a holding program to minimize the danger of spread until more was 
known about the size of the infested area and the biology of the scale. 
The first application reduced the infestation considerably, and there was 
some reason to hope that a continued oil-spray program might eradicate 
the scale. However, after a certain level of infestation was reached, spray- 
ing caused no further reduction. Scales that were exposed to spray seemed 
easy to kill, but there were always scales in deep crevices in the bark and 
other protected places that could not be reached by oil spray. The addition 
of DDT or rotenone to the oil failed to increase the kill appreciably. The 
addition of parathion to oil sprays greatly increased their effectiveness, but 3 
applications of a spray containing 1.75 to 3 percent of light-medium o1l 
plus 1 pound of 25-percent parathion per 100 gallons of oil did not eradt- 
cate the scale. 
FUMIGATION 
As soon as it was evident that the scale could not be eradicated with oil 
sprays, fumigation with hydrocyanic acid was tried. It seemed likely that 
uniform distribution of high, sustained concentrations of this gas would be 
necessary to reach the scales in protected places with a lethal dose. In 1942 
the possibility of using gastight tents and hydrocyanic acid applicators 
equipped with blowers to insure improved distribution and retention of gas 
used in citrus fumigation, were being studied at the Whittier (Calif.) 
laboratory of the Bureau of Entomology and Plant Quarantine. Findings 
in this research were applied in the Hall scale project. 
In preliminary tests in February 1943 trees were fumigated with 54 ml. of 
liquid hydrocyanic acid per 100 cubic feet under nearly gastight and semi- 
gastight tents. The fumigant was introduced with a blower applicator. 
Between March and September about 50,000 scales were examined, and all 
were dead. | 
These results indicated that fumigation with gastight tents might eradi- 
cate the scale. Such questions as the best dosage from the standpoint of 
effectiveness and tree tolerance, diurnal and seasonal time of fumigation, 
andlength of exposure were then investigated. Finally, techniques were 
developed for handling high concentrations of residual gas and for covering 
large trees having many pruning ends and fruit spurs, without causing 
excessive damage to the tents. 
On the basis of these tests, it was decided to use a dosage of 40 cc. of liquid 
hydrocyanic acid per 100 cubic feet under gastight tents for 50 minutes, to 
make the fumigations in the daytime during the winter when the host trees 
were dormant, and to use suction blowers to evacuate the residual gas 
before removing the tents from treated trees. 
Covering dormant deciduous trees with fumigation tents is much more 
difficult than covering citrus trees. To avoid tearing the tents in moving 
them over trees lacking a protective shell of foliage, it is necessary to lift 
