60 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA. 
will destroy all insects except a few of the toughest of the matured 
scales. It will also destroy a large percentage of the eggs. 
In fumigating for the black scale the treatment should be applied at 
the time when the insects are largely or all, if possible, in the early stages 
of development. Close watch of the condition of the scale in his 
orchard will enable the grower to determine the proper time. There 
is a general breeding period during late summer or early autumn 
when this condition usually exists. Wait until the eggs of the old 
brood have all hatched, even if by this time some of the earliest 
issuing insects are becoming leathery. This is the time at which 
fumigation is advised and a three-fourths schedule (see fig. 11, p. 59) 
should be used. 
If it be desirable to fumigate at a time when the insects are in all 
stages, including the mature and tough individuals as well as those 
of the younger generations, a full schedule No. 1 (see fig. 9, p. 34) 
should be used. 
It would be difficult to advise a dosage sufficient to destroy the 
eggs, because of the wide variability in the action of such a dosage. 
The writer has seen the eggs of some species of scale insects destroyed 
by employing schedule No. 1, whereas in other instances the eggs 
have been unaffected even where a very much heavier dosage was 
used. This is partly attributable to the closer attachment to the 
host plant of the different individual scales and partly to a small 
parasitic insect (Scutellista cyanea Motsch.) which attacks the eggs 
of the black scale very freely. The larvae of this parasite cements 
the edges of the scale to the host plant, making it more difficult for 
the gas to penetrate to the eggs under such scales than under those 
not parasitized. The exposure should last one hour. 
THE YELLOW SCALE. 
The yellow scale is comparable in almost all respects with the red 
scale except that it is much less destructive. The region in and about 
Redlands, San Bernardino County, is the principal section in southern 
California in which special effort for the control of this insect alone 
is required. 
During September, 1909, 2 to 3 acres of orange trees at Redlands 
infested with the yellow scale were treated with dosage rates of from 
one-half to 1J of schedule No. 1. The insects were destroyed on all 
trees except those on which a one-half schedule was used. A very 
few survived on these. No difference in effect was apparent between 
an exposure of 45 minutes and one of an hour. 
A three-fourths schedule (see fig. 11, p. 59) is recommended for 
the yellow scale. The section in which the yellow scale is most 
serious is about Redlands, where a great acreage is on terraced land. 
In such fumigation it is believed that a full schedule No. 1 should be 
