22 BULLETIN 1426, U. S. DEPARTMENT OF AGRICULTURE 
Males accompanying females in the egg galleries are common dur- 
ing the whole period of reproduction, and it is believed that the female 
is fertilized more than once during her reproductive period. 
Only one case has been observed where eggs were laid in the walls 
of what appeared to be a hibernation chamber or last year’s larval 
mine. This, a case of three eggs (two in one pocket), in pockets 
leading from an old central boring, was recorded at Yamhill, Oreg., 
May 13, 1915. 
INCUBATION PERIOD 
The incubation period of the eggs is apparently dependent on the tem- 
perature to which they are exposed. In determining the length of 
this period under northwestern conditions the writer found that his 
records varied to an unusual extent from those of workers in other 
parts of the country, as well as from those of earlier authors, both 
American and European. Schmitt (39, p. 394) stated “Schon nach 
8 Tagen findet man die kleinen Liarvchen.’’ The American authors, 
Riley (37), Webster (45, 46, 47, 48), and Davis (9) apparently ac- 
cepted Schmitt’s egg period, as their papers on this insect contain 
no intimation that they attempted to ascertain the incubation period. 
H. L. Parker made several incubation experiments at Hagerstown, 
Md., placing root-borer eggs on moist blotting paper in tin salve 
boxes, the location of which during the incubation is not stated. 
The egg period under these conditions was 10 or 11 days; ‘eggs from 
burrow,” May 18 to May 29; “‘deposited since yesterday,’ May 19 
to May 29; and “‘eggs of June 8,” June 8 to June 18, 1916. He re- 
corded a large percentage of eggs as dead in from 12 to 17 days. 
Incubation chambers identical, or nearly so, with those used by 
Parker were used in the writer’s experiments, carried on at Forest 
Grove. Most of the salve boxes contained plaster of Panis cells, 
which were moistened as needed. In a few cases the eggs were left 
in position on a piece of clover root which was then placed on a moist 
blotter or in a plaster cell in a tin salve box. All of the writer’s 
records given here refer to these types ef incubation apparatus, which 
were placed outdoors, in the shade on the north side of a building, 
beside a thermograph, or on a bench in a screened insectary. The 
egos used were of unknown age, dissected from roots collected in the 
field. With a little experience in observing such eggs, it was easy 
to distinguish between a fresh or recently laid egg and one laid several 
days previously. The records secured for eggs collected in the field 
soon after oviposition began may be considered approximately correct 
for the egg period at that season. Later records, when the freshness 
of eggs could not be assumed a priori but was necessarily estimated 
from their appearance, would be less reliable. On the other 
hand, of the same lot, the eggs taking longest to hatch under exactly 
identical conditions as compared with others hatching earlier are as- 
sumed to have been the freshest of the lot when the experiment was 
started. These records are given in Table 5, together with the mean 
temperatures during the period and asummation of the effective tem- 
peratures as estimated according to Pierce (35) and Sanderson and 
Peairs (38). The estimated zero of effective temperature is 45° F., 
and the approximate temperature constant is 300. 
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