CONTROL OF ARGENTINE ANT IN CALIFORNIA. 37 
ber 15 had spread to 41. By the middle of December this ant had 
established colonies throughout the orchard, with the exception of 
the corner where the Argentine species still maintained a foothold. 
Several instances of like incursions, but in lesser degree, occurred in 
orchards at Upland in 1918 and at Alhambra in 1919. 
Relation of Breeding to Control. 
The most rapid and conspicuous increase in numbers of ants is 
during the months of July, August, and September, and this is ac- 
counted for by the high mean temperatures prevailing during that 
period and the abundance of available food. High temperatures 
stimulate the queen to maximum egg production, and the life cycle 
of workers from egg to maturity under these most favorable weather 
conditions approaches four to five weeks, instead of two to four 
months, as during the coldest weather. Since the Argentine ant has 
been observed to slacken activity at temperatures below 60° F., it is 
apparent that during the spring and autumn not only does the egg 
production become greatly reduced, but the period of development 
is lengthened. Therefore, poison distributed during the spring and 
autumn should prove most effective. 
During the hot summer months when all stages are present and the 
development is so swift the rapid destruction of all workers present 
at the time of sirup distribution would not clean up the infestation 
because additional workers would be daily emerging from the pupae 
and these could care for the undeveloped larvae. Eradication of the 
young during the summer by poisoning of workers is less likely at 
this time than during the period of slow development. 
CONTROL ON MARGINAL TREES. 
Ant eradication has been most difficult on the outside rows of or- 
chards, particularly so when bordered by ditches or strips of uncul- 
tivated land. In clean-cultivated orchards, as stated elsewhere in 
this bulletin, there is little movement between trees, yet the marginal 
rows are commonly attended by ants breeding in the strips of border- 
ing ground. If this adjacent territory is heavily colonized by ants, 
the latter will be attracted by sirup on these trees. The greatest 
difficulty has been experienced at times in eradicating ants from these 
trees, and ants have been eradicated from entire large orchards with 
the exception of outside trees. The difference in rapidity of eradica- 
tion between the marginal two rows of trees and the rest of a 10-acre 
orchard by the sirup method is shown in figure 17. Orchards from 
which ants have been entirely eradicated one year have been known 
to have the outside two to three rows reinfested the following year 
from adjacent ant-sustaining territory. As a matter of precaution it 
is sometimes advisable to keep containers of sirup on the outside row 
