NUMBER OF COLONIES 
1971] 
Erickson 
Displacement of Native Ant 
261 
Figure 2. The number of colonies in the entire field of P. calif ornicus (o) 
and 1. humilis (®) from October 1963 to October 1968. 
of more favorable habitats (Wilson, 1971). Colonies of I. humilis 
contain a large number of queens with thousands of workers ( Smith, 
1947) and proliferate by swarming of detachments of workers who 
accompany secondary queens out of the nest (Wheeler, 1933 ; Wynne- 
Edwards, 1963; Crowell, 1968). They are highly omnivorous but 
tend to seek sweet or fatty foods (Eckert and Mallis, 1937,’ Creigh- 
ton, i960; Cook, 1953), and tend aphids and scale insects in orchards 
and gardens (Skaife, 1961). 
In contrast with I. humilis , the California harvesters are large 
ants (4-6 mm long) which are primarily seed gatherers, but are also 
known to be slightly omnivorous (Van Pelt, 1966). Colonies of 
P. calif ornicus are small in comparison to I. humilis and contain 
only one queen. Proliferation takes place by large swarms of winged 
reproductives. The California harvester ant tends to nest in dryer 
semi-desert habitats and can tolerate much higher temperatures than 
I. humilis (Wheeler, 1926; Cole, 1932, 1968; Michener, 1942; 
Erickson, 1972). 
The relative reproductive potential of I. humilis is probably much 
higher than P. calif ornicus. This is most likely due to the large num- 
