1961] 
Insect Control Programs 
85 
colonization ahead of the main infested area by queens and colony 
fragments transported by vehicular traffic. Nursery stock used to be 
a prime source of new infestations, but since nursery treatments and 
quarantine regulations have come into effect, fertilized females acci- 
dently carried in automobiles are probably responsible for most 
colonization. 
Wherever the red phase has expanded to overcome the dark phase, 
the two extreme forms have interbred to produce a series of inter- 
mediates, and in most cases the red form soon comes to predominate 
by a process of genetic swamping coupled with its greater success in 
warfare between nests. In fact, it may not be too extravagant a 
speculation to conclude that it was the injection of the red-form genes 
into the existing dark population that sparked the spectacular spread 
of the species in the last three decades. At present, the North Ameri- 
can population consists mainly of light reddish ants, the dark phase 
surviving mainly in peripheral situations and cool swamplands. 
Wherever it spreads, S. saevissima tends to replace the populations 
of S. xyloni and S. geminata in its path, though this is less true of 
the dark-colored geminata occupying woodlands in Florida and per- 
haps elsewhere 26 ; saevissima in the U.S. generally avoids shaded situa- 
tions. The imported fire ant is able to build up remarkably dense 
populations. I have seen pastures in eastern Mississippi in which it 
was literally possible to walk for a considerable distance by stepping 
from mound to mound without touching a foot to the ground between. 
Such situations are exceptional, and usually mark the entry of the 
species into a new area, or else follow control measures that have 
knocked out a stable population of old, large nests. When the old 
nests are eliminated, large numbers (up to 185 per acre) of smaller 
new ones take their places, but as they grow, nests are gradually 
eliminated until the density is again relatively low (10-50 nests per 
acre usually). 
Studies made to date have not been critical enough to detect possible 
widespread population fluctuations in untreated areas, but about a 
century ago, Bates noted a radical change in a native population of 
S. saevissima in the Amazon Basin. 
A small number of parasites of this ant are known in its native 
habitat, including several known or suspected inquilinous species of 
ants and a phorid fly, but no real study has ever been made of this 
phase of the ant’s biology. These parasites have been lightly dismissed 
as a control possibility by previous writers, but it seems to me that the 
whole subject of parasitism should be looked into. Parasites might do 
