The mechanisms of interspecific competition among an invasive and two nativ
e Solenopsis fire ant forms were investigated in a series of laboratory exp
eriments. In separate trials each with a different food resource, the nativ
e S. geminata x xyloni retrieved the greatest amount of a protein- and lipi
d-rich artificial food resource and a high protein natural food resource, a
nd the native S. geminata retrieved the greatest amount cf a high carbohydr
ate food resource. In trials investigating aspects of interference competit
ion at the colony level, the invasive SI invicta proved to be initially mor
e aggressive than S, geminata, but less aggressive than S. geminata x xylon
i Solenopsis invicta eventually controlled more of the foraging arenas agai
nst both native forms when colonies were equivalent by worker biomass, but
not when colonies were equivalent by worker number. When paired with S. inv
icta, S. geminata suffered a significantly greater proportional reduction i
n both workers and entire colonies when colonies were initially standardize
d by worker biomass, but not when colonies were standardized by worker numb
er. When paired with S. invicta, a significantly greater proportional reduc
tion of workers occurred in S. geminata x xyloni, regardless of how colonie
s were standardized. Ln pains ise trials at the individual level. majors al
ways exhibited significantly less mortality than miners, regardless of the
Solenopsis form. The majors of both native Forms suffered significantly les
s mortality than those of S. invicta. Superiority in colony-level interfere
nce ability appears to be an important mechanism allowing S. invicta to dis
place native Solenopsis forms. The ability of S. invicta to reach high popu
lation densities, because of intrinsic biological characteristics or an esc
ape from natural enemies, plays an important contributory role. Similar mec
hanisms may underlie the success of other invasive ant species.