Doņa Ana County, New Mexico was quarantined in 1998 for red imported fire ant, Solenopsis invicta. This is a very aggressive ant species that displaces native ant populations, disrupts natural ecosystems, interferes with agricultural operations, and is a health hazard to livestock, wildlife and people. Despite regular surveys, S. invicta has not been identified in the county during the past three years. The predominant fire ant is the southern fire ant, S. xyloni. One speculation is that the native S. xyloni was misidentified because variation of distinguishing characteristics is common within this species. To quantify variability of these characteristics, a morphological comparison of 10 individuals from 10 colonies of both species was conducted using scanning electron microscopy (SEM). The clypeal tooth, striations of the mesopleuron, length of antennal scape, the petiolar process, number and size of mandibular teeth, and color were examined. Measurements and observations of the characteristics suggest that morphological variability is greater in S. xlyoni than in S. invicta. Due to potential overlap of these characteristics care should be taken when these two species coexist.
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