Ips calligraphus (Germar), I . ponderosae Swaine and I . interstitialis (Eichhoff) represent one variable species with the oldest name, Ips calligraphus (Germar) taking precedence. Ips calligraphus can always be recognized by the six spines on each side of the declivity. All other species in North America have less than six spines on each side.
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G. R. Hopping (1965) studied this question.
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