The larvae grow very slowly, spending two years in the larval stage, and pupate in the fall in niches in the outer bark.

Mating is short, and the copulation precedes a chase and a firm grasp of the female with the male mandibles on her pronotum.

In pursuit of their prey, they are also very quick and very skilled at moving the bark beetle corridors, where they can run backwards.

[4] Thanasimus formicarius feeds on 27 bark-beetle species belonging to 15 genera (Agrilus, Dendroctonus, Dryocoetes, Hylastes, Hylesinus, Hylurgops, Hylurgus, Ips, Leperesinus, Orthotomicus, Pityogenes, Pityokteines, Polygraphus, Scolytus, Tomicus and Trypodendron) which infest coniferous (pine, spruce, larch, Douglas fir, and others), and broad-leaved trees (oak, ash, poplar, and others).

[3][7] Different experiments suggest that ant beetles prefer Pinus trees over Abies or Picea for oviposition.

Some experiments suggested that bark thickness could be a critical factor limiting pupation of T. formicarius on spruce.