Wood packaging material (WPM) is a major pathway for international movement of bark- and wood-infesting insects. ISPM 15, the first international standard for treating WPM, was adopted in 2002 and first implemented in the United States in 2006. ISPM 15 allows bark to remain on WPM after treatment, raising concerns that insects could infest after treatment, especially if bark were present. We conducted field studies to evaluate insect infestation of green logs and lumber with varying amounts of bark after heat treatment. In a log study, Cerambycidae and Scolytinae (ambrosia beetles and bark beetles) readily infested and developed in logs with bark after heat treatment. In a lumber study, Cerambycidae and bark beetles laid eggs in all sizes of bark patches tested (≈25, 100, 250, and 1,000 cm²) after heat treatment but did not infest control or heat-treated lumber without bark. Cerambycidae completed development only in boards with bark patches of 1,000 cm², whereas bark beetles completed development on patches of 100, 250, and 1,000 cm². Survival of bark beetles was greater in square patches (10 by 10 cm) versus rectangular patches (2.5 by 40 cm) of the same surface area (100 cm²). In surveys at six U.S. ports in 2006, 9.4% of 5,945 ISPM 15-marked WPM items contained bark, and 1.2% of 563 ISPM 15-marked WPM items with bark contained live insects of quarantine significance under the bark. It was not possible to determine whether the presence of live insects represented treatment failure or infestation after treatment.