Overwintering behavior of Tomicus piniperda (L.) was studied in a Scotch pine (Pinus sylvestris L.) Christmas tree plantation in Indiana (1992–1994) and a plantation in Michigan (1994). In general, adults feed inside shoots during summer, then move to overwintering sites at the base of trees in autumn. In early autumn, adults were most often found in shoot-feeding tunnels that were still surrounded by green needles, whereas few were in tunnels surrounded by yellow or brown needles. For all years and sites combined, the range in the percentage of recently tunneled shoots that contained live T. piniperda adults decreased from 89 to 96% in mid-October, to 15–66% in early November, to 2–10% in mid-November, and to 0–2% by late November to early December. In each year, the first subfreezing temperatures in autumn occurred in October, before most adults left the shoots. Of 1,285 T. piniperda-tunneled shoots, one to seven tunnels (mean = 1.6) and zero to three adults were found per infested shoot. Of these 1,285 attacked shoots, 55% of the shoots had one tunnel, 33% had two, 9% had three, 3% had four, and <1% had five to seven tunnels each. When two or more tunnels occurred in a single shoot, adults were most commonly found in the innermost (most basal) tunnel. For the 2,070 tunnels found in the 1,285 shoots, average shoot thickness at the tunnel entrance was 6.0 mm, average distance from the tunnel entrance to the shoot tip was 6.3 cm, and average tunnel length was 2.3 cm. Four Scotch pine Christmas trees were dissected in January 1993. Eighty percent of the tunneled shoots were in the upper quarter of the tree crown and 98% were in the upper half. For the four trees inspected in January, one live adult was found in a shoot and 85 adults were found in the outer bark along the lower trunk from 1 cm below the soil line to 19 cm above the soil line. No overwintering adults were found outside the trunk in the duff or soil near the base of each test tree. Implications of these results are discussed in terms of surveying, timing the cutting of Christmas trees, and cutting height for Christmas trees. [ABSTRACT FROM PUBLISHER]