Selection of Overwintering Sites by Tomicus piniperda (Coleoptera: Scolytidae) during Fall Shoot Departure

Selection of overwintering sites at the base of Scotch pine, Pinus sylvestris L., trees by Tomicus piniperda (L.) adults was monitored in northwestern IN. We monitored adult movement five times during the period of 22 October 1998 to 31 January 1999 by dissecting shoots and trunks of four trees per sample date. For all adults collected, 100% were found in shoots on 22 October, 81% on 4 November, 10% on 18 November, 2% on 3 December, and <1% on 31 January. No overwintering adults were found on the lower trunks of trees sampled on 22 October. Some adults (N = 16) were found on 4 November, 1 day after the first subfreezing air temperatures were recorded. For all adults (N = 448) collected from lower trunks during the three sample periods in November and December, the mean height along the trunk where adults overwintered was 2.6 cm above the duff line. Mean overwintering height did not differ significantly among sample dates (P > 0.21). When distance along the trunk was divided into 5-cm intervals relative to the duff line, most adults were found either 0 to 5 cm above the duff line (48%) or below the duff line (32%). Overall, 98% of the overwintering adults were found at or below the 10-cm height level from the duff line and 99% were at or below the 25-cm height level. A significantly lower percentage of adults was found below the duff line on 4 November as compared to 18 November and 3 December. However, percentages of adults found in each 5-cm interval above the duff line did not differ significantly among sample dates. Overwintering adults were found above 20 cm from the duff line on 18 November and 3 December, but no adults were found above 8 cm on 4 November. Results suggest that the first adults to arrive at the tree base prefer to overwinter within the first 10 cm of the duff line but do not necessarily prefer the area closest to or below the duff line.