Verbenone reduces mountain pine beetle attack in lodgepole pine

Mountain pine beetle, Dendroctonus ponderosae, is the most common cause of mortality of mature lodgepole pine (Pinus contorta var. latifolia). In 2000-2002 the antiaggregative compound verbenone was applied annually to the same lodgepole pine stands in campgrounds and resort facilities at the Sawtooth National Recreation Area in central Idaho to assess its ability in deterring mountain pine beetle attack through the course of a multiyear outbreak. Verbenone was applied at the rate of forty 5g pouches/ac releasing 25-35 mg/24h at 20degreesC. Significantly fewer trees were attacked and killed in the verbenone plots during 2000 and 2001. However, of the plots containing verbenone, a higher percentage of large trees were attacked in the second year of treatment, suggesting that the efficacy of verbenone may diminish under increasing beetle pressure. In 2002, there were nearly twice as many trees attacked and killed in the verbenone plots as in the untreated plots. It is hypothesized that the change in the performance of verbenone may be due to the large beetle population overwhelming the treatment or because mountain pine beetles undergo a change in their response to verbenone that may be attributed to small diameter host trees.