ATTACK DYNAMICS OF IPS-PINI (SAY) AND PITYOGENES-KNECHTELI (SWAINE) (COL, SCOLYTIDAE) IN WINDTHROWN LODGEPOLE PINE TREES

The pine engraver, Ips pini Say, is a pest of lodgepole pine, Pinus co ntorta var. latifolia Engelman. It usually infests dead or weakened tr ees but at high population densities can kill standing trees. Pityogen es knechteli Swaine often co-exists with I. pini, but is not economica lly important. Our objective was to test the hypothesis that competiti ve interactions between P. knechteli and I. pini may adversely affect the success of I. pini in lodgepole pine. This hypothesis was tested b y: 1) investigating the relationship between emergence hole of the two species in naturally-infested hosts; and 2) comparing the attack dyna mics of both species alone and together in naturally-infested windthro wn trees. There was a strong, non-linear inverse relationship between emergence holes of I. pini and P. knechteli. Emergence hole densities of I. pini and P. knechteli were positively and negatively correlated, respectively, with tree diameter. These results suggest that high num bers of one species are associated with low numbers of the other, and that beetles in the two species tend to partition the shared resource according to its diameter. There was a slight negative effect on the s uccess of beetles in both species in naturally-infested, co-attacked t rees. It appears that I. pini and P. knechteli can co-exist within the same host tree in part because of a lack of exploitative competition, and also because beetles in the two species partition the phloem reso urce so as to minimize competitive interactions. While P. knechteli wo uld be of limited practical potential in competitive displacement of I . pini, its ability to coinfest hosts with I. pini may enhance the adv erse effect of I. pini on the mountain pine beetle, Dendroctonus ponde rosae Hopkins.