Competitive interactions between the mountain pine beetle and the pine engraver in lodgepole pine

The pine engraver (PE), Ipspini (Say), often coexists with the mountain pine beetle (MPB), Dendroctonusponderosae Hopk., in lodgepole pine, Pinuscontorta var. latifolia Engelm. The PE colonizes the upper bole, while the MPB infests the lower bole. We investigated the hypothesis that interspecific interactions between the PE and the MPB within trees can adversely affect MPB progeny production or survival. In lodgepole pine bark attacked by both species, PE and MPB emergence holes had a strong negative interrelationship, suggesting that high numbers of one species resulted in lower numbers of the other. In the laboratory, attacks by PEs on lodgepole pine logs resulted in significantly decreased numbers of MPB progeny, particularly when MPBs and PEs were allowed to attack logs simultaneously. Reductions in MPB progeny were 92.8 and 96.2% when the ratios of attacking beetles were 100 PE: 50 MPB per square metre and 200 PE: 50 MPB per square metre, respectively, compared with that in control logs with 50 MPB per square metre. In an August field experiment, pheromone-induced attack by the PE on trees just attacked by the MPB resulted in a 72.5% reduction in mean MPB progeny production compared with that in MPB-attacked control trees. Possible reasons for the reduced success of MPB when it and the PE coinhabit the same host may be exploitation competition, interference competition, or introduction of fungal competitors deleterious to the MPB. Induced infestation by PEs of MPB-infested trees may have the potential to reduce or control MPB populations.