Host selection behavior of bark beetles (Coleoptera: Scolytidae) attackingPinus ponderosa, with special emphasis on the western pine beetle,Dendroctonus brevicomis.

Detection of weakened hosts from a distance by bark beetles through olfaction was investigated in field experiments. No significant numbers of Scolytidae were attracted to anaerobically treated pine bolts, stem disks, or sugar and ponderosa pine bark including phloem. Treatment of living trees with cacodylic acid induced attacks byDendroctonus brevicomis, D. ponderosae, Ips latidens, Gnathotrichus retusus, andPityophthorus scalptor, beginning two weeks after treatment. There was no significant difference between landing rates ofD. brevicomis andD. ponderosae on screened treated trees and screened controls. There was a significant increase in landing rates ofG. retusus andI. latidens, because both species had penetrated the screen and produced pheromones. Tree frilling alone did not increase the landing rate of bark beetles. Freezing of the lower trunk with dry ice did not increase significantly the landing rate ofD. brevicomis, D. ponderosae, G. retusus, orI. latidens on screened trees, whereas unscreened frozen trees were attacked by all four species. There was no significantly higher landing rate byD. brevicomis, D. ponderosae, I. paraconfusus, I. latidens, G. retusus, orHylurgops subcostulatus on screened trees evidencing symptoms of severe infection by the root pathogenVerticicladiella wagenerii, than on symptornless trees. These experiments show thatD. brevicomis, D. ponderosae, I. paraconfusus, I. latidens, andG. retusus land, apparently indiscriminately, on healthy and stressed hosts. Thus, in these species host discrimination must occur after landing and prior to sustained feeding.