The role of olfactory stimuli in the location of weakened hosts by twig-infesting Pityophthoros spp.

1. Senescing, shade-suppressed, or broken branches of Monterey pine Pinus radiata are infested by twig beetles in the genus Pityophthorus (Coleoptera: Scolytidae). The studies reported here tested whether twig beetles can discriminate between healthy and pitch canker-diseased branches, whether diseased branch tips produce more ethylene than undamaged controls, and whether ethylene and other volatiles, produced by the plant in response to tissue damage, are utilised by twig beetles in host location. 2. Significantly greater numbers of twig beetles were reared from pitch cankersymptomatic than from pitch canker-asymptomatic branches of Monterey pine collected in the field. 3. Needles of Monterey pine branches inoculated with the pitch canker fungal pathogen Fusarium circinatum produced significantly higher levels of ethylene than needles of control branches, and this was evident just prior to, and during, symptom expression. 4. In trapping studies in which pheromone production was prevented, there was no evidence of attraction of twig beetles to a source of ethylene alone, to cut host branches, or to cut branches treated with the ethylene-releasing compound, ethephon. The results suggest that twig beetles identify weakened branches after landing.