Influences of host chemicals and internal physiology on the multiple steps of postlanding host acceptance behavior of Ips pini (Coleoptera: Scolytidae)

A series of bioassays were performed to evaluate the effects of host tree monoterpenes on initial entry, within-tissue orientation, and gallery construction by the bark beetle Ips pini (Say). Monoterpenes at concentrations present in host trees affected all stages of postlanding host acceptance behavior. However, the effects of each monoterpene varied with the particular stage of the orientation sequence. The number of beetles entering a phloem-based medium decreased with increased concentrations of most monoterpenes. An exception occurred with beta-pinene, which elicited increased beetle entry at moderate concentrations. Once beetles began to form galleries within phloem-based media, higher monoterpene concentrations increased the likelihood that they would move from amended to nonamended sections of the substrate. However, low to moderate concentrations of alpha-pinene arrested beetles within treated regions of the arena. The total gallery length generally decreased with increasing monoterpene concentrations. However, gallery lengths were consistently higher at intermediate concentrations of alpha-pinene. The enantiomeric composition of alpha-pinene did not influence entrance or gallery formation by I. pini. Moreover, the concentration of monoterpenes appears to be more important than the type of monoterpene in affecting postlanding host selection behavior. There was a positive correlation between host entry and total percentage of lipids within beetles. We describe a new bioassay that evaluates initial host entry by bark beetles. a critical qualitative decision in their life history, and relates this decision to quantitative aspects of host chemistry. Our results indicate that postlanding host acceptance is a multistep process shaped by environmental conditions and internal physiological state of the insect.