Sex attractants in frass from bark beetles.

Shortly after Kangas et al. (1) reported that a-terpineol was the major component of a primary host attractant for Blastophagus piniperda L., Silverstein, Rodin, and Wood (2) revealed the identity of three other terpene alcohols that play a role in the orientation of bark beetles. However, Silverstein et al. concluded, from laboratory bioassay, that a mixture of the three terpene alcohols they had isolated and identified from the frass produced by male Ips confusus (LeC.) represents "the sex attractant that is responsible for the mass attack following initial boring activity." This terminology and conclusion largely ignore present knowledge in the behavior of bark beetles and should be held in doubt, since reactions of walking insects in laboratory bioassay do not necessarily reflect the behavior of flying populations (3). In fact, frequently, materials that elicit response in laboratory bioassay are found inactive under field conditions. Furthermore, the term "sex attractant" would appear not to fit material that is active at different combinations of various hostand insect-produced components, does not stimulate mating in any obvious way, and attracts both sexes and probably other insects as well. Instead, it has been determined by field biossay that the attractant principle responsible for population aggregations of Ips confusus originates in the hindgut of the male when the beetle feeds in new host material, as well as in the phloem tissue of nonhost species; it has also been shown, under field conditons, that hindguts and fecal matter deriving from mature and fed males are the ultimate source of the attractant (4). None of these findings, which are now accepted, remained unchallenged, for controversial evidence is easily obtained from laboratory observations (5). The complex nature of the phenomenon of population aggregation of bark beetles (6), with its various activities, such as directed flight and landing and arrest in search of suitable breeding places, must involve a whole chain of reactions, among them reactions to various olfactory stimuli. In order to derive legitimate conclusions concerning the attractant principle that causes directed flight, the behavioral elements in population aggregation that allow meaningful bioassay of candidate substances