Discrimination of airborne pheromones by mate-searching male western black widow spiders (Latrodectus hesperus): species- and population-specific responses

Males of many web-building spiders abandon their webs at maturity to search for a potential mate. Since wandering can be very risky, and females are often widely distributed, males should use any cues that might ensure rapid and accurate location of conspecific females. Although it has long been assumed that mate-searching male spiders locate females using species-specific airborne pheromones released from webs, few studies have experimentally examined this phenomenon in the field. Our results show that male western black widow spiders (Latrodectus hesperus Chamberlin and Ivie, 1935) are attracted to females' webs by an airborne cue released from the web, can distinguish between conspecific and heterospecific females, and can discriminate between webs produced by conspecific females from different geographical populations. The latter result demonstrates a partial premating block to fertilization between populations at the edges of the species range. Complementary interpopulation laboratory matings suggest that there may also be a postmating block to fertilization, as these copulations did not result in viable offspring. This study provides experimental field evidence of male attraction by airborne pheromones released from females' webs, shows the potential importance of these pheromones in species discrimination in black widow spiders, and suggests that northern and southern populations of L. hesperus may be incipient biological species.