Evidence for a Substrate-Borne Sex Pheromone in the Parasitoid Wasp Trichogramma brassicae

While the use of chemical cues in host location is well known in parasitoid insects, little is known about their role in sexual communication. Sex pheromones were investigated in the egg parasitoid Trichogramma brassicae, and the evidence for a substrate-borne pheromone was demonstrated. Mated and unmated males concentrated their walking in an area where virgin females (<1 or 5 days old) had walked. Males were tested 15 min after females left the area. Male response decreased over time but was always significant during the 8 min of tracking. This suggests that females deposit chemicals on the substrate for mate finding. The pheromone is soluble in hexane, as shown by two kinds of extracts: female body rinsing and rinsing of the flask where females had walked. Hexane extracts resulted in a positive response by males. The response to different doses showed a saturation above a threshold of 4–8 female equivalents. The results do not exclude the possibility that a volatile part of the pheromone attracts males over long distances. Substrate-borne pheromones may increase mating chances for virgin females searching for oviposition sites, and may also increase mating chances on emergence sites by reducing male dispersion.