Parasitoids use volatiles to recognize and locate suitable hosts. Numerous studies have investigated parasitoid host location from a pest management perspective, but comparatively little is known regarding parasitoid-pollinator interactions. Previous research has shown that parasitoids of some native bees respond to volatiles emitted by host frass and cocoons. We used a Y-tube bioassay to test whether two parasitoid species (Monodontomerus torchioi and Melittobia acasta) are attracted to volatiles associated with their host, the blue orchard bee, Osmia lignaria. Specifically, we tested attraction to (1) cocooned adult O. lignaria females (2) O. lignaria frass (3) methanol and (4) acetic acid. Both parasitoid species were attracted to host frass and acetic acid. Although M. acasta showed a strong attraction to volatiles from cocooned O. lignaria females, it was repelled by methanol, a chemical constituent of Osmia cocoons. In contrast, M. torchioi showed no response to cocooned O. lignaria and only a slight attraction to methanol. Both parasitoid species appear to be differentially attracted to specific host-associated volatiles, suggesting that they may respond to distinct olfactory cues when locating potential bee hosts.
