Eavesdropping parasites: Do blue orchard bee (Osmia liganria) nest volatiles attract parasites?

Over the last several decades, commercial honeybee populations (Apis mellifera) have sharply declined, prompting increased research interest in native pollinator species. The native blue orchard bee (Osmia lignaria) is a promising candidate for supplementing pollination within fruit and nut orchards in Central California. However, our knowledge of its interactions with parasites remains limited. Specifically, few studies have examined whether parasites are attracted to host nest volatiles. To investigate parasite attraction to bee nest volatiles, we placed O. lignaria nesting tubes in a conventionally farmed orchard in Waterford, CA as well as in a nearby ravine with a natural population of O. lignaria in February 2014. At both locations we randomly applied a commercially available O. lignaria nest volatile extract to half of the nesting tubes, while the other tubes received a control treatment of ethyl alcohol. In a second experiment, we applied nest volatile extract to baited sticky traps to isolate the effect of the chemical signal. Nesting tubes and sticky traps to which nest volatile extract had been applied did not show higher rates of parasitism than control nesting tubes and sticky traps. However, the sticky traps treated with nest volatile extract showed higher diversity of trapped insects in comparison to the control sticky traps. Based on our results, O. lignaria nesting volatiles appear to act as a chemical attractant for a broad diversity of insects. Further study is necessary to investigate potential implications of O. lignaria nest volatiles and orchard pollination management.