Do blue orchard bees (Osmia lignaria) exhibit plastic behavior in response to parasitism by the non-native houdini fly (Cacoxenus indagator)?

With continued global connectivity of people, trade and food distribution, parasites are being redistributed internationally and have already begun to threaten bee populations. Recently, the houdini fly, Cacoxenus indagator, was accidentally introduced from Europe to the northwestern United States. Houdini flies are kleptoparasitic on cavity nesting bee species and are widespread in blue orchard bee, Osmia lignaria, nests in Washington and northern Oregon. However, it is unclear how houdini flies may impact O. lignaria.

To combat native kleptoparasites, female bees spend more time inside their nest, which reduces foraging time. This results in smaller pollen provisions and shifts in offspring sex ratio within nests towards males. We determined if O. lignaria is able to respond plastically to houdini flies by exhibiting the same behaviors despite not being co-evolved with the parasite. To test this, we partnered with Rent Mason Bees, a Washington-based company, that supplies O. lignaria to thousands of individual home gardeners and farmers throughout the western United States. We leveraged their network of homeowner-partners to deploy standard sentinel nest-blocks at 74 sites in California, Oregon, and Washington. Individual nests were dissected from each site to determine parasitism rate, offspring size, and sex ratio. The results from this study will determine how an economically important bee is impacted by non-native parasite introduction. Many bee species exhibit similar strategies to O. lignaria, therefore this work will provide an important baseline for the responses of cavity-nesting bees to introduced kleptoparasites.