Effects of Humidity on Native and Non-Native Cavity Nesting Solitary Bees

Invasive species are defined as those that are not native to an ecosystem but may cause harm to the environment or to human health. Osmia cornifrons, a species introduced to the United States, and Osmia lignaria, a native species, are cavity-nesting solitary bees. Although not officially classified as invasive, O. cornifrons presents competition for the niche that O. lignaria inhabits. O. cornifrons has established populations throughout the East Coast and the Pacific Northwest, while O. lignaria is found all throughout the contiguous United States. This interspecies difference in geographical range exists despite their similar temperature ranges. To determine if relative humidity (RH) was an influential factor in this difference, we placed 315 of both O. cornifrons and O. lignaria propinqua into one of three relative humidity treatments (10%, 50%, or 75%) and measured their weight change and survival. We expected to see O. cornifrons die more quickly and lose more weight in lower humidity conditions, with no apparent change in hours to death or weight change among O. lignaria propinqua groups. We found that females of both species died more quickly when subjected to a low humidity treatment than those at higher conditions. Female O. cornifrons in the 10% RH condition also lost a higher percentage of total body mass than those in either of the other RH groups. However, female O. lignaria propinqua showed only a difference between the 10% group compared to the 50% and 75% treatments. O. lignaria females outlived O. cornifrons females in every humidity treatment we tested. Males of both species showed no difference in percent weight loss or time to death between treatment groups. Our study showed that humidity is a limiting factor on O. cornifrons geographical range, and that O. lignaria lignaria is at higher risk of population decline due to niche overlap with O. cornifrons in the eastern U.S. With further research, our results indicate that O. cornifrons can be used safely as a managed pollinator in more arid climates without risk of it becoming invasive in such regions. It could also indicate expansion of O. cornifrons geographical range in the U.S. if climate change increases relative humidity.