Trees, Degrees, Wasps and Bees: The Microclimatic Effects of Forest Cover in Agricultural Land and Impacts on the Nesting Success of Solitary Cavity-Nesting Bees and Wasps

Abstract

Forests buffer temperatures to create cooler, more stable thermal environments, or 'microclimates'. Agricultural landscapes can be severely lacking in thermal-buffering features (like forest cover) and can become very hot environments in summer. Solitary cavity-nesting bees and wasps occur in agricultural landscapes and provide crucial ecosystem services (pollination, pest control). It is well documented that high temperatures disrupt the reproduction and development of insects, but there remain unanswered questions about how solitary, cavity-nesting bees and wasps that occur in agricultural landscapes are impacted by forest microclimates and high temperatures, especially in terms of their nesting success. In this study, I investigated the impacts of forest cover in agricultural landscapes on temperature inside bee and wasp cavity-nests in margins adjacent to crop fields, and the impacts of forest cover plus nest temperature on the nesting of solitary cavity-nesting bees and wasps. I observed two scales of forest cover: at the scale of 1 km-radius landscapes and at the local, field-margin scale, where margins were either forested or open. My first objective was: (1) to assess the effect of forest cover in agricultural landscapes on nest temperatures. Next, I investigated the impact of forest cover and nest temperatures on (2) nest abundance and richness, (3) brood cell number and (4) developmental success. Daily maximum, mean and range of nest temperatures were significantly lower in forested field margins, but forest cover at a larger scale (in 1-km radii) had no effect on nest temperature. Cavity-nests in forested margins had greater probability of developmental success, but forest cover (neither scale) and nest temperature were not associated with any changes in nest richness or brood cell number. Cavity-nesting bees were more associated with open margins, while wasps were more associated with forested margins. These findings point to the potential of incorporating both forested and open habitat types in agricultural landscapes to better support solitary cavity-nesting bees and wasps.