Article: Susannah B. Lerman, Alexandra R. Contosta, Joan Milam, and Christofer Bang. “To mow or to mow less: Lawn mowing frequency affects bee abundance and diversity in suburban yards.” Biological Conservation 221 (2018): 160-174. Doi: https://doi.org/10.1016/j.biocon.2018.01.025
Suburban Green Spaces and Bee Refuges?
We all know that bees are important pollinators, and many of us are probably aware that several pollinator populations have been declining due in part to habitat loss. However, previous studies have shown that cities can support a rich and diverse bee population in green spaces such as public parks. But what about one of the largest ‘green spaces’ in the Unites States – private lawns?
In terms of ecosystem services, lawns can have a good and a dark side. On the positive, lawns store carbon, can help regulate moisture to reduce the suburban heat island effect, and are a pervious surface which helps absorb water run-off. However, some lawn care practices include high water use (irrigation), chemical applications (herbicides and fertilizers), and can emit greenhouse gases through mowing.
As this study points out, lawns without a high use of herbicides can also have a rich nectar source in the form of lawn flowers such as white clover. So is it possible that alternative lawn mowing practices could help support bee populations, even in a suburban environment?
The Methods: Mowing, flower counting, and bee catching
Sixteen single family homes in the suburban town of Springfield, Massachusetts were used to test three different lawn mowing behaviors over two years from May through September. Each participating home was restricted from watering, applying herbicide, or installing a pollinator garden. The participating residences were assigned to mow the lawn every week, every 2 weeks or every 3 weeks. Each mowing event used the same mower, set the same grass height, and left the grass clippings on the lawn.
Every 3 weeks, before mowing would occur, bees were sampled and identified. To attract bees the researchers set up pan traps with nectar or used small handheld nets. Additionally, the total number of each type of flower was counted for the entire yard. Lastly, the grass height was measured before each lawn was mowed.
Susannah Lerman and colleagues then looked at the differences in the bee abundance, bee species diversity, the floral abundance, and the grass height for the three different lawn mowing treatments.
What did they find?
There were differences in both bee and floral abundance between the three different lawn mowing treatments. For the bees, the ‘mow every 2 week’ treatment had significantly more bees, however, it also had lower bee richness and evenness compared to the ‘mow every 1 and 3 week’ treatment. The high bee abundance in this 2-week mowing treatment, but lack of diversity and evenness, was due to the dominance of only a handful of generalist bee species. In fact, throughout the study, only 10 bee species made up 78% of all bees captured and observed!
In terms of floral abundance, the ‘mow every 3 weeks’ regime had significantly higher floral abundance compared to the ‘mow every 1 and 2 weeks’ treatments. However, ‘mow every 3 weeks’ treatment also had significantly taller grass, as one might expect. A total of 54 different species of flowers were identified, with white clover being the most abundant.
The researchers hypothesized that the ‘mow every 3 weeks’ treatment may have had a lower bee abundance (which was not expected), despite having more floral resources, due to sampling bias. In other words, there was so much nectar to pick from, the pan trap bait did not seem tempting. Alternatively, it could be that the shorter grass in the ‘mow every 1 and 2 weeks’ treatments provided easier access to the flowers for the bees.
What Does this Mean for your Lawn Mowing Habits?
Ultimately, Lerman et al. suggest applying a ‘lazy lawn-mowing’ approach to your own yard. The two week treatment seemed to be the best compromise between a pollinator-friendly lawn, but not an unkempt mess like the 3-week treatments (as decided by the residents and neighbors of the 3 week treatments). Overall, even in a suburban environment, you can have an abundant bee community – all the while saving time and money by mowing less.