Signs of Summer 6: More on Bees

Photo by I.Tsukuba, Flickr

I had an email from a friend a few weeks ago asking me about the impact of lawn mowing on bees. Since I regularly mow a one acre field that is a complex mix of grasses and flowering plants, I have had some experience adapting my mowing to the populations of bees (and other pollinators) that visit the flowers throughout the summer. I also wanted to check the literature to see if there had been any published papers about the impact of mowing on bees.

It is important to note that most people mowing suburban lawns won’t have any significant interaction with bees or other pollinators because their lawns are pollenational deserts. It’s a good news/bad news thing! The expanses of  single grass species that are so carefully and expensively manipulated into flat green carpets have no plant species that would attract (or feed and nurture) pollinating insects. So for any of you managing one of these ecological disaster zones, you don’t need to worry about your mower interacting with bees.

For people like me, though, who allow a rich mixture of grasses and “weeds” (and I use that word in its most non-pejorative sense possible!), there are times of abundant flowering (of dandelions, violets, clovers, and many other breathtakingly beautiful “weeds”) when bees and other pollinators might be out in the lawn. So the question is: is a mowing a recognized hazard for bees?

I poked around in the literature trying to find any studies that examined or quantified the impact of lawn mowing on bees. I could not find a single article. I did find, though, some recent, very comprehensive studies and compilation papers that described the collective matrix of stresses that are negatively impacting bees.  The figure below summarizes the 2013 findings of the OPERA Research Center of the Universita Cattolica del Sacro Cuore in association with the European Union:

Lawn maintenance practices that generate small field sizes, monocultures of grasses, or that involve the utilization of broadly applied pesticides would definitely fit into the bee stress/impact matrix, but negative impacts of grass mowing practices don’t ever come up in the scientific discussion.

What do I do when mowing my mixed field of grasses and “weeds?” When the clover blooms I just don’t mow. I skip a week or two to let the bees have their fill of the clover nectar and pollen. The yard, the bees, the clover and the mower (me) are all benefited by a week or so off (there are many other things to do in the summer instead of mowing grass)! When I do mow and there are bees out and about on the scattered, flowering weeds, I go slow enough so that the bees (who are exquisitely sensitive to sounds, vibrations, and even electrical fields (see discussion below)) can get out of the way. I also keep my mowing blade quite high and often don’t even cut the clover flowers as I pass through the grass.

Photo Public Domain, Pixabay

In summer the “field bees” that are out gathering pollen and nectar have very short life spans. Worker bees only live for about six weeks spending just the last half of their lives flying out of the hive to visit flowers. During the long weeks of the summer, then, there will be wave after wave of field bees heading out to find flower sources. On their flights they are exposed to potential predators, to sudden changes in environmental conditions, and to other physical dangers (including mowing). Fortunately, the robust reproduction of a healthy hive can keep the field bees coming!

So the answer to my friend concerned about lawn mowing and bees is: the overall impact is probably small and there are some simple things that the mower can do to reduce that impact even more.

There have been several other articles about bees over the past few months. A study by the United Nations’ Intergovernmental Science-Policy Platform on Biodiversity and Ecosystems Services published online back in March enumerated the potential economic cost of the worldwide decline in pollinators. This study stated that both vertebrate and invertebrate pollinators are needed for 35% of the global crop production that sustain human consumers. The value of these crops is estimated to be $577 billion per year, and many of these pollinating species (including birds, bats, and 20,000 species of bees) are facing significant population declines and even extinction.  Causes of these declines include the loss of wild plants and their essential nectar and pollen foods, exposure of of the pollinators to pesticides, and the rising levels and increasingly rapid word-wide distribution of pathogens and parasites. The OPERA study chart (above) summarizes the UN’s findings on pollinator stresses very well.

A team of researchers at Penn State just published a paper in the journal Atmospheric Environment describing the impact of air pollution on the the ability of honeybees to detect (and follow) scent molecules being generated by flowering plants. Rising levels of ozone greatly inhibited the chemical sensory systems of honeybees and led to longer, less efficient foraging and pollinating patterns!

Photo by B. Moisset, Wikimedia Commons

In some happier research, a study published in June in the American Midland Naturalist examined the pollen types gathered by mason bees (Osmia species) around the Rocky Mountain Biological Laboratory in Colorado. A number of the mason bees studied specialized in gathering sunflower pollen from their surrounding vegetative habitats. Sunflower pollen, though, is very low in nutrient quality compared to other types of available pollen. The mason bees, though, that gathered the sunflower pollen had much lower levels of parasitic wasps affecting their larvae. There was a distinct, overall survival benefit, then, of reduced parasitism when the nutrient poor pollen was used nearly exclusively to fill the brood chambers and feed the mason bee larvae! Looking at bumblebees, though, another Penn State research group just published a paper in the Proceedings of the National Academy of Sciences in which they determined that these types of bees preferentially gather the highest nutritional quality of pollen from the flowering species in their habitats.

Photo by Trounce, Wikimedia Commons

And finally, a study published at the end of May also in the Proceedings of the National Academy of Sciences demonstrated that bumblebees have specialized body hairs that sense electric fields! This type of an electrical sensory system is quite uncommon in terrestrial organisms, but may be used by bumblebees to find flowers or to avoid dangers (like lawn mowers perhaps?). The researchers also speculated that these electrical sensory hairs may be present on many type of insects and may explain some anecdotal observations of insect (especially pollinating insect!) disruptions in areas of human generated “electric smog” (i.e. locations with power lines, and dense concentrations of radio waves and wireless communication networks).

So, your cell phone may be affecting your honey bees, everyone! Keep your calls short!

Summer is racing past! I hope that everyone is out enjoying it!