Conserving native bees for their vital pollination services is of national interest. It turns out that an excellent way to save bees is to not treat lawns with herbicides (weed-killers) or any other pesticides. It turns out that lawns with "flowering weeds" (think wildflowers or flowering plant species such as dandelions and clover) are valuable nectar and pollen sources for all sorts of species of bees (and also for butterflies).
And as much research shows, pesticides (insecticides, herbicides, fungicides) have all sorts of negative health effects - on humans, pets, and wildlife (here, here, here, and here).
Chemically treated lawns are "dead zones" with an absence of normally occurring plant species. In contrast, a study found 63 different flowering plant species in 17 suburban lawns that were NOT treated with any herbicides or pesticides over the 2 year study period. The bee species they found tend to travel only short distances and so lived within close proximity of the lawns with the flowering weeds.
So do your bit for both human health and to protect bees, and don't treat lawns with chemicals. View what we now call lawn weeds as "spontaneous flowering plants" and that they are wildlife habitats. Of course using an organic fertilizer is OK, but just "mowing and leaving grass clippings" is also an excellent way to fertilize the lawn.
From The University of Massachusetts at Amherst (press release): To Help Bees, Skip Herbicides and Pesticides, Keep Lawns Naturally Diverse
Declining populations of pollinators is a major concern to ecologists because bees, butterflies and other insects play a critical role in supporting healthy ecosystems. Now a new study from urban ecologists at the University of Massachusetts Amherst suggests that when urban and suburban lawns are left untreated with herbicides, they provide a diversity of “spontaneous” flowers such as dandelions and clover that offer nectar and pollen to bees and other pollinators.
Private lawns make up a significant part of urban lands in the United States, an estimated 50 percent of city and suburbs, say Susannah Lerman and co-author Joan Milam, an adjunct research fellow in environmental conservation. They write, “Practices that support nesting and foraging opportunities for bees could have important implications for bee conservation in suburban areas.”
Lerman, an adjunct UMass Amherst faculty member who is also with the U.S. Forest Service, says, “We are still surprised at how many bees we found on these untreated lawns.” In this study of lawns in suburban Springfield, Mass., she and Milam found that “spontaneous lawn flowers could be viewed as supplemental floral resources and support pollinators, thereby enhancing the value of urban green spaces.”
For this study, supported by the National Science Foundation, the researchers enlisted owners of 17 lawns in suburban Springfield. Between May 2013 and September 2014, the homeowners did not apply chemical pesticides or herbicides to lawns. “We documented 63 plant species in the lawns, the majority of which were not intentionally planted,” the authors report. Lerman and Milam visited each yard six times per year for two years, collecting a total of 5,331 individual bees representing 111 species, of which 97 percent were native to North America.
Conserving native bees for their vital pollination services is of national interest, Lerman and Milam point out.....Overall, one of their main findings, say Lerman and Milam, is that “when lawns are not intensively managed, lawn flowers can serve as wildlife habitat and contribute to networks of urban green spaces.”
Further, “developing outreach to homeowners and lawn care companies to encourage, rather than eliminate, lawn flowers such as dandelions and clover and thin grass cover or bare spots could be a key strategy for urban bee conservation programs targeting private yards.”
Original study from the Annals of the Entomological Society of America: Bee Fauna and Floral Abundance Within Lawn-Dominated Suburban Yards in Springfield, MA
Private yards comprise a significant component of urban lands, with managed lawns representing the dominant land cover. Lawns blanket > 163,000 km2 of the United States, and 50% of urban and suburban areas. When not treated with herbicides, lawns have the capacity to support a diversity of spontaneous (e.g., not planted) flowers, with the potential to provide nectar and pollen resources for pollinators such as native bees.....We collected 5,331 individual bees, representing 111 species, and 29% of bee species reported for the state. The majority of species were native to North America (94.6%), nested in soil (73%), and solitary (48.6%).
We recorded 63 different flowering plant species in 17 lawns during 2013 and 2014. Dandelion (Taraxacum officinale) was the most widespread flower, found in all lawns in both years (Table 4). White clover (Trifolium repens), purple violet (Viola sororia), yellow wood-sorrel (Oxalis stricta), Canadian horseweed (Conyza canadensis), annual fleabane (Erigeron annuus), dwarf cinquefoil (Potentilla canadensis), and Pennsylvania smartweed (Polygonum pensylvanicum) were recorded in at least 60% of all sites for the two years (Table 4). In 2013, horseweed, hairy rock cress (Arabis hirsute), and white clover represented more than 67% of all flowers, whereas in 2014, white clover, yellow wood-sorrel, purple smartweed and purple violet were the most abundant species.