Go Natural And Organic
One of the biggest threats to bumblebees (and all bees) is the use of chemical pesticides. Unfortunately, these toxic chemicals are most commonly associated with lawn care and the lawn care industry — and these toxic compounds are being applied by the ton to the millions of acres of land covered by lawns.
Toxins to avoid: Systemic neonicotinoids are especially harmful and widely sold at the "big box" stores and uninformed nurseries and garden centers. Often overlooked is the use of agricultural chemicals on the soil, such as diazinon to "control" ground-dwelling insects and their grubs, pre-emergent herbicides, and fungicides.
For so many reasons, if you have a lawn, care for it organically! If you do need to protect your lawn from beetle grubs, the primary target of diazinon, use milky spore, a natural grub control. For above-ground insects, diatomaceous earth is a safe, natural alternative control.