As pollinator habitat disappears, through development of native habitats, pesticide use, and other stresses, there has been a large body of research and instruction on how to support pollinators (and other beneficial insects) with native plants and overwintering techniques. Many sites recommend leaving dead plant material through the winter to help pollinators and other insects overwinter, and removing it in spring. Yet, spring happens over an extended period of time, weather warms and cools repeatedly, plants bloom early one year and other years, so I find the term “spring” to be rather vague.
Are there indicators we can use as guidelines for ecological garden maintenance? I found it difficult to find practical information about when it is relatively safe to remove overwintered debris in spring, but these sources about native bees that I’m sharing here provide a lot of information that can guide our recommendations and practice.
Bulletin #715 from the University of Maine Understanding Native Bees states, “The most likely place to find bees is in the flowers of native plants, when the day is sunny, relatively calm, and the temperature is above 70°F. To be active, fly and feed, bees need to be warm. A few species are active below 60°, but most prefer temperatures above 72°.” And, “Although some species may be active by late February if temperatures are unusually warm, the vernal bee species (those present in the spring) generally become active by mid-April. You may observe them on early blooming flowers, such as willow catkins and dandelions. Some native bee species continue their activities into the autumn until the last asters, dandelions and autumn dandelions die.”
“A suitable habitat for any bee species must contain a minimum of productive floral resources and patches, as well as nesting sites, and each of these must be within flight range of each other. While bees can often exploit many different floral resources, solitary bees often have specific species that they utilize for their pollen sources. If these bees require specific pollen sources, their life cycle will often revolve around the blooming period of the specific plants. The adult bees will emerge when the plants are in bloom, so that they are able to maximize pollen collection (Cane, 2001) UVic Restoration Project. For instance, the blue orchard Mason bee nests in stems or manmade nesting tubes and their emergence coincides with the blossoming of most fruit trees, including apples, pears, and stone fruits (Home Orchard Society).
Despite being from California, the general guidelines from entomologist Robbin Thorp should be applicable elsewhere: “It’s probably best to frame the bee calendar in context of the bloom of various plants,” Thorp points out. “Manzanita (a CA native) is one of the first flowering shrubs and when they come in to bloom that is the time to look for queens of our two early bumble bee species, Bombus melanopygus and B. vosnesenskii. Some of our large digger bees like Habropoda and some Anthophora come on during that bloom. In the vernal pools, early flowering starts in late February and some of our solitary ground nesting mining bees, Andrena start about then. When the red bud comes into bloom about mid-March the Blue Orchard Bee (BOB), some other species of bumble bees, and some sweat bees come out. Leafcutting bees (Megachile) and some long-horned digger bees (Melissodes and Svastra) start their activity about mid-May. ”
And another great resource is Heather Holme’s website Restoring the Landscape with Native Plants. The website provides very
accessible information on a wide variety of plants and insects native to the Midwest (many that the Northeast has in common) and discusses how the plants and insects interact with each other. The website for her book Pollinators of Native Plants includes links to downloadable full-color pdfs of native plants for specific site conditions, the nesting habits of native bees, and common host plants of butterflies, skippers, and moths.
And for those on the West coast: