Long ago, bees were hunters (sound familiar?), with protein the core of their diet. With the evolution of flowers, bees became gatherers of pollen and nectar, which is why the diversification of flowers is so important.
Worldwide there are twenty thousand species of bees, represented by 9 families – 6 of which are found in Minnesota. Within these 6 families are at least 320 different species! And 98-99% of them are solitary bees. meaning they do not live in social congregations like honey bees do. 30% of them are tunnel nesting bees (Mason, leaf cutter, yellow faced, carpenter East Coast bees, and others). The other 70% of solitary bees are ground nesting bees (sweat bees, miner bees, plaster bees, digger bees, etc,). The remaining 1-2% live in social communities as social cavity nesters (honey bee democracies). Mind boggling?
Animal transportation is the dominant form of pollen movement (as compared to wind, and other mechanisms), and accounts for 78% of it in temperate climates, and 94% of pollen movement in tropical climes. When we remember that 35% of the global food supply relies on pollination, this critical mode of transport is underscored. Enter wild bees: their densely haired bodies are designed for this task. Their branched hairs increase surface area and are statically charged to attract oppositely charged pollen, which sticks to the hairs and is carried off by nature’s gatherer.
Bees get their nutrition from nectar and pollen and have distinguishing palates – all nectar and pollen is not created equally from a bee’s perspective. Bee diversity helps fill many niches across the spectrum of pollination, as different bees are adapted to harvesting from different sized and configured plants given different tongue lengths, or by preferring differing times of the season or day, flying under different weather conditions, etc. Alfalfa is suited to leaf cutters and sweat bees, with honey bees needing to chew into the flower from the sides so as to avoid getting drops of nectar on their heads; bumble bees can unhinge their flight mechanism and vibrate sounding the note “C” when working tomatoes and blueberries, etc). Potential crop yields of almonds and sunflowers demonstrate that species will change their behaviors around each other; studies of strawberry and blueberry pollination reveal altered foraging strategies that change with the weather. The bottom line: synergistic activity to maximize pollination.
|Rusty Patch Bumblebee|
Photo courtesy of Sarina Jepsen
The Xerces Society
And now “The Troubles.” A recent Illinois study has shown >50% decline in wild bee populations compared to 1916, with only 54 of 109 species being re-discovered. Joel Gardner, one of Ian’s colleagues in the Dept. of Entomology, has demonstrated 3 new species, but 11 species NOT re-discovered since 1937 in his recent research at Lake Itasca in MN. A glaring example is the 87% decline in Bombus affinis, the Rusty Patched Bumble Bee.What are the causes of these declines? Mainly urbanization and agricultural practices. There is strong evidence that natural habitats support greater diversity (kind and quality of plants) and abundance (more food for bees).As noted previously, bees prefer different pollens as well as different nectars. Some bees are generalists (polylectic); some requiring specific types (oligolectic); and some that can utilize only one type (monolectic – like oil bees [Macropis steironematis], who specialize on fringed loosestrife of the primrose family). There are 53 species of olig- and monolectic specialist bees – so the importance of diversity is critical to them!
Nectar is also quite different, and not all bees can drink of the same cup due to anatomic variations such as tongue length: bumble bees have all lengths of tongues, digger bees have the longest, honey bees are mid-range at 6mm. Due to differences in corolla length on flowers, and the viscosity of the nectar, bees are limited as to which nectar they can harvest. Honey bees search for water, but many wild bees do not – which is why viscosity matters - again, reinforcing the need for diversity and variety of plants.
That being said, the duration of the phenological season is key – the shorter the season, the more poorly bees will fare. Our main goal should be to avoid “phenological gap” by providing season-long blooms. This helpful chart comes courtesy of Dan Palmer.
|Week:||1 2 3 4||1 2 3 4||1 2 3 4||1 2 3 4||1 2 3 4||1 2 3 4|
|Silver Maple||X X X|
|Crab Apple||X X|
|Sugar Maple||X X|
|White Dutch Clover||X|
|Yellow Sweetclover||X X X|
|Birdsfoot Trefoil||X||X X||X X|
|White Sweetclover||X||X X X X||X|
|Spotted Knapweed||X||X X|
|Purple Loosestrife||X X|
|Beggars Ticks/Stick tights||X X||X|
|Data courtesy Dan Palmer|
Bees also demonstrate “floral constancy,” where, in the case of a highly abundant and nutritious plant, they will intensely focus on it to conserve on foraging time. This may indicate that monoculture “patches” can also be good for some generalists (polylectics). Ian’s own research on clover in Minneapolis has shown that both honey bees and bumble bee visitation responses top out at about 70 flowering heads per 50 square meters. Other excellent plants are hyssop (abundant nectar production, 44% sugar) and monarda (40% sugar in nectar, great for bumble bees secondary to the long corolla but not so much for honey bees – resulting in decreases competition, which cuts energy costs to make each visit).
Nesting sites are also linked to diversity. These include requirements for mud, leaf pulp, gravel, plant resin, and other building materials. Solitary bees and bumble bees need their nests to be close to flowers, in a range from 1/5 to 1/2 mile. We can help by providing nesting resources like sand, not over-mulching, and reconsidering our “American lawns” with an eye to Dutch white clover, dandelions, wild thyme, and to our seeding practices.
Pesticides, diseases, climate change (affecting the timing of flowering) – all of these also impact our wild bee populations. We can be mindful of species relationships, floral niches, improved nutrition, and use dense plantings to do our parts in helping our wild winged ones live long and prosper.
|Bee on borage photo courtesy of Marcie Forsberg|
The University of Minnesota Bee Lab has a pollinator outreach garden. A list of plants in the garden comes from the Eloise Butler Garden (www.friendsofthewildflowergarden.org ). See also Eric Mader’s “Conserving Pollinators: A Primer for Gardeners” at www.extension.org. Elaine Evans, Ian Burns, and Marla Spivak have a publication “Befriending Bumble Bees” which is a wonderful resource http://befriendingbumblebees.com/
Thanks, Ian, for a provocative, informative discussion!
bg with production assistance from elw :-)