Living in a world with rapidly declining biodiversity, how does the gardener intent on mitigating this loss, at least in her little corner of the world, measure success? Keeping in mind that biodiversity in the garden ecosystem is measured as the total number of unique species in that place, any sort of direct count would be beyond the gardener’s reach. Most of us lack the training, tools, and time to identify every living creature which inhabits the place we call The Garden, particularly if we include soil life in the census.
You can, however, get a real sense of garden biodiversity by monitoring key groups of highly visible organisms. For example, how many bird species use your garden for food, shelter, or nesting? If you are willing to give caterpillars in the garden their due, the number of bird species you see feeding on those larvae are an indication of high diversity in both birds and insects. Those moth-eaten tree leaves at the end of summer are another signal of success.
Even better, perhaps, would be a running tab on the number of different insect species found foraging for nectar and pollen on garden plants. This subgroup of total biodiversity would include not only pollinators but also hoverflies, tachinid flies, and other insects that as adults feed on pollen and nectar, while their larvae prey on aphids and other herbivores.
This group of nectar and pollen foragers constitutes insect diversity that is particularly relevant to the gardener. And while the pollination services rendered by this group may be most appreciated in the vegetable garden, needed pollen and nectar must be available from ornamental plants early and late in the gardening year in order to keep the pollinators in the area.
The potential for learning more about pollinator diversity in the garden ecosystem is great. Helpful resources abound, including an online series of regional publications, “Selecting Plants for Pollinators”, published by the Pollinator Partnership and the North American Pollinator Protection Campaign (http://www.pollinator.org/guides.htm), and the University of Maine Cooperative Extension (UMCE) Bulletin #7153, “Understanding Native Bees, the Great Pollinators: Enhancing Their Habitat in Maine” (http://umaine.edu/publications/7153e/). These and other publications make you aware of the high diversity of pollinators that can inhabit your garden if you attend to their primary needs: food, water, and nesting sites.
Native bees alone constitute a large group of pollinators, more than 270 species in Maine, a number too daunting to embrace. The Pollinator Partnership/NAPPC publication does a nice job of breaking this number down to 11 native bee groups, while the UMCE bulletin does a bit more lumping to come up with 6 groups. Nesting requirements for these various groups of native bees is summarized as follows.
Bumble Bees (of which there are 16 species in Maine) nest in abandoned mouse nests, rodent burrows, and under human-made cavities such as old boards and upside-down flower pots. In Marjorie’s Garden there is an old tree stump on the edge of the perennial bed where, from April to October, bumblebees can be seen entering and leaving their nest through a hole at ground level. This is evidence of a simple rule in gardening for insects: don’t be too tidy.
Plasterer bees, also called “polyester” or “cellophane” bees because the females line their brood cells with a cellophane- or polyester-type substance, range in length from 0.3–0.6 inches with the yellow-faced plasterer bees the smallest. Most species, other than the yellow-faced bees, are black with a white pile on their head and thorax (the region bearing wings and legs). These bees make their nests in bare ground banks, or cliffs.
The yellow-faced plasterer bees nest in twigs, plant stems, and wooden bee nest boxes. The smallest bees in the this group, they have hairless black bodies with yellow or white markings on their faces. They are often mistaken for tiny wasps.
Called “sweat bees” because of their attraction to human sweat, these bees are among the smallest, ranging from 0.1–0.5 inches in length. Some species are a bright metallic green, others brown or black. They nest in sunny areas of compacted soil void of vegetation, and also in rotten wood. They are fond of composite flowers such as rudbeckias, cosmos, and sunflowers.
Miner and Sand Bees
These bees, also called andrenid bees, nest in sunny bare ground, sand, soil, under leaf litter, or in banks and cliffs. Under ideal conditions, large numbers can be found in a single nest. Most are black, 0.3–0.6 inches in length, sometimes with abdominal stripes. These are among the first bees to emerge in spring, so you are likely to find them foraging dandelions.
Leafcutter Bees and Mason Bees
This group includes the all-important Osmia bees that are important in the pollination of Maine’s lowbush blueberry crop. Most species are covered with hairs adapted for collecting pollen and the thick layers of pollen on the bodies of females make them easy to identify. Bees in the genus Osmia are shades of metallic blue, or blue-black. Those in the genus Megachile are shades of gray-brown.
Most species in this group nest in old borings in trees made by other insects, such as beetles. Bees in this group also accept wooden bee nesting houses.
Aptly named, these bees nest in sandy soil and in gardens where pumpkins, gourds, or squash grow. They can be found at work before dawn and often sleep in the wilted flowers of pumpkins and squash.
From this list, it is evident that gardeners who want to invite native bees to nest in their gardens will be more successful if they leave a few old boards or upended flower pots lying about, allow one or two logs to rot slowly at the edge of the garden, leave leaf litter to accumulate in some areas, allow for a few sunny spots of bare ground, and minimize tilling to avoid disturbing in-ground nests. Above all, use absolutely no pesticides. All of these efforts will pay off in a greater diversity of native bees that provide pollination services through the growing season.
Providing Water for Garden Insects
Garden insects must have a clear, reliable source of water. It is very important that the water source have a shallow or sloping side so that insects can easily approach the water without drowning. This can be accomplished by partially filling a small saucer with sand so that the depth of added water in the middle is less than an inch deep.
A high diversity of insects foraging for nectar and pollen is a sure sign that your garden is in tune with nature. It matters little if you are able to identify every unique creature by its species name, but by recognizing each species as unique in the time of day you find it at work in the garden, in its choices of flowers to forage, and in its size and markings, you can count it as a unique species. And there is nothing more gratifying than finding a new species hard at work in your garden.
Author’s Note: Readers interested in attracting pollinators should read last week’s Bangor Daily News column, “Annuals That Attract Pollinators and Other Beneficial Insects”. Next week’s column will deal with garden shrubs that attract pollinators and other beneficial insects.