I decided some time ago to keep a list of each unique species of plant and animal that finds a home in Marjorie’s garden, a log of the garden’s biodiversity. Last week the list grew by two species. The first was a black-throated green warbler feeding on insects in the compost pile. I had noticed this species before, flitting about the canopies of spruce and fir trees, but this new sighting provided evidence that it is a thread in the garden food web.

The second was a catbird wrestling an earthworm in the shade of a katsura tree. The worm was as long as the bird and it took a minute for the catbird to gain the advantage before flying into the woods, perhaps to its nest. Marjorie had heard catbirds singing, but this was the first evidence of its use of the garden to earn a living.

If you start thinking about your garden as a bastion of biodiversity, as a refuge for wildlife imperiled by habitat destruction, particularly insects (“the little things that run the world,” as ecologist E.O. Wilson calls them), then you want to see aphids and other “pests” on your garden plants, to count them among the garden’s diversity of wildlife. For without these pests, there will be no predator insects, no hoverflies, lacewings, parasitic wasps or ladybugs, the predators that keep populations of garden pests in balance.

Without a diversity of insect species in the garden, there will be no warblers or catbirds, and no cardinals, purple finches or other bird species that we enjoy at our seed feeders. They feed insects to their young.

If you resort to toxic chemicals to control insect pests, you break down the food web that creates biodiversity. Even botanical pesticides, rotenone for example, kill beneficial insects as well as pests.

Aphids in the garden are good news, assuming that you can also count their predators as part of the garden’s wildlife. Hoverflies, lacewings and ladybugs all prey on aphids. Hoverflies also eat mealybugs and other soft-bodied pests, while lacewings eat scales and mites.

Populations of these beneficial insects can be cultivated by creating an insectary planting in your garden. Insectaries are small patches of plants that attract the predatory insects by providing the pollen and nectar resources that they require. For example, Marjorie and I enjoy nasturtiums and place them in pots about the garden. Aphids like nasturtiums too, so to make sure that the aphids do not get out of hand we plant cosmos about the garden, a plant favored by both hoverflies and lacewings. The predators control the aphids to the point where we seldom notice any actual plant damage.

Other plants to include in your garden insectary include members of the carrot family such as fennel, coriander, dill and Queen Anne’s lace; they are used by a wide variety of predators. Composite flowers, including sunflower, cosmos, yarrows and daisies, will attract lacewings, ladybugs and parasitic wasps, while alyssum, a member of the brassica family, will catch the attention of hoverflies.

Plan your insectary for successive bloom from early spring through fall, providing nectar throughout the season. This will not only satisfy the needs of many beneficial insects, but also provide color in the garden.

A garden insectary should be thought of as a long-term permanent component of your garden. Results are not instant but the benefits to your garden are cumulative. As resident populations of beneficial insects become established, your garden will become a balanced environment with a complex food web of plants, insects, birds, and, of course, the gardener.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.