When I was a boy growing up on the central Georgia-Alabama border, all doors and windows were screened and a fly swatter hung on a nail in most rooms. Some folks, like Dr. Lenoir, head of the small biology department where I studied as an undergrad, eschewed the swatter in favor of catching a fly on the wing, without interrupting our discussion, and throwing it with sufficient force against the nearest wall.
Until I started gardening and paying much closer attention to the creatures in Marjorie’s Garden, these memories formed the substance of my understanding of flies. I had no real sense of the diversity within the insect order Diptera, nor of the role of some fly species as pollinators or the work of other species as predators of aphids, caterpillars, and other garden herbivores. All I had to do was stop and look to see flies in a totally different way.
Hoverflies in the Garden
Among the most frequent visitors to flowers in our garden are adult hoverflies, also called flower flies or syrphids, the later name in reference to their family, the Syrphidae, a group of about 6,000 species within 200 genera. The common name of “hoverfly” comes from their habit of hovering over flower heads before alighting to feed. In our garden, we find them feeding on the nectar and pollen of white and yellow flowers, including the white flower clusters of Queen Anne’s Lace (Daucus carota) and mapleleaf viburnum (Viburnum aceriifolium), the single white blossoms of crabapples, and the golden flower clusters of yarrow (Achillea sp.) They are also attracted by odors, including the sweet-smelling pink spikes of summersweet clethra (Clethra alnifolium) and the nectar-rich blue flowers of annual plants such as lobelia (Lobelia sp.) and bachelor’s buttons (Centaurea cyanus).
Hoverflies bear little resemblance to houseflies except for the presence of only two wings, a characteristic that separates all Dipterans from the four-winged bees and wasps (Hymenoptera). Some hoverflies mimic the bold warning colors of bees and wasps in an effort to ward off potential predators, thus the presence of only one pair of wings helps to distinguish them, along with antennae typically shorter than those of bees or wasps.
Pollinating hoverflies are typically not as hairy as most bees, a factor which makes them less efficient pollinators. Some species compensate for this, however, by making more frequent visits to flowers. Certain hoverflies are important pollinators for specific plants in natural ecosystems while others are occasional pollinators of vegetable garden plants.
Hoverfly larvae are either saprotrophs, feeding on decaying plant and animal matter in the soil, or insectivores, feeding on aphids, thrips, and other plant-sucking insects. In both cases, they perform a valuable garden service. Hoverflies in the genus Volucella are unique in that they use their mimicry to enter the nests of bees where they lay eggs. The larvae act as nest cleaners, feeding on dead bees and detritus.
Tachinid Flies in the Garden
Flies in the family Tachinidae, commonly called tachinid flies, are the most important form of parasitoid fly providing biological control of garden herbivores. (Parasitoids ultimately kill their host while parasites do not.) They resemble houseflies except for very stout bristles at the tips of their abdomens. There are over 1,300 known species of tachinid flies in North America.
Tachinid larvae are internal parasitoids of a large variety of immature insects, including the larvae of beetles, butterflies, moths, sawflies, and the nymphs of earwigs, grasshoppers, and true bugs. As a group, they control a wide variety of garden herbivores, including cabbage loopers, Japanese beetles, army worms, cutworms, tent caterpillars, squash bugs, Colorado potato beetles, and many others. The adults of some tachinid species glue their eggs to the host larvae while others lay eggs on the foliage where the host larvae will eat them. Still other species inject their eggs directly into the host’s body.
To date we have identified three species of tachinid flies in our garden. The larvae of one of these, Belvosia borealis, found foraging as a adult on Queen Anne’s lace, are parasitoids of moth larvae that feed on catalpa, ash, privet, and lilac. The larvae of Hystricia abrupta, a common tachinid that as an adult forages on the flowers of summersweet clethra, are parasitoids of tussock moth larvae and fall webworms.
Perhaps most important to gardeners is the work of a small tachinid, Istocheta aldrichi, imported from Asia in 1922 and released into the wild to combat agriculturally-important herbivores. The larva of this tachinid is an internal parasite of the adult Japanese beetle. Each adult female fly produces up to 100 tiny eggs over a two-week period, gluing them to the thorax of any adult Japanese beetles it encounters. Upon hatching, each maggot bores into the beetle’s body, killing it before it has a chance to reproduce. Early each summer, as we knock Japanese beetles off of grape leaves into a pail of soapy water, we quickly extract any that harbor the lethal eggs, tossing them onto the ground so that the fly larvae can finish them off.
So, keep a watchful eye this coming garden season. If you find a Japanese beetle with one or more small whitish dots just behind its head, don’t destroy it! Let it live out its short doomed existence so that the tachinid eggs will hatch and the larvae will consume the beetle on their way to becoming adult flies.
Reports on the effectiveness of tachinid fly parasitization of Japanese beetles in northern New England range from 20 to 40 percent. Sounds encouraging, but hold on, there are some interesting wrinkles preventing the fly from becoming the be-all and end-all.
First, the presence of the tachinid fly depends on the presence of the adult fly’s major food source, aphid secretions on Japanese knotweed, a non-native invasive plant that many of us are trying hard to eliminate. So, to combat the Japanese beetle we need to encourage an Asian fly which depends for its existence on an invasive plant species from Japan.
Also, the tachinid fly’s life cycle is not well synchronized with that of the beetle. The flies emerge several weeks before the beetles and thus only lay eggs on the first emerging beetles, then disappear before the peak of beetle emergence. Still, I think most gardeners will agree, we will take all the help we can get!
Take the time to search them out and you too will come to understand that flies are much more that targets for swatters and the swift of hand. They are pollinators, predators, and recyclers of organic matter. They are creatures that help us succeed as gardeners. Most important, they are part of garden wildness.