I have plenty of opportunities this winter to stare into the canopies of trees. Mia the Beagle insists on taking me for a walk up the lane and back three times every day, regardless of the weather. Her idea of a walk and mine are at odds, as she is prone to follow a newly discovered scent from one side of the road to the other, back and forth, around in circles, until I push the issue. And since we are mutually tethered by a leash, I often find myself standing on the side of the road looking up into the trees.
This past Monday, on a midday walk with Mia, I noticed a flock of black-capped chickadees in a mixed grove of birches and maples, each bird flitting from branch to branch, pecking industriously at each stop. I estimated a dozen chickadees at work in this one spot along the road. My first thought was that this explained their frequent absence at my porch feeders – chickadees cannot live by sunflower seeds alone. My second thought was that they were eating insect eggs, something I see them doing every winter day on the rhododendron outside the kitchen window.
Later that day, I received an email from a national garden association, an organization that sends out regional “this month in the garden” advice. One of the recommended garden chores for January was worded as follows:
“Take a tour around the garden on a mild winter day and nip insect problems in the bud. Look for the tan egg masses of gypsy moths on the trunks and branches of trees, especially oaks; scrape them off and destroy them. If you see the stick-covered “bags” of bagworms on arborvitae and junipers, remove them to destroy overwintering eggs. The egg masses of Eastern tent caterpillars are most often found on apples, crabapples, cherries and hawthorns. Prune out and destroy the shiny, black, iridescent egg cases that encircle small twigs.”
The juxtaposition of these two events in the same winter day illustrates a dichotomy in thinking about the role of the gardener in our changing world. The garden association writer takes what I will call an anthropocentric (“human-centric”) view of the garden, the view that whatever the gardener can do to maximize productivity or minimize damage to garden plants, he should do. My own view, and that of a growing cohort of gardeners, is that we should adopt an “ecosystem centric” view, allowing the garden food web to function unhindered. We should leave well enough alone.
Chickadees, nuthatches, and other birds need the protein supplied by insect eggs in their winter diets. And those insect eggs that are not eaten by birds will, hopefully, hatch into larvae (caterpillars) that will provide essential protein to nesting birds in spring. Of course, those larvae will do their share of leaf munching before being caught by a bird and, if all goes well, some larvae will survive predation to pupate. Adult insects will develop from those pupae that survive predation and some of these adults, those not captured by predators, will start the cycle anew.
Or so it should be. When the gardener looks at a tattered oak leaf in late summer, she should smile, knowing that all is well in her garden.
Questions came to mind as I pondered the two events of last Monday. Just how do gypsy moths, bagworms, and Eastern tent caterpillars fit into the garden food web? Does the imported gypsy moth, like the Japanese beetle, have no natural predators, leaving control in the garden up to the gardener? Do chickadees eat gypsy moth eggs? What are the natural predators of bagworms and tent caterpillars? So, I spent a couple of hours on the Internet.
The gypsy moth, Lymantria dispar, an herbivore native to Europe that eats the foliage of oaks, maples, apples, birch, aspen, willow, mountain ash, pine, and spruce, first appeared in the United States in the late 1860s. Since that time, a variety of natural predators have been discovered, including over 20 insect predators and parasitoids (insects, usually small flies and wasps, that lay their eggs on their victims, the larvae hatching from those eggs devouring the host), small mammals, birds, a virus and a fungus species. Among the insect predators is the forest caterpillar hunter beetle (Calosoma sycophanta), imported to New England from Europe in 1905, that will climb the tree in order to eat both the larvae and pupae of gypsy moths. Active only when large gypsy moth larvae are present, this beetle does not pose a threat to native butterflies and moths. Other insect predators of gypsy moth include a parasitoid fly, a predatory stinkbug species, and two parasitoid wasp species. Among the birds that feed on gypsy moth eggs is – you guessed it – the chickadee, along with nuthatches who also peck at the egg masses in winter to extract the eggs. Predators of gypsy moth larvae other than insects include birds, rodents, frogs, toads, skunks, and raccoons.
Now, you tell me, with all of this help in the ecologically-functional garden, do we really need home gardeners walking about the winter garden with putty knives, scraping gypsy moth eggs off of plants? Yet this is exactly where such human-centric, “nip it in the bud” advice can lead. Sooner than later, any insect egg mass becomes a target for destruction, particularly when the offered description, “tan egg mass”, is so nebulous. Where would this leave chickadees and the other predators that depend on insect eggs for winter protein, or those birds that require the protein in caterpillars during nesting season? Not in your garden, for sure.
My research revealed similar stories for both bagworms (Thyridopteryx ephemeraeformis) and Eastern tent caterpillars (Malacosoma americana). Natural predators of bagworms, an herbivore that feeds on arborvitae and junipers, include a parasitoid wasp, populations of which can be enhanced in the garden by planting daisies and other members of the sunflower family. Bagworms are attacked by at least 11 species of parasitic wasps, as well as white-footed mice and sparrows.
Birds, including orioles, jays, chickadees, and nuthatches, are predators of Eastern tent caterpillars. Add to this list 127 insect parasitoids, 28 insect predators, frogs, reptiles, squirrels, skunks, mice, bats, and bears. One bear, in a single day, can eat 25,000 tent caterpillars. I’m not going to lose any sleep over a few webs along the stems of our highbush blueberries or in the top of one of our birches. This is what bird food looks like!
In the ecologically-functional garden, the gardener can spend winter walking with the dog, watching chickadees in the tree tops, and leave herbivore control in the hands of the garden food web.