Gardening for Nature
Save our Essential Insects!
LAST MONTH I HAD AN OPPORTUNITY to listen to a talk by Doug Tallamy, Professor and Chair of the Department of Entomology and Wildlife Ecology at the University of Delaware in Newark, Delaware, and author of Bringing Nature Home: How You Can Sustain Wildlife With Native Plants (Timber Press, 2009). Tallamy laid out the case for utilizing native plants in landscape and shared his conviction that if we in the U.S. could devote half of our planted grass lawns to mixed native plants, we would add natural areas to the landscape greater than the area of all of our national parks combined. I think that’s a wonderful, though quixotic, idea—but then again I’m not the possessor of a fine, green lawn that has cost me much time and money. Tallamy will have an uphill fight to get traction with this.
His observations are sobering. If we’ve paid any attention at all we’ve noticed the number of insects and birds steadily decreasing around us. Fifty years ago when driving on a long trip on a summer’s evening, you would have to constantly stop and wash the slime of squished and encrusted “bugs” off the windshield. This past summer when I was driving in the evening in N. E. Ohio for a long distance, past a reservoir and farm fields and through parkland, I think I had to clean my windshield twice—just twice in about 80 miles—and then only when I was close to one of the reservoirs. Where did all the insects go? And why so few birds?
Tallamy points out that breeding birds in the summer depend heavily on insects to feed their young—especially on those bags of juicy food that we recognize as caterpillars—and native plants are where the birds will find them. The lack of native plants and the ubiquitous use of insecticides and herbicides do away with this food source and the birds have to adapt. Most of them either move to another location—if they are able to—or simply don’t breed. Especially for birds that migrate, nature has worked it out that if food isn’t available to raise a brood, the birds will not try and instead will conserve their resources for the journey at the season’s end. On Staten Island we’re not so aware of this happening because we’re blessed with so much woodland. But on a larger scale, bird and insect numbers are way, way down.
Tallamy’s illustration of the value of native flora was the ginkgo tree. That Chinese tree with fan-shaped leaves had been thought extinct until it was rediscovered in Japan (by the Western world) around 1700. Now it’s commonly planted in temperate areas around the world in both public and private landscapes. How many kinds of caterpillars does a gingko support in our country? Tallamy is an entomologist so he checks on these things. The answer is one—just one species of insect caterpillar feeds on the leaves of the gingko. This is of course great news for a landscaper. Plant a gingko and you don’t have to worry about your tree looking (literally) moth-eaten. But it’s not so good for birds.
Tallamy’s comparison was native oak trees. If I recall Tallamy’s number correctly, over 500 different insects have been found to feed on oaks. There must be a preferred bug among them for a large number of foraging songbirds. Indeed I learned from his talk that a pair of chickadees needs to collect about 7,500 caterpillars to raise an average brood of chicks.
The same kind of value applies pretty much to herbaceous plants.
Entomologists have noted that most insects are very faithful to a particular plant food source not by choice, but by inheritance. In any one place plants and insects have competed over the ages and evolved together. The plants don’t want to be eaten and the insects want to eat them. Over eons plants developed chemicals to deter enough herbivory that they can survive and reproduce, and the companion insects developed enough resistance that they too can safely consume enough plant matter to reproduce. It’s a complicated system that sometimes goes awry, but usually some balance is achieved. The flowering plants especially need the herbivores—insects to pollinate, birds and mammals to spread seed, etc.—so they don’t produce toxins that will just kill off all the bugs.
The insect resistance of alien herbs provides them an advantage outside of the woodlands also. From the days of the earliest explorers wealthy people all over the world imported and displayed new plants. It was a status thing. As shipping became faster and easier and nurseries developed, both middle-class folk and the wealthy began to grow new plants from south of the equator and from overseas. If those new plants were insect resistant all the better, and that desirable trait would give many of those plants a head start if they made the leap from yard to field or forest.
I was mulling over some of this on my last few forays into the woods. Standing at the edge of a stiltgrass meadow opposite Moses’ Mountain, I couldn’t help but think how much that dense cover of alien grass has reduced the number of creatures that meadow could support. When we look at the grass, we see almost no evidence that anything has been eating it. Several bumblebees flew about visiting the wild blackberry that had recently come into flower and a few dark dragonflies hawked about. But beyond a few amber-colored skippers, I didn’t see much prey for the dragonflies—assuming that dragonflies would take such a large insect. An unexpected visitor was a ruby-throated hummingbird perched high up on the tallest branch of an isolated snag in the middle of the meadow. It must have been some 20 feet up and it didn’t stir for many minutes. I wondered what it was feeding on because none of the nectariferous, trumpet-shaped flowers we associate with hummingbirds were in bloom here and there certainly weren’t any feeders on that hillside. Finally, it lifted off, hawked something out of the air, and returned to its lookout. Hummingbirds, I find, aren’t just nectar feeders. They are happy with bugs when they can get them, and in fact depend more on insects they catch than nectar. I wondered if more stiltgrass implies fewer hummingbirds.
While I was there I glanced about at the trees with leaves and was happy to note that almost all—barring a small Norway maple—were native. Many of the adjacent trees were oaks, and I saw that the largest, a black oak, had quite perforated leaves. A few branches looked like someone had blasted them with a shotgun. This tree at least was providing a feast for our native bugs and, beyond those few branches, really didn’t look too much the worse for wear. But I can see that in an urban yard someone who was meticulous about appearances might prefer to plant an import “guaranteed” to be bug resistant. Think again.