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UD scientist fears insect food supply affected by non-native plants

NEWARK, DE.--Ever wonder about an insect’s preferred cuisine? Not likely, unless, of course, the critter in question is an uninvited guest chewing on your prize rose. Yet Doug Tallamy, University of Delaware professor of entomology and applied ecology, is concerned.

In a recent study, he found that insects largely ignore the leaves of non-native plants, opting instead to eat the leaves of native species. But with non-native plants choking out native growth, the insects’ food supply is disappearing rapidly. At issue is the availability of insect food, a fact on which the entire food chain rests.

“Why should the disappearance of insects concern people who spend millions every year getting rid of them in the garden?” Tallamy asks. “Plants are at the base of the food chain, and insects feed on plants. If insects disappear, so do the creatures that depend on them.
“More than 90 percent of insects are restricted in their diets, because they do not have the enzymes required to digest the leaves of non-native plants, sometimes referred to as exotics,” he explains. “Fewer plant food sources, fewer insects, wildlife vanishes.”

Over several months this year, Tallamy and Rebekah Baity, an undergraduate researcher in the UD College of Agriculture and Natural Resources, began to measure the amount of leaf area eaten by insects on more than two dozen plants and trees. His research site was 10 acres of long uncultivated farmland in Chester County, Pa., where non-natives—specifically, oriental bittersweet, autumn olive, Japanese honeysuckle and multiflora rose—have crept into unmowed fields once inhabited by native plants.

Evaluating the food preferences of native insects, Tallamy found that the insects consumed 239 square centimeters of the leaves on black oak, a native American tree, as compared with 12 square centimeters of Norway maple, a highly invasive species rapidly spreading throughout eastern North America.

The UD entomologist says that although a number of studies have been conducted on ecological problems caused by non-native plants, the focus has been the economics of unchecked invasion and the biological threat of displacing native plants. Tallamy said he believes his is the first study to quantify the effects of non-native plants on the food supply of insects, relate it to the food supply of birds and the ultimate consequence to ecological balance.

“Fast-growing, non-native vines such as bittersweet, Japanese honeysuckle and climbing bushes such as multiflora rose can actually strangle a healthy tree—even a native mulberry like this one,” Tallamy says, pointing to a huge tree trapped under a jumble of honeysuckle vines.

“All plants vie for sunshine for photosynthesis, yet vines have an advantage,” he says. “Before long, the tree weakens, first losing its ability to bear leaves, seeds and fruit, then declining to the point that a wind or ice storm finishes it off. The result in a loss of plant food for insects and protein and shelter for birds.”

Tallamy points to the multiflora rose as an example of a shrub introduced into this country as an ornamental plant. This rose offers excellent nesting sites for birds and berries for them to forage in fall, so marketers purposely appeal to people who encourage wildlife in their yards, gardens and farms. But now this aggressively growing bush has overtaken creek banks and roadsides all over the United States, effectively choking out native plant species.
“Yes, it does provide nesting and berries for winter-feeding birds,” notes a frustrated Tallamy, “but what is overlooked is that native birds also nest in native plants. More important, when raising their young in the spring, birds depend upon a supply of insects and larvae to feed the hatchlings. If the insects are eliminated, so are future generations of birds.”

According to Tallamy, when people see green open spaces they think nature has taken over. What they don’t recognize is that the fields of green in many parts of southeastern Pennsylvania and Delmarva are overrun with 90 percent non-native growth.

“A green field in which the native plants have been choked out by invasive non-native ones is no more productive than a parking lot when it comes to providing food for insects, birds and other animals,” the scientist says.

“The non-native butterfly bush is another case in point. People plant butterfly bushes in the mistaken idea that they are helping nature,” he says. “Okay, the bush attracts butterflies for its flower nectar, but they will not lay their eggs on these leaves, because the foliage offers no nutrition to the hatching larva.

“Planting a native species, such as viburnum, milkweed, Joe Pye weed or purple cone flower for every butterfly bush—now that would really help nature. “

Tallamy is optimistic that the data he and Baity have collected will attract the attention of birders, who may be able to influence the garden marketplace.

“I hope the scientific evidence gathered in this study will help spur people into action,” says Tallamy. “Just imagine the future impact on the bird populations and our natural heritage if homeowners replaced the non-native ornamental plants on their property with plant species historically native to the area.”

The research is being funded by the Adkins Arboretum of Ridgely, Md., and the University of Delaware Science and Engineering Scholars Program.

Contact: Patricia McAdams, (302) 831-1356, mcadams@udel.edu
Date: Aug. 2, 2001

Junk Food for Insects:

  • Multiflora rose
  • Oriental bittersweet
  • Japanese honeysuckle
  • Autumn olive
  • Mile-a-minute weed
  • Garlic mustard
  • Norway maple
  • Ox-eye daisy
  • Red clover

Foods Insects Like:

  • Viburnum
  • Hickory
  • Oak
  • Black walnut
  • Black willow
  • Elderberry
  • Goldenrod
  • Black cherry
  • Red maple