URBANA - Although mass plantings of red and yellow tulips in city parks are beautiful and a few well-placed trees surrounded by mulch in a parking lot provide shade for a car or two while breaking up the monotony of the asphalt, according to Lawrence Hanks, a University of Illinois entomologist, unless they are ruthlessly sprayed with insecticides they are sitting ducks for insect infestations.
"Controlling the insect population without the use of insecticides is a simple concept, really. It's something that's been looked at since the 60s," said Hanks. "The more plant diversity, the more stable the environment will be."
Hanks began a study in 2000 with the hypothesis that planting a variety of flowering plants would attract natural enemies like insects to an area, keeping the insect population in check without the use of insecticides. The test plots have already demonstrated that the method works. His ultimate goal is to provide landscapers with a list of flowering trees, plants and ground covers that will make a sustainable landscape for insecticide-free maintenance.
"What we're recommending is that landscapers create a little plant community, a mini-wildlife refuge with plants that support insects providing food for natural enemies like ground beetles and crickets," said Hanks. He said that rather than islands of greenery in a parking lot, he would like to recommend that landscapers design more of a swath of green space -- one that perhaps links with a larger nearby natural area or field.
"They may have to give up a couple of parking spaces to make it happen but they can also make an economic argument because in the tiny areas they have to replace the trees more often due to infestation and they have to spray more."
He explained that planting a tree with mulch around it is a popular landscape design for parking lots and along city sidewalks, but it is a pretty bleak ecosystem. If you plant a shrub next to the tree, you're better off than having the tree by itself, but Hanks would like to see even more plant diversity to control the insect population. The alternative is regularly spraying with insecticides.
"People want greenways in their cities. They want flowers and birds but not the pests," said Hanks. "They don't want grasshoppers chewing up their greenery. But they also don't want to use insecticides. If they use insecticides, they won't attract birds and their own domestic pets and children may suffer from contact."
Hanks said that if people can accept having a variety of insects in their greenscapes, the population of one pest won't get out of control. "Pine needle scales are terrible pests of ornamental plants," said Hanks. "We noticed that in settings with diverse landscaping, you can hardly find one scale. But in a small parking lot green space -- the kind you find next to a fast food restaurant -- there was a very high density of scale infestation."
The ecological balance sustains the populations of natural enemies.
"The next step in the study is to identify the mechanism -- that is, what is it about the combination of plants that works? Is it the flower or the green part of the plant? Our study this summer will look at plots with the flowers intact and plots with the flowers snipped off to see which is more effective at controlling the insect population."
The overall goal is to reduce the use of insecticide. Hanks said that it might mean that some people will have to change their mind to accept this strategy. But it would mean that instead of an insect explosion that comes in and destroys the landscape, there would be much lower densities in an ecological balance that encourages other types of animal life.
"So we're saying that for the sake of an ecologically-balanced environment, without the need for weekly spraying, accept the insects and along with it you'll get more of what you want like birds and butterflies."
This research is being conducted in collaboration with Clifford Sadof of Purdue University, and is supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture. Hanks is also affiliated with the Department of Natural Resources and Environmental Sciences in the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.