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The Honolulu Advertiser
Posted on: Monday, March 22, 2004

Plants' anti-pest qualities debatable

By Jan TenBruggencate
Advertiser Columnist

Q. Many of us are supplanting lawns for groundcover because it saves time. There are ground covers that also keep pests at bay. I understand certain oregano does this with the Japanese beetle. Do you know of any?

— Lloyd Fischel, Ha'iku, Maui.

A. Many plants have developed techniques to protect themselves, and the strong smells and tastes of many herbs are believed to be forms of protection against insects.

There is an entire field of gardening called companion planting that deals with extending the natural protections of one kind of plant to a neighboring species.

Rodale Publishing, at its Web site, organicgardening.com, suggests chives next to roses will repel rose pests. Tomatoes may help repel certain cabbage pests, and dill may attract wasps that feed on cabbage worms. Marigolds may work against some flying insects, but there is better evidence that marigolds make life miserable for nematodes, those underground worms that attack many plant roots.

The City of Carlsbad, Calif., has a Web site, www.ci.carlsbad.ca.us/cserv/wdpest.html, that says mints repel ants, geraniums repel leaf hoppers, radishes repel stink bugs and marigolds repel thrips.

But some of the information may be based on folklore, which may or may not be accurate. Also, common terminology is unspecific. A certain mint variety might indeed work against certain ant species but not against others.

The alliums — the group that includes onions, garlic and chives — seem to be among the most celebrated of insect-repelling plants. They are variously claimed to repel beetles, cabbage worms, aphids and mites, not to mention rabbits and mice. But the literature suggests they have their own pests that could be problems.

With respect to the groundcover idea, University of Hawai'i vegetable crop specialist Hector Valenzuela said that planting all one species over a large area is generally not a good idea.

"Something that helps is growing different types of plants, perhaps in rows. Diversity helps. If you have companion plants, you might distract some pests," he said.

He also suggested that gardeners experiment. What works well in one area might not work as well in another.

As for oregano and the Japanese beetle, maybe it works and maybe not. The good news, Valenzuela said, is that the Japanese beetle, a pest across on the Mainland, is not yet believed to be in Hawai'i.

If you have an issue, question or concern about Hawai'i's environment, write to Jan TenBruggencate, The Advertiser's Kaua'i Bureau chief and science and environment writer. Reach him at jant@honoluluadvertiser.com, (808) 245-3074, P.O. Box 524, Lihu'e, HI 96766.