In many ways plants and insects are intimately related. Possibly the most widely appreciated is the 150 million-year-old mutual dependence of flowering plants and honey bees. Without honey bees, many of our crops would not be pollinated.
Plants also have developed strategies to discourage the multitude of insects that want to eat them. Plants keep insects at bay by using physical obstructions, such as tough leaves, spines and hairs, and chemical warfare. Chemicals called essential oils are believed to have evolved in plants mainly as a defense against insects. Touch the leaves of plants like rosemary or mint, and you will smell the essential oils designed by nature. Humans have taken advantage of these fragrant oils and used them in many products, including laundry and dish detergents, toothpastes, perfumes, air fresheners, drinks, cooking oils and even mosquito repellents.
In the development of insecticides, natural products have a big advantage over synthetic chemicals because they often are generally regarded as safe (GRAS) for humans by the Environmental Protection Agency. Chemical companies can spend more than $100 million in the development and registration of an insecticide, while a chemical on the GRAS list can cost less to produce and be put on a fast track to registration. Vetiver grass and termites
Vetiver grass contains a natural repellent to the Formosan subterranean termite. Vetiver grass is a type of bunch grass related to sugarcane and lemon grass. Because of its massive and fast-growing root system, it is known around the world for its ability to reduce erosion. It is native to India and has been grown in Louisiana for about 200 years. The fast-growing roots contain essential oils used in citrus sodas, soaps and perfumes. Early Louisianians place the dried roots in closets and armoires for its pleasing fragrance and to repel moths.
In the late 1990s, a Louisiana entrepreneur, Don Heumann, and his wife, Judy, brought vetiver plants to the attention of LSU AgCenter researchers, who found that a major substance – vetivone – was well-known to be the major insect-repelling chemical in vetiver grass extracts. This initiated an investigation into similar chemicals that might affect termites. One such chemical was found – nootkatone – which became a lead compound in termite control research. Scientists tested the roots and found them to contain other repellent and toxic compounds effective against Formosan termites in the laboratory.
The AgCenter scientists also were able to extract and identify from the roots several other components especially active in the repelling and killing of termites. Tests are under way to determine if the plant can be used as a natural repellent against termite invasions into homes by planting them as a hedge. Additionally, the U.S. Army Corps of Engineers is now considering the use of vetiver grass along levee systems in New Orleans to reduce erosion during hurricanes. This may also affect the termite infestations known to be at high levels on some of the levees.
Several AgCenter patents from this work have been licensed for development into commercial insecticides. Sesame seeds and ants
Sesame seeds have long been used as an ant repellent in kitchens in Egypt. Sesame oil is reported to control chinch bugs, spider mites, aphids, leaf rollers, leaf miners, white flies, fungus gnats and scale insects. AgCenter investigations of the components of unroasted sesame seed oil against Formosan termites showed that sesamol – a natural compound in the seeds – is an effective repellent at low concentrations and remains active for several months. Sesamol is relatively inexpensive and shows promise as a natural alternative pesticide. Patchouli oil
Patchouli oil has a nice woody smell detectable if you rub the leaves between your fingers. Its heyday in the United States marketplace was as a natural deodorant in the 1970s. The oil remains an important natural material in the perfumery and food industries and has been used in Asia historically to repel clothes moths and as a cold remedy. Using gas chromatography, AgCenter scientists determined that the oil consists mainly of patchouli alcohol (40 percent) and alpha-patchoulene (15 percent). Treatments of food sources with the oil or alcohol could dramatically stop termite feeding. The alcohol is about twice as effective as the oil itself. Naphthalene and 2-acetonaphthone
LSU AgCenter researchers discovered that high levels of naphthalene occur in Formosan subterranean termite nests. This led to the testing of a related compound – 2-acetonaphthone – a natural oil of corn. The performance of 2-acetonaphthone on the Formosan termite was evaluated via topical application, nochoice and choice treated filter paper assays, and in sand barrier assays. It proved highly effective in the topical application assays and in a sand barrier at concentrations as low as 8 parts per million. The strong repellency observed at concentrations much lower than some other naturally occurring chemicals has led to a patent submission by the AgCenter. Ku Shen and termites
A member of the legume family known as sophora root or Ku Shen contains alkaloids that possess broad biological activity. This ancient Chinese herb used in traditional medicines grows wild throughout China in sandy areas, sunny meadows and hillsides. About two dozen alkaloids have been identified in Ku Shen, with matrine and oxymatrine being the major components of the dried root. Tests with wood treated with matrine and oxymatrine against Formosan termites showed antifeedent, a chemical agent that causes an insect to stop eating, and toxic properties that remain effective for at least a year. Acknowledgments
Research associated with these studies would not have been completed without the assistance of the following individuals: Sanaa A. Ibrahim, Huxin Fei, Savi Kambham, Poornima Jayasimha, Karen Nix, Betty Zhu, Feng Chen and Lara Maistrello. Gregg Henderson, Paul K. Adams Professor, and Lixin Mao, Postdoctoral Researcher, Department of Entomology, LSU AgCenter, Baton Rouge, La., and Roger A. Laine, Professor, Department of Biological Sciences, LSU, Baton Rouge, La. (This article was published in the fall 2007 issue of Louisiana Agriculture.)