Methyl bromide and phosphine are the most widely used fumigants for controlling pests in grain and dry, stored food products and quarantine insects in cut flowers for export. Some stored-product insects have reportedly developed resistance to phosphine. Phosphine also can cause corrosive damage if buildings and equipment are not built with noncorrosive materials. With the proposed phaseout of methyl bromide in 2001, growers are searching for alternatives.

Many aromatic plants, spices, and herbs can synthesize chemicals, such as essential oils, that kill or repel many insects but don't harm mammals. The volatility and insecticidal efficiency of the oils make them good prospective fumigants. So scientists are studying essential oils as alternative natural fumigants to methyl bromide and phosphine.

Eli Shaaya, a professor with the Department of Stored Products, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, is a visiting scientist at USDA's Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, Florida. He and his colleagues conducted laboratory studies to evaluate the potential of these compounds.

"We used the most common stored-product insects—rice weevil (Sitophilus oryzae), lesser grain borer (Rhyzopertha dominica), sawtoothed grain beetle (Oryzaephilus surinamensis), and red flour beetle (Tribolium castaneum)—as test insects. We also used two quarantine insects that attack cut flowers for export, the whitefly (Bemisia tabaci) on dianthus and thrips (Frankliniella occidentalis) on roses," says Shaaya.

The essential oils used in the tests were obtained from fresh plants, herbs, and spices, using steam distillation. Three types of tests were performed to evaluate the biological activity of oils. The first test was in space fumigation in chambers of 3.4 liters. Second, the highly active oils were tested in 600-milliliter fumigation chambers filled to 20 percent or 70 percent by volume with wheat. Finally, pilot tests were carried out in simulation columns 10 centimeters in diameter by 20 centimeters in height, filled to 70-percent volume with wheat.

Shaaya and his colleagues found that the oils under the code name ZP51 and SEM76 were the most effective against stored-product insects. In space fumigation, a concentration of only 1.4 to 4.5 g/m³ of air was enough to obtain 90 percent adult mortality of all the insect species tested with a 1-day exposure time. For most of the other oils tested (about 50), a concentration of over 15 g/m³ was needed to obtain 50-percent kill of S. oryzae and T. castaneum.

"In the second laboratory test, we evaluated the effectiveness of ZP51 against S. oryzae and T. castaneum in 600-milliliter chambers filled to either 20-percent or 70-percent volume with wheat," says Shaaya. "With 20-percent fill, concentrations of 3 and 10 g/m³ of ZP51, and 1-day exposure were enough to cause 100-percent mortality of S. oryzae and T. castaneum, respectively."

"We found that with 70-percent fill, a concentration of 30 and 20 g/m³ and exposure times of 2 and 3 days, respectively, were required to obtain 100-percent mortality for S. oryzae. A concentration of 40 g/m³ and an exposure time of 4 days were needed to obtain 100-percent mortality of T. castaneum," says Shaaya.

In the final test, the scientists found that in columns filled 70 percent with wheat, a concentration of 50 g/m³ and 5 days exposure were needed for 100-percent control of S. oryzae and T. castaneum. As for R. dominica and O. surinamensis, 7 days were needed for 100-percent mortality. And, if the concentration was increased to 70 g/m³ with 4 days exposure time, 85–100 percent mortality of all insect species studied could be achieved.

"We demonstrated that the concentration of essential oils decreased gradually and only negligible amounts were found 3 months after treatment," Shaaya notes.

"A number of essential oils were also found active against cut-flower insects. Against the whitefly, a concentration of 10 g/m³ and exposure of 2 hours were enough to obtain 100-percent mortality. In the case of thrips, a concentration of 20 g/m³ and exposure time of 4 hours were needed to obtain 100-percent mortality. No phytotoxicity was observed 7 days after fumigation.

"Based on our studies, these two essential oils, ZP51 and SEM76, show great potential as fumigant alternatives to methyl bromide and phosphine. A concentration as low as 50 grams of oil/m³ is enough to obtain effective control of stored-product insects, compared with the recommended concentration of 30–50 g/m³ methyl bromide. In the case of cut flowers, a number of potential oils were also identified," says Shaaya.

Shaaya presented his research results at the Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions held in San Diego, November 3–5, 1997.