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Critter Chemistry

August 5, 2002

Bug Sprays

Formulations for Killing and Repelling Insects Include Natural and Synthetic Products

Mitch Jacoby

Barbecued chicken flavored with hickory smoke was the evening's fare, and I was doing the cooking. But as I stepped inside the backyard shed to gather a handful of wood chunks, I heard a lot of buzzing. As the door closed slowly behind me, out of the corner of my eye I could see that the fist-sized thing on the inside of the door, heading straight for my back, was a busy wasp nest.

I crawled out unscathed--with the hickory chunks--then took several photos, finished cooking, and ran to a nearby store for a can of insecticide. With small children frequenting the yard, I needed to destroy the nest. The product I selected worked as advertised. After just one squirt from 15 feet away, the wasps began "dropping like flies," and I was left wondering--what is this stuff?

In the past, organophosphates and carbamates were used widely as quick knockdown agents, according to Michael G. Waldvogel, an entomologist at North Carolina State University, Raleigh. A specialist in residential and industrial pests, Waldvogel notes that the odor, volatility, and toxicity to mammals of those substances eventually led to their replacement in the consumer and professional pest-control markets.

Nowadays, pyrethroids, which are synthetic versions of chrysanthemum-derived pyrethrins, are used in many insecticide formulations. For example, the house-brand wasp and hornet killer I used lists two active ingredients: tetramethrin (a pyrethroid ester) and a phenoxybenzyl cyclopropanecarboxylate. Raid's product includes tetramethrin and a chlorinated carboxylate.

Other manufacturers blend tralomethrin, a brominated pyrethroid ester, and allethrin, a related compound that does not contain halogens, in products marketed as stinging-bug killers. But those same products, in nearly the same concentrations (no more than 0.05%), are also found in some roach and ant killers and spider killers. A key difference between the crawling- and flying-critter control agents, however, is packaging. Antiroach products are applied as gentle mists. In contrast, wasp exterminator comes in cans with propellants and nozzles designed to deliver the poison in a 20-foot stream--just in case a couple of angry bugs get away.

Although a great variety of insects are knocked down quickly by pyrethroids, pyrethrins, and other compounds, some of them get back up after a while. To keep bugs down for good, chemists have long blended poisons with compounds described as "synergists." These substances lack significant insecticidal character on their own, but team up with primary pest-control agents to produce potent insecticide formulations.

ONE SUCH COMPOUND is piperonyl butoxide, a piperic acid derivative. Known for blocking the action of insect detoxification enzymes, the synergist is found in retail flying-insect killers, mosquito- and termite-control products, and some veterinary pesticides. Piperonyl butoxide is also a key component in over-the-counter products for treating head and body lice. In addition to boosting insecticide potency, synergists reduce costs by lowering the concentration of expensive active ingredients needed to make effective pest-control formulations.

Biting and stinging bugs are also controlled by other materials and methods. One pesticide maker markets a line of products under the trade name Victor, whose active ingredient is mint oil. According to the manufacturer, the product kills insects by blocking their breathing holes and overstimulating their nervous systems.

Other manufacturers, whose products are advertised as safe, natural, and environmentally friendly, blend a variety of plant oils to produce pesticides. For example, Bioganic's wasp and flying-insect killers include clove oil, sesame oil, and wintergreen oil. Other products made by the company contain thyme and citronella oils. Company scientists note that the products kill insects by blocking neural pathways not found in mammals. They say that, by attacking octopamine neuroreceptors, the oils interrupt insect movement and metabolism.

With store shelves lined with pest-control products, you might think that traditional methods for getting rid of yellow jackets and other stinging bugs would be a thing of the past. Not so, Waldvogel points out. The entomologist follows Internet gardening discussions and says that it's still common for participants to advise one another "to pour gasoline down the nest opening, light it, and run. We discourage the practice," he says, "but people do it anyway."

Repelling pests, as opposed to killing them, is another way to deal with the nuisance and dangers they pose. Insect-borne diseases can be deadly. And itchy and irritated skin, though not life-threatening, can easily spoil a camping trip or evening stroll. Many repellent products--some containing synthetic compounds and others based on natural components--are available to consumers. Which should you use?

Researchers at the University of North Carolina, Chapel Hill, and the University of Florida, Vero Beach, recently compared the effectiveness of commercial insect repellents in preventing mosquitos from biting people. Under controlled test conditions, the group found that products formulated with more than 20% DEET (N,N-diethyl-m-toluamide), the well-known synthetic repellent, protected test subjects from being bitten for roughly five hours. In contrast, many botanical products offered less than 20 minutes of protection. And wristbands impregnated with repellents offered no protection [N. Engl. J. Med., 347, 13 (2002)].

Looks like I'm barbecuing again tonight. Maybe this time I'll try a little mesquite or applewood. Either way, I need to go into the shed to get the grilling supplies. But this time, I'm going to watch my back, and I'm going in armed with a can of bug spray.

Chemical & Engineering News
ISSN 0009-2347
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