RESEARCH
Symposium covers chemical insect control Isolating and synthesizing natural sex attractants and finding man-made lures receive major attention million was attributed to the boll wee vil. Chemist James H. Tumlinson at the U.S. Department of Agriculture's Boll Weevil Research Laboratory, State College, Miss., and coworkers Dr. Paul A. Hedin, who directed the USDA work, Dr. Richard C. Gueldner, Dr. Dick D. Hardee, and Dr. A. C. Thompson as well as Dr. James D. Minyard of Mississippi State Uni versity, State College, Miss., report that they have isolated the compo nents of the male boll weevil's sex attractant. Moreover, they have syn thesized three of its four compounds in biologically active forms. They are now working to synthesize the fourth compound and to produce eco nomical synthetic compounds for use in boll weevil control. The compounds that make up the
A G SL F O O D
While use of DDT and other pesti cides is coming under increasingly heavy attack (C&EN, Jan. 27, page 12), other chemicals to help control insect pests are emerging from the laboratory. Many of these new chem icals are synthetic reproductions of insects' natural secretions. The latest information about them surfaced at the division Symposium on Chemicals Controlling Insect Behavior. One of the most important insect pests in the U.S., in terms of crop loss economics, is the boll weevil. Al though $75 million was spent last year to control this pest on 10 million acres of cotton, a crop loss of $133
sex attractant were isolated from both fecal material and insects by steam distillation and a series of chromato graphic separations, Mr. Tumlinson explains. None of the compounds had previously been found in natural prod ucts nor had they been synthesized or characterized. On the basis of chem ical and spectral evidence four ter penoid compounds were identified. They are I, 2 - isopropenyl- 1 -methylcyclobutaneethanol; II, cis-3,3-dimethyl-A'^-cyclohexaneethanol; III, cis3,3 - dimethyl - A'»a-cyclohexaneacetaldehyde; IV, *rans-3,3,-dimethyl-A''a cyclohexaneacetaldehyde. In fecal ma terial, the compounds were present in concentrations of 0.76, 0.57, 0.06, and 0.06 p.p.m., respectively. Compounds II, III, and IV have been synthesized (see box for synthesis scheme).
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Laboratory tests show that the syn thetic compounds are equally as at tractive as the natural substances to female boll weevils, Mr. Tumlinson says. However, to produce a sub stance attractive to females it is neces sary to combine compounds I and II with either III or IV, he explains. Ab sence of either of the alcohols or both of the aldehydes gives an inactive mixture. Also, Mr. Tumlinson points out, live male boll weevils attract equal num bers of males and females in field tests in spring and fall. This indicates that these compounds may act as an ag gregating pheromone as well as a sex pheromone, he says. (A pheromone is a substance secreted by an animal to influence the behavior of other ani mals of the same species.) Thus, they are potentially even more valuable for controlling the boll weevil. Production of the sex attractant by boll weevils appears to be diet related, he says, since males who have been fed fresh cotton squares are more at tractive than those that were given an artificial diet. 'This leads to the postulation that the weevil may be converting a plant constituent into the sexually attractive compounds. This is even more appealing when one con siders that both trans-/?-ocimene and
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myrcene are found in cotton bud es sential oil and that their monoterpene alcohol or pyrophosphate ester pre cursors are therefore in plants." The USDA scientists have postu lated a pathway by which a terpene alcohol—a precursor of myrcene— could be converted to all four active compounds (see box for hypothetical biosynthetic scheme). Mr. Tumlinson quickly points out that this is purely speculative and that there is no sup porting evidence at this time. "How ever," he adds, "considering the rather unusual structures of the active com pounds and the related circumstances, we think this proposal is very reason able." Sex pheromones. Much of the cur rent work on chemical control of in sects centers on sex pheromones. USDA chemist Martin Jacobson gives a measure of the increased amount of activity in this field: Between 1837 and 1965 there were 425 literature references on the subject, he says. In the past four years, more than 400 ad ditional references on insect sex phe romones have appeared. "This is due in no small measure to the fact that these substances, produced by insects as a requisite to their reproduction, may be used for their destruction." These sex at tractants are detected
by insects in minute amounts, Mr. Jacobson points out, and are undoubt edly among the most potent physiolog ically active substances known. In sects detect sex pheromones by sense organs located mainly in their anten nae. Thus far, their main practical use has been in insect surveys where traps are baited with the attractants. The catches indicate the size and location of infestations; economical use of in secticides can then be determined. However, recent studies have shown that insecticides can be effectively used in the traps baited with sex attractants. This eliminates the need to spread the insecticide over entire fields and helps prevent killing useful insects and other animals. Attractants have proved ef fective for at least 400 feet downwind in field tests. Instead of orienting insects to a trap, a different method of controlling their sex life is to keep them from finding each other. Sex pheromone masking can be done in two ways, Mr. Jacobson explains. One technique is to perme ate the atmosphere with the female sex pheromone. This prevents the males from orienting to and inseminating fe males. Field trials have indicated that economic control of some insects over large areas may be possible with this method, he says. The second masking method is to use other chemicals. "One study has shown that the action of the female polyphenus moth sex attractant on the male can be completely and reversibly blocked by the vapors of formalde hyde," he says. "Also, the action of Jrans-2-hexenal, which is needed to trigger production of this attractant, can be masked by numerous volatile agents, including Chanel No. 5." At the ACS national meeting in Sep tember 1965, Mr. Jacobson predicted that the sex attractants of many insect species, especially the lepidoptera— butterflies and moths—would prove to be long-chain unsaturated alcohols or their esters (C&EN, Sept. 20, 1965, page 42). At that time, the sex phe romones of only three species of lep idoptera had been identified. Since then, four other sex phero mones of the lepidoptera have been identified. As predicted, they are longchain, unsaturated acetates. Also, the presence of sex pheromones in 28 other species of lepidopterous insects has been demonstrated. These phero mones are produced by adult females in glands located in the last few ab dominal segments. "A pattern appears to be unfolding in the love life of this family of insects," Mr. Jacobson says. "Although it is still too early to predict the specific pheromone structure for a given species, even that may be possi ble before too long," he comments. APRIL 28, 1969 C&EN
97
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38 C&EN APRIL 28, 1969
BOLL WEEVIL. J. Tumlinson uses gas chromatography in boll weevil work
The structure of the sex attractant is very critical to the insect's receptiveness to it, he says. As an example, Mr. Jacobson explained an experiment he did with propylure (10-propylfrans-5,9-tridecadien-l-ol acetate) earlier this year. As little as 10% of the cis isomer mixed with the trans form strongly depressed response of male pink bollworm moths. And with 15% of the cis isomer, male response was completely nullified. Also, males exposed to the trans isomer within 15 minutes after exposure to the cis form failed to respond. A complete response was obtained each time to consecutive exposures to the trans isomer alone. Synthetic attractants. Besides isolating natural attractants and then synthesizing them, USDA also has a program to find synthetic attractants or lures for economically important insect pests. Dr. Morton Beroza, USDA chemist and symposium chairman, explains: "In this approach a large number of chemicals are tested to find a lead—a weak attractant. When a lead is found, the chemist then uses his ingenuity and chemical virtuosity to synthesize more potent, related compounds." USDA has also devised an early warning detection system to prevent new infestations of insects into the U.S. For example, Dr. Beroza says, the Mediterranean fruit fly infested a million acres of Florida in 1956. The insect feeds on citrus and other subtropical crops. Using attractant-baited traps to determine the location and density of the infestation, the Government was able to eradicate the fly by spraying the infested areas with an insecticide-bait formulation. The cost was steep—about $11 million—he says. However, it has been estimated that it would cost about $20 million a year in crop loss to live with the insect. The great amount of international trade and traffic makes these acciden-
tal imports of insects inevitable, Dr. Beroza explains. However, USDA now has attractant-baited traps deployed about U.S. ports of entry, and new infestations are rapidly detected and eradicated before they can spread. USDA estimates that between 1958 and 1964 the early warning detection system saved the Government and taxpayers more than $9 million in potential eradication costs. The U.S. Government, the major user of attractants, uses from 200 to 2000 pounds of each of some 10 synthetic attractants a year for detection. Olfaction. One area of research in controlling insects chemically has much wider applications: studies of the olfactory process. One theory of olfaction states that the stereochemical features of a molecule determine its odor. Another, the Dyson-Wright theory, says that vibrations of the molecule in the far infrared region (500 to 50 cm.-1) trigger the olfactory response. In one experiment with cue-lure, which attracts the melon fly, the hydrogen atoms were systematically and, in some cases, almost completely replaced with deuterium atoms by Dr. R. E. Doolittle, Dr. Beroza says. The heavier deuterium atoms shifted the infrared absorption maxima to lower frequencies. This would be expected to change the odor of cue-lure according to the Dyson-Wright theory, he points out. However, with extensive tests involving more than 2600 counts of responding insects, there was no difference in attraction between cue-lure and any of the deuterated compounds. "We had to conclude that our data did not support the infrared absorption theory for odor," Dr. Beroza says. "Proponents of the molecular shape theory may find the data useful since deuteration of the molecule does not change its shape." At least in the foreseeable future, attractants, repellents, and hormonelike materials described at the symposium will not supplant the use of insecticides to control insects, Dr. Beroza says. "Insecticides are a practical necessity for providing the food and fiber the world needs. Despite occasional reports on the deleterious effects of pesticides on ecological systems or the concern expressed about human safety, the precautions taken these days to assure safe use of insecticides are most thorough/' Attractants will make it possible to control insects with less insecticide by showing where and when they should be applied, he says. "Thus, insect attractants will not eliminate insecticides, but they will enable us to use insecticides wisely and assure the safety of our people, our food supply, environment quality, and wildlife."
