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INSECTICIDES BASED on INSECT HORMONES Bull SESSION X INSECTICIDES BASED ON INSECT HORMONES Bull. Org. mond. Sandte 11971, 44, 381-389 Bull. Wld Hlth Org. Insect Hormones and Their Derivatives as Insecticides WILLIAM S. BOWERS' The hormonal control of moulting, reproduction, and diapause in insects has little or no relationship to any similar phenomena in other animals, and the hormones involved in these processes are unlike any known hormones of vertebrates. The availability ofpure chemicals with high biological activity has permitted an asto- nishing increase in research on insect hormones. At present, understanding of insect endocrinology is far too incomplete to justify much speculation about the possibility of using insect hormones as insecticides. However, the preliminary studies discussed in this paper give reason for hope, and the results justify further effort. Man's contest with insects for food and fibre HORMONAL REGULATION OF INSECT DEVELOPMENT has been on a nearly equal footing. In the past, the omnivorous characteristics and nomadic habits Fig. 1 illustrates hormonal regulation of insect of man, together with a low population density, metamorphosis as exemplified by the life-cycle of enabled him to cope with insect-mediated famine the yellow mealworm, Tenebrio molitor L. Each and pestilence. immature moult (larva to larva) takes place in the Today the nutritional needs of an expanding presence of the moulting hormones (ecdysones) population, existing for the most part in urban and the juvenile hormones. The ecdysones are concentrations, must be satisfied by the intensive necessary for all moulting to occur, but the juve- cultivation or rearing of relatively few domesticated fruits, grains, and animals. This specialization has Fig. 1 generated an abundance of high quality food but has also permitted maximum competition by insect Hormonal regulation of insect development * pests. Cultural, biological, and chemical methods of Ad insect control have become a major concern of the CD agricultural scientist. To date, the use of toxic 02OQo chemicals has been the basis of our most successful efforts to control insects. However, the potential environmental hazards of many of these chemicals, together with the rapid development of resistance ADUJLT LARVAE to them by insects, necessitates a more enlightened approach to chemical methods of control. If more satisfactory chemical insecticides are to be developed, they must, in addition to being effective, meet certain new criteria. They must not pose immediate or long-term hazards to human X~PUPA populations, domestic animals, or wildlife. Ideally their insecticidal effects should be specific-that is, they should act only against a given target pest. 1 Senior Insect Physiologist, Insect Physiology Laboratory, Entomology Research Division, Agricultural Research Ser- vice, US Department of Agriculture, Beltsville, Md., USA. * JH = juvenile hormone; MH = moulting hormone 2651 381- 382 W. S. BOWERS Fig. 2 Structure of juvenile hormones * K 'N 'N H20H / OOCH3 / OOCH3 "N0 'N CH20CH3 IO / \ OOCH3 'N 'N 'N H2N(C2H5 X X - ~~X'CI,Brx N H20H r /\ A f ~~H20CH13 I~ ~ ~ ~ V OOCKH3 Mr 0 / OOCH3 VI - H20C H2CH20C H2CH20C4Hg K H2CH2CH3 KIU -0C H(OC H2C H2)2OC H2 C H3 XXI 0 P OC H2C!CH o / n OCH2CH2CH3 K ~~~~~xxn 0 --CH20 / K 0 0 / 0 0-- C°X-X\\0 0v CH302C WHO 10235 * References: compounds l-lll, Schmialek (1961); compounds IV & V, Bowers (1963); compound VI, Bowers (1965); compound VII,'Bowers (1966); compound Vil, Cerny (1967); compounds IX & X, Slama et al. (1968); compound XI, Romanuk (1967); compounds XII, Roller (1967); compound XIII, Meyer et al. (1968); compounds XIV-XVIII, Bowers (1968); compounds XIX- XXIII, Bowers (1969). INSECT HORMONES AND THEIR DERIVATIVES AS INSECTICIDES 383 nile hormones are present only when the genetic Fig. 3 "6programming" of the insect requires growth without Juvenile-hormone-active para-substituted maturation or differentiation. Thus, juvenile hor- aromatic terpenoid ethers mones (JH) prevent metamorphosis during immature life. When the insect reaches the point of metamor- R phosis to the adult reproductive stage, it moults .o R' to a pupal form and finally to the adult. Both of R=CH3,CH2C I3 these moults must take place in the absence of JH. iorCOOCH3 The original idea of a hormone-based insecticide was R -C H31C H2 C H3 C)_-1 founded on the discovery of Williams (1956) that treatment of the pupal stage of an insect with a hormone extract would disrupt morphogenesis another naturally occurring analogue of juvabione during the pupa-adult moult and produce an inter- (VIII) and synthesized several aromatic analogues mediate form of insect incapable of survival. (IX, X) with similar activity for these insects. Com- Although additional lethal effects of JH on insects pound XI was isolated (Romanuk et al., 1967) have been discovered, the original idea of disrupting from a hydrochlorination reaction mixture (Law adult development by treatment with JH remains et al., 1966) and was found to be selectively active valid and is still the basis of most biological assays in the nanogram range against pyrrhocorids. Roller for these hormones. et al. (1967) characterized one of the natural juve- nile hormones from the cecropia moth (XII) and CHEMISTRY OF JUVENILE HORMONES Meyer et al. (1968) characterized a second homo- logous juvenile hormone from the same moth. The first chemical discovered to have JH activity During an investigation of the possibility of was famesol (I, Fig. 2), which was extracted from increasing the activity of JH compounds with the faeces of Tenebrio by Schmialek (1961). Subse- conventional insecticide synergists, we found that quently, the methyl ether (II) and the diethyl amine several synergists-such as piperonyl butoxide (XIV), (III) of farnesol were found to have greater biolo- sesamex (XV), and Niagara 16388 (XVI)-possessed gical activity than the parent alcohol (Schmialek, appreciable JH activity. 1963). In our laboratory we began structural Since the chemistry of the synergists could in no modification work and found that while hexahydro- way be related to the previously known active farnesol (IV) and its methyl ether were active, several terpenoids, we combined chemical features of the simple saturated alcohols and their methyl ethers synergists with the terpenoids to see if the JH activity were active, especially dodecyl methyl ether (V) could be increased. The aromatic ethers of epoxy (Bowers & Thompson, 1963). Continuing these farnesol were prepared (XVII, XVIII) and were studies, we determined sufficient of the chemical discovered to be about as active as the synergists and biological properties of the JH in cecropia (Bowers, 1968). However, when the terpenoid extract to permit the synthesis of trans,trans-10,11- portion was shortened by one isoprene unit (sesamyl epoxy methyl farnesenate (VI), which we discovered geranyl ether epoxide, XIX) the activity was in- to be extremely active in all of our biological creased tremendously. Working on the hypothesis assay systems (Bowers et al., 1965) and which we that the ethyl branches of the cecropia hormones feel predicted the structure of the natural cecropia might result in increased activity, we prepared juvenile hormones. Following the discovery of compounds XX, XXI, and XXII. These " hybrid" JH activity in conifers (Slama & Williams, 1965) combinations proved to be the most active of all we isolated from the balsam fir and identified a the compounds studied and in our biological monocyclic sesquiterpenoid compound (VII), which assays they were uniformly active at subnanogram we called juvabione (Bowers et al., 1966), and which levels (Bowers, 1969). Several non-methylenedioxy shows a relatively high specific JH activity only aromatic derivatives were prepared and were found against members of the Pyrrhocoridae, an insect to possess relatively high JH activity. Fig. 3 shows family that contains important pests such as the the structures of several para-substituted aromatic cotton stainers. Subsequently, other investigators geranyl ethers with significantly high juvenile (Cerny et al., 1967; Slama et al., 1968) isolated hormone activity. 2 384 W. S. BOWERS LETHAL EFFECTS OF INSECT HORMONES a pupa-adult intermediate. Many of the biological assays for JH are based on this phenomenon- It is clear that the presence or absence of JH at for example, the Tenebrio genitalia assay developed certain critical periods of insect development pro- in our laboratory is based on the retention of foundly affects the direction of this development. pupal genitalia in an otherwise normal adult beetle During the moulting process the presence of JH following the topical application of the candidate prevents cellular differentiation and, hence, matu- hormonal compound to the abdomen of the pupa. ration. In the absence of JH morphogenesis and For ultimate sensitivity, the hormone treatment maturation proceed towards the adult stage. The must be applied during the early phase of pupal application of a JH chemical to a stage of develop- development, since JH chemicals appear to exert ment that is due to undergo maturation results in their effects by preventing the expression of genetic the formation of an intermediate that is incapable information dealing with metamorphosis. Even of further development and dies. Therefore, the large doses of JH cannot reverse differentiation. classical notion of a JH insecticide was based Therefore, in anticipation of the use of a JH chemical simply upon supplying the hormone to the insect for control purposes the time
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