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Chapter 21
Bioorganic Chemistry on Sex Pheromones Secreted by Lepidopteran Insects and Their Application for Plant Protection Tetsu Ando Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology (TUAT), Koganei, Tokyo 184-8588, Japan (
[email protected])
Lepidopteran sex pheromones have been identified from more than 500 species. The pheromones in the most predominant group (Type I) are composed of unsaturated C - C18 straight-chain compounds with a terminal functional group, such as bombykol produced by the silkworm moth. In addition to them, females in some evolved families produce C - C polyunsaturated hydrocarbons and the epoxy derivatives, constituting a second major group (Type II). While some synthetic pheromones have already been utilized for plant protection on the basis of their strong attractive activities for male moths, many bioorganic chemical studies are currently underway on this exciting topic. This paper addresses recent research conducted mainly in the Chemical Ecology Laboratory in TUAT and explores the future of pheromone studies and potential applications. 10
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© 2005 American Chemical Society In New Discoveries in Agrochemicals; Clark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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227 The sex pheromones of Lepidoptera are usually produced by female moths and attract males. The pheromone is a main factor for reproductive isolation, so it must be species-specific. Lepidoptera is one of the biggest insect groups which has been established for over 100 million years since die Mesozoic era. To date, lepidopteran sex pheromones have been identified from nearly 540 species. Additionally, sex attractants of mother 1,240 species have been found by field tests with synthetic pheromones and their related compounds (/, 2). Although the chemical structures of the pheromones are very simple, the variety of the chemical structures and blending of multiple components cause their diversity. About 75 % of these compounds consist of unsaturated fatty alcohols and their derivatives with a C to C straight chain, such as bombykol (E10,Z12-16:OH) (Figure 1). These Type I compounds with a terminal functional group have been identified from many groups in Lepidoptera. Another 15 % of the natural pheromones consist of polyunsaturated hydrocarbons mid their epoxy derivatives with a C to C 3 straight chain, such as (Ζ,Z,Z)-3,6,9-nonadecatriene (Z3,Z6,Z9-19:H) and cis(£2)-3,4-epoxy-6,9-nonadecadiene (epo3,Z6,Z9-19:H) secreted by the Japanese giant looper, Ascotis selenaria cretacea (Figure 1). These Type II compounds do not have a terminal functional group and have been identifiedfromthe species in highly evolved insect groups such as the family of Geometridae (2). w
i8
i 7
Type I
2
Type II :
/\/= ^sA/^/^/\/v
0H
bombykol (E10,Z12-16:OH)
\y=^y=\/^^ N
/^v /=\/=^^ v
Z3,ze,z9-19:H epo3,Z6,Z9-19:H
Figure L Reprsentative lepidopteran pheromones and their abbreviations. Studies on lepidopteran pheromones are underway at the Chemical Ecology Laboratory in TUAT. The work is divided into four areas. (A) The systematic synthesis of pheromone compounds and their field evaluation to find new attractants; (B) Development of new analytical techniques and identification of natural pheromones using the synthetic compounds as an authentic sample; (C) Pest control applications for synthetic pheromones; (D) The biosynthesis and olfactory perception system of the sex pheromones. This paper deals with the last three topics (B - D).
Identification of Natural Sex Pheromones GC-EAD Technique In addition to the observation of male responses in a flask or a wind tunnel, the activity of pheromone components in the gland extract can be detected using an electrophysiological technique. The electroantennogram
In New Discoveries in Agrochemicals; Clark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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228 (EAG), a recording of the potential changes measured between the base and tip of an insect antenna as a result of chemical stimulation, has played an important role in the bioassay system (J). One advantage of the system is that the EAG recording is easily accomplished with an antenna of an unconditioned male moth in a bright room. Because the activities of several compounds can be successively measured after short intervals, all active components in a pheromone blend are detected by one injection into gas chromatography combined with an EAG détecter (GC-EAD) (4). Recently, the capability of amplifiers has increased so much that the sensitivity of the ΕAD against a pheromone component is higher than that of an FID. Type I Pheromones with a Conjugated Diene System The persimmon fruit moth, Stathmopoda masinissa, is a microlepidopteran species in the family of Oecophoridae and is a well-known harmful pest of persimmon fruits in Japan. The tiny larvae feed on only a small part of the fruits, but the infected fruits drop to the ground before maturity. Since spraying of an insecticide is effective in a very limited term, when the larvae move from the unfolding buds to the fruits for feeding on the core, the sex pheromone would play an important role as a monitoring tool. By GC-EAD analysis of an n-hexane extract of the pheromone glands removed from the females in the scotophase, three distinct EAG-active components were detected. GC-MS analysis revealed the EAG-active compounds consisting of an aldehyde, an acetate, and an alcohol with a conjugated diene system in a C . chain (J). The mass spectra of the natural components are different from the authentic samples including the diene system at a higher position than the 9position, which had been synthesized before (6); on the other hand, the aldehyde component produced a unique base peak at m/z 84 indicating a 4,6diene structure. However, die contents in the pheromone glands are very low, and the position of the diene system has not been confirmed by a chemical dérivât ization. In order to confirm the double-bond positions, four geometrical isomers of 4,6-hexadecadienyl compounds were synthesized in addition to the other positional isomers including the conjugated diene system between die 3- and 10-positions (7), and then, their GC-MS data revealed that the natural components are 4,6-dienes with a (£,Z)-configuration; i.e., E4,Z6-16:Ald, E4,Z6-16:OAc, and E4,Z6-16:OH. While the base peaks of the alcohol and acetate including the 4,6-dienyl structure have been recorded at m/z 79, it has been confirmed that die 4,6-hexadecadienal shows a characteristic base peak at m/z 84, which is expected to have a stable 2,3-dihydropyranyl structure ([CsHO]"*") probably formed by a cyclization and hydrogen rearrangement after 6
In New Discoveries in Agrochemicals; Clark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
229 Table I. Base Peaks in the El-Mass Spectra of Type I C Compounds Including a Conjugated Diene System i6
Base peak (m/z) 6,8-16-10,12-16
11,13-16
Alcohol
67
79
79
67
95(B)
81(C)
67(D)
Acetate
79
79
79
67
95(B)
81(C)
67(D)
Aldehyde
—
84(A)
80
67
95(B)
81(C)
67(D)
«
67 ^
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3,5-16
4,6-16
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5,7-16
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81 81 ^
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