Overview of popular insect pheromones in Vietnam.
The negative impacts of conventional pesticides on health, environment, and organisms have involved strong development of integrated pest management (IPM) strategies. The use of insect pheromones becomes an effectively alternative selection in agricultural and forest pest control. Pheromone researches in Vietnam started in the last few decades and in addition to technical factors, recent achievements in the Vietnamese agriculture have an important direct link to the pheromone developments. In this chapter, we review the pheromone researches related to synthesis and field trials of several especial insect pheromones, in which Vietnamese scientists have mainly participated or collaborated with foreign research groups. First, we will discuss an overview of popular insect pheromones in Vietnam, a lot of species of which are also found around the world, as an important reference for scientists who would have especial consideration in this field. Further, synthetic routes of pheromones are summarized with various structures including chiral, racemic, mono- and poly-olefinic pheromones where some schemes have become standard methodologies for synthesis of similar structural compounds. Finally, field evaluations of the pheromones of numerous species are discussed in detail.
- insect attractant
- pheromone trap
- pest control
- integrated pest management
- field application
Nowadays, numbers of pests are becoming increasingly resistant due to the conventional pesticides which cause damage to useful parasites and imbalance in the ecosystem, creating environmental pollution and adverse effect on the economy . This leads to the concept of IPM being rapidly developed to solve the problems of pesticide use. IPM allows safer insect control and poses the least risks while maximizing benefits and reducing costs. Although many methods have been employed, the use of pheromones obviously is one of the most effective approaches in pest control which can be achieved by mass trapping and killing the harmful pests selectively . Moreover, the use of pheromone traps minimizes risks to human health and reduces destruction of the living environment.
Because of the tropical climate, Vietnam is generally favourable for many typical pests with rapid breeding which cause damage to the crops and forest trees throughout the year . Since Nguyen group's first report in early 1980s , the demand of pheromone for IPM in Vietnam is steadily growing and that generally leads to a strong development of synthesis and field trial of pheromones. Success of the Vietnamese agriculture in the recent years has had important contribution from use of this technique. At the same time, the numerous reports on pheromones in Vietnam from other groups, such as groups of Can Tho University, have been increasing rapidly during the last decades.
Herein, our aim is to review synthesis and field trials of insect pheromones from research groups in Vietnam. The chapter consists of three main parts including introduction of pheromones of popular insects in Vietnam, an overview of synthetic methodology presented in structurally typical order, and finally field trials of several important insect pheromones.
2. Popular insect pheromones in Vietnam
Due to the big difference of weather conditions between the regions of Vietnam, for instance the South of Vietnam having no winter season but the North having four clearly identifiable seasons, numerous insect species have been found in Vietnam . Known pheromones of popular species in the country are summarized in Table 1 where only major component of pheromones identified at ratios of more than 65% in their mixture is listed with a respective reference. Species specificity is commonly achieved by the use of blends of several pheromone components. The data reveal that Lepidoptera is the best studied insect order related to pheromone, with data available for about 27 species. The pheromones of this order usually consist of alcohols, acetates, or aldehydes of long chain containing double bonds while the other orders mostly possess pheromone molecules bearing chiral carbons.
|Order, family and species||Common name||Major component of pheromone*||Ref.|
|Cnaphalocrocis medinalis G.||Rice leaf folder||Z13-18:Ac or Z13-18:Ald||[5, 6]|
|Chilo suppressalis W.||Rice stem borer||Z11-16:Ald|||
|Hellula undalis F.||Cabbage webworm||E11E13-16:Ald|||
|Crocidolomia binotalis Z.||Cabbage head caterpillar||Z11-16:Ac|||
|Conogethes punctiferalis G.||Yellow peach moth||E10-16:Ald|||
|Scirpophaga nivella F.||Sugarcane top borer||E11-16:Ald|||
|Proceras venosatus W.||Striped sugarcane borer||Z13-18:OH|||
|Sesamia inferens W.||Pink stem borer||Z11-16:Ac|||
|Helicoverpa armigera H.||Cotton bollworm||Z11-16:Ald|||
|Spodoptera exigua H.||Beet armyworm||Z9E12-14:Ac|||
|Spodoptera litura F.||Oriental leafworm||Z9E11-14:Ac|||
|Chrysodeixis eriosoma D.||Green garden looper||Z7-12:Ac||[17, 18]|
|Ctenoplusia albostriata B. & G.||Eastern streaked plusia||Z7-12:Ac|||
|Argyrogramma signata F.||Green semilooper||Z5-10:Ac|||
|Ctenoplusia agnata S.||-||Z7-12:Ac|||
|Zonoplusia ochreata W.||-||Z7-12:Ac and Z5-12:Ac|||
|Spodoptera pectinicornis H.||Water lettuce moth||Z7-12:Ac|||
|Phyllocnistis citrella S.||Citrus leaf miner||Z7Z11E13-16:Ald|||
|Conopomorpha cramerella S.||Cocoa pod borer||E4E9Z10-16:Ac and|
|Agrius convolvuli L.||Convolvulus hawk moth||E11E13-16:Ald|||
|Homona coffearia N.||Tea tortrix||12:OH|||
|Archips atrolucens D.||Citrus leaf roller||Z11-14:Ac and E11-14:Ac||[18, 23]|
|Adoxophyes privatana W.||Apple leaf-curling moth||Z11-14:Ac|||
|Meridemis furtiva D.||-||Z11-14:Ac|||
|Etiella zinckenella T.||Pea pob borer||Z11-14:Ac|||
|Prays endocarpa M.||Citrus pock caterpillar||Z7-14:OH and Z7-14:Ald|||
|Parasa lepida C.||Nettle caterpillar||Z7,9-10:OH|||
|Cylas formicarius elegantulus S.||Sweet potato weevil||Z3-dodecen-1-yl E2-butenoate|||
|Phyllotreta striolata F.||Striped flea beetle||6R,7S-Himachala-9,11-diene|||
|Cosmopolites sordidus G.||Banana root borer||1S3R5R7S-sordidin|||
|Oryctes rhinoceros L.||Coconut rhinoceros beetle||Ethyl 4-methyloctanoate|||
|Rhynchophorus ferrugineus O.||Red palm weevil||4S,5S-Ferrugineol|||
|Xylotrechus quadripes C.||Coffee white stemborer||2S-Hydroxydecan-3-one|||
|Aphis glycines M.||Soybean aphid||1R4aS7S7aR-Nepetalactol|||
|Nezara viridula L.||Southern green stink bug||Trans-1,2-Epoxy-Z-α-bisabolene|||
|Myzus persicae Sulzer||Green peach aphid||E-β-Farnesene|||
|Brevicoryne brassicae D.||Cabbage aphid||4aS7S7aR-Nepetalactone|||
|Planococcus citri R.||Citrus mealybug||Planococcyl acetate|||
|Pseudococcus comstocki K.||Comstock mealybug||2,6-Dimethyl-1,5-heptadien-3-yl|
|Dysdercus cingulatus F.||Red cotton bug||S-Linalool|||
|Thrips palmi K.||Melon thrips||R-Lavandulyl 3-methyl-|
|Orseolia oryzae W.||Asian rice gall midge||2S,6S-Diaxetoxyheptane|||
3. Synthesis of pheromones
As discussed above, most of the insect pheromones found in Vietnam possess chiral and olefinic structures. Chiral pheromone is defined as a compound containing at least one asymmetric carbon atom, while olefinic attractant bears one or more double bonds C=C in the carbon chain. Generally, insects are attracted more efficiently by a typical optical or/and configurative isomer than by a mixture of its isomers. Hence, an unambiguous understanding of production of these pheromones is particularly necessary for their application. Herein are summarized important synthetic approaches of pheromones which have been used for field trials in Vietnam over three decades. Synthetic approaches are divided into three main categories as described below.
3.1. Chiral pheromones
3.1.1. 4R,8R-dimethyldecanal (4R,8R-1, 4R,8R-Tribolure, 1)
Suzuki et al.  described the identification and synthesis of tribolure (1) as the aggregation pheromone of the Tribolium flour beetles, in which the (4R,8R)-1 isomer is a major component of the natural pheromone [43, 44]. A simple way for synthesis of (4R,8R)-1 has been reported by Nguyen and co-workers [45–47]. The selective peroxidation of (S)-3,7-dimethylocta-1,6-diene (2) gave an important intermediate diol 3 which was as an initial material for synthesis of both components, the tosylate 5 and the Grignard reagent 6. The Wurtz condensation of the two components in presence of lithium cuprate, followed by simple conversions, affords the pheromone (4R,8R)-1 (Scheme 1).
3.1.2. (4S, 6S, 7S)-7-hydroxy-4,6-dimethylnonan-3-one (Serricornin, 7)
(4S, 6S, 7S)-7-hydroxy-4,6-dimethylnonan-3-one (7) named Serricornin is the female-produced sex pheromone of the small tobacco beetle, Lasioderma serricorne, that has been isolated and identified by Chuman et al. [48, 49] The key step in synthesis of this pheromone was reaction between 8 with ethyl triisopropoxytitanium according to Cram's rule to obtain a ratio of isomer (3S,4S)-9 with ee 85% . In order to isolate individual optical isomers, (S)-1-phenylethylamine (S-PEA) was treated with acid 10 to afford a mixture of diastereomeric salts . After repeated crystallization, the (3S, 4S)-10 exhibited ee 92%. The compound 10 was converted into the lactone by a two-step procedure, followed by Grignard coupling that completed synthesis of the target pheromone (Scheme 2).
3.1.3. (10R)-methyldodecyl acetate (10R-11)
The smaller tea tortrix moth, Adoxophyes sp., is a widespread and economically important pest of the tea plant. It has been demonstrated that the male-produced sex pheromone consists of four components, in which 10R-11 was identified as the minor component . Tamaki et al.  showed that the 10R-11 was more bioactive than the S-isomer in field test. The synthesis of R-11 has been reported through two approaches which employed the Grignard coupling between the achiral and chiral units as a key step . The latter unit was obtained from the chiral diol 3 converted into the tosylate 12 or the above bromide 6. Coupling of these compounds with the corresponding Grignard reagents of protected aldehydes and subsequent esterification completed synthesis of the pheromone 10R-11 (Scheme 3).
3.2. Racemic pheromones
3.2.1. Ethyl 4-methyloctanoate (13)
Hallett et al.  described identification of the aggregation pheromone of Rhinoceros beetles, Oryctes rhinoceros L., the most important destructive pest of coconut, oil, and other palms in tropical Southern Asia, Pacific islands, and Indian islands as ethyl 4-methyloctanoate (13). The racemic pheromone [55, 56] has been straightforwardly synthesized from natural citronellol with one-step oxidation of 2,6-dimethyl-2-decene by KMnO4-FeCl3 as the key step, followed by esterification in overall yield of 45% (Scheme 4).
3.2.2. 5,9-Dimethylpentadecane (14) and 5,9-dimethylhexadecane (15)
Francke et al.  identified and synthesized the major and minor components of sex pheromone of female leaf miner moths, Leucoptera coffeella, a pest of coffee trees as 5,9-dimethylpentadecane (14) and 5,9-dimethylhexadecane (15), respectively. Synthesis of these racemic components has been described from citronellol by Doan et al. . Grignard coupling reactions with tosylated intermediates under ultrasound irradiation, which has been efficiently employed in literature , were the key steps in the synthetic strategy. The alkene derivatives 16 were oxidized and then reduced to afford the important tosylated synthon 17. Grignard reaction of the corresponding tosylates 17 with 2-methylhexylmagiesium bromide furnished the racemic pheromones, 14 and 15, under accelerating ultrasound irradiation in yields over 90% (Scheme 5).
3.2.3. 8-Methyldec-2-yl propanoate (18) and 10-methyl-2-tridecanone (19)
8-Methyldec-2-yl propanoate (18) was identified as sex pheromone of northern corn rootworm, Diabrotica longicornis Say  and western corn rootworm, Diabrotica virgifera virgifera Le Conte  while the sex pheromone of southern corn rootworm, Diabrotica undecimpunctata howardi Barber, was isolated and identified as 10-methyl-2-tridecanone (19) by Guss et al. . A method for the synthesis of mixture (2R/S,8R)-18 with a chiral centre at C-8 was performed using three chiral substrates using Grignard coupling as a key step . The important ketone synthon was reduced with NaBH4, followed by esterification to obtain the pheromone in total yield over 50% (Scheme 6).
In a similar fashion, synthesis of the sex pheromone 10R-19 from the chiral material was based on the successive reaction of 1-tosyloxy-4R-methylheptane and the Grignard reagent of 1-bromo-5,5-ethylenedioxyhexane . A straightforward approach to the synthesis of racemic mixtures of 18 and 19 has been recently reported from diol derivatives using the Grignard coupling of protected bromohydrins as a key step . The important intermediate aldehydes, which have a similar structure in both pheromones, were synthesized by oxidation reaction of corresponding alcohols using PCC as an oxidation reagent. Pheromones 18 and 19 were obtained in overall yields of 35% and 29%, respectively (Scheme 7).
3.3. Olefinic pheromones
3.3.1. (14R)-Methyl-8-hexadecenal (14R-21)
The geometrical isomers of (14R)-methyl-8-hexadecenal (14R-21) were identified as major components of the pheromone of Khapra beetle, Trogoderma granarium. Both pheromone isomers were found in a Z: E ratio of 92 : 8 and named (Z) and (E)-trogodermal . Mori and coworkers [67, 68] demonstrated that the R-enantiomers, 14R,8Z-21 and 14R,8E-21, revealed bioactivity on male dermestid beetles, T. glabrum, T. inclusum and T. variabile. Nguyen et al. [69, 70] described synthesis of the both R-isomers based on the chiral bromide substrate 20 and employed substitution coupling as a key step. Scheme 8 shows synthetic route of the E-isomer from acrolein using SN2′ substitution of Grignard reagent of 20 to acyclic allyl acetate to afford the E-isomer with purity of 90%, containing a small amount of branched product. At the same time, the Z-isomer was efficiently prepared via three steps in overall yield 24% from commercially available Z-2-buten-1,4-diol. The key step was condensation of Z-disubstituted primary allyl acetate with Grignard reagent according to the nucleophilic substitution mechanism (SN2) to furnish the pheromone 14R,8Z-21 as a major product (Scheme 9).
3.3.2. (Z)-7-Dodecenol (23), (Z)-7-dodecen-1-yl acetate (24), (Z)-7-tetradecenol (25), (Z)-7-tetradecen-1-yl acetate (26), (Z)-7-tetradecenal (27)
Berger et al.  isolated and identified (Z)-7-dodecen-1-yl acetate (24) as the male-produced pheromone of cabbage loopers, Trichoplusia ni Hubner, a destructive pest of peas and weed plants in Asia while the precursor of this pheromone, (Z)-7-dodecen-1-ol (23) was reported as an inhibitor of this sex pheromone by Tumlinson et al. . Vang et al.  described the identification of three compounds, (Z)-7-tetradecenol (25), (Z)-7-tetradecen-1-yl acetate (26), and (Z)-7-tetradecenal (27) as the sex pheromone of the citrus pock caterpillar, Prays endocarpa. The compound 26 has also been found as sex pheromone of other species such as the citrus flower moth (P. citri and P. nephelomina) [73, 74] and the olive moth (P. oleae Bern) [75, 76]. These pheromones have been synthesized from the available commercial diol 22 via the SN2 mechanism between Grignard reagent prepared from protected bromohydrins with Z-allyl acetate derivatives in presence of CuI catalyst as a key step . The Z-isomers were purified by column chromatography using Silica gel impregnated with AgNO3 as the stationary phase (Scheme 10).
3.3.3. (Z)-3-Dodecen-1-yl (E)-2-butenoate (28), (Z)-11-hexadecenol (29), (Z)-11-hexadecenyl acetate (30), (Z)-11-hexadecenal (31)
The sweet potato weevil, Cylas formicarius elegantulus S., and Diamondback moth, Plutella xylostella L., are prevalently serious insects in Vietnam. Heath et al.  identified and first synthesized (Z)-3-dodecen-1-yl (E)-2-butenoate (28) as the female-produced sex pheromone of the sweet potato weevil. The female of Diamondback moth secretes the pheromone to attract the males identified as (Z)-11-hexadecenyl acetate (30) and (Z)-11-hexadecenal (31) in a ratio of 1:1 to 3:1 . Yamada and Koshihara  found (Z)-11-hexadecen-1-ol (29) synergizing the attractiveness of the pheromone mixtures of 30 and 31.
The efficient synthetic pathway of these pheromones from commercially unsaturated acid derivatives has been described . Acrylic acid or 1-undecenic acid was straightforwardly converted into the corresponding triphenylphosphonium salt of methyl esters which reacted with 1-alkanal in presence of dibenzo-18-crown-6 to afford (Z)-ester derivatives. Reduction of these derivatives with LiAlH4 was done to obtain the corresponding alcohols which were subsequently oxidized with PCC into the aldehyde pheromone (31) or esterified with crotonyl chloride and anhydride acetic into pheromones 28 and 30, respectively (Scheme 11). Another way for synthesis of 29–31 based on (Z)-2-buten-1,4-diol similar to the description in Scheme 10 has also been reported by Nguyen .
3.3.4. 7,11,13-Hexadecatrien-1-ol (32) and 7,11,13-hexadecatrienal (33)
Ando et al.  have identified (Z7,Z11)-hexadecatrienal (Z7,Z11-34) as an sex attractant of the citrus leaf miner females, Phyllocnistis citrella Stainton, a harmful citrus pest in Asia. However, two other research groups [83, 84] demonstrated that a mixture of (Z7,Z11,E13)-hexadecatrienal (Z7,Z11,E13-33) and Z7,Z11-34 at a ratio of 3:1 strongly attracted the citrus leaf miner in Brazil and California. Vang et al.  have described synthesis and comparison of biological test of two geometrical isomers, Z7,Z11,E13-33 and Z7,E11,E13-34. The isomers were synthesized, the key steps being Wittig reaction of the protected ylides using a base NaN(SiMe3)2 to furnish (Z)-isomer as the major products in overall yield of 3%. The pure individual isomers, (Z7,Z11,E13)-33 and (Z7,E11,E13)-33, were obtained by isolation from a mixture of corresponding alcohols 32 (2:1) using preparative HPLC methodology, followed by oxidation with PCC (Scheme 12).
4. Pheromone trap
Pheromone trap is a useful tool for management of insects. Numerous trap types are being used efficiently for IPM such as board trap, tube trap, and pitfall trap (Figure 1). The board traps are most commonly used for trapping moths damaging vegetables or forest trees, while the tube traps are used efficiently for trapping fruit fly. These traps are either hung from branches or nailed to infested trees. Pitfall trap is usually employed to collect beetles and weevils. This trap consists of a container with a lot of small windows buried in the ground where its windows are at surface level . The use of trap generally baited with a lure containing the corresponding pheromone is significantly dependent on individual insect characteristics. The pheromone is dissolved in a suitable solvent, usually hexane and then placed on a rubber septum or septa to protect the compounds from degradation. After evaporation of the solvent, the lure is pinned at centre of the traps which would be put on the field at suitable intervals. Lures are designed to release pheromones at a constant or near-constant rate during the course of the experiment. Release rate is dependent on the molecular, physical and chemical properties of the lure matrix and environmental conditions such as temperature and local weather. Traps are checked and insects are recorded over time, usually for several days or weeks.
5. Control of harmful insects in Vietnam
5.1. Diamondback moth, Plutella xylostella
Lure type: synthesized pheromone, 29 (>98%, GC), 30 (99.2%, GC) and 31 (95.5%, GC) in a ratio of 1:10:1 and lures provided from AgriSense Co.
Trap type: sticky board traps (Figure 2).
Areas: vegetable fields in Tu Liem (HaNoi, north of Vietnam), Da Lat (Lam Dong province, central highland region of Vietnam), Cu Chi and Hoc Mon District (Ho Chi Minh City, South of Vietnam).
Diamondback moth attacks only cruciferous vegetable crops in the family Cruciferae including cabbage, broccoli, cauliflower, collard, etc. Plant damage is caused by larvae feeding on foliar tissue and this is particularly damaging to the seedlings, resulting in disrupted head formation. A comparison study of field test using the synthesized pheromones and commercial lures was carried out between years 1994 and 1995 in many areas of Vietnam . The summary of the results is presented in Table 2. It reveals that distributing density of Diamondback moths is clearly different among geographic areas in Vietnam. For example, the mean males captured by all lures in Da Lat and Cu Chi are significantly higher than the pests in Tu Liem and Hoc Mon district. The authors also demonstrated stronger male response when dosage of the pheromone mixture steadily increases and the captured males by lures containing 5 mg of the synthesized pheromone are similar to the commercial lures.
|Captured mean males/trap/night±S.E.|
|Tu Liem (Jan. 2 to Feb. 2, 1994)||Da Lat (Apr. 20 to May 20, 1994)||Cu Chi (Mar. 1 to Apr. 1, 1994)||Hoc Mon (Jan. 4 to Feb. 4, 1994)||Cu Chi (Mar. 1 to Apr. 1, 1995)||Hoc Mon (Jan. 4 to Feb. 4,1995)|
5.2. Sweet potato weevils, Cylas formicarius elegantulus
Lure type: lures baited with the synthesized pheromone 28 (96%, GC) and products provided by Department of Entomology, CIP Aptartodo 5969, Lima 1, Peru.
Trap type: tube trap made from plastic flask with six windows (Figure 3).
Area: sweet potato fields in Hoc Mon district (Ho Chi Minh City).
Sweet potato weevil, C. formicarius is the most serious pest of sweet potato which mainly causes damage to fields not only in Vietnam but also around the world. The adult female deposits a single egg near the juncture of stem and tuber and the hatched larvae burrow directly into the tuber of plant. Management of this pest is particularly important in agriculture and storage. Nguyen et al.  carried out field tests of the weevils in sweet potato fields of Hoc Mon district of Ho Chi Minh City. The results revealed that the weevil capture increased with an increase of pheromone dosage and two pheromone sources, synthesized and commercial, are comparable in their attractiveness to this weevil (Table 3).
|Cultivation season||Captured mean males/trap/night±S.E|
5.3. Screening sex attractants for moths in Mekong Delta of Vietnam
Lure type: synthesized and commercial compounds
Monoenyl alcohols: Z7-12:OH (23), Z11-14:OH and E11-14:OH
Monoenyl acetates: Z5-10:Ac, Z5-12:Ac, Z7-12:Ac (24), Z9-12:Ac, Z9-14:Ac, Z11-14:Ac, E11-14:Ac
Dienyl acetates: Z9,E11-14:Ac and Z9,E12-14:Ac
Monoenyl aldehyde: Z11-14:Ald
Trienes: Z3,Z6,Z9-18:H, Z3,Z6,Z9-19:H, Z3,Z6,Z9-20:H and Z3,Z6,Z9-21:H
Epoxydienes: racemic mixtures of epoxy3,Z6,Z9-18:H, Z3,epoxy6,Z9-18:H, Z3,Z6,epoxy9-18:H and their C19-C21 homologs
Trap type: sticky board trap (30 × 27 cm)
Area: orchards of Chinese apple, guava and longan in Can Tho City from December 1998 to November 1999 and orchards of plum and guava from January to December in 2000
Hai et al.  screened six typical attractants to obtain evaluation of the harmful pest population in Mekong Delta of Vietnam. The field tests found male attraction of nineteen Lepidopteran species including nine taxonomically identified species and ten other taxonomically unidentified species. Compound Z11-14:Ac and its mixture were identified as important attractants of Tortricid species such as A. privatana, A. atrolucens, M. furtive, while numerous Noctuid species such as A. signata, C. eriosoma, C. agnate, C. albostriata, A. ochreata, and S. pectinicornis were attracted by lures baited with Z7-12:Ac (24) as a major component. In addition, the seasonal effect clue for male catch of Noctuid species was also observed. For example, the flights were primarily captured in the dry season, from January to March, and in the latter half of the rainy season, September to December whereas the male catch was rarely observed from April to July. These results could help to depict effective ecological behaviour of the pests in these areas.
5.4. Rhinoceros beetle, Oryctes rhinoceros
Lure Type: synthesized compound 13 and the host material, kairomone, which was extracted from fresh coconut tissue in solvents mixture (ethanol (68%), ethyl acetate (27%) and pentane (5%)).
Trap type: pitfall traps made from 20-liter plastic buckets with window size about 3 × 8 cm (Figure 4).
Area: coconut fields with the 3–10 years old trees in Hau Giang and Ben Tre province (Mekong Delta).
The coconut rhinoceros beetle causes extensive damage to economically important wild and plantation palms in Vietnam and Southeast Asia region. The adults eat the leaves and burrow into the crown leading to stunted plant development. Dang et al.  investigated the beetles in two provinces of Mekong Delta between two seasons, dry and rainy, in years 2004 and 2005 using lures baited with pheromone and kairomone. The authors showed that synergism of kairomone leads to an increased beetle response to their pheromone in all trials and the beetle catches were not too different between the two provinces at the same time. The result revealed that the insect density in the rainy season was higher than in the dry season in both areas (Table 4). In addition, steady increase of the beetle catches with increasing release rate of the pheromone was also observed (Table 5).
|Lure (50 mg)||Captured mean pests/trap/2 weeks±S.E.|
|Hau Giang||Ben Tre|
5.5. Citrus leaf miners, Phyllocnistis citrella
Pheromone: two geometrical isomers of synthesized compounds 32 and 33 and commercial isomers of 34.
Trap type: sticky board trap (30 × 27 cm).
Area: citrus orchards in Can Tho City from November 21, 2005 to March 12, 2006 and in Ogasawara Islands, Japan from November 17, 2005 to April 5, 2006.
The citrus leaf miner, a widespread Asian species, is found to be dangerous to all citrus in many areas around the world such as, in East and South Africa. The larvae make serpentine mines on young leaves or shoots, resulting in leaf curling and inducing a serious plant disease. Vang et al.  described field trials of the moths in citrus orchards in Vietnam and Japan using isomeric mixture of compounds 44–46. The data in Vietnam revealed that the citrus leaf miner males were not captured by a lure baited only with Z7,Z11-46 but were efficiently attracted with a mixture of Z7,Z11,E13-45 and Z7,Z11-46 in a ratio of 3:1, whereas similar synergistic effect on moth catches was not observed in Japan. The authors, therefore, concluded that the sex pheromone of Vietnamese citrus leaf miners is different from their pheromone in Japan.
5.6. Citrus Pock Caterpillar, Prays endocarpa
Pheromone: synthesized compounds 25, 26 and 27.
Trap type: sticky board trap (30 × 27 cm).
Area: pomelo orchards of villages in Vinh Long province in 2007 and from April 4 to June 10, 2008.
The citrus pock caterpillar is an economically serious pest of the pomelo (Citrus grandis L.) orchard in Vietnam and Southeast Asia. The larvae attack and mine into the peel of the fruit which would either develop into tumours or even drop if the attack is in the early stage of development. Vang et al.  reported the analysis of gas chromatography-mass (GC-MS) from pheromone gland extract of female moths finding three monoenyl-C14 derivatives, 25–27 in a ratio of 10:3:10. Field data showed that the male moth was attracted by only aldehyde 27, while both other compounds were not attractive as well as no synergism with this aldehyde in the adult catches was found. The authors also demonstrated that achieving efficiency with mass trapping experiment was similar to use of a pesticide in suppression of the pest in pomelo orchard.
5.7. Citrus leaf rollers (Archips atrolucens, Adoxophyes privatana and Homona sp.)
Pheromone: synthetic and commercial compounds Z11-14:Ac, E11-14:Ac, Z9-14:Ac, Z9-12:Ac and 14:Ac.
Trap type: sticky board trap (30 × 27 cm).
Area: orange orchards in Hau Giang province and Can Tho City in 2011 and 2012.
Recently, the natural pheromones of three citrus leaf rollers, Archips atrolucens, Adoxophyes privatana and Homona sp., which have been known as serious defoliators of citrus trees in Vietnam, were identified using analysis techniques GC-EAD and GC-MS . The results showed that three components Z11-14:Ac, E11-14:Ac and 14:Ac in a ratio of 64:32:4 were identified as the sex pheromone of A. atrolucens while two other couples Z11-14:Ac and Z9-14:Ac (92:8); Z11-14:Ac and Z9-12:Ac (96:4) as the sex pheromones of A. privatana and Homona sp., respectively. However, their field test revealed that amount of the minor components in tested pheromones of all three species was slightly greater than those in the natural pheromones. For instance, the best male catches were found for A. atrolucens, a blending mixture of Z11-14:Ac, E11-14:Ac and 14:Ac at a ratio of 2:1:1; for A. privatana, a blending mixture of Z11-14:Ac and Z9-14:Ac at a ratio of 9:1 and similarly for Homona sp., a blending mixture of Z11-14:Ac and Z9-12:Ac at a ratio of 9:1.
In this chapter, we review the researches related to synthesis and field evaluation of insect pheromones which have been done by Vietnamese researchers or collaborators since 1980s. Pest control by the use of attractant traps promises to obtain an accurately quantitative estimation of insect population. This important information would help farmers to get more effective protection of the plants in Vietnam. In some cases, using the pheromones could reduce damage to the plants. In addition, pheromones were made possible by advances in synthetic organic chemistry and most of the synthetic approaches mentioned earlier are novel and employable to prepare at a large scale. Some schemes of the synthetic routes have become a standard methodology for synthesis of the similarly structured molecules such as synthesis of enantiomers from (S)-3,7-dimethylocta-1,6-diene or synthesis of Z and E-monoenyl from (Z)-2-buten-1,4-diol and acrolein, respectively. Moreover, this study is motivated by the prospect of gaining better understanding of ecological relationships to develop IPM in Vietnam. Also, it helps organic chemists, entomologists and authorities to get sharp orients for further projects in developing an efficient environmentally benign tool. We believe that pheromones are going to become essential materials for the durable development of a green agriculture in the near future.