Journal of Chemical Ecology, Vol. 12, No. 12, 1986
P R E D I C T I O N OF R E L E A S E RATIOS OF MULTICOMPONENT PHEROMONES FROM RUBBER SEPTA 1
R.R. H E A T H , P . E . A . T E A L , J.H. T U M L I N S O N , and L.J. MENGELKOCH Insect Attractants, Behavior, and Basic Biology Research Laboratory Agricultural Research Service, USDA Gainesville, Florida 32604
(Received October 18, 1985; accepted December 31, 1985) Abstraet--A method has been developed to predict the release ratio of the components of blends of alcohols, acetates, and/or aldehydes from rubber septa. The calculations of predicted release ratios are based on the relative vapor pressures of the components. The relative vapor pressures of the compounds were calculated from their retention indices on a liquid crystal capillary gas chromatographic column. The correlation between the theoretically predicted and experimentally determined ratios was very good. Thus, formulations can be prepared that will release a desired ratio of the components of a multicomponent pheromone blend. Key Words--Pheromone blends, formulatien on rubber septa, relative vapor pressure, liquid crystals.
INTRODUCTION The sex pheromones of many lepidopteran insect species have been identified, and the practical value of several o f these has been demonstrated in programs designed to monitor or suppress pest insect populations. Additionally, extensive studies have been conducted to analyze and evaluate insect behavior in the laboratory and in the field. Several materials have been adapted or developed to release pheromones at controlled rates to increase the effectiveness o f these chemicals in monitoring or control programs. Chemical analyses of pheromone gland constituents and volatiles collected from calling females, in addition to Mention of a commercial or proprietary product does not constitute an endorsement by the USDA. 2133 0098-0331/86/1200-2133505.00/0
9 1986 Plenum Publishing Corporation
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behavioral studies and field trapping experiments, have established that most lepidopteran pheromones consist of blends of two or more compounds. Usually a precise ratio of the components of a blend is required to elicit optimum response. However, thus far there has been very little research to develop an accurate method to predict and control the precise ratio and rate at which a pheromone blend is released from a formulation. There have been several investigations in which the ratio of components in a pheromone blend loaded onto a controlled release substrate was empirically adjusted to give optimum trap captures. Additionally, the release rate of individual pheromonal compounds from different substrates has been measured. Butler and McDonough (1979, 1981) and McDonough and Butler (1983) determined the half-lives of several n-alkyl and alkenyl acetates and alcohols loaded individually on rubber septa by quantitating the amount of a compound remaining on a septum after various periods of time. Similarly, Leonhardt and Moreno (1982) measured release rates of several pheromones and pheromone blends from laminated Hercon | dispensers by gas-liquid chromatographic (GLC) analysis of quantities of the materials remaining in the dispensers after they were aged varying amounts of time in a greenhouse and in the field. Baker et al. (1980) collected and quantified Oriental fruit moth pheromone deposited on the wall of a glass flask and determined that (Z)-8-dodecen-l-ol (Z8-12 : OH) was emitted three times faster than (Z)-8-dodecenyl acetate (Z8-12 : Ac) from rubber septa. Using an improved version of Regnault's (1845) gas saturation technique, Hiroka and Suwanai (1978) measured the vapor pressures of 12 alkyl compounds including alkanes, acetates, and an alcohol. Their investigation of a binary mixture of lauryl and myristyl acetate demonstrated that a binary sex pheromone system behaves approximately according to Raoult's Law, i.e., the mole fraction of a compound in the vapor above a binary liquid mixture correlated closely with the mole fraction of the compound in the liquid mixture and the relative vapor pressures of the two compounds. Thus, two compounds with equal vapor pressure would occur in the same ratio in the vapor as in the liquid. Other references pertaining to pheromone release rates from various substrates include the work of Weatherston et al. (1982) and several chapters in books edited by Mitchell (1981) and Kydonieus and Beroza (1982). The information developed so far allows the determination of the average release rate of a pheromone over a period of days and, in some cases, may allow the approximate average ratio to be determined and controlled. Unfortunately, there is as yet no information that will allow one to predict and control with a reasonable degree of accuracy the precise ratio and rate at which the components of a pheromone blend will be released during a period of 1 hr or less under a given set of conditions. Our interest in analyzing the behavioral responses of males of several species of Heliothis, Spodoptera, and other Lep-
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idoptera to their respective pheromone blends requires a method to predict and control release rates and ratios for blends of compounds that might vary considerably in chain length and functionality. In a previous paper, we demonstrated that the retention time of a compound on a liquid crystal capillary column could be correlated with the compound's reported vapor pressure and/or reported half-life (Heath and Tumlinson, 1986). The correlation of the retention time expressed in equivalent chain length units (ECLU) of seven acetates and the natural logarithm of their reported vapor pressures (Olsson et al., 1983), resulted in a coefficient of determination (r 2) of 0.999. Attempts to correlate the retention times of the seven acetates on capillary columns coated with isotropic phases (apolar OV-101 | and polar CW-20) with their vapor pressures resulted in drastically reduced coefficients of determination of 0.724 and 0.446, respectively. Correlation of the retention time (in ECLU) on the liquid crystal column with the reported half-lives (Butler and McDonough, 1979, 1981; McDonough and Butler, 1983) of five saturated acetates (10-14 carbons), four monounsaturated acetates, and four monounsaturated alcohols (12-16 carbons), resulted in a high degree of correlation (r 2 of 0.995). Based on these findings, we proceeded on the hypothesis that relative vapor pressure and evaporation rates of many compounds could be approximated based on the compounds' retention times on a liquid crystal column. This information could then be used to predict the release ratios of components in pheromone blends. We report in this paper the results of our investigation into the prediction of the release ratios of various combinations of acetates, aldehydes, and alcohols from rubber septa based on retention indices of the compounds on a liquid crystal column.
METHODS AND MATERIALS
Chemicals used for this investigation (Table 1) were obtained from Chemical Samples Co. (Columbus, Ohio). In several instances synthetic modification of the functional group was required to give the desired compound. All compounds were purified by AgNO 3 liquid chromatography or recrystallized from pentane, distilled in a short-path distillation apparatus, and analyzed on three different capillary columns to ensure a high degree of purity. Capillary columns were drawn and coated in our laboratory by previously described techniques (Heath et al., 1980, 1981). The three columns used were: 46 m x 0.25 mm ID cholesteryl-p-chlorocinnamate (a cholesteric liquid crystal phase); 46 m x 0.25 mm ID OV-101; and 42 m • 0.25 mm ID SP-2340 | Columns were operated in either the split or splitless mode with helium as the carrier gas at a linear flow of 18 cm/sec. The columns were installed in a Varian | 3700 gas
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HEATH ET AL, TABLE 1. RELATIVE VAPOR PRESSURE AND HALF-LIVES CALCULATED FROM
EQUIVALENT CHAIN LENGTH ( E C L U ) OF COMPOUNDS CHROMATOGRAPHED ON LIQUID CRYSTAL CAPILLARY G L C COLUMN
Compounda
ECLU b
Relative vapor pressure"
Relative half-lives (days)
Sat-12 : Ac Z9-14:A1 Sat-14 : A1 Z 9 - 1 4 : OH Sat-13 : Ac Zll-14:OH Z 9 - 1 4 : Ac Sat-14 : Ac Z 1 1 - 1 6 : AI Sat-16 : Al Z i 1-16 : OH Sat- 15 : Ac Z 11-16: Ac Sat- 16 : Ac
1200 1213 1250 1296 t300 1306 1366 1400 1413 1450 1447 1500 1562 1600
0.4633 0.4062 0.2804 0.1769 0.1700 0.1601 0.0877 0.0624 0.0549 0.0379 0.0390 0.0229 0.0123 0.0086
38.1 43.4 63.1 100.4 104.6 111.1 203.7 287.1 327.4 475.8 461.0 788.4 1474.7 2164.6
aExample: (Z)-9-tetradecenal = ( Z 9 - 1 4 : A 1 ) , (Z)-9-tetradecen-l-ol = ( Z 9 - 1 4 : O H ) , (Z)-9-tetradecenyl acetate = ( Z 9 - 1 4 : Ac), (Z)- 11-hexedecenal = (Z 11-16 : AI), and (Z)- 11-hexadecenyl acetate = (Z 11-16 : Ac). bEstablished with saturated acetates. CCalculated from equations I and 2 in text.
chromatograph equipped with flame ionization detectors with a lower limit of detection of ca. 0.2 ng for hexadecane. Rubber septa (Cat. No. 8753-D22, A.H. Thomas Co., Philadelphia, Pennsylvania) were exhaustively extracted with CH2C12 for 24 hr and air dried a minimum of three days before use. A septum was loaded by depositing the desired blend, dissolved in 100/zl of hexane, into the cup of the large end. The quantity of pheromone loaded varied somewhat, depending on the number of blend components but was maintained between the limits of 400/zg and 600 #g per septum. The collection apparatus is an adaptation of a Grob and Zficher (1976) design and includes modifications by P.S. Beevor and coworkers (Tropical Products Institute, London, England) to collect pheromones from insects. We have adapted and further modified this method (Tumlinson et al., 1982). Briefly, it consists of a small charcoal trap prepared by sealing 3-5 mg of charcoal (Bender and Hobein AG, Zurich, Switzerland) between two 325-mesh stainlesssteel frits in a 6-ram OD, 3.7-mm ID Pyrex | tube. Air is delivered through the silanized glass aeration apparatus at a flow rate of 0.5-1 liter/rain. The septa are placed ca. 2 cm upwind from the charcoal trap and aerated for 1-2 hr. When
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aeration is complete, the filter is rinsed with six aliquots (15-20 /zl each) of distilled dichloromethane, the aliquots are combined, an internal standard is added, and the solution is concentrated to about 5-10/zl by gently warming. Isooctane or another solvent of choice is added for analysis by capillary GLC with splitless injection. In a preliminary experiment, a backup charcoal trap was connected to the outlet of the primary trap. Extraction and analysis of the secondary trap resulted in no detectable amount of material. Additionally, recoverability of compounds from the charcoal trap was determined by depositing a known amount of the compound (usually as a blend with three other compounds) onto the charcoal directly in 20- to 50-ng amounts. The compounds were extracted after solvent evaporation and analyzed as previously described. Before each experiment, the aeration chamber was cleaned by rinsing with hexane, acetone, water, acetone, and hexane. The chambers were dried overnight at ca. 100~ The charcoal traps were cleaned by rinsing with ca. 4 ml CH2C12 and CS2. The traps were then dried using a flow of nitrogen. Determination of relative vapor pressure and half-lives of compounds used in this investigation are based on the previously determined equations (Heath and Tumlinson, 1986). The equation for the relative vapor pressure of a compound is (ln) vapor pressure = (ECLU - 1123)/-99.9
(1)
and the equation for a compound's half-life is (In) half-life = - 8 . 4 8 + 0.0101 9 ECLU
(2)
where ECLU is equal to the retention time of a compound on a cholesteryl-pchlorocinnamate capillary column relative to saturated acetates (Swoboda, 1962). To derive a formula for determining the release ratio from rubber septa of components in pheromone blends, several assumptions were made. We assumed that the relative vapor pressure of the solute in rubber septa could be approximated by using ECLU values obtained by chromatography on a liquid crystal stationary phase (isotopic phases showed no correlation) (Heath and Tumlinson, 1986). Thus in this assumption, the proportionality constant as required by Henry's Law is empirically determined, and the rate of evaporation of a compound from rubber septa is also correlated with the retention time of the compound in the liquid crystal phase. The ratio of a component in the vapor from a multicomponent blend is the percent of the compound in the liquid times its evaporation rate, divided by the sum of each of the components (in %) in the liquid multiplied by their respective evaporation rates. It is well documented that the evaporation rates of pheromones from rubber septa can be determined by measuring their half-lives and is a first-order equation (Butler and McDonough, 1979, 1981; McDonough and Butler, 1983). If we assume that the
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HEATH ET AL.
evaporation of one component will not affect the evaporation rate of another component, our formula for release ratio is the same as that using relative vapor pressure. However, in this case evaporation rate is in days, and we must convert all half-lives to their reciprocals for calculation of release ratios using equation (3). The calculation of the percent release of a compound when formulated in a blend on rubber septa is based on use of the calculated relative vapor pressure of the compound or calculated half-life of the compound. The equation for calculating percent release of a compound is RI =
LjP~ r LiP1 + L2P2 + L,,P~ r
(3)
where L is the percent load of the compound and pr is the reciprocal of the relative half-life (evaporation rate) or the relative vapor pressure of the compound. RESULTS AND DISCUSSION
The equivalent chain length, calculated relative vapor pressures, and calculated relative half-lives for all compounds included in this study are presented in Table 1. Various blends were prepared and the theoretical release ratio of each component was determined using equation 3. The volatiles released by the blend when formulated on rubber septa were trapped and analyzed. Prior to analysis of charcoal-trapped volatile blends, the aeration apparatus and charcoal traps were evaluated to determine the amount of material lost due to adsorption on glass surfaces or irreversible adsorption on the trap. The percent of each compound recovered when blends containing ca. 50 ng of each component were deposited directly on the trap and then reextracted without aeration is given in Table 2. The recoveries ranged from 74 to 94%, and data obtained in subsequent experiments were corrected to account for the losses on the trap. Analysis of the solvent rinses of the internal glass surfaces of the apparatus after experiments indicated that no measurable material was adsorbed on the silanized glass surfaces. This is probably because the septa were placed within 2 cm of the trap, and thus there was very little glass surface downwind of the septum for the volatiles to contact. Analysis of backup traps connected in series to the primary traps indicated no significant breakthrough of compounds from the first trap occurred. The various blends of compounds that were formulated and aerated are listed in Table 3. Different ratios of the same blend of compounds were analyzed to test the hypothesis that this method would be valid over a wide range of ratios. Volatiles from the septa were analyzed over a seven-day interval. Analyses were obtained on 1- to 2-, 3- to 4-, and 5- to 7-day-old septa (after
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TABLE 2. PERCENT RECOVERY OF COMPOUNDS FROM CHARCOAL FILTER
Compound"
Recovered (%)
Standard deviation
Replicate
Sat-12 : Ac Z 9 - 1 4 : AI Sat-14 : A1 Z 9 - 1 4 : OH Sat- 13 : Ac Z 1 1 - 1 4 : OH Z 9 - 1 4 : Ac Sat- 14 : Ac ZII-16:A1 Sat- 16 : AI Z 11-16:OH Zll-16:Ac Sat-15 :Ac Sat- 16 : Ac
91 78 76 76 94 80 86 87 76 74 79 85 91 87
8.5 14.19 9.5 8.7 8.4 1 ! .0 7.6 8.2 7.0 16.2 9.4 7.6 10.2 7.8
6 6 6 6 6 8 6 6 6 6 10 6 10 6
aExample, (Z)-9-tetradecenal = Z 9 - 1 4 : A1, (Z)-9-tetradecen-l-ol = Z 9 - 1 4 : OH, (Z)-9-tetradecenyl acetate = Z 9 - 1 4 : Ac), (Z)-11-hexadecenal = Z 11-16 : AI, and (Z)-11-hexadecenyl acetate = Z11-16:Ac.
the three-day aging process). The analysis of each blend at a given time interval was replicated at least three times. Because there was no significant change in the measured ratio during the seven-day interval, the data from all analyses were averaged and compared to the calculated theoretical release ratio. For blends that contained compounds for which the half-lives had been reported by Butler and McDonough (1979, 1981) and McDonough and Butler (1983), we calculated the predicted release ratio based on reported half-lives and compared the data with the predicted release ratios based on relative vapor pressure and with the measured release ratios (Table 3). An attempt was made to analyze a representative number of different combinations of molecular weights and/or functional groups to correlate as closely as possible with actual pheromone blends found in a variety of insects. The results obtained when C12-C ~4 saturated acetates were formulated as three-component blends in approximately equal amounts showed that the release ratio of the acetates could be predicted with less than 4% error (Table 3). When the 16-carbon acetate was formulated with other saturated acetates, a large difference was found between percent recovered and percent theoretical. McDonough and Butler (1979) found similar discrepancies with the half-life of the 16-carbon acetate when compared to shorter chain length acetates and suggested that the deviation was due to the inability of the larger acetate molecules to penetrate the rubber matrix. Another possible explanation is that the 16-
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HEATH ET AL.
TABLE 3. COMPARISON OF PERCENT LOAD, PERCENT RATIO CALCULATED, MEAN PERCENT FOUND AND STANDARD DEVIATION
Blend a
Load (%)
% Calculated using vapor pressure or calculated half life
Reported half-life b
Found (%) Mean"
SD
Sat- 12 : Ac Sat- 13 : Ac Sat-14 : Ac
41.1 28.8 30.1
73.8 18.9 7.2
74.4 19.8 5.6
70.1 21,5 8.5
2.2 1.4 0.9
Sat-13 : Ac Sat-14 : Ac Sat- 15 : Ac
37.7 31.1 31.1
70.6 21.5 7.9
77.5 17.8 4.6
70.2 21.4 8.5
1.5 1.6 1.9
Sat- I4 : Ac Sat- 15 : Ac Sat-t6:Ac
36.9 33.2 29.3
69.2 23.2 7.6
54.8 19.4 26.6
5.9 2.2 8.0
Z 9 - 1 4 : A1 Sat- 14 : A1 Zll-16:A1 Sat- 16 : A1
27.3 23.7 29.5 19.6
55.1 33.1 8.1 3.7
51.9 34.7 8.0 5.0
3.2 1,3 2.1 1.8
Z 9 - 1 4 : A1 Sat-14 : A1 ZII-16:AI Sat- 16 : Al
19.0 17.0 37.0 27.0
49.9 30.7 12.9 6.5
45.1 31.4 10.1 10.5
2.9 1.5 2.0 3.1
Z9-14:A1 Sat- 14 : AI Z 1 1 - 1 6 : A1 Sat-16 : AI
6.0 6.0 52.0 36.0
29.2 20.2 34.2 16.4
28.7 23.0 28.2 20.1
4.0 3.5 3.8 2.9
Z 9 - 1 4 : OH Sat-13 : Ac Zll-16:OH Sat- 15 : Ac
23.8 31.9 18.3 25.9
38.5 49:6 6.5 5.4
34.4 55.2 7.2 3.2
38.6 49.9 4.7 6.7
3.8 3.2 4.2 1.2
Z 9 - 1 4 : OH Sat- 13 :Ac Z 11-16 : OH Sat-16 : Ac
14.7 19.1 28.8 37.4
33.2 41.5 14.3 11.0
30.3 47.0 16.1 6.7
33.6 42.2 10.6 13.6
3.6 1.9 2.9 2.1
Z 9 - 1 4 : OH Sat- 13 : Ac Z I 1-16 : Ot-I Sat-15 : Ac
5.2 6.5 41.4 45.9
19.6 23.6 34.3 22.4
18.5 27.6 39.8 14.1
20.5 24.7 26.2 28.7
1.8 1.6 4.1 2.6
Z9-14:A1 Sat-13 : Ac ZII-16:AI Sat- 15 : Ac
26.3 25.6 25.3 22.7
53.1 35.7 6.9 4.3
50.9 32.8 12.2 4.1
2,4 3.2 6.1 2.2
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TABLE 3. Continued
Load (%)
% Calculated using vapor pressure or calculated half life
Z9-14 : A1 Sat- 13 : Ac Z11-16 : AI Sat-15 :Ac
15.2 15.0 39.7 29.9
45.0 30.8 15.9 8.3
48.2 26.8 16 9
3.8 4.8 3.1 2.0
Z9-14:A1 Z9-14:Ac ZII-16:A1 Z11-16 : Ac
2.7 66.4 3.6 25.7
16.5 76,8 2.6 4.1
17.3 (13.6)d 76.7 (76.9) 3.1 (6.7) 2.9 (3.4)
6.2 6.9 1.1 0.7
Z9-14 :A1 Z9-14 : Ac Z11-16: AI Z11-16: Ac
7.4 58.8 8.4 25.4
36.6 55.2 4.9 3.3
32.4 (32.0) 60.6 (60.3) 4.3 (5.8) 2.6 (1.9)
4.1 3.8 0.6 1.1
Blenda
Reported half-lifeb
Found (%) Mean"
SD
aExample: (Z)-9-tetradecenal = zg-14:A1, (Z)-9-tetradecen-l-ol = Z9-14:OH, (Z)-9-tetradecenyl acetate = Z9-14 : Ac, (Z)-I l-hexedecenal = Z 11-16 : AI, and (Z)-I 1-hexadecenylacetate = Z11-16:Ac. bCalculatedfrom reported half-lives(Butlerand McDonough, 1979, 1981; McDonoughand Butler, 1983). CMean is calculated from the average obtained from the 1- to 2-, 3- to 4-, and 5- to 6-day-old samples. Each mean represents a minimum of nine analysis. dAnalysisobtained on samples left in the field in parentheses. carbon acetate is a solid at room temperature, and our inability to predict the release rate may be due to the establishment of a solid-liquid phase equilibrium. The relative vapor pressure data obtained for the aldehydes using the retention time data obtained on the liquid crystal column enables us to predict relative release rates for the aldehyde blends with less than 6% error for any given component in the three blends investigated (Table 3). Greater variability occurred between calculated and measured released ratios when blends were composed of acetates and alcohols or acetates and aldehydes (Table 3). There are at least two possible explanations for this. The aldehydes and the alcohols are adsorbed more strongly by the charcoal and glass surfaces, and thus small but immeasurable differences could contribute to greater error when analyzing blends composed of compounds with two or more types of functional groups. Also, solutions of compounds with more than one type of functional group may differ more in physical chemical properties than do compounds with one type of functionality. Further investigation of this phenomenon is required to determine the actual reason for these discrepancies. Nonetheless, as a first approximation, the experimental results are very close to the calculated values, and this
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HEATH ET AL.
is the most accurate method developed thus far for predicting release ratios of components in pheromone blends. A greater disparity between the predicted release ratio and measured release ratio was obtained when reported half-lives (Butler and McDonough, 1979, 1981; McDonough and Butler, 1983) were used in the calculations (Table 3). It should be noted that the error is significantly reduced (equal to that observed using relative vapor pressure values) if the values for the half-lives are determined using equation 2. As an example of the practical application of this study, we formulated blends containing two aldehydes and two acetates (Z)-9-tetradecenal (Z914 : A1), (Z)-9-tetradecenyl acetate (Z9-14 :Ac), (Z)-I 1-hexedecenal (Z1116:A1), and (Z)-I 1-hexadecenyl acetate (Z11-16 :Ac) in two different ratios. All septa were analyzed in the laboratory on the first or second day after loading. Then one batch was placed in traps in the field for four days while an identical batch was aged in the laboratory during the same period and analyzed again at the end of three to four days. At five to seven days after loading, release ratios of the blends were again measured from both lab- and field-aged septa. The result of this study (Table 3) indicated that the placement of septa in the field for a five-day period did not increase significantly the variation of the release ratio of the compounds when compared to the lab septa. CONCLUSIONS
The purpose of this investigation was to explore the hypothesis that relative release rates (half-lives) and relative vapor pressure of individual compounds could be used to calculate the approximate release ratios of the compounds formulated as a blend in rubber septa. Our results indicate that, at least to a first approximation, our hypotheses enabled us to determine release ratios. Certain refinements in measurement techniques or more accurate correction factors for recoveries of alcohols and aldehydes may yield more accurate data. Additionally, our investigation was based on an empirical approach and does not provide fundamental reasons for the existence of the correlation of the retention time of a compound on a liquid crystal column and its predicted release ratio when formulated in a blend on rubber septa. Thus there exists a need to further define the mechanism of evaporation of pheromones from rubber septa. However, the use of the method described should greatly facilitate the determination of the percent load required to provide the desired release ratio of components in pheromone blends. REFERENCES BAKER, T.C., CARDE, R.T., and MmLER, J.R. 1980. Oriental fruit moth pheromone component emission rates measured after collection by glass surface adsorption. J. Chem. Ecol. 4:749758.
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BUTLER, L.I., and McDoNOUGH, L.M. 1979. Insect sex pheromones: Evaporation rates of acetates from natural rubber septa. J. Chem. Ecol. 5:825-837. BUTLER, L.I., and McDoNOUGH, L.M. 1981. Insect sex pheromones: Evaporation rates of alcohols and acetates from natural rubber septa. J. Chem. Ecol. 7:627-633. GROB, K., and Z(3CHER, D. 1976. Stripping of organic trace substances from water. Equipment and procedure. J. Chromatogr. 117:285-294. HEATH, R.R., and TUMLINSON, J.H. 1986. Correlation of retention times on a liquid crystal capillary column with reported vapor pressures and half-lives of compounds used in pheromone formulations. J. Chem. Ecol. 11:2081-2088. HEATH, R.R., BURNSED, G.E., TUMLINSON, J.H., and DOOLITTLE, R.E. 1980. Separation of a series of positional and geometrical isomers of olefinic aliphatic primary alcohols and acetates by capillary gas chromatography. J. Chromatogr. 189:199-208. HEATH, R.R., and TUML1NSON,J.H. 1986. Correlation of retention times on a liquid crystal capillary column with reported vapor pressures and half-lives of compounds used in pheromone formulations, J. Chem. Ecol. 12:2081-2088. HIROKA, Y., and SUWANAI,M. 1978. Role of insect sex pheromones in mating behavior. II. An aspect of sex pheromone as a volatile material. Appl. Entomol. Zool. 11:38-43. LEONHARDT, B.A., and MORENO, P.S. 1982. Evaluation of controlled release laminate dispensors for pheromones of several insect species, pp. 159-173, in B.A. Leonhardt and M.A. Beroza (eds.). Insect Pheromone Technology: Chemistry and Applications. American Chemical Society Symposium Series 190. American Chemical Society, Washington, D.C. KYDONIEUS, A.F., and BEROZA, M.A. 1982. Insect Suppression with Controlled Release Pheromone Systems. CRC Press, Boca Raton, Florida. McDoNOUGH, L.M., and BUTLER, L.I. 1983. Insect sex pheromones: Determination of half-lives from formulations by collection of emitted vapor. J. Chem. Ecol. 9:1491-1502. MITCHELL, E.R. 1981. Management of Insect Pests with Semiochemicals. Plenum Press, New York. OLSSON, A.M., JONSSON, J.A., THEL1N, B., and LILJEFORS,T. 1983. Determination of the vapor pressures of moth sex pheromone components by a gas chromatographic method. J. Chem. Ecol. 9:375-385. REGNAULT,H.V. 1845. Determination of vapor pressure using gas saturation. Ann. Chem. 15:129134. SWOBODA,P.A.T. 1962. Qualitative and quantitative analysis of flavour volatiles from edible fats, pp. 273-291, in M. van Swaay (ed.). Gas Chromatography 1962, Butterworth, London. TUML1NSON,J.H., HEATH,R.R., and TEAL, P.E.A. 1982. Analysis of chemical communications systems of Lepidoptera, pp. 1-27, in B.A. Leonhardt and M. Beroza (eds.). Insect Pheromone Technology: Chemistry and Applications. American Chemical Society Symposium Series 190. American Chemical Society, Washington, D.C. WEATHERSTON, J., GOLUB, M.A., and BENN, M.H. 1982. Release rates of pheromones from hollow fibers, pp. 146-157, in B.A. Leonhardt and M.A. Beroza (eds.). Insect Pheromone Technology: Chemistry and Applications. American Chemical Society Symposium Series 190. American Chemical Society, Washington, D.C.
P R E D I C T I O N OF R E L E A S E RATIOS OF MULTICOMPONENT PHEROMONES FROM RUBBER SEPTA 1
R.R. H E A T H , P . E . A . T E A L , J.H. T U M L I N S O N , and L.J. MENGELKOCH Insect Attractants, Behavior, and Basic Biology Research Laboratory Agricultural Research Service, USDA Gainesville, Florida 32604
(Received October 18, 1985; accepted December 31, 1985) Abstraet--A method has been developed to predict the release ratio of the components of blends of alcohols, acetates, and/or aldehydes from rubber septa. The calculations of predicted release ratios are based on the relative vapor pressures of the components. The relative vapor pressures of the compounds were calculated from their retention indices on a liquid crystal capillary gas chromatographic column. The correlation between the theoretically predicted and experimentally determined ratios was very good. Thus, formulations can be prepared that will release a desired ratio of the components of a multicomponent pheromone blend. Key Words--Pheromone blends, formulatien on rubber septa, relative vapor pressure, liquid crystals.
INTRODUCTION The sex pheromones of many lepidopteran insect species have been identified, and the practical value of several o f these has been demonstrated in programs designed to monitor or suppress pest insect populations. Additionally, extensive studies have been conducted to analyze and evaluate insect behavior in the laboratory and in the field. Several materials have been adapted or developed to release pheromones at controlled rates to increase the effectiveness o f these chemicals in monitoring or control programs. Chemical analyses of pheromone gland constituents and volatiles collected from calling females, in addition to Mention of a commercial or proprietary product does not constitute an endorsement by the USDA. 2133 0098-0331/86/1200-2133505.00/0
9 1986 Plenum Publishing Corporation
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HEATHET AL.
behavioral studies and field trapping experiments, have established that most lepidopteran pheromones consist of blends of two or more compounds. Usually a precise ratio of the components of a blend is required to elicit optimum response. However, thus far there has been very little research to develop an accurate method to predict and control the precise ratio and rate at which a pheromone blend is released from a formulation. There have been several investigations in which the ratio of components in a pheromone blend loaded onto a controlled release substrate was empirically adjusted to give optimum trap captures. Additionally, the release rate of individual pheromonal compounds from different substrates has been measured. Butler and McDonough (1979, 1981) and McDonough and Butler (1983) determined the half-lives of several n-alkyl and alkenyl acetates and alcohols loaded individually on rubber septa by quantitating the amount of a compound remaining on a septum after various periods of time. Similarly, Leonhardt and Moreno (1982) measured release rates of several pheromones and pheromone blends from laminated Hercon | dispensers by gas-liquid chromatographic (GLC) analysis of quantities of the materials remaining in the dispensers after they were aged varying amounts of time in a greenhouse and in the field. Baker et al. (1980) collected and quantified Oriental fruit moth pheromone deposited on the wall of a glass flask and determined that (Z)-8-dodecen-l-ol (Z8-12 : OH) was emitted three times faster than (Z)-8-dodecenyl acetate (Z8-12 : Ac) from rubber septa. Using an improved version of Regnault's (1845) gas saturation technique, Hiroka and Suwanai (1978) measured the vapor pressures of 12 alkyl compounds including alkanes, acetates, and an alcohol. Their investigation of a binary mixture of lauryl and myristyl acetate demonstrated that a binary sex pheromone system behaves approximately according to Raoult's Law, i.e., the mole fraction of a compound in the vapor above a binary liquid mixture correlated closely with the mole fraction of the compound in the liquid mixture and the relative vapor pressures of the two compounds. Thus, two compounds with equal vapor pressure would occur in the same ratio in the vapor as in the liquid. Other references pertaining to pheromone release rates from various substrates include the work of Weatherston et al. (1982) and several chapters in books edited by Mitchell (1981) and Kydonieus and Beroza (1982). The information developed so far allows the determination of the average release rate of a pheromone over a period of days and, in some cases, may allow the approximate average ratio to be determined and controlled. Unfortunately, there is as yet no information that will allow one to predict and control with a reasonable degree of accuracy the precise ratio and rate at which the components of a pheromone blend will be released during a period of 1 hr or less under a given set of conditions. Our interest in analyzing the behavioral responses of males of several species of Heliothis, Spodoptera, and other Lep-
PREDICTION OF RELEASE RATIOS
2135
idoptera to their respective pheromone blends requires a method to predict and control release rates and ratios for blends of compounds that might vary considerably in chain length and functionality. In a previous paper, we demonstrated that the retention time of a compound on a liquid crystal capillary column could be correlated with the compound's reported vapor pressure and/or reported half-life (Heath and Tumlinson, 1986). The correlation of the retention time expressed in equivalent chain length units (ECLU) of seven acetates and the natural logarithm of their reported vapor pressures (Olsson et al., 1983), resulted in a coefficient of determination (r 2) of 0.999. Attempts to correlate the retention times of the seven acetates on capillary columns coated with isotropic phases (apolar OV-101 | and polar CW-20) with their vapor pressures resulted in drastically reduced coefficients of determination of 0.724 and 0.446, respectively. Correlation of the retention time (in ECLU) on the liquid crystal column with the reported half-lives (Butler and McDonough, 1979, 1981; McDonough and Butler, 1983) of five saturated acetates (10-14 carbons), four monounsaturated acetates, and four monounsaturated alcohols (12-16 carbons), resulted in a high degree of correlation (r 2 of 0.995). Based on these findings, we proceeded on the hypothesis that relative vapor pressure and evaporation rates of many compounds could be approximated based on the compounds' retention times on a liquid crystal column. This information could then be used to predict the release ratios of components in pheromone blends. We report in this paper the results of our investigation into the prediction of the release ratios of various combinations of acetates, aldehydes, and alcohols from rubber septa based on retention indices of the compounds on a liquid crystal column.
METHODS AND MATERIALS
Chemicals used for this investigation (Table 1) were obtained from Chemical Samples Co. (Columbus, Ohio). In several instances synthetic modification of the functional group was required to give the desired compound. All compounds were purified by AgNO 3 liquid chromatography or recrystallized from pentane, distilled in a short-path distillation apparatus, and analyzed on three different capillary columns to ensure a high degree of purity. Capillary columns were drawn and coated in our laboratory by previously described techniques (Heath et al., 1980, 1981). The three columns used were: 46 m x 0.25 mm ID cholesteryl-p-chlorocinnamate (a cholesteric liquid crystal phase); 46 m x 0.25 mm ID OV-101; and 42 m • 0.25 mm ID SP-2340 | Columns were operated in either the split or splitless mode with helium as the carrier gas at a linear flow of 18 cm/sec. The columns were installed in a Varian | 3700 gas
2136
HEATH ET AL, TABLE 1. RELATIVE VAPOR PRESSURE AND HALF-LIVES CALCULATED FROM
EQUIVALENT CHAIN LENGTH ( E C L U ) OF COMPOUNDS CHROMATOGRAPHED ON LIQUID CRYSTAL CAPILLARY G L C COLUMN
Compounda
ECLU b
Relative vapor pressure"
Relative half-lives (days)
Sat-12 : Ac Z9-14:A1 Sat-14 : A1 Z 9 - 1 4 : OH Sat-13 : Ac Zll-14:OH Z 9 - 1 4 : Ac Sat-14 : Ac Z 1 1 - 1 6 : AI Sat-16 : Al Z i 1-16 : OH Sat- 15 : Ac Z 11-16: Ac Sat- 16 : Ac
1200 1213 1250 1296 t300 1306 1366 1400 1413 1450 1447 1500 1562 1600
0.4633 0.4062 0.2804 0.1769 0.1700 0.1601 0.0877 0.0624 0.0549 0.0379 0.0390 0.0229 0.0123 0.0086
38.1 43.4 63.1 100.4 104.6 111.1 203.7 287.1 327.4 475.8 461.0 788.4 1474.7 2164.6
aExample: (Z)-9-tetradecenal = ( Z 9 - 1 4 : A 1 ) , (Z)-9-tetradecen-l-ol = ( Z 9 - 1 4 : O H ) , (Z)-9-tetradecenyl acetate = ( Z 9 - 1 4 : Ac), (Z)- 11-hexedecenal = (Z 11-16 : AI), and (Z)- 11-hexadecenyl acetate = (Z 11-16 : Ac). bEstablished with saturated acetates. CCalculated from equations I and 2 in text.
chromatograph equipped with flame ionization detectors with a lower limit of detection of ca. 0.2 ng for hexadecane. Rubber septa (Cat. No. 8753-D22, A.H. Thomas Co., Philadelphia, Pennsylvania) were exhaustively extracted with CH2C12 for 24 hr and air dried a minimum of three days before use. A septum was loaded by depositing the desired blend, dissolved in 100/zl of hexane, into the cup of the large end. The quantity of pheromone loaded varied somewhat, depending on the number of blend components but was maintained between the limits of 400/zg and 600 #g per septum. The collection apparatus is an adaptation of a Grob and Zficher (1976) design and includes modifications by P.S. Beevor and coworkers (Tropical Products Institute, London, England) to collect pheromones from insects. We have adapted and further modified this method (Tumlinson et al., 1982). Briefly, it consists of a small charcoal trap prepared by sealing 3-5 mg of charcoal (Bender and Hobein AG, Zurich, Switzerland) between two 325-mesh stainlesssteel frits in a 6-ram OD, 3.7-mm ID Pyrex | tube. Air is delivered through the silanized glass aeration apparatus at a flow rate of 0.5-1 liter/rain. The septa are placed ca. 2 cm upwind from the charcoal trap and aerated for 1-2 hr. When
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2137
aeration is complete, the filter is rinsed with six aliquots (15-20 /zl each) of distilled dichloromethane, the aliquots are combined, an internal standard is added, and the solution is concentrated to about 5-10/zl by gently warming. Isooctane or another solvent of choice is added for analysis by capillary GLC with splitless injection. In a preliminary experiment, a backup charcoal trap was connected to the outlet of the primary trap. Extraction and analysis of the secondary trap resulted in no detectable amount of material. Additionally, recoverability of compounds from the charcoal trap was determined by depositing a known amount of the compound (usually as a blend with three other compounds) onto the charcoal directly in 20- to 50-ng amounts. The compounds were extracted after solvent evaporation and analyzed as previously described. Before each experiment, the aeration chamber was cleaned by rinsing with hexane, acetone, water, acetone, and hexane. The chambers were dried overnight at ca. 100~ The charcoal traps were cleaned by rinsing with ca. 4 ml CH2C12 and CS2. The traps were then dried using a flow of nitrogen. Determination of relative vapor pressure and half-lives of compounds used in this investigation are based on the previously determined equations (Heath and Tumlinson, 1986). The equation for the relative vapor pressure of a compound is (ln) vapor pressure = (ECLU - 1123)/-99.9
(1)
and the equation for a compound's half-life is (In) half-life = - 8 . 4 8 + 0.0101 9 ECLU
(2)
where ECLU is equal to the retention time of a compound on a cholesteryl-pchlorocinnamate capillary column relative to saturated acetates (Swoboda, 1962). To derive a formula for determining the release ratio from rubber septa of components in pheromone blends, several assumptions were made. We assumed that the relative vapor pressure of the solute in rubber septa could be approximated by using ECLU values obtained by chromatography on a liquid crystal stationary phase (isotopic phases showed no correlation) (Heath and Tumlinson, 1986). Thus in this assumption, the proportionality constant as required by Henry's Law is empirically determined, and the rate of evaporation of a compound from rubber septa is also correlated with the retention time of the compound in the liquid crystal phase. The ratio of a component in the vapor from a multicomponent blend is the percent of the compound in the liquid times its evaporation rate, divided by the sum of each of the components (in %) in the liquid multiplied by their respective evaporation rates. It is well documented that the evaporation rates of pheromones from rubber septa can be determined by measuring their half-lives and is a first-order equation (Butler and McDonough, 1979, 1981; McDonough and Butler, 1983). If we assume that the
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HEATH ET AL.
evaporation of one component will not affect the evaporation rate of another component, our formula for release ratio is the same as that using relative vapor pressure. However, in this case evaporation rate is in days, and we must convert all half-lives to their reciprocals for calculation of release ratios using equation (3). The calculation of the percent release of a compound when formulated in a blend on rubber septa is based on use of the calculated relative vapor pressure of the compound or calculated half-life of the compound. The equation for calculating percent release of a compound is RI =
LjP~ r LiP1 + L2P2 + L,,P~ r
(3)
where L is the percent load of the compound and pr is the reciprocal of the relative half-life (evaporation rate) or the relative vapor pressure of the compound. RESULTS AND DISCUSSION
The equivalent chain length, calculated relative vapor pressures, and calculated relative half-lives for all compounds included in this study are presented in Table 1. Various blends were prepared and the theoretical release ratio of each component was determined using equation 3. The volatiles released by the blend when formulated on rubber septa were trapped and analyzed. Prior to analysis of charcoal-trapped volatile blends, the aeration apparatus and charcoal traps were evaluated to determine the amount of material lost due to adsorption on glass surfaces or irreversible adsorption on the trap. The percent of each compound recovered when blends containing ca. 50 ng of each component were deposited directly on the trap and then reextracted without aeration is given in Table 2. The recoveries ranged from 74 to 94%, and data obtained in subsequent experiments were corrected to account for the losses on the trap. Analysis of the solvent rinses of the internal glass surfaces of the apparatus after experiments indicated that no measurable material was adsorbed on the silanized glass surfaces. This is probably because the septa were placed within 2 cm of the trap, and thus there was very little glass surface downwind of the septum for the volatiles to contact. Analysis of backup traps connected in series to the primary traps indicated no significant breakthrough of compounds from the first trap occurred. The various blends of compounds that were formulated and aerated are listed in Table 3. Different ratios of the same blend of compounds were analyzed to test the hypothesis that this method would be valid over a wide range of ratios. Volatiles from the septa were analyzed over a seven-day interval. Analyses were obtained on 1- to 2-, 3- to 4-, and 5- to 7-day-old septa (after
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TABLE 2. PERCENT RECOVERY OF COMPOUNDS FROM CHARCOAL FILTER
Compound"
Recovered (%)
Standard deviation
Replicate
Sat-12 : Ac Z 9 - 1 4 : AI Sat-14 : A1 Z 9 - 1 4 : OH Sat- 13 : Ac Z 1 1 - 1 4 : OH Z 9 - 1 4 : Ac Sat- 14 : Ac ZII-16:A1 Sat- 16 : AI Z 11-16:OH Zll-16:Ac Sat-15 :Ac Sat- 16 : Ac
91 78 76 76 94 80 86 87 76 74 79 85 91 87
8.5 14.19 9.5 8.7 8.4 1 ! .0 7.6 8.2 7.0 16.2 9.4 7.6 10.2 7.8
6 6 6 6 6 8 6 6 6 6 10 6 10 6
aExample, (Z)-9-tetradecenal = Z 9 - 1 4 : A1, (Z)-9-tetradecen-l-ol = Z 9 - 1 4 : OH, (Z)-9-tetradecenyl acetate = Z 9 - 1 4 : Ac), (Z)-11-hexadecenal = Z 11-16 : AI, and (Z)-11-hexadecenyl acetate = Z11-16:Ac.
the three-day aging process). The analysis of each blend at a given time interval was replicated at least three times. Because there was no significant change in the measured ratio during the seven-day interval, the data from all analyses were averaged and compared to the calculated theoretical release ratio. For blends that contained compounds for which the half-lives had been reported by Butler and McDonough (1979, 1981) and McDonough and Butler (1983), we calculated the predicted release ratio based on reported half-lives and compared the data with the predicted release ratios based on relative vapor pressure and with the measured release ratios (Table 3). An attempt was made to analyze a representative number of different combinations of molecular weights and/or functional groups to correlate as closely as possible with actual pheromone blends found in a variety of insects. The results obtained when C12-C ~4 saturated acetates were formulated as three-component blends in approximately equal amounts showed that the release ratio of the acetates could be predicted with less than 4% error (Table 3). When the 16-carbon acetate was formulated with other saturated acetates, a large difference was found between percent recovered and percent theoretical. McDonough and Butler (1979) found similar discrepancies with the half-life of the 16-carbon acetate when compared to shorter chain length acetates and suggested that the deviation was due to the inability of the larger acetate molecules to penetrate the rubber matrix. Another possible explanation is that the 16-
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HEATH ET AL.
TABLE 3. COMPARISON OF PERCENT LOAD, PERCENT RATIO CALCULATED, MEAN PERCENT FOUND AND STANDARD DEVIATION
Blend a
Load (%)
% Calculated using vapor pressure or calculated half life
Reported half-life b
Found (%) Mean"
SD
Sat- 12 : Ac Sat- 13 : Ac Sat-14 : Ac
41.1 28.8 30.1
73.8 18.9 7.2
74.4 19.8 5.6
70.1 21,5 8.5
2.2 1.4 0.9
Sat-13 : Ac Sat-14 : Ac Sat- 15 : Ac
37.7 31.1 31.1
70.6 21.5 7.9
77.5 17.8 4.6
70.2 21.4 8.5
1.5 1.6 1.9
Sat- I4 : Ac Sat- 15 : Ac Sat-t6:Ac
36.9 33.2 29.3
69.2 23.2 7.6
54.8 19.4 26.6
5.9 2.2 8.0
Z 9 - 1 4 : A1 Sat- 14 : A1 Zll-16:A1 Sat- 16 : A1
27.3 23.7 29.5 19.6
55.1 33.1 8.1 3.7
51.9 34.7 8.0 5.0
3.2 1,3 2.1 1.8
Z 9 - 1 4 : A1 Sat-14 : A1 ZII-16:AI Sat- 16 : Al
19.0 17.0 37.0 27.0
49.9 30.7 12.9 6.5
45.1 31.4 10.1 10.5
2.9 1.5 2.0 3.1
Z9-14:A1 Sat- 14 : AI Z 1 1 - 1 6 : A1 Sat-16 : AI
6.0 6.0 52.0 36.0
29.2 20.2 34.2 16.4
28.7 23.0 28.2 20.1
4.0 3.5 3.8 2.9
Z 9 - 1 4 : OH Sat-13 : Ac Zll-16:OH Sat- 15 : Ac
23.8 31.9 18.3 25.9
38.5 49:6 6.5 5.4
34.4 55.2 7.2 3.2
38.6 49.9 4.7 6.7
3.8 3.2 4.2 1.2
Z 9 - 1 4 : OH Sat- 13 :Ac Z 11-16 : OH Sat-16 : Ac
14.7 19.1 28.8 37.4
33.2 41.5 14.3 11.0
30.3 47.0 16.1 6.7
33.6 42.2 10.6 13.6
3.6 1.9 2.9 2.1
Z 9 - 1 4 : OH Sat- 13 : Ac Z I 1-16 : Ot-I Sat-15 : Ac
5.2 6.5 41.4 45.9
19.6 23.6 34.3 22.4
18.5 27.6 39.8 14.1
20.5 24.7 26.2 28.7
1.8 1.6 4.1 2.6
Z9-14:A1 Sat-13 : Ac ZII-16:AI Sat- 15 : Ac
26.3 25.6 25.3 22.7
53.1 35.7 6.9 4.3
50.9 32.8 12.2 4.1
2,4 3.2 6.1 2.2
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P R E D I C T I O N OF R E L E A S E R A T I O S
TABLE 3. Continued
Load (%)
% Calculated using vapor pressure or calculated half life
Z9-14 : A1 Sat- 13 : Ac Z11-16 : AI Sat-15 :Ac
15.2 15.0 39.7 29.9
45.0 30.8 15.9 8.3
48.2 26.8 16 9
3.8 4.8 3.1 2.0
Z9-14:A1 Z9-14:Ac ZII-16:A1 Z11-16 : Ac
2.7 66.4 3.6 25.7
16.5 76,8 2.6 4.1
17.3 (13.6)d 76.7 (76.9) 3.1 (6.7) 2.9 (3.4)
6.2 6.9 1.1 0.7
Z9-14 :A1 Z9-14 : Ac Z11-16: AI Z11-16: Ac
7.4 58.8 8.4 25.4
36.6 55.2 4.9 3.3
32.4 (32.0) 60.6 (60.3) 4.3 (5.8) 2.6 (1.9)
4.1 3.8 0.6 1.1
Blenda
Reported half-lifeb
Found (%) Mean"
SD
aExample: (Z)-9-tetradecenal = zg-14:A1, (Z)-9-tetradecen-l-ol = Z9-14:OH, (Z)-9-tetradecenyl acetate = Z9-14 : Ac, (Z)-I l-hexedecenal = Z 11-16 : AI, and (Z)-I 1-hexadecenylacetate = Z11-16:Ac. bCalculatedfrom reported half-lives(Butlerand McDonough, 1979, 1981; McDonoughand Butler, 1983). CMean is calculated from the average obtained from the 1- to 2-, 3- to 4-, and 5- to 6-day-old samples. Each mean represents a minimum of nine analysis. dAnalysisobtained on samples left in the field in parentheses. carbon acetate is a solid at room temperature, and our inability to predict the release rate may be due to the establishment of a solid-liquid phase equilibrium. The relative vapor pressure data obtained for the aldehydes using the retention time data obtained on the liquid crystal column enables us to predict relative release rates for the aldehyde blends with less than 6% error for any given component in the three blends investigated (Table 3). Greater variability occurred between calculated and measured released ratios when blends were composed of acetates and alcohols or acetates and aldehydes (Table 3). There are at least two possible explanations for this. The aldehydes and the alcohols are adsorbed more strongly by the charcoal and glass surfaces, and thus small but immeasurable differences could contribute to greater error when analyzing blends composed of compounds with two or more types of functional groups. Also, solutions of compounds with more than one type of functional group may differ more in physical chemical properties than do compounds with one type of functionality. Further investigation of this phenomenon is required to determine the actual reason for these discrepancies. Nonetheless, as a first approximation, the experimental results are very close to the calculated values, and this
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HEATH ET AL.
is the most accurate method developed thus far for predicting release ratios of components in pheromone blends. A greater disparity between the predicted release ratio and measured release ratio was obtained when reported half-lives (Butler and McDonough, 1979, 1981; McDonough and Butler, 1983) were used in the calculations (Table 3). It should be noted that the error is significantly reduced (equal to that observed using relative vapor pressure values) if the values for the half-lives are determined using equation 2. As an example of the practical application of this study, we formulated blends containing two aldehydes and two acetates (Z)-9-tetradecenal (Z914 : A1), (Z)-9-tetradecenyl acetate (Z9-14 :Ac), (Z)-I 1-hexedecenal (Z1116:A1), and (Z)-I 1-hexadecenyl acetate (Z11-16 :Ac) in two different ratios. All septa were analyzed in the laboratory on the first or second day after loading. Then one batch was placed in traps in the field for four days while an identical batch was aged in the laboratory during the same period and analyzed again at the end of three to four days. At five to seven days after loading, release ratios of the blends were again measured from both lab- and field-aged septa. The result of this study (Table 3) indicated that the placement of septa in the field for a five-day period did not increase significantly the variation of the release ratio of the compounds when compared to the lab septa. CONCLUSIONS
The purpose of this investigation was to explore the hypothesis that relative release rates (half-lives) and relative vapor pressure of individual compounds could be used to calculate the approximate release ratios of the compounds formulated as a blend in rubber septa. Our results indicate that, at least to a first approximation, our hypotheses enabled us to determine release ratios. Certain refinements in measurement techniques or more accurate correction factors for recoveries of alcohols and aldehydes may yield more accurate data. Additionally, our investigation was based on an empirical approach and does not provide fundamental reasons for the existence of the correlation of the retention time of a compound on a liquid crystal column and its predicted release ratio when formulated in a blend on rubber septa. Thus there exists a need to further define the mechanism of evaporation of pheromones from rubber septa. However, the use of the method described should greatly facilitate the determination of the percent load required to provide the desired release ratio of components in pheromone blends. REFERENCES BAKER, T.C., CARDE, R.T., and MmLER, J.R. 1980. Oriental fruit moth pheromone component emission rates measured after collection by glass surface adsorption. J. Chem. Ecol. 4:749758.
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2 143
BUTLER, L.I., and McDoNOUGH, L.M. 1979. Insect sex pheromones: Evaporation rates of acetates from natural rubber septa. J. Chem. Ecol. 5:825-837. BUTLER, L.I., and McDoNOUGH, L.M. 1981. Insect sex pheromones: Evaporation rates of alcohols and acetates from natural rubber septa. J. Chem. Ecol. 7:627-633. GROB, K., and Z(3CHER, D. 1976. Stripping of organic trace substances from water. Equipment and procedure. J. Chromatogr. 117:285-294. HEATH, R.R., and TUMLINSON, J.H. 1986. Correlation of retention times on a liquid crystal capillary column with reported vapor pressures and half-lives of compounds used in pheromone formulations. J. Chem. Ecol. 11:2081-2088. HEATH, R.R., BURNSED, G.E., TUMLINSON, J.H., and DOOLITTLE, R.E. 1980. Separation of a series of positional and geometrical isomers of olefinic aliphatic primary alcohols and acetates by capillary gas chromatography. J. Chromatogr. 189:199-208. HEATH, R.R., and TUML1NSON,J.H. 1986. Correlation of retention times on a liquid crystal capillary column with reported vapor pressures and half-lives of compounds used in pheromone formulations, J. Chem. Ecol. 12:2081-2088. HIROKA, Y., and SUWANAI,M. 1978. Role of insect sex pheromones in mating behavior. II. An aspect of sex pheromone as a volatile material. Appl. Entomol. Zool. 11:38-43. LEONHARDT, B.A., and MORENO, P.S. 1982. Evaluation of controlled release laminate dispensors for pheromones of several insect species, pp. 159-173, in B.A. Leonhardt and M.A. Beroza (eds.). Insect Pheromone Technology: Chemistry and Applications. American Chemical Society Symposium Series 190. American Chemical Society, Washington, D.C. KYDONIEUS, A.F., and BEROZA, M.A. 1982. Insect Suppression with Controlled Release Pheromone Systems. CRC Press, Boca Raton, Florida. McDoNOUGH, L.M., and BUTLER, L.I. 1983. Insect sex pheromones: Determination of half-lives from formulations by collection of emitted vapor. J. Chem. Ecol. 9:1491-1502. MITCHELL, E.R. 1981. Management of Insect Pests with Semiochemicals. Plenum Press, New York. OLSSON, A.M., JONSSON, J.A., THEL1N, B., and LILJEFORS,T. 1983. Determination of the vapor pressures of moth sex pheromone components by a gas chromatographic method. J. Chem. Ecol. 9:375-385. REGNAULT,H.V. 1845. Determination of vapor pressure using gas saturation. Ann. Chem. 15:129134. SWOBODA,P.A.T. 1962. Qualitative and quantitative analysis of flavour volatiles from edible fats, pp. 273-291, in M. van Swaay (ed.). Gas Chromatography 1962, Butterworth, London. TUML1NSON,J.H., HEATH,R.R., and TEAL, P.E.A. 1982. Analysis of chemical communications systems of Lepidoptera, pp. 1-27, in B.A. Leonhardt and M. Beroza (eds.). Insect Pheromone Technology: Chemistry and Applications. American Chemical Society Symposium Series 190. American Chemical Society, Washington, D.C. WEATHERSTON, J., GOLUB, M.A., and BENN, M.H. 1982. Release rates of pheromones from hollow fibers, pp. 146-157, in B.A. Leonhardt and M.A. Beroza (eds.). Insect Pheromone Technology: Chemistry and Applications. American Chemical Society Symposium Series 190. American Chemical Society, Washington, D.C.
