Controlled Release of Sex Pheromone from Ester Wax/Polymer Bag Dispenser for Prevention of Grapholita molesta
Introduction
Grapholita molesta insects (GM) inhabit fruit trees (e.g., peach, apple, and pear), and their larvae destroy fruit.1 Although various pesticides have been used for prevention of GM, the larvae cannot be prevented from entering fruit.1 Sex-pheromone-mediated mating disruption (MD) has been implemented to reduce the larval population by various types of MD dispensers.2 The principle is that male insects follow ghost females with artificial pheromones released by an MD dispenser instead of actual female insects (Fig. 1).2 Because male insects have only one chance to mate during their life, an MD dispenser can steal the one mating opportunity.2
GM is active from April to September, and they produce a new generation four or five times per year.2 Therefore, a dispenser is required to release GM pheromone constantly for 6 months. Various types of MD dispensers have been commercialized such as reservoir, microemulsion, and wax.3 However, the duration of pheromone release from the MD dispensers is short and is insufficient to cover all generations of GM insects, and the release rate varies with climate changes.3
Paraffin wax and polyethylene polymer have been reported as the dispenser materials for sustained release of GM pheromone.4 Generally, the release rate of the active
Figure 1. (A) GM insect generations in 1 year. (B) The principle of mating disruption by pheromone dispenser. The male may either fly randomly or go to a dispenser instead of finding a female.6 Reproduced by permission from the Utah State University Extension IPM Program.ingredient can be controlled by the solubility with carrier materials or using a diffusion barrier.4 Thus, ester waxes (beeswax, Japan wax, and carnauba wax) that have a similar chemical structure with GM pheromone were studied to prove that the solubility of GM pheromone affects the release pattern of GM pheromone. In this study, four kinds of waxes and polymer film bags were screened as dispenser materials compared with paraffin wax and polyethylene film bag for controlled release of GM pheromone, and the best suited wax/ polymer bag group containing GM pheromone was selected to apply to a peach orchard in South Korea.2,3
Experimental
Paraffin wax, beeswax, and Japan wax were melted at 60°C, and carnauba wax was melted at 90°C; GM pheromone ((Z)-8-dodecenyl acetate) (5%w/w) was added to each melted wax while stirring. Each wax mixture containing GM pheromone was poured into a wax mold, cooled slowly at room temperature, and stored at 4°C until use. The melting point of each wax mixture was measure with a differential scanning calorimeter (DSC).
Four polymer film bags (polyethylene [PE], polypropylene [PP], nylon/polyethylene [NY/PE], and polyethylene terephthalate/ polyethylene [PET/PE]) were screened to select the optimal container in which to hold a wax mixture containing GM pheromone. The film thicknesses were as follows: PE and PP, 60 µm; NY/PE, 15 µm/60 µm; and PET/PE, 12 µm/60 µm. These films were used for control of GM pheromone release and protection against irregular weather.
The wax mixtures containing the GM pheromone were placed in headspace vials at 30°C. After 7 days, the quantities of GM pheromone in the air and wax matrix were measured by GC-MS. A GM pheromone release test was conducted in each wax matrix at 30°C in an incubator for 102 days in the laboratory. The quantity of GM pheromone released was determined by measuring the quantity of GM pheromone residue in each wax mixture over time using a GC system.2,3
The Japan wax/PP film dispenser containing GM pheromone was applied as an MD dispenser to a peach orchard in Hwaseong, Kyung-gi, South Korea. The MD effect of the dispenser was assessed by equation 1 based on the captured GMs in traps.2 A low number of captured males indicates effective control of GM in that plot.
MD effect (%) = [1 – (X/Y)] × 100 (1)
X = the number of males captured in MD dispenser plots
Y = the number of males captured in control plots
Results and Discussion
The solubility of the GM pheromone with each wax and degradation of the GM pheromone in the wax mixtures were confirmed by DSC, as shown in Figure 2A.3 The melting points of the paraffin wax and beeswax were 38.22–56.5 and 54.2°C, respectively, and Japan and carnauba waxes were 56 and 83.7°C, respectively. The melting point of the pure GM pheromone was –62.7°C. Paraffin wax mixture containing 5% GM pheromone showed peak separation of the GM pheromone and paraffin wax at –62.7 and 40–56°C, respectively. On the other hand, the GM pheromone was miscible in the beeswax and Japan wax mixture without any separated peaks. Although carnauba wax has similar chemical structure to GM pheromone, so that the solubility with GM pheromone was good, two small separated peaks were shown at –62.7 and –47.9°C. These two peaks were the degradation peaks of GM pheromone, which occurred because of high preparation temperature of the carnauba wax mixture, 90°C, rather than solubility.3
A headspace analysis was conducted before the GM pheromone release test, as shown in Figure 2B.3 The ratios of the quantity of GM pheromone in the air/wax matrix were calculated as 0.23, 0.16, 0.11, and 0.04 for the paraffin wax, beeswax, Japan wax, and carnauba wax mixture groups, respectively. Therefore, the order of the release rate was paraffin wax > beeswax > Japan wax > carnauba wax. Figure 3 shows the GM pheromone release rate of each MD dispenser.2,3 Each wax mixture containing 5% (w/w) GM pheromone was packed in four kinds of polymer film bags (PE, PP, NY/PE, and PET/PE). All wax/polymer bag groups were studied to find the optimal MD dispenser group for sustained release of GM pheromone under laboratory conditions. Female GM release the pheromone for 2–3 h/day at a rate of 8.48–25.3 ng/h.5 Therefore, the suitability of the wax matrix as a dispenser was determined by comparison with GM pheromone release rates between female GM and each wax/polymer bag group. In all NY/PE and PET/PE groups, a small
Figure 2. (A) Differential scanning calorimetry heating curves for the paraffin wax (PP), beeswax (BP), Japan wax ( JP), and carnauba wax (CP) mixtures including 20% (w/w) GM pheromone. (B) GM pheromone weight ratio in the air/wax matrix of a headspace vial at 30°C measured by gas chromatography– mass spectroscopy (each wax mixture contains 5% [w/w] GM pheromone). Reproduced by permission from the Polymer Society of Korea.3amount of GM pheromone that could not influence the MD effect was released and caged in the film bags. On the other hand, most of the wax/PP bag group showed a
Figure 3. (A) The release rate (µg/cm2 per day) of GM sex pheromone according to dispenser materials at 30°C. (B) Slope and average of pheromone release rate of A. Reproduced in accordance with the Creative Commons attribution non-commercial license.2zero-order release rate of GM pheromone, whereas the PE group showed an irregular release rate. Thus, the PP polymer film bag was chosen as the dispenser film bag. Although both beeswax/PP and Japan wax/PP dispensers showed the same zero-order release rate per hour (3.1 ± 0.3), the slope of the pheromone release rate of beeswax was –0.590 and Japan was –0.295 in a naked wax group test, as shown in Figure 3B. From the result, Japan wax showed a more zero-order release pattern than beeswax, so that was selected as the wax material carrier with PP film. As a result, the Japan wax/PP group was applied to the peach orchard for MD of GM.
Figure 4A shows an MD dispenser ( Japan wax/PP) and the control (delta trap containing GM pheromone), and the application was designed as shown in Figure 4B.2 A negative control group that had only a delta trap without GM pheromone showed no male insects on the sticky sheet of the delta trap. On the other hand, the control group captured 169 ± 17 male insects. In the control + MD group, 2 ± 1 male insects were captured. From these results, the Japan wax/PP dispenser showed a 98% MD effect for 5 months in a peach orchard using equation 1.
Conclusions
Figure 4. Application to peach orchard. (A) Mating disruption dispenser (MD, Japan wax/ PP) and the control (delta trap with GM pheromone). (B) The average trapped number (mean ± SE) of GM male insects in the negative control (only delta trap without GM pheromone), control, and control + MD groups for 5 months. Reproduced in accordance with the Creative Commons attribution non-commercial license.2Japan wax showed good solubility with GM pheromone through a single DSC peak without any separation compared with paraffin wax. The high solubility was correlated with sustained release of the GM pheromone; thus, Japan wax showed a zero-order release pattern. The release pattern of GM pheromone in the PP bag group was also consistent with time. Therefore, the Japan wax/PP group was selected as a dispenser material for MD of GM in this study and showed an MD effect of 98% for 5 months in a peach orchard.
Acknowledgements
This work was carried out with the support of “Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ01021302),” Rural Development Administration, Republic of Korea.
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