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Fruit Notes |
Two Odor Compounds Hold Promise for Increasing Trap Effectiveness for Plum Curculio |
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Fruit Notes |
Tracy Leskey, Max Prokopy, Anthea
Yanopolous, Margaret Young, Brian Hogg, Fidelma Boyd, and
Ronald Prokopy Larry Phelan |
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Fruit Notes |
There has been no trap developed for plum curculios (PCs) that successfully detects the beginning of PC activity in orchards each spring. Traps for other species of weevils such as the cotton boll weevil and the sugar cane weevil use a combination of attractive compounds present in host plant odors and weevil-produced pheromones to increase trap effectiveness. PCs are attracted to odors produced by their host fruit over short distances as reported in the 1996, 1997, and 1998 Winter Issues of Fruit Notes. Under field conditions, PCs are attracted to host fruit odors at distances up to 3 yards. Eller and Bartelt of Illinois found that male PCs produce an aggregation pheromone called grandisoic acid. Therefore, we decided to screen 18 of the 19 compounds present in plum odor that were identified by Larry Phelan's lab at Ohio State University in hopes of finding attractive compounds that could be used in combination with grandisoic acid to improve trap performance. Compounds were tested in the laboratory and in the field to identify those that were most attractive to PCs. Materials & Methods A profile of volatile compounds that comprises the odor of freshly picked plum fruit (2 weeks after bloom) was completed in the laboratory of Larry Phelan using a gas chromatograph and mass spectrometer. The compounds listed in Table 1 plus phenol were identified as comprising plum odor. All compounds were evaluated as potential attractants for PCs in laboratory and field experiments with the exception of phenol, which is highly toxic to mammals. Compounds were tested in the laboratory at three concentrations (1, 0.1, and 0.01%) and in the field at two concentrations (5 and 0.5%). PCs used in laboratory bioassays were collected from unsprayed wild plum and apple trees. For all laboratory tests, PCs were starved 24 hours prior to testing. Tests were conducted at the beginning of darkness. A 75 ul aliquot of the compound (the treatment) was pipetted onto a small cotton wick placed next to one of the two pipette tips that served as ports into a Petri dish test chamber. Either 75 ul of hexane or water (depending on the solubility of the compound) was used as a solvent control and was pipetted onto a second cotton square placed next to the other pipette port. Handling of PCs was kept to a minimum. A single PC was placed gently in the center of each Petri dish test chamber. Each time a specific compound was tested, 12 PCs were tested singly in individual dishes and kept together on a tray. Dishes were then moved immediately to the testing room. All bioassays lasted 2 hours, and all compounds were tested at least four times at each concentration (12 individual PC per tray x 4 trays = 48 individual PCs tested per compound and concentration). To measure the level of attraction to a particular compound (the treatment), we used a Response Index (RI). The RI was calculated by subtracting the number of PCs responding to the control (C) from the number responding to the treatment (T), dividing this amount by the total number of PCs tested each time, and multiplying by one hundred. Thus RI = [(T - C) / 12] x 100. The greater the RI value, the more attractive was the stimulus. In the field, green boll weevil traps were placed on the ground beneath the canopy of unsprayed apple trees approximately one yard from the trunk at each cardinal point. Compounds being tested were diluted in mineral oil to a 5% concentration and applied to a 3-inch piece of cylindrical cotton wick that was wrapped in aluminum foil and attached by a wire to a boll weevil cone-shaped trap top. One end of the wrapped foil cylinder was clipped to permit dissemination of odor. For each tree, two traps were baited with an individual compound and placed at north and south positions, and two traps baited with mineral oil only (the control) were placed at east and west positions. After 48 hours, the number of PCs captured in each of the traps was counted, traps were baited with fresh wicks, and positions of traps were rotated around the tree so that compound-baited traps were in east-west positions and control-baited traps were in north-south positions. This procedure was repeated 12 times. A second experiment was conducted using only the most and least attractive compounds at two concentrations: 5 and 0.5%. In this experiment, procedures were nearly the same as in the first experiment except this procedure lasted only 24 rather than 48 hours, and was repeated only 10 times. To measure the attractiveness of a particular compound, a Field Response Index was created by subtracting the number of PCs responding to the control (C) from the number responding to the treatment (T), dividing by the total number of PCs captured in the treatment and control traps, and then multiplying by 100. Thus, RI = [(T - C )/ (T + C)] x 100. The greater the RI, the more attractive the compound. Results Laboratory Results. For compounds at 1%, none of the RI values were significantly (Table 1). At 0.1%, ethyl isovalerate provided a positive and significant RI. Other compounds did not result in significant RIs at 0.1%. At 0.01%, ethyl isovalerate and limonene resulted in positive and significant RIs, and all others resulted in nonsignficant RIs. Field Results. In the first field experiment, the only significant RIs were recorded for 5% solutions of ethyl isovalerate and limonene (Table 2). In a second set of experiments testing ethyl isovalerate and limonene (the most attractive compounds from the first field experiment) and 3-hexanol (the least attractive compound from the first field experiment), significant RIs were recorded for 5% solutions of ethyl isovalerate and limonene, but not 3-hexanol at 5% or any of the three at 0.5% (Table 2). Conclusions Data obtained from our laboratory and field experiments are in agreement. Two compounds, ethyl isovalerate and limonene, proved significantly attractive to PCs under the test conditions described here. For the future, we plan to test these two attractive compounds alone or in combination with male-produced pheromone as baits for the pyramid traps described in 1997 and 1998 Winter Issues of Fruit Notes, and also for alternative trap designs including a circle trap and a twig-mimicking black cylinder trap to see if trap efficacy is increased. Acknowledgements This work was supported by USDA Hatch funds and by the New England Tree Fruit Growers Research Committee. We thank Jim Hardigg for allowing us to use his orchard for part of this work. |
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