 |
|
|
||
|
Fruit Notes |
Plum Curculio Responses to Host Fruit and
Conspecific Odors |
|||
|
Fruit Notes |
Tracy Leskey, Amy Wiebe, Susan Nixson, and Ronald
Prokopy |
|||
|
Fruit Notes |
Many species of weevils are attracted to host plant odors and use them in host finding. Further, many species of weevils produce aggregation and/or sex pheromones. Plum curculios (PCs) have been shown to be attracted to host fruit odors in the laboratory over short distances and in the field at distances up to 3 yards. Further, a male-produced aggregation pheromone, grandisoic acid, was recently identified in PCs by Eller and Bartelt of Illinois. At present, a reliable monitoring system for detecting adult PC entry into orchards from overwintering sites does not exist. However, if attractive odors such as those from host fruit and/or pheromones were employed in conjunction with a trap that was also visually attractive to adult PCs, then a reliable monitoring device could be created as has been done for other species of weevils. In the 1996 and 1997 Winter issues of Fruit Notes, we reported on results of laboratory Petri-dish bioassays that addressed responses of adult PCs to odors emitted from McIntosh apple trees. Here, we present results from bioassays conducted in large Plexiglas arenas designed to assess PC attraction not only to fruit odors but also to odors emitted by other PCs. Materials and Methods Large clear Plexiglas arenas with dimensions of 24x24x12 inches with Plexiglas lids were used as still-air arenas for the following experiments. Materials to be tested as emitting potentially attractive odors were placed in small cotton bags hung in the upper corners (one per corner) of each box. Originally, we tried testing PCs with cotton bags placed in lower corners of arenas, but found that because of the natural tendency of PCs to crawl upwards, hanging the bags in upper corners was a more effective means of testing PCs. Either ten male or ten female PCs starved for 24 hours and chilled 30 minutes prior to testing were released into the center of a box at the beginning of darkness. Numbers of PCs that crawled to within one-half inch of an odor source held within a cotton bag were recorded after 1 hour. Each experiment was repeated three more times, each time rotating the position of cotton bags containing odor sources. Treatments tested as potentially emitting attractive odors included five freshly picked wild plums, five wild plums plus five male or female PCs, five male or female PCs alone, five wild plums alone, and five punctured plums. Punctured plums were used to simulate plums that had been fed upon by PCs because we wanted to learn if plums that had been punctured released odors that may be attractive to PCs. Each plum was punctured twice, one puncture made one hour before and one puncture made immediately before an experiment. An empty cotton bag served as the control in each experiment. Results presented reflect the mean number of PCs captured for each treatment over the four replications. Results Male Responses. In experiment 1 (Table 1), we used two arenas to test male response to the following treatments. In arena one, treatments included males, males plus wild plums, and wild plums placed in a small cotton bag, with an empty cotton bag serving as the control. Here, males responded in significantly greater numbers to cotton bags containing plums alone or males plus plums compared to males alone or the empty bag. Treatments tested in arena two included females, females plus plums, plums, and a control. In this case, males were attracted to females plus plums in significantly greater numbers than to any other treatment. In experiment 2 (Table 2), in arena 1 we then evaluated male responses to punctured plums compared to plums without punctures with two empty cotton bags serving as controls. Here, we wanted to learn if males were responding to odor emitted by punctures made in plums by feeding PCs. We saw no difference in response of males to punctured plums compared to plums not punctured, although there was a numerically greater response to the former. In arena two, we compared male responses to females plus plums, males plus plums and to two empty bags serving once again as controls. Significantly more males responded to females plus plums than to any other treatment, indicating that males may be responding to a female-produced odor. Female Responses. In experiment 3 (Table 3), we repeated treatments for female responders that we tested with male responders in experiment 1. In arena one, female responders were attracted to males plus plums in significantly greater numbers than to males alone or the control bag, with intermediate attraction to plums alone. In arena two, significantly more females were attracted to plums alone than to controls, with intermediate attraction to females plus plums. In experiment 4 (Table 4), we repeated the same treatments with female responders that we had tested with male responders in experiment 2. In arena one, females responded in nearly equal numbers to punctured plums and plums that had not been punctured. In arena two, numerically more females were attracted to males plus plums compared to females plus plums or to control. Conclusions We conclude that the Plexiglas arena bioassay system is an effective way to test PC attraction to host fruit odors as well as odors emitted by other PCs at distances of approximately 16 inches under still-air conditions. Both sexes were attracted to odors emitted by freshly picked wild plums. Perhaps the most important result from these experiments is an indication of the existence of a female-produced sex pheromone, as evidenced by the strong attraction of male PCs to the treatment that included females plus wild plums. Further, this putative female-produced pheromone may have been synergized or enhanced in attractivity to males by the presence of wild plum odor. Although we cannot rule out the possibility of sounds made by PCs as influencing these results, we could smell a unique odor in arenas that included treatments containing females plus plums. We plan to use this bioassay system extensively in 1998 to confirm these preliminary findings and in tests aimed at identifying the chemical nature of these attractive compounds. Acknowledgments This work was supported by Hatch funds and by the New England Tree Fruit Growers Research Committee. |
|||