Adequacy of hand-defoliation of tansy ragwort (Senecio jacobaea L.) as a simulation of defoliation by cinnabar moth (Tyria jacobaeae (L.)) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4x51hm883

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  • Hand-defoliation was evaluated for its ability to simulate herbivory by cinnabar moth larvae, Tyria jacobaeae (L.) (Arctiidae) on the weed tansy ragwort, Senecio jacobaea L. (Asteraceae). The evaluation was done on a field population of flowering ragwort, for three different timings (early, middle, and late season) of damage. In the insect-defoliation treatments third and fourth instar larvae were introduced to plots of ragwort and allowed to consume foliage and flower heads over a 12- day period. In the hand-defoliation treatments leaf laminae were stripped by hand from the petioles, and all floral material was picked off during a single day that corresponded with the end of the insect-defoliation period. Both hand- and insect-defoliation resulted in low (0-20%) survival rates similar to that of undefoliated plants. Larvae sometimes left small amounts of foliage and flower heads on the plants, but these did not affect the regrowth response of the plant. Both damage methods yielded similar effects on the amount of secondary (regrowth) foliage, the timing of reproduction, and the number of secondary capitula (flower heads). There were small but significant differences between the 2 methods in the initial rates of regrowth, in stem and capitula height, and in biomass of stems. Extending the time period of hand-defoliation resulted in a stem height like that of insect-defoliated plants. Where simulation of herbivory over a wider range in times of attack is desired, provision must be made for plant parts that escape damage in very early (e.g.", basal leaves) and very late (e.g., mature capitula) times of attack. Observation of a natural population of cinnabar moth larvae revealed that the few larvae remaining late in the season may damage regrowth as it appears on some plants; repeated damage to regrowth could also be simulated by hand-defoliation. Hand-defoliation was judged to be adequate in simulating the effects of cinnabar moth damage on parameters affecting the birth and death rates of ragwort. The tests comparing the defoliation methods were sufficiently sensitive to detect the effect of the timing of damage on the number of secondary capitula and amount of secondary foliage.
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