The relative impact of single vs. multiple agents on the biological control of Tansy ragwort (Senecio jacobaea) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0v838334s

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  • Field experiments were done to test the relative effectiveness of single and multiple biological control agents in reducing weed populations using two insects, the cinnabar moth, Tvria iacobaeae (L.), and the ragwort flea beetle, Lonqitarsus iacobaeae (Waterhouse). These two insects were released for biological control of the biennial weed tansy ragwort. Together, they had a greater impact on ragwort, at the experimental site, than either alone. Artificial defoliation simulated the cinnabar moth feeding behavior of defoliating and defoliating bolting plants in the early summer. Flea beetle larvae and adults were sampled to determine their seasonality and the plant parts and stages they most frequently fed on. Larvae fed internally in the leaf petioles and roots from October to July. Adults fed on leaves year round with their greatest activity occurring in late summer and fall. Flea beetles alone reduced vegetative plant densities by about 95%, but only with simulated cinnabar feeding did they affect flowering plants, reducing regrowth of leaves and flowers. Cinnabar moths alone had an effect only on flower and seed production. This combination of herbivores with little nicheoverlap, spread out attack over time, plant stages, and parts, and yielded stronger host population depression than either insect yielded alone.
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