Graduate Thesis Or Dissertation


Alternative Controls for Stephanitis pyrioides in the Pacific Northwest. Public Deposited

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  • The azalea lace bug (Stephanitis pyrioides Scott) is a global pest of rhododendrons and azaleas (Rhododendron spp.). It is originally from Asia, has been established in the eastern U.S. for the past century, and was recently detected in the Pacific Northwest in 2008-2009. Stephanitis pyrioides feeds on the underside of leaves, removing the chlorophyll from the mesophyll layer causing leaf chlorosis. It leaves unattractive residues on the underside of leaves as a result of frass deposition and molting. Rhododendron is an economically important genus for the nursery and landscape industries in the state of Oregon. Presently S. pyrioides is controlled using systemic insecticides, but the possible environmental impacts and possible development of resistance compel efforts to find alternative controls. Previous research has offered regional solutions which cannot be used at a large scale in Oregon. The research presented in this thesis aimed to add to the body of knowledge about alternative ways to control this pest. We set out to determine 1) if S. pyrioides infestations can be controlled using plant volatiles, a blend of herbivore-induced plant volatiles (HIPV) and floral volatiles, to attract naturally-occurring green lacewing, a known voracious lace bug predator, and 2) if Rhododendron spp. resistance to S. pyrioides can be enhanced with supplemental silicon. In the first study, two out of three different blends of plant volatiles attracted green lacewing compared to a control in farm landscapes, none in urban landscapes. The blends comprised of methyl salicylate + acetic acid +2-pheylethanol, and acetophenone + acetic acid + 2-phenylethanol. One out of four experiments that used the volatile blends to attract lacewing effectively controlled S. pyrioides using the blend with acetophenone (above). Other natural enemies were also monitored for attraction: the blends with methyl salicylate or acetophenone (above) recruited some predators and the floral blend with phenylacetaldehyde + methyl salicylate + acetic acid recruited some parasitoids, but associated reductions of S. pyrioides were not large enough to draw any correlations. In past studies, increasing host plant resistance by supplementing plants with elemental silicon has enhanced the defense systems of monocots and some dicots. The silicon may increase cell wall strength making it more difficult for herbivores to feed on or, may affect plant chemistry and palatability. Rhododendrons supplemented with silicon had reduced numbers of S. pyrioides eggs and frass spots in choice experiments with whole plants and detached leaves albeit post-supplementation tests showed no increase in silicon content. If rhododendrons are unable to absorb and accumulate supplemental silicon, a potential topical effect of foliar applications is conceivable but the mechanism for similar results obtained with soil applications is unclear. Tri-trophic interactions in varying landscapes are complex and examples of using plant volatiles to successfully manipulate natural enemies for effective biological control are still scant. This study is another example of that complexity. A more practical tool for the control of S. pyrioides in Oregon may be the use of silicon to enhance host resistance. Further research in this area is needed to bridge the gap between small scale experimental success and large scale, practical alternative controls.  
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  • Graham, K. V. 2020. Alternative controls for Stephanitis pyrioides in the Pacific Northwest. Master's thesis, Oregon State University, Oregon.
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  • Funding was provided by USDA CRIS 2072-22000-040-00D, OSU ARCO-Swallow Horticulture Research Fellowship and American Rhododendron Society.
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