Plant architectural barriers to feeding site selection by the meadow spittlebug, Philaenus spumarius (L.) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x346d778s

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  • While current theories describing the insect-plant interaction have emphasized the biochemical aspects of the relationship, morphological components can also play a significant role in determining which plants or tissues are susceptible to insect attack. Xylem sap on which spittlebugs feed may lack many of the plant compounds responsible for host selection and preference, and for this species architectural barriers may be more significant in restricting host plant utilization. This investigation examined the role of plant anatomical structures as barriers to the selection of feeding sites by the meadow spittlebug, Philaenus spumarius (L.). The distribution pattern of f_. spumarius on Anaphalis margaritaceae (D.C.) suggested that trichomes on the stem may restrict the first through third instar nymphs to feeding on the leaves, while tissue hardness may prevent nymphs from feeding on the lower stem. Fifth instar nymphs feeding on Medicago sativa (L.) may also be confronting a tissue hardness barrier on the lower stem. Caging experiments on hirsute vs. shaven stems confirmed that trichomes were a barrier to the first three instar nymphs at the apex of the plant. Depth of xylem elements and tissue hardness were not significant barriers to feeding. The mechanism of resistance appeared to be that trichome height exceeds the length of the nymphs' beak, and thereby interferes with the initiation of stylet penetration. Fewer nymphs were able to feed when caged at increasing distance below the terminal bud (DBTB). For A. margaritaceae, tissue hardness and the trichome layer were the barriers to feeding; for M. sativa, tissue hardness and decreased availability of xylem vessels reduced feeding. Stem segments within the cages were sectioned to determine which tissues were impeding stylet penetration. In A. margaritaceae the progressive lignification of the bundle cap and interfascicular region with increasing DBTB were the main tissues preventing stylets from reaching the xylem. In M. sativa on the other hand, the bundle cap and the interfascicular parenchyma were penetrable at maturity, but the increasing number of lignified fibers in the xylem prevented the stylets from reaching a xylem element. The predictive capability of a needle penetrometer was assessed by correlating feeding ability of fifth instar nymphs with tissue hardness measurements on the two hosts. Penetrometer measurements on the lower stem lacked sensitivity to tissues impeding stylet penetration, and were a poor indication of feeding potential in this region. Preference tests in the absence of trichome and tissue hardness barriers showed that the nymphs fed on normally restricted areas of the plant. Gradients in two parameters-of the spittlebugs' food niche, xylem sap tension and the concentration of amino acids in the sap, indicated that the preferred stem was more favorable in terms of xylem sap tension. While tissue hardness restricted nymphs from a portion of their preferred range of feeding sites, the uniform distribution of nymphs suggested that they either did not respond to variation in these parameters, or responded to a combination of parameters with opposite gradients.
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  • description.provenance : Approved for entry into archive by Deborah Campbell(deborah.campbell@oregonstate.edu) on 2013-08-13T21:44:05Z (GMT) No. of bitstreams: 1 HoffmanGeorge1983.pdf: 1708063 bytes, checksum: c6ae3eedf99212b77f8d08d55c3ac19e (MD5)
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