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Plant Phenotype and Interspecific Competition between Insects Determine Sawfly Performance and Density

Mopper, Susan, Whitham, Thomas G., Price, Peter W.
Ecology 1990 v.71 no.6 pp. 2135-2144
Dioryctria, Neodiprion, Pinus edulis, fecundity, field experimentation, herbivores, host plants, interspecific competition, models, mortality, moths, phenotype, population growth, sawflies, sex ratio, trees
Host plants not only influence herbivore performance, they can also mediate interactions between herbivores. We conducted a 5—yr field study to test the effects of pinyon pine (Pinus edulis) phenotype and competitive interactions on the colonization success, mortality, fecundity, and sex ratios of a foliage—feeding sawfly, Neodiprion edulicolis. Our experiments revealed four major patterns. (1) Sawfly survival was significantly higher on susceptible trees than on resistant trees. (2) In contrast, sawfly fecundity was significantly lower on susceptible trees than on resistant trees. (3) Interactions with a stem moth, Dioryctria albovitella, caused significantly reduced sawfly fecundity on susceptible trees. (4) Sawfly mortality, rather than sawfly fecundity, was the dominant factor influencing population growth rates: after four generations, sawflies transferred to resistant trees. A model developed with performance information from our field experiments accurately described patterns of population increase in the field: sawflies rapidly attain high densities on susceptible tress, and accumulate slowly on resistant trees. Trees displaying a susceptible phenotype are therefore better hosts than resistant phenotypes, despite the negative competitive interactions that occur between sawflies and moths on heavily infested susceptible trees.