Relative Importance of Abiotically Induced Direct and Indirect Effects on a Salt-Marsh Herbivore

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Few studies have attempted to determine experimentally the relative importance of direct and indirect effects of host plants on herbivorous insects in the field. This study identifies important direct and indirect effects of a coastal plant on its most common insect herbivore and assesses the relative importance of those effects. The direct effects of increased interstitial soil salinity and nitrogen on the abundance of Pissonotus quadripustulatus (Homoptera: Delphacidae), which feeds on Borrichia frutescens (Asteraceae), are reported. Also reported are the indirect effects of these treatments on parasitism of P. quadripustulatus eggs laid in plant stems by the fairyfly parasitoid Anagrus sp. nr. armatus (Hymenoptera: Mymaridae). Soil salinity was experimentally elevated by the addition of salt pellets, and plant foliar nitrogen was increased by the addition of fertilizer.Both salt and fertilizer increased the abundance of P. quadripustulatus. There was a significant salt X fertilizer interaction, suggesting that salt stress may be more important when plants are more nitrogen limited. Salt, by increasing the frequency of tough B. frutescens stems, decreased the rate at which Anagrus parasitized P. quadripustulatus eggs. Fertilizer, by increasing the frequency of B. frutescens stems that were softer and easier to penetrate, increased the rate of parasitism. Changes in parasitism, however, did not affect P. quadripustulatus density. Tests for density dependence showed that the results reported here were attributable to application of the treatments and not to spatial density dependence. This study suggests that, in a stressful salt-marsh system, the direct effects of plant quality on herbivores are more important than indirect effects of plant morphology mediated by natural enemies.Key words: abiotic factors; Borrichia frutescens; community structure; direct vs. indirect effects; egg parasitism; herbivores, affected by host-plant variation; host-plant nitrogen; Pissonotus quadripustulatus; plant morphology, affected by salinity; plant quality, effects of salinity and nitrogen; saltmarsh community; soil salinity. of its most common herbivorous insect and assess their relative importance. Using the coastal salt-marsh plant Borrichia frutescens (Asteraceae), the direct effects of plant quality on the sap-sucking insect Pissonotus quadripustulatus (Homoptera: Delphacidae) were studied. At the same time, we investigated the indirect effects of host-plant morphology on these herbivores via plant-mediated changes in parasitism rates of herbivore eggs by the parasitoid Ana...

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