Relative importance of abiotically induced direct and indirect effects on a salt-marsh herbivore

Few studies have attempted to determine experimentally the relative importa nce 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 relati ve importance of those effects. The direct effects of increased interstitia l soil salinity and nitrogen on the abundance of Pissonotus quadripustulatu s (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 fairy fly parasitoid Anagrus sp. nr. armatus (Hymenoptera: Mymaridae). Soil salin ity 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. Th ere was a significant salt X fertilizer interaction, suggesting that salt s tress 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 incre asing the frequency of B. frutescens stems that were softer and easier to p enetrate, increased the rate of parasitism. Changes in parasitism, however, did not affect P. quadripusrtuatus density. Tests for density dependence s howed that the results reported here were attributable to application of th e treatments and not to spatial density dependence. This study suggests tha t, in a stressful salt-marsh system, the direct effects of plant quality on herbivores are more important than indirect effects of plant morphology me diated by natural enemies.