RELATIVE IMPORTANCE OF ABIOTICALLY INDUCED DIRECT AND INDIRECT EFFECTS ON A SALT-MARSH HERBIVORE

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 × 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.