A HERBIVORE PARADOX - WHY SALT-MARSH APHIDS LIVE ON POOR-QUALITY PLANTS

Plant variability and higher trophic levels can directly affect herbivore population dynamics, although syntheses of the relative contributions of these factors are uncommon. We studied the relationship between a New England salt marsh aphid, Uroleucon ambrosine (Thomas), its host plant in the marsh, Iva frutescens L., and its ladybird beetle predators. Bimonthly censuses on four different types of Iva revealed that aphids were most abundant on shorter plants that were physically disturbed by floating rafts of plant debris and ice and physiologically stressed by frequent saltwater flooding. Conversely, tall plants higher in the intertidal had fewer aphids even though these plants were more productive. Experimental manipulations showed that aphids had higher population growth rates on tall plants compared to all other plant types but that predation was greatest on the better-quality tall plants. This difference was simply due to ladybird beetles' landing on tall structures more often than on shorter structures. As a consequence of covarying plant quality and predator abundances, predators influenced herbivores directly by excluding them from tall plants as well as indirectly by restricting them to low-quality resources.