ECOLOGY AND POPULATION BIOLOGY Effects of Aggregation Size and Host Plant on the Survival of an Ant-Tended Membracid (Hemiptera: Membracidae): Potential Roles in Selecting for Generalized Host Plant Use

Insect herbivores that are tended by ants exhibit a range in host plant use from specialists to extreme generalists. Potential factors that may inßuence relative suitability of different host plants include the presence or absence of ants and the size of aggregations formed by nymphs feedingonahost.WeusedamembracidÐantmutualism to testwhetherpresenceof ants or aggregation size (from 20 to 320 nymphs per plant) resulted in variable selection for host use by the generalist membracid Publilia modesta (Uhler) (Hemiptera: Membracidae) that feeds on host species Chrysothamnus viscidiflorus (Hook.) Nutt. and Wyethia spp., among others. Survival of nymphs as midinstars and the percentage of nymphs developing into adults on C. viscidiflorus were greater for nymphs in small than in large aggregations. Survival of nymphs asmid-instars onWyethia spp. was also greater in smaller aggregations, but the percentage of nymphs developing into adultswas independent of aggregation size. The presence of the tending ant, Formica obscuripes Forel, had no effect on the percentage of nymphs developing into adults during this experiment. The survival of membracids on C. viscidiflorus was always greater than survival of membracids on Wyethia spp., regardless of aggregation size or presence of ants; the favored host did not vary in a way that would explain the observed generalizedhost plant use.C. viscidifloruswas a superior host plant, because it senesced after nymphs became adults, whereasWyethia spp. senesced early enough that most nymphs died before reaching adulthood.