Niche Partitioning in Spittlebugs (Homoptera: Cercopidae) Sharing Shelters on Host Plants

Abstract

Resource exploitation was measured for two spittlebugs using shelters on host plants. Approximately 80% of the older nymphs of Philaenus spumarius and Lepyronia quadrangularis lived cradled in leaf axils. Manipulative experiments showed the preference for leaf axils over adjacent internodes is related to shelter rather than food, and that spittlebugs favor wide over narrow axils. Preference for a host species was correlated with the quality of shelter provided by the host's axils. Leaves reduce runoff and evaporation of spittle, the frothy excrement that surrounds and protects the nymph. Sampling of old—field vegetation indicated that the two spittlebug species concentrated in shelters at different heights in the vegetation; the larger species (L. quadrangularis) extended beyond the vegetation height range of the smaller (P. spumarius), and overlap between the species was greatest on short plants (<25 cm). Overlap was greater on tall (>50 cm) than intermediate (25—50 cm) stems. Tall stems offer less available stem length because hard lower stem tissues make stylet penetration difficult. The supply of shelters far exceeded demand even after excluding axils made useless by narrow width, trichomes, tissue hardness, and distance from the occupied site. Despite the abundance of empty axils, individuals of the same or different spittlebug species often lived together in the same axil. Crowding did not delay development or stunt growth; in field samples, the instar and dry mass of nymphs were independent of density over the observed range in density among spittlemasses (1—12 nymphs per spittlemass) and among stems (1—17 nymphs per stem). In accord with recent reviewers who conclude that competition for food rarely occurs among insect herbivores, this study demonstrates that use of shelters is similarly characterized by high overlap, undersaturation, and absence of density—dependent effects on growth and development.