A STUDY OF THE ABUNDANCE AND PATCHINESS OF CICADA NYMPHS (HOMOPTERA, TIBICINIDAE) IN A NEW-ZEALAND SUB-ALPINE SHRUB-GRASSLAND

Cicada emergence skins in a subalpine shrub grassland have been sample d during 1969-75 to determine the abundance and spatial distributions of nymphs feeding on plant roots. A guild of six cicada species is pri marily associated with two forms of vegetation: shrubs (Dracophyllum a nd Cassinia) and tall tussock (Chionochloa). Skin locations were mappe d relative to dominant vegetation species, litter zones, and soil and rock pavements over a range of aspects, altitudes, and vegetation type s, and sampling methods were scaled at four levels: the locality, plot , quadrat, and individual plant. There were significant differences in skin counts over four years, and different measures of mean skin dens ities are given for the four sampling scales. The two primary vegetati on types had cumulative 1969-72 mean densities of 5.2 +/- 4.0 and 12.9 +/- 10.0 skins/quadrat (2.3 m2), and the 1969-72 mean productivities of the upper 25% of quadrats (adjusted for percent ground cover) were, respectively, 5.5 and 35.5 skins/m2. These productivities are believe d to be conservative estimates of the maturing nymph numbers per indiv idual host plant over the span of one cicada generation. Over a 17-yea r span, such productivities lie within the upper range of mean densiti es recorded for 17-year periodical cicadas in the United States. As th e dominant subalpine vegetation species are very slow-growing, it is s uggested that high densities of nymphs feeding on root sap may affect plant vitality, although 1971/1987 comparisons of vitality in 52 Chion ochloa tussocks could not positively demonstrate a correlation across all data. Skin dispersion analyses indicated significant levels of pat chiness, in agreement with other nymphal studies and with known cicada oviposition behaviour. No single dispersion model fitted the data com prehensively, and it is suggested that a gradual shifting of the centr es of cicada aggregation may occur over a cumulative period of several generations.