Population Cycles Over a 16-Year Period in an Upland Race of Strophingia ericae (Homoptera: Psylloidea) on Calluna vulgaris

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Abstract
A psyllid, S. ericae feeding on phloem sap of C. vulgaris has a 2-yr life cycle in which the cohorts overlap at high altitudes. The dynamics of this species have been studied for 16 yr on the Moor House National Nature Reserve [UK]. Overlapping cohorts which appear to constitute separate gene pools coexist on the same Calluna shoots. Small individuals of each new cohort are likely to be exploiting the same resource as large individuals of the previous cohort. Each cohort set (odd or even years) showed 3 regularly spaced peaks of abundance during the 16 yr. When recruitment to a new cohort ended mortality became density independent up to the onset of winter. Mortality was very low during winter and there was little change in age structure from Nov.-March. Mortality from the 1st spring to the 2nd winter was density dependent. Again during the 2nd winter there was little mortality or change in age structure. Mortality from the 2nd spring to the end of the life cycle was density dependent. Log increase in biomass of a new cohort during its 1 full year was negatively correlated with the log total biomass of psyllids in both cohorts, but not significantly correlated with the log biomass of the new cohort. This interaction appeared to be the cause of oscillation in biomass from one year to the next. The density of psyllids on a 100 m2 area of blanket bog was halved in 1978 by a single application of malathion. Subsequent recovery of the population was compared with a nearby control area. Although experimental and control populations converged in density afer 1 yr, it took 2-3 generations before densities and age structures converged. S. ericae at Moor House shows population regulation which is largely due to interactions within and between cohorts and these appear to have a cyclical effect on population density.

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