Predicting numbers of an insect (Neophilaenus lineatus: Homoptera) in a changing climate.

Measurements of the density of an upland population of the spittlebug Neophilaenus lineatus (L.) (Homoptera, Auchenorrhyncha) were made from 1961 to 1997. Population change from year to year is shown to be largely density-independent. Analyses of meteorological records over the 37-year period showed that the weather variable with the highest correlation with population change was mean minimum temperature in September. A simple model was constructed relating numerical changes from 1961 to 1992 to temperature, and this was found to explain 75% of the change in this population. The model was used to predict numerical change from 1993 to 1997 over which period it explained 70% of the change. The model predicts that a projected rise in mean temperature of 1°C would increase the mean population density of N. lineatus by 50%. Experimental cloches at the upland site which raised mean temperature by 1°C, had N. lineatus densities which were 157% higher than in adjacent control quadrats. An experimental raising of field temperature at a lowland site where population change had been shown to be density dependent did not result in density changes, although dates of hatching were earlier.